Hi everyone, and welcome to Miss Estric Biology. And in this video, I'm going through the entire module 2 for OCR Biology, which is Foundations in Biology. And if you do need even more help, either with remembering the information or making notes, then I actually have a full set of OCR notes already made for you with all of the key terms included and highlighted. Well, now get yourself comfy, get something to write with, and let's get into it.

So first of all, there are three different types of microscopes that you need to learn about. Starting with the light microscope. And light microscopes have the lowest or poorest resolution. And that's because they have the longest wavelength of what creates the image, and that is light. Light has the longest wavelength compared to electrons. But the plus side is you can get living samples that you can examine, and you can get a color image as well.

The two electron microscopes are transmission and scanning. And for a transmission electron microscope, it has a high magnification and resolution. And to create these images, the electrons pass through the specimen to create it. The scanning electron microscope also has a high magnification and resolution, slightly lower than the transmission, but much higher than the light microscopes. And in these ones, the electrons bounce and scatter off the surface of the specimen to create a 3D image.

The resolution is the minimum distance between two objects in which you can still view them as separate. And in a light microscope or optical microscope, this is determined by the wavelength of the light. Whereas electron microscopes, it's determined by the wavelength of an electron. Magnification refers to how many times larger the image is compared to the actual object that you are viewing.

So with the light microscopes or optical microscopes, there are four types of slide preparation that you need to be familiar with: dry mount, wet mount, squash slide, and smear slide. The dry mounts are when thin slices or even the whole organism or specimen are viewed. And what you would do is place that specimen on top of your glass slide with just a cover slip placed on top. So this might be used if you're examining a really thin slice of plant tissue, or you might have tried this before with your own hair.

Wet mounts are more common, and you would have done this potentially in lower school when you looked at maybe a swab of your cheek cells or onion cells. And this is when the specimens are added to water or maybe a stain before you have the cover slip lowered on top with a mounted needle to prevent air bubbles forming. And if you were going to examine living aquatic organisms, you would have to do that as a wet mount.

Now, in A-level, you might be more familiar with a squash slide. So these are wet mounts, but then you would push down on the cover slip to ensure that you have a really thin layer of cells so that the light can pass through. So when you did your root tip squash practical, you would have taken a very thin slice from the tip of the root of an onion or garlic, placed the stain on top, squashed it by pressing down, and that way you should get light passing through. So you could visualize and see the chromosomes.

And then finally, we have the smear slides. These are created by placing a drop of the sample at one end of the slide and then using the edge of another slide held at an angle to smear the sample across that initial slide. A cover slip is then placed on top, and this is used when examining blood cells in a blood sample, for example.

Next, we move on to this math skill: the eyepiece graticule and calibrating it. So inside a light microscope, in the eyepiece, there is actually a scale that you can insert on a glass disc, and that is what the eyepiece graticule is. And the point of it is that when you look through the microscope, you can use that scale to measure the size of the object you're looking at. But there are different lenses on your microscope which are going to be causing different magnifications. So you might have times 10, times 40, times 100. And at each magnification, the value of one of the divisions on your eyepiece graticule will be different. So you have to know how to calibrate it at every magnification to work out what one division on your eyepiece graticule is worth.

So this is how you would do that. You would need to use something called a stage micrometer. And this is a glass slide which essentially has a ruler on it. And you would place that on the stage of your microscope. Look through your eyepiece and then move your stage micrometer so that it is aligned right next to your eyepiece graticule. So on this image, the circles are representing your field of view when you're looking down the microscope. The bottom scale is the scale on your stage micrometer, and that has been aligned against the eyepiece graticule, which is already within the eyepiece of the microscope.

Once you've then got them aligned, step two is to count how many divisions on the eyepiece graticule fit into one of the divisions on the stage micrometer. In this case, we can see two divisions fit into one of the stage micrometers. Now, on a stage micrometer, each division is worth 10 micrometers. That's why that value there says 10. It's one division, but that one division is worth 10 micrometers. So we can then use that to work out at this magnification, what is one division worth on the eyepiece graticule? So if we know that one of these divisions is worth 10, and two divisions fit into one of those, it's 10 divided by two. So we know that at this magnification, one of these divisions on the eyepiece graticule is worth 5 micrometers. So you can then measure your specimen to work out what is the actual size of your specimen.

Something else that you can do in terms of calculations with the light microscopes, but even with electron microscopes as well, is the magnification calculation. So you need to know that to work out the magnification, it's the size of the image divided by the size of the real object. And you might have to rearrange that formula to work out one of the other components of this equation. And it's never usually that straightforward. Often, you have to have changing of units involved as well. So we usually measure the size of an image in millimeters, and the size of the real object is usually in micrometers. So to convert your millimeters into micrometers, you would have to multiply by a thousand. So it's normally that additional step as well.

So staining is the next thing you need to know about. And some cell components are really difficult to see under the microscope unless you add a stain to make it a much more obvious color so it stands out. And differential staining is a technique which involves many chemical stains being used to stain different parts of the cells different colors, again, just to make it visually more obvious what you're looking at. So, crystal violet or methylene blue are two stains that are commonly used. And they are positively charged stains, and that means they'll be attracted to anything that is negative. So, negatively charged components of the cell, and they will stain those parts of the cell. You could then also add nigrosin and congo red, which are negatively charged, and therefore they actually can't enter the cells because the cytoplasm would repel it, and that would create a stained background a different color, and the unstained cells will then stand out.

Observations of structures from under a microscope can be recorded in the form of scientific drawings. And there are set rules that you need to know for how you create a scientific drawing, because these are very different to your typical artistic, shaded style drawings. So, number one, it should always be in pencil. Title the diagram to indicate what the specimen is. You have to state what magnification was used for the microscope and that you have drawn the image at. You should be labeling the key features as well. So for a high-power plan, this would be limited to just the nucleus, cytoplasm, and plasma membrane. You also have to annotate cell components, cells, and sections of tissues that are visible also, and stating what colors and shapes they are. So not necessarily their functions. Um, so then point six is the bit where we're saying it's not like an artistic diagram. So you don't do sketches. So you should use solid lines that touch, but there are no overlaps between those lines. Also, there should be no coloring or shading in. And then lastly, the label lines should be horizontal, drawn with a pencil and ruler, and not have arrowheads on them. So essentially, the aim of a scientific drawing is to show the size, location, and proportion, and annotating some of these structural details.

You could also be asked to do the scientific skill of scientific drawings, and these are very different artistic drawings. They have a complete set of rules, and it includes when you are looking at your specimen under the microscope, you would then try and draw it using, first of all, it has to be a pencil, a really sharp pencil so you can be accurate. You have to include a title of what you've drawn. You have to state the magnification you, or you could include a scale. You should always annotate the cell components, or the cells themselves, or the section of tissue that is visible that you've drawn. It shouldn't be sketchy lines. It has to just be solid lines that don't overlap with no gaps. And you mustn't do any coloring or shading. So essentially, the aim of these diagrams is to show the size, location, proportion. They are not artistic. So that's why we don't have any sketching, shading, or coloring. It's all about the facts: the size, shape, position, and labeling the structures. And although I've said this for microscopes, when you come on to do dissections, you could be asked to do scientific drawings of what you observe in your dissection as well.

So then we move on to the electron microscopes. And a beam of electrons is used here to create the image. And electrons have a very short wavelength. And that is why electron microscopes have a higher resolution. And because they've got a high resolution, small organelles and internal structures can be visualized. The image is created using an electromagnet. So that magnet is used to focus the beam of electrons. Now, you can't actually use living samples with electron microscopes. And that's because the air would absorb the beam of electrons. So your electron microscopes have to be used with a vacuum. So your specimen has to be in a vacuum so that the air doesn't absorb the electrons. The image is also in black and white. So you do have to add a stain to add any color, or you can do that artificially afterwards.

So transmission electron microscopes are one type of electron microscope. And for these ones, your specimen has to be very, very thin. You're using an extremely thin slice through your specimen. They're stained and then placed in a vacuum. An electron gun will then be used to release a beam of electrons. The electromagnet will focus that beam, and these transmit or pass through the specimen. Some parts of the specimen will absorb the electrons, and that will make them look darker in the image, and some parts won't, and they look lighter. So you get a 2D image because the electrons are passing through. And these are really useful for being able to see the internal structures of cells.

Scanning electron microscopes. In these ones, the specimen doesn't have to be thin because the electrons don't pass through the specimen. Instead, the electrons are beamed onto the surface, and the electrons scatter in different ways. So, they reflect backwards in different ways depending on the contours of your specimen. And this is what produces a 3D image of the surface of the specimen.

So the next bit of spec then is knowing the organelles you find in eukaryotic and prokaryotic cells. You need to know the structure and function. And here are the 13 organelles that you need to know for eukaryotic cells. And eukaryotic cells include animals, plants, and fungi. So here's a cross-section of an animal cell showing some of those structures. And here's a cross-section of the plant cell. So you can see the location and general shape of these organelles, but we're going to go through each of them in detail.

Starting with the nucleus. The key structures are that you have a nuclear envelope, which is this double membrane layer, and the holes that we can see within it are the nuclear pores, and that is what the mRNA can pass out of after transcription. There's also nucleoplasm, which is a granular jelly-like material in the middle. The chromosomes are found inside of the nucleus, and these are protein-bound. So wrapped around histone proteins, and they are linear. Lastly, we have the nucleolus, which is this smaller sphere inside of the nucleus, which is the site of RNA production, and it makes ribosomes. So the function then is, first of all, it's the site of DNA replication and transcription, and transcription is the first stage of protein synthesis when mRNA is created. It also contains the DNA for each cell, and it's the site of ribosome synthesis.

Flagella are not actually found on all eukaryotic cells. They're found on some, so for example, sperm cells, and it's this whip-like tail structure, and the function is for mobility. It is also sometimes as a sensory organelle for chemical stimuli.

Cilia again are not on all cells, and these are hair-like projections that come out of the cell itself, and they might be stationary cilia, or they might be mobile. A mobile cilia move to help sweep substances along, and that could be in the trachea, for example, they'll be moving in this wave-like motion to help sweep the mucus up and out of your trachea to prevent lung infection. Stationary cilia are important in sensory organs such as your nose.

Centrioles. These are made up of microtubules, which we can see here in this image, the microtubule, and they occur in pairs to form a centrosome. So that's what we're looking at here. We've got them as a pair forming that centrosome. Now, these are involved in the spindle fiber formation, which is essential in organizing the position of chromosomes in mitosis and meiosis.

The cytoskeleton is a network of fibers found within the cytoplasm all over a cell, and it consists of microfilaments, microtubules, and some intermediate fibers as well. Now, the function of this is it provides mechanical strength to cells. It helps to maintain the shape and stability of a cell, and many organelles are actually bound to that cytoskeleton to hold in a fixed place. Microfilaments are responsible for cell movement, and microtubules are responsible for creating scaffold-like structures. The intermediate fibers help to provide mechanical strength.

We then move on to looking at the endoplasmic reticulum. In terms of the structures, there's two types. We have rough and then we have smooth endoplasmic reticulum. Both of them have folded membranes which are called cisternae. But the key difference is the rough have ribosomes attached to the outside, whereas the smooth doesn't. The difference in function then links to the fact that the rough has the ribosomes because the rough endoplasmic reticulum has the function of protein synthesis for proteins destined to leave the cell, and the proteins are transported through the rough endoplasmic reticulum and into secretory vesicles. The smooth endoplasmic reticulum is where lipids and carbohydrates are synthesized and stored.

Next then we look at the Golgi apparatus and the secretory vesicles. This is the Golgi apparatus and the secretory vesicles here, which I've seen many students describe as looking a bit like a Wi-Fi symbol, and that's how they recognize it in comparison to the smooth endoplasmic reticulum. But what it is is folded membranes making the cisternae. You then have secretory vesicles that pinch off from that cisternae. The function then is it's where we have the addition of carbohydrates to proteins to form glycoproteins. You also have the production of secretory enzymes. Carbohydrates are secreted. We have the transport, modification, and storage of lipids. We get lysosomes forming. Molecules are labeled with their destination. And then finally, the finished products are transported to the cell surface membrane, which we can see here in those secretory vesicles, where they fuse with the membrane and the contents are released.

Lysosomes. So we just said lysosomes can be created by these Golgi apparatus. And lysosomes are vesicles, um, and they're bags or vesicles of digestive enzymes, and they can contain lots of different enzymes. Some functions include the hydrolysis of bacteria. So you have lysosomes fusing with phagosomes in phagocytosis, that releases the digestive enzyme to hydrolyze and destroy pathogens. They're also involved in completely breaking down dead cells. And then once they have hydrolyzed and digested whatever it is that they're breaking down, the lysosome will then fuse with the cell membrane, and that will release its contents to the outside of that cell.

Mitochondria are double membrane-bound organelles. So we have an outer membrane, and the inner membrane is shown here in yellow. It folds in to create the cristae. The mitochondria is the site of the fluid center in the middle is called the mitochondrial matrix, which is the site of some of the stages of aerobic respiration. And they actually also contain their own ribosomes and loops of DNA so that they can create the enzymes necessary for respiration inside of the organelle itself. So it's the site of aerobic respiration specifically. Therefore, it's the site of ATP production. And as I've already said, it contains the DNA to code for the enzymes needed for respiration. So the ribosomes it contains are the 70S ribosomes, which are much smaller, which are also the type that are found in prokaryotic cells.

So that leads us into ribosomes. Ribosomes are small. They're very, very small. You can see like these tiny dots in the microscope images. They're made up of two subunits of protein and RNA. So those are the two molecules it contains. There are 80S ribosomes, which are the larger ribosomes found in the cytoplasm of eukaryotic cells, or 70S ribosomes, which are much smaller, found in prokaryotic cells and also in mitochondria and chloroplasts for eukaryotes. And the function is it's where protein synthesis occurs.

Chloroplasts are found in plant cells, which are eukaryotic organisms. They also have a double membrane. And then inside, you have these foldings of even more membrane which are called the thylakoids. And these foldings stack up to look a bit like coins. And we call those stacks the grana for plural, or granum for singular. And you also have this fluid, which is shown in this beige-orange color, which contains lots of enzymes needed for photosynthesis. So this is the site of photosynthesis.

Cell walls are found in plants and fungi of eukaryotic organisms, not in animal cells. And in plants, you have microfibrils of the cellulose polymer. And fungi, you have chitin instead. So they're made up of two different molecules. Plants have cellulose for structural strength, whereas fungi have chitin, and that is still a polysaccharide, but it also contains nitrogen. The function of the cell wall, though, is to provide structural strength.

The plasma membrane is found in all cells, and this would form the cell surface membrane. It's made up of this phospholipid bilayer, and within that bilayer, you have other molecules embedded, such as channel proteins, carrier proteins, proteins on the outside which might act as receptors, glycoproteins which might act as receptors. You have cholesterol, which affects the fluidity and therefore the permeability of the membrane as well. And all of these structures together help to control what can enter and exit the cell, or it could be the organelle, because you do have these membranes on the outside of some organelles as well.

So, some of those organelles are involved in the production of proteins, and you do get long answer questions where it's in this topic, but they want you to link together everything you've learned to say which organelles are involved in the production and secretion of proteins. So, let's summarize that. Number one, the polypeptide chains are synthesized on the rough endoplasmic reticulum, or you could say the ribosomes, because those are found on the outside. Those polypeptide chains move to the cisternae in the ER, and they get packaged and and they get folded and packaged into vesicles to be sent to the Golgi apparatus for further modification. Now, they have to move through the cytoskeleton to get to that Golgi apparatus. Once they're inside the Golgi apparatus, the proteins are modified further and packaged into the vesicles, and those secretory vesicles carry the proteins to the cell surface membrane. The vesicle then fuses and releases the protein by exocytosis.

So looking at the prokaryotic cells then, in comparison to the eukaryotic cells, here are the key differences. First of all, prokaryotic cells are much smaller in size. They don't contain any membrane-bound organelles. Both have ribosomes, but prokaryotic cells have the smaller 70S ribosomes, whereas eukaryotic cells have the larger 80S ribosomes. The DNA in a prokaryotic cell is not contained within the nucleus. And prokaryotic cells do have cell walls, but they're made up of peptidoglycan. Whereas in eukaryotic cells, in plants, it'd be cellulose, and in fungi, it'd be chitin. Now, some prokaryotic cells have additional features, which are the plasmids, a capsule around the cell, and the flagella.

So let's have a look at some of those key differences. So we said there's no membrane-bound organelles. So that means they don't have their circular DNA in a nucleus. They don't have mitochondria, chloroplasts, a Golgi apparatus, and endoplasmic reticulum. They're much more basic organisms. The ribosomes, we said they do still have ribosomes, but they're much smaller. So they are the 70S ribosomes compared to the 80S ribosomes found in eukaryotic cells. They don't have a nucleus. So instead of a nucleus, they have their single circular DNA just free within the cytoplasm, and it is not attached to proteins. And that is what this image here is showing. Now, that is not the same thing as plasmids. Plasmids are additional loops of DNA that some bacteria have. They don't all have them, and they only have a few genes on them, and it's where you'd find the genes for antibiotic resistance. And bacteria either don't have them at all, or they might have them, but they have them in varying numbers.

The cell wall of a prokaryotic cell is made up of peptidoglycan, which is a glycoprotein. In comparison, eukaryotic cells, animals don't have a cell wall, but plants and fungi, which are eukaryotes, do. And plants have a cell wall that's made up of microfibrils of the cellulose polymer. And fungi have a cell wall made up of chitin, which is a nitrogen-containing polysaccharide.

The capsule, which again, only some bacteria would have. This is a slimy layer made of protein on the very outside, and the function is to prevent the bacteria from drying out or desiccating, but it also helps to cover the antigens to make it harder for the host's immune system to detect the bacteria.

The flagellum, which some bacteria have, and some might have multiples, some might only have one, some won't have any. Just like a flagellum in a eukaryotic cell, it rotates, and its function is to move the bacterium.

Part of this chapter is 2.12 Biological Molecules. Biological molecules all contain carbon, but below shows the elements that each molecule contains. So in addition to carbon, carbohydrates have hydrogen, oxygen, as do lipids, but just in a different proportion. Proteins also have nitrogen and sometimes sulfur. And nucleic acids also have nitrogen and phosphorus. Ions also play a really important role. And here is a long list of the ones that you need to know. 2025 update here. I have actually just edited some of the uses. So just check through those carefully to be mark scheme specific. So if you're going to be making notes or flashcards, definitely just double check that you're using the information from this slide. So the cations, ions which are the positive ions, and then the anions, which are the negative ions. The best thing to do to learn this would be to turn all of these into a flashcard. So have cation calcium ions on one side, and then have the information on the other side to show what the function of that ion is or what important role it plays. So for this bit, I definitely recommend you pause, turn this into flashcards, and then this will be the best way to get your head around all of this content.

So then let's take a look at water as a biological molecule. It's a polar molecule due to the uneven distribution of charge. And we can see here we've got delta negative on the oxygen and the delta positive on the hydrogen atoms, which means a slight negative and a slight positive charge. Now, because of that uneven distribution, that enables the formation of hydrogen bonds between the oxygen and a hydrogen atom between two different water molecules. And that's what we're seeing here, a hydrogen bond forming between the hydrogen and the oxygen of different water molecules. And a hydrogen bond is actually pretty weak in terms of bonds. But collectively, they do provide quite a lot of strength. And it's that structure and those hydrogen bonds which form all the different properties of water that we're going to have a look at. So these are the four that you need to know: as an important solvent in reactions, a transport medium, a coolant, and for providing habitats. So let's take a look at the details for each one.

Starting with water as a solvent. Because water is a polar molecule, that means that it can also interact with other polar molecules. And the reason for this is the slight positive charge on the hydrogen atoms will attract any negative solutes that have dissolved. And the slight negative charge on the oxygen atoms of water will attract any positive ions in the solutes. So if we think about, for example, sodium chloride, the chloride would be negative, the sodium would be positive, and that is why it dissolves so readily in water. Non-polar or hydrophobic molecules cannot dissolve in water and instead are actually repelled, and that's things like lipids. The cytosol in eukaryotic and prokaryotic cells is mainly water. So this ensures many solutes can dissolve within the cell and then be easily transported, which is why this is such an important property.

This leads us into the importance of water as a transport medium. Once those solutes have dissolved, they can then be readily transported around plants or animals. And if it's in animals, it'd be within the blood, and if it's in plants, it could be in the xylem or the phloem. Now, for this bit of the topic, it would mainly be the xylem that it's referring to, because that would be all of those ions that we saw earlier on, but glucose, sucrose, those sugars do also dissolve and they're transported in the phloem. Now, it is possible to transport these dissolved solutes in the xylem due to the cohesion that we saw in the water molecules. So we said that hydrogen bonds form between the hydrogen and oxygen atoms in different water molecules, and due to that, the water molecules stick together, and they form a continuous column of water. This is an advantage because as water evaporates out of the stomata in the leaves, that leaves a negative space, a negative pressure, and that negative pressure pulls on the continuous column of water, and it is easily transported up the stem because it is all stuck or cohesed together.

The next one is water as a coolant, and this is due to two different properties of water. First of all, water has a high specific heat capacity, and this means it takes a lot of energy to increase the temperature of water. You do learn that in more detail in chemistry, but that's sufficient for biology. The reason it requires so much energy is because energy is needed to break the hydrogen bonds between the water molecules to increase the temperature. Now, that's an advantage because it means internal temperatures of plants and animals remain relatively constant. So even if there is a fluctuation in the air temperature, that should mean that the temperature inside of an organism and in the cells remains relatively constant. And that's good because then enzymes are not going to denature, or if it gets really cold, they're not going to reduce inactivity too far. So essentially, water buffers temperature changes.

Now, the other property is the fact that water has a large latent heat of vaporization. And this means a lot of energy is required to convert water in its liquid state to a gaseous state, water vapor. And again, that's due to the hydrogen bonds. Energy is required to break the hydrogen bonds between water molecules to turn it into a gas. And that provides a cooling effect when animals sweat, for example, or even when plants transpire.

Water also provides a habitat. And we said previously that it acts as a buffer for temperature changes, and that is useful for any aquatic organism as well, because living in a body of water, if the temperature is buffered, then it should mean that your enzymes aren't going to denature if there are any significant fluctuations in temperature. The other property links back to the cohesion provided by the hydrogen bonds. Those hydrogen bonds not only create a continuous column of water in a xylem, but they also create a surface tension to the top layer of water molecules, and that enables small invertebrates to be able to move and even live on the surface, and that provides them with a habitat away from the predators within the water. Finally, ice is actually less dense than liquid water due to the hydrogen bonds, and as a result, ice floats on top of bodies of water, and that can provide a surface habitat for animals such as polar bears, and it also insulates the water below, keeping it liquid for aquatic organisms.

So now we're moving on to the next bit in this topic, which is looking at monomers and polymers. And mono means one, poly means many. So our literal definition is monomers are smaller units which can bind together to create larger molecules, or in fact polymers, and a polymer is made up of lots of monomers bonded together. Here are our examples of monomers and polymers that you need to know in this topic: glucose is a monomer which forms the polymer starch, cellulose, and glycogen. Amino acids is the monomer that forms the polymer protein, and nucleotides form the DNA and RNA polymers. 2025 edit, just to make you aware before you see me go through this whole flow diagram, you actually only need to know glucose and ribose on your spec. So although I'm going to talk about others, glucose and ribose are the two monomers or monosaccharides that you need to know for OCR 2025.

So if we have a look at the carbohydrates first, this is showing you an overview of all the carbohydrates that you need to know about. We already said that carbohydrates contain carbon, hydrogen, and oxygen. And we can split them into monosaccharides, disaccharides, and polysaccharides. Mono meaning one, saccharide is sugar. So it's just one sugar unit. And that would be your glucose, fructose, and galactose. And glucose is the main one that you learn about. Disaccharides. Di meaning two, is two sugars bonded together. And you'll be learning about sucrose, maltose, and lactose. Finally, polysaccharides means many sugars. And what you actually have is at least three sugars bonded together. In reality, it's going to be far more than three sugar units. And you'll be learning about starch, cellulose, and glycogen.

So here's alpha-glucose, one of the isomers, and it's C6H12O6. So it contains six carbon atoms, 12 hydrogen atoms, and six oxygens. And here is your structure. And this is the level of detail that you need to be able to draw it in. So the way that I always remember it is start off by having your hexagon, and you have an oxygen in the top right. Every other bend or angle in that hexagon is a carbon. Coming off carbon one, you have hydrogen on top, hydroxyl. Same for carbon two. Same for carbon four, but it's carbon three, which is the opposite way around. So the hydroxyl groups on top, hydrogen is on the bottom. Then we get to carbon five and six. So you only have a hydrogen coming off carbon five, and then carbon six is branched off the top, C6. You then would have a hydrogen here, hydrogen there, and a hydroxyl there, or in other words, CH2OH. So you do need to know how to draw alpha-glucose in this level of detail.

If we then have a look at our beta-glucose, which is our other isomer. We can see that for beta-glucose, which is over on this side, it's exactly the same with the exception of this. The hydroxyl and the hydrogen on carbon one, which is this first carbon in the ring, are the other way around. The hydroxyls on top this time. Now, that slight difference has a big impact on the position that bonds can form and therefore the overall shape, structure, and function of the polysaccharides.

Then we have the structure of ribose, one of the other monosaccharides you need to know. And monosaccharides can be categorized according to how many carbon atoms they contain. Now, ribose is classed as a pentose sugar because it contains five carbons. And we can see where we've got these numbers, that's indicating where the carbons are in the structure. So we've got carbon 1, 2, 3, 4, and 5. Glucose has six carbon atoms. That would be known as a hexose sugar. So where you see -ose on the end, that means it's a sugar. And then the prefix in front is to indicate how many carbons it contains. And here is our structure of ribose, the pentose sugar, which is a monosaccharide that you need to know the details of.

So next we move on to the disaccharides. Di meaning two, and we said it's made up of two sugar units or two monosaccharides. They're joined together by a glycosidic bond, which is formed during a condensation reaction. And a condensation reaction is when a water molecule is removed. A chemical bond is formed to create a larger molecule. And that's what we can see here. Glucose plus glucose forms a larger molecule, the disaccharide maltose, and water is produced. The other two disaccharides you need to know about are lactose and sucrose. All three of the disaccharides are made up of one monomer of glucose, and then it's the second one which is different for all three. Maltose is made up of an extra glucose. Lactose is made up of galactose. Easier to remember because lactose is in the name. And then sucrose, the second monosaccharide it's made up of is fructose.

The last extra bit of information that you do need to know about disaccharides is for the glucose molecule that is used to make each of these disaccharides, you need to know the isomer. So for maltose, it's two lots of alpha-glucose. Lactose, it's beta-glucose and also galactose. And sucrose, it's alpha-glucose plus fructose. So we said that disaccharides are made in a condensation reaction. That is a reaction we're going to see over and over in the biological molecules topic when making polymers and also when making other molecules. So, it's a key definition to learn, definitely one to put on a flashcard. So, a condensation reaction is joining two molecules together by removing water, and a chemical bond is formed. Hydrolysis reaction is the splitting apart of molecules through the addition of water, and a chemical bond is broken. Just an extra detail to make this really mark scheme specific with 2025 is you need to say that it is a water molecule for the condensation reaction and a water molecule for hydrolysis. Not just the addition of water or the removal of water. It is one water molecule per reaction.

Let's have a look at a condensation reaction in action. We've got a generic monosaccharide and another generic one over here. We aren't really shown any of the extra details, only where the bond is going to form, because that is the part of interest. So we can see here the hydroxyl group, that is where the water is going to be eliminated from, and in removing that water molecule, a bond forms between the carbon, oxygen, and carbon, and that in this case is a glycosidic bond. And we name the glycosidic bond by the position it is found. So a 1 to 4 glycosidic bond means it's a bond found between carbon 1 and carbon 4. And the way we number them is always starting from the first carbon after the oxygen in the ring. So this would be carbon 1, 2, 3, 4, 5, and 6. And same on this one, 2, 3, 4, 5, and 6. So this is a 1-4 glycosidic bond. The opposite of that, hydrolysis, would be breaking that bond through the addition of water. And in doing that, the water is added back in to those hydroxyl groups. And now we go back to having those two monosaccharides.

Next, we move on to our polysaccharide. And this is created by many condensation reactions between many glucose monomers. So just as a basics to start, starch is found in plants, and it's a store of glucose. Cellulose is also found in plants, but the function is different. It's for structural strength. Glycogen is found in animals, and this one is a store of glucose also. So this summary table goes through the structure and function of all three of those carbohydrates. Again, this would be a great slide to pause and turn into multiple flashcards. Each of these individual boxes could be a flashcard.

So let's have a look at starch first. The monomer is alpha-glucose. That is the isomer of glucose that this molecule is made up of. And because it's got alpha-glucose, it's able to form both 1-4 and 1-6 glycosidic bonds. Amylose is one of the polymers in starch, and that only has 1-4 glycosidic bonds. And due to that, amylose forms these long straight chains that then coil up to make a helix. So overall, it's got a helix shape. Whereas amylopectin is made up of 1-4 and 1-6 glycosidic bonds. And as soon as there's a 1-6 glycosidic bond, that results in a branch coming off. So amylopectin is a branched molecule. These two we'd already said on the slide before. So I'm going to skip on now to how the structure links to the function. So we said it's a store of glucose. The fact that the amylose forms a helix means you can compact the molecule to fit a large amount of glucose in a small space. The fact that amylopectin is branched increases the surface area for enzymes to attach onto the end glucose molecules, hydrolyze them off, breaking the bond, and that glucose can then be used in respiration. The final one is actually the same for all three because it is so large. It's insoluble in water, and that means it can be stored within a cell and not affect osmosis. So it's not going to cause the cell to swell and burst.

Cellulose is the next one. This is also found in plants, but the difference here is it is for structural strength. It has a very different structure, and that is all down to the fact that the monomer is beta-glucose. Because it's beta-glucose, we get 1-4 glycosidic bonds only. And that results in long straight chains of this beta-glucose molecule. Those long straight chains are held in parallel to each other, and many hydrogen bonds form between all of those chains. One hydrogen bond is weak, but because there are so many, it provides this collective strength, and those then form microfibrils, which combine to form a cellulose fiber. So the function then, linking to that structure, is the fact that there are so many hydrogen bonds provides that collective strength, and that is why cellulose within the cell wall helps to give structure and prevent the cell from bursting.

Lastly, our glycogen is also made of alpha-glucose, and it's very similar in structure to amylopectin in starch. It is composed of 1:4 and 1:6 glycosidic bonds, but it actually has far more 1-6 glycosidic bonds than amylopectin. So that means it's even more highly branched. Now, this is important because the fact that there's even more branches means there's an even greater surface area for the rapid hydrolysis back into glucose. So the glucose can be used in respiration. And because this is the store of glucose in animals, mainly in muscle and liver cells, if that glucose can be rapidly released, if an animal does need to run to either protect itself or to hunt, then it will have that glucose.

Our next biological molecule are the lipids. So these are macromolecules, but they're not polymers. They're non-polar molecules, so they don't have a charge. For that reason, they're insoluble in water, but they do dissolve in organic solvents such as ethanol. And they are hydrophobic, meaning they are repelled by water. Lipids are made up of two molecules: fatty acids and glycerol, and they do not form polymers. The two key lipids that you need to know about are the triglycerides shown here on the left, and phospholipids shown on the right. They are very similar in structure. They both have a glycerol molecule here, which is sometimes described as a glycerol backbone, and they then have fatty acids attached to them. A triglyceride has three fatty acids, whereas a phospholipid has two fatty acids, and instead of the third, it has a phosphate group attached to it. So the way that those molecules are made is very similar, but this one is just focusing on the triglycerides. That glycerol molecule binds to the three fatty acid chains through a condensation reaction. So water is eliminated, a bond is formed, and we've made a larger molecule. The bond that forms is an ester bond. So let's have a look at how that occurs. Here's the glycerol molecule, and then we have three fatty acids. Condensation reaction is the removal of water. So we need to have a look at where that water comes from. So it's coming from the hydroxyl group from the glycerol and from the fatty acid. When that is removed, we then have a triglyceride plus three water molecules that were removed. And here is the ester bond. And we have three ester bonds. We have one for each fatty acid that has been bonded on.

Fatty acids can be either saturated or unsaturated. A saturated fatty acid is when the hydrocarbon chain has only single bonds between the carbon atoms. Unsaturated fatty acid is when the hydrocarbon chain has at least one double bond between the carbon atoms. The properties of triglycerides do link to the structure. So first of all, the function is that they can transfer energy, and this is due to the large ratio of energy-storing carbon to hydrogen bonds compared to the number of carbon atoms. So a lot of energy can be transferred if that was to be broken down due to that high ratio of hydrogen to oxygen atoms. They can also act as a metabolic water source. And this is because triglycerides can release water if they are oxidized. And this is essential in animals such as camels that live in the desert where there's very little water. As lipids are large, hydrophobic molecules, they are also insoluble in water, and that means they're not going to affect osmosis. They're also relatively low in mass, so a lot can be stored in an animal without increasing the mass as much as muscle would, and therefore making them so heavy it could impact their movement.

So phospholipids are made up of one glycerol molecule, two fatty acids, and a phosphate group. The way they're formed is very similar. It will just be two condensation reactions instead of three, but we still get ester bonds forming, but it'll be two instead of three that we saw in the triglycerides. Now, because of this phosphate group, it results in very different properties. That phosphate group has a charge. It's a negative charge, and that means that the head, which is what we call this structure at the top, is hydrophilic. And if it's hydrophilic and it's got a charge, it can interact and attract water. The fatty acid tails are non-polar. So they don't have a charge. And for that reason, they're described as hydrophobic, and they repel water but mix with fats. Now, because of these two different charged regions, they are able to form the phospholipid bilayer. When you add phospholipids to water, because the heads are attracted to water, they will spin to be exposed on the outside. The tails are repelled by water. So they'll spin to be on the inside where they can interact with the other fatty acids, but they're not exposed to the water. And that's how we get the phospholipid bilayer that makes up cell surface membranes, but also some organelle membranes.

Cholesterol is our next biological molecule, and it is very different in structure because it is a steroid. Steroids have four carbon rings and a hydroxyl group at one end, and they have both hydrophobic and hydrophilic regions. Now, these are embedded in the cell membranes to impact the fluidity. They help reduce the fluidity of membranes at high temperatures, and they increase fluidity at low temperatures, and in that way, they help to control the movement across a cell membrane.

Proteins are our next biological molecule, and they are large polymers, and they are described as macromolecules. They're made up of the amino acid monomers, and here is our general structure of an amino acid. We have a central carbon with an amine group, a carboxyl group, a hydrogen, and then the R group is the variable group. And that means it is the part that changes in all 20 different amino acids. Proteins are organized into four different levels. They are first made by the ribosome and creates this polypeptide chain. And then we're going to look at how that gets folded, processed with different bonds holding the structures together at each of these levels of organization. So the primary structure is the order or the sequence of amino acids in a polypeptide.

chain. And you do have to emphasize sequence or order to get that mark.

The secondary structure is the further folding of the primary structure. And you can either get it folding into an alpha helix or beta pleated sheets. And these shapes are held in place by hydrogen bonds. We can see those just here. The hydrogen bonds form between the oxygen in a caroxile group and the hydrogen on an amine group. And that is what we can see down here. So it's between different amino acids between those two atoms and that holds that bet sheet and alpha helix in place.

The next level of organization is the tertiary structure and this structure is the further folding to form a unique 3D shape held in place by four different types of bonds. There are the hydrophobic and hydrophilic interactions and these are very weak interactions. The hydrogen bonds which we already saw and again those are quite weak. Ionic bonds are stronger bonds and those form between the R groups of the different amino acids and the dulfide bonds which are only sometimes present because there has to be a sulfur in the R group between two amino acids for this to form. So the ionic and the dulfide bonds form between the R groups of different amino acids. Whereas the d and the dulfide bond as we said only sometimes occurs if there is an R group that contains sulfur between two amino acids.

The last level of organization is the quarterary structure and this is when a protein is made up of more than one polyeptide chain which is shown here by the different colors. So we've got four polyeptide chains collected together to form one protein. And the key example that you learn is hemoglobin which is made up of four polyeptide chains. Now hemoglobin also has a prosthetic group attached to each polyeptide chain and that is the hem group and a prosthetic group is one that is not made up of amino acids and for a hemoglobin molecule the prosthetic group doesn't contain amino acids but instead it contains the ion. A protein that has a prosthetic group such as hemoglobin can be described as a conjugated protein which simply means a nonproin group has been added to it.

You can also group your proteins as either fibrous or globular based on their final 3D shape in the tertiary structure. A fibrous protein has polyeptide chains that form long twisted strands that link together. They're very stable structures insoluble in water and all of those three points collectively help to give structural properties. So fibrous proteins provide structural strength. For example, collagen which is found in your bones and keratin which is in your hair.

Globular proteins on the other hand are normally spherical in shape. They're relatively unstable. They are soluble and involved in metabolic functions. So for example, enzymes and antibodies and some hormones. And if you think about enzymes, we do say they're sensitive to certain conditions such as changes in pH and temperature. And that's what we mean by relatively unstable.

For the 2025 specs change, I've just added in some extra details here linked to the level of detail you need to know for fibrous proteins and globular proteins. So if you are making notes or flashcards from this video, use this slide for the level of detail that is required. So three key examples of fibrous proteins that you need to know are collagen, keratin and elastin. Collagen forms part of the skin, the tendons, cartilage, ligaments, bone and connective tissues in the bronkey, bronchioles and trachea. It's a quarterary structure protein because it contains more than one polyeptide chain and it contains three polyeptide chains and these are wound around each other like rope. The chains are held by hydrogen and covealent cross links between those molecules and the cross links are staggered for strength and chains like close to each other due to 35% of its amino acids being glycine which is the smallest amino acid. It's a very flexible molecule but it's not stretchy. Keratin is used to form hair, skin, and nails which all protect the body. It's important that it's insoluble. So these structures are not broken down by water in the environment. Elastin is our final example and it's a common fibrous protein and makes up the elastic fibers around the alvoli and blood vessels. For example, in the walls of the arteries, arterials, venules, and veins. It also allows these structures to stretch and recoil to their original shape and size.

The three globular proteins you need to know about are hemoglobin, pepsin, and insulin. Hemoglobin has been described previously. So it's a quarterary structure protein because it's got more than one polyeptide chain and in fact it has four. It has two alpha chains and two beta chains and each one has a hem group attached to it which is a prothetic group which is where the oxygen binds. Enzymes are also globular proteins. Pepsin is an example of an enzyme found in the stomach. It is a protease enzyme which can digest proteins using its specific shaped active site which is complimentary in shape to its substrate. Lastly then insulin which is also a globular protein and the hormone produced by the beta cells in the pancreas to lower blood glucose concentration. Its specific 3D shape is complmentary to the receptors on the cell surface membrane of the target cells which for insulin are liver and muscle cells.

Next, we're moving on to the biochemical tests for those biological molecules. Starting with the test for starch. To test for starch, you would add iodine solution, which is this orangey brown color. And if starch is present, then it would go bluey black color.

To test for a reducing sugar, you need to add Benedict solution and heat for 5 minutes at 80° C. And you do need to know the duration and the temperature. A positive test of observation would be that that solution goes from this blue color to either yellow, green, orange, or brick red. And the more red the solution is, the higher the concentration. You can actually also use reagent test strips. And this can be used to test for the presence and concentration as well of reducing sugars.

For non-reducing sugars, you would do this following a negative Benedict's test result, which basically means the Benedict solution remains blue. You would add hydrochloric acid and boil. And this is acid hydraysis. You'd be splitting apart or hydraying any disaccharides into the monossaccharides. You would then cool the solution and add an alkali such as sodium hydroxide to neutralize. You would then add Benedict's solution and heats for 5 minutes at 80° C. If it was a positive test observation, the solution turns from blue to green, yellow, orange, or brick red. The more red, the higher the concentration. You usually get at least orange or brick red because if you did have a non-reducing sugar such as sucros, you originally had just one sugar, sucros. But once you've hydrayed it, you've split it into two sugars. So you now have double the quantity of sugar present, which is why it should go a more red color. Ignore the fact that it says one next to all of these. It should be 1, 2, 3, 4, 5.

To test for proteins, you would add buret solution and make sure you are spelling that correctly. Buret, not buret, like a piece of apparatus using chemistry titrations. And then the positive test result would be it goes from a blue solution to that purple color solution.

To test for lipids, you would use the emulsion test. And to do this, you would dissolve your sample in ethanol first of all. Then you'd pour the sample on top of distilled water. And then finally a positive test observation would be a white emulsion forming.

You can also use calorimeters to get quantitative data instead of qualitative. So if you were using a calorimeter first of all you'd need to set the filter in the calorimeter. So set it to a particular wavelength of light. Then you'd calibrate to zero using distilled water to basically show that this is the amount of light that's absorbed when there is nothing in there that is providing a color. So you're calibrating it to show what zero is. Then you would insert your samples from your biochemical test. So for example, different concentrations of glucose with Benedict solution and filter to then remove the precipitate and what is left is what you would put in. You'd measure the percentage transmission of light and then finally draw a calibration curve using the results from known concentrations of glucose.

You can also use bio sensors and this is when you'd have a single strand of DNA or protein which is complmentary to the test sample and that is immobilized. When the sample is added, it will bind to the immobilized DNA or protein. This binding causes a change in a transducer and as a result an electrical current is released and this current is processed to determine the concentration of sample present.

Chromatography can also be used and these practical investigations are looking at the separation of proteins, carbohydrates, vitamins or nucleic acids conducted using thin layer chromatography or TLC or paper chromatography. In chromatography there's a stationary phase. So for example the silica covered plate or paper that does not move and there's a mobile phase and this is the solvent which does move. A concentrated sample of the biological molecule is placed 1 cm from the end of the stationary phase. The stationary phase is then placed in a beaker with less than 1 cm depth of the mobile phase which is a solvent. As the solvent moves up the stationary phase, it has an affinity for the biological molecules and dissolves them. And this carries them up the stationary phase. Molecules that are the most soluble in that solvent will be carried the furthest. And the molecules that are the least soluble in that solvent won't be carried as far. Once the solvent is carrying no more of the molecule, the plate or the paper is removed from the solvent and the distance traveled by the molecules is measured. So to identify which biomolelecules you have present on your chromatography paper, you can work this out by calculating the retention factor or RF value. And here's the formula for this which you need to know. It's the distance moved by the solute divided by the distance moved by the solvent. And by comparing the calculated RF values with RF values of known molecules in that same particular solvent, the biological molecules that you have present can be identified. So this is a technique that can also be used to test for biological molecules but also for drugs and contaminants in food.

Next move on to nucleotides and nucleic acids. So first of all nucleotides are the monomers from which nucleic acids such as DNA and RNA are formed and those nucleic acids contain nitrogenous bases which can be categorized according to their structure how many rings they have. So a purine has two carbon rings and that would be asinine and guanine. As we can see here in the diagram we've got the first ring and then the second ring. Whereas perramdines those are nitrogenous bases which only contain one ring within their structure and that would be cytosine and thamine and DNA and uricil for RNA. The rest of the nucleotide is made up of pento sugar which is ribos for RNA and deoxyibbos for DNA and then finally both would have a phosphate group. So the thymine or uricellin RNA is complmentary to the base adanine whereas guanine is complimentary to the base cytosine. And what that means is you'll always have a purine and a perramdine opposite each other in the double strand for DNA. And that ensures that the two strands are always equal width apart.

Both DNA and RNA nucleotides undergo condensation reactions to go from having the nucleotide to the polymer chain. And it's a phosphodiester bond that forms between the adjacent nucleotides to create that polymer. The polymer of these nucleotides is called a poly nucleotide. And that phosphodiester bond is a really strong covealent bond. And that forms between the pentos sugar which would be deoxyibbos and DNA or ribos and RNA and the phosphate of a different nucleotide. So it' form between the phosphate group and the pento sugar.

Next we have a look at ATP and this is very similar in structure to a nucleotide. It contains a pento sugar which is always ribos and it contains a nitrogenous base which is always adinine. Adinine and ribos together make adenosine. And that's why ATP stands for adenosine triphosphates because we also have three phosphate groups attached. And those three phosphate ions or groups play a significant role in energy transfer. And that's why ATP is essential for metabolism. It's an immediate energy source providing energy for a whole range of different reactions. Now ATP is made during respiration both aerobic and anorobic but most of it is made in aerobic respiration and this is through a condensation reaction using the enzyme ATP synthes. So here we see ADP plus an inorganic phosphate. That's what PI stands for. And that forms ATP plus a water molecules released because it's a condensation reaction. This is a reversible reaction because ATP can be hydraized using the enzyme ATP hydraase. And in that case, the enzyme plus the addition of water would split one of the bonds between the phosphate groups that releases a small amount of energy and therefore we get ADP plus PI. Now, as well as releasing a small amount of energy, the inorganic phosphate group that's been released can then be bonded onto a different compound. And in doing that, it makes the compound it's bonded to more reactive. And we call that phosphorilation. And this is actually an example of phosphorilation. ADP is phosphorolated to form ATP. And that makes ATP more reactive. It has more energy.

So going back to DNA, deoxy ribboucleic acid codes for the sequence of amino acids in the primary structure of a protein. And it's the primary structure of a protein which determines the final 3D shape and structure of a protein. The polymer, the DNA polymer forms a double helix made of two antiparallel strands and those two strands are joined together by hydrogen bonds that form between the complimentary bases. So if we have a look at how the DNA structure relates to its function, it's a very stable structure and that is because of the phosphodiestester bonds that form between the adjacent nucleotides creating what we call the sugar phosphate backbone and those covealent bonds mean that the whole structure is very stable. The fact that it's double stranded is advantageous when it comes to DNA replication means that both of those strands can be used as a template. The fact that there are weak hydrogen bonds between the complimentary bases is an advantage for DNA replication as well because it means that very little energy is required to break those bonds, separate the two strands and therefore have those two strands acting as a template. It's also a large molecule and that means it can carry a lot of information. Finally, the fact that there is those complimentary base pairing between adinine and thymine and guanine and cytosine means that identical copies can be created when DNA is replicated.

Now, DNA can be precipitated out of cells to be examined. And that's one of the methods that you need to be aware of. So, you can extract it from plant material using this method. First of all, you'd have to homogenize the cell. So you'd have to break it open with a blender and also a detergent. That will break open the cells and cell membranes will also be broken open by the detergent and that releases the contents of the cells. You would then filter to remove any large debris. Add salt to break hydrogen bonds between the DNA and water molecules. Add proteas to digest the proteins, those histone proteins associated with the DNA. and then add ice cold ethanol to precipitate out the DNA from the solution. And the DNA appears as white strands like we can see just here in the image.

So next we move on to more details on RNA and the polymers of RNA. There are three types M, T and R. And if we start with R, the R stands for ribosomeal. And that ribosomal RNA is what ribosomes are made up of. It's the main bulk of a ribosome. The ribosomes are also made up of protein. So those are the two components rna and proteins.

mRNA is a copy of a gene from DNA. It's created in the nucleus and then it leaves via the nuclear pore to carry a copy of the genetic code of one gene to a ribosome in the cytoplasm. Now it's much shorter than DNA and the reason for that is it's only a copy of one gene. Human DNA, for example, consists of approximately 23,000 genes. Whereas mRNA is only a copy of one of those genes. So, it's going to be much shorter and therefore it's small enough to come out of the pores in the nuclear envelope. It is short-lived though and that's because it is leaving the nucleus and entering the cytoplasm. And in the cytoplasm, there are enzymes that can hydrayze the polymer. And that is why we do not want the DNA leaving the nucleus because if it did, your hard copy of a genetic code is at risk of being hydrarolyed and broken down. Whereas mRNA, it's shortlived. It just needs to survive long enough to be involved in protein synthesis. Then it's digested by the enzyme and those nucleotides can be recycled to make another mRNA strand. So the mRNA is a copy of one gene of DNA but it's a single strand and every three bases in the sequence codes for one specific amino acid and these three bases are known as a codon. So a codon is three bases on mRNA that codes for a specific amino acid.

tRNA is found in the cytoplasm. It's also a singlestranded molecule, but it gets folded to create what we call a cloverleaf shape and it's held in this cloverleaf shape by hydrogen bonds. Its job is to transfer or bring specific amino acids to the ribosome. And that's what we have here, an amino acid attached at the top to this binding site. Now, it's specific because it is determined by the three bases on the bottom of the tRNA called an antic-codon. And these three bases are complementaryary to a particular codon on the mRNA sequence.

Next we move on to semiconservative DNA replication. DNA replication is described as semicconservative because in replication one entire strand of DNA is conserved and one entire new strand is created from new nucleotides. Copy errors in DNA replication can occur but they occur randomly spontaneously and they result in a change to the DNA base sequence and that's what a mutation is. That would be an example of a gene mutation. DNA replication occurs in Sphase within interphase of the cell cycle.

And when describing the DNA double helix the top and the bottom of each strand have a particular term we call it the three prime and this symbol here means prime. So threep prime end or the fivep prime end. And this is determined by which carbon within the deoxyibbbo sugar of the nucleotide is closest to the top or the bottom. So this side we're told it is the three prime end. If we zoom in to see that in more detail, what we mean by that is the number carbon that's most exposed. We've got carbon one, carbon 2, carbon 3, carbon four, and carbon five up here. So the bottom of this chain, carbon three would be described as being the most exposed. So this would be the three prime end of this chain. Whereas this chain, it's this carbon most exposed at the end, which is our carbon five. So that's what this three prime five prime refers to. And it's a way to describe the top and the bottom but take into account the chains are antiparallel so they do look slightly different.

The enzyme that catalyzes DNA replication is complimentary in shape to the three prime end. And that is the relevance of this to semicconservative DNA replication. That enzyme can only bind at the three prime end of a chain and then it will move along towards the five prime end. So the key stages in DNA replication are first of all the enzyme DNA helilicase breaks the hydrogen bonds between the complimentary bases of the two DNA polymer chains and that causes the two chains to separate apart. Both of those strands then act as a template for the DNA replication and free floating DNA nucleotides will align opposite their complimentary bases on both of those template strands. And within the nucleus there are free floating DNA nucleotides. That's where they come from. Hydrogen bonds will then form between the complimentary bases of this new strand and the old strand. And then DNA polymerase is the enzyme that will attach to join together adjacent DNA nucleotides and that means it is forming the phosphodiestester bond between the nucleotides to create the new polymer chain.

Properties of the genetic code then there are three special features. It's degenerate, it's universal and it's nonover overlapping. Degenerate means that amino acids are coded for by more than one triplet of bases on DNA. Universal means the same triplet of bases codes for the same amino acid in all organisms. And non-over overlapping means each base in a gene is only part of one triplet of bases that codes for one amino acid. So each codon or triplet of bases is read as a discrete unit. Now the reason these different properties are advantageous are first of all the fact that the genetic code is degenerate means even if a gene mutation occurs changing one of the bases in a triplet it might mean it still codes for the same amino acid and therefore the mutation would have no impact on the final sequence of amino acids in the polyeptide chain and therefore the protein shape and function. Universal is advantageous in genetic engineering. So it means that we are able to remove a human gene for example the human gene for insulin and insert it into the plasmid of a bacterium and therefore the bacterium will make human insulin. Non-over overlapping is also advantageous linking to the concept of mutations. The fact that each base is only part of one codon means that if there was a mutation in that codon, which meant it now coded for a different amino acid, the mutation will only affect one codon. So in your whole sequence of amino acids that get coded for, only one would be incorrect. And that should reduce the overall impact that the mutation has.

And that leads us into this concept of protein synthesis. Proteins are created on the ribosomes of the rough endopplasmic reticulum in two stages. Transcription happens first and this is when mRNA is created from a copy of one gene on DNA and then translation is the second stage and that's where the mRNA has left the nucleus. It attaches to a ribosome and it is used then to create a polyeptide chain. And that's what we're going to have a look at these two processes in detail. There are a couple of key terms though that you need to be familiar with to fully understand this topic. And the first is intron and exxons. Intron are the sequences of bases in a gene that do not code for amino acids and therefore they don't get coded for to create anything in the polyeptide chain. And in fact, they get removed out of the mRNA after it's been transcribed. and we call that splicing. The introns get spliced out of the mRNA. Exxons are sequences of bases in a gene that do code for sequences of amino acids. So those are the coding sections on your mRNA. Then we have start and stop codons. And at the start of every gene, there is a start code on meaning three bases. And those bases enable the ribosome to attach to the mRNA sequence and that initiates translation. At the end of every gene, there are three bases that do not code for an amino acid and that is the stop code on. And when the ribosome reaches those three bases, because it doesn't code for an amino acid, there's no corresponding tRNA molecule and it causes the ribosome to detach from the mRNA and therefore it ends translation.

Transcription is the first stage in protein synthesis and this is the process in which a complimentary mRNA copy of one gene on the DNA is created in the nucleus and that can then leave the nucleus and be used in the next stage translation. So let's go through what the key marking points would be for the process of transcription which happens in the nucleus. First of all, for our DNA molecule, hydrogen bonds are broken between the DNA bases and that causes the double helix to unwind like we can see here and the two strands to separate and one of those strands then acts as the template for the creation of the new mRNA strand. Then we have these three mRNA nucleotides within the nucleus and they will align opposite the template strand according to their complimentary base pairing. But bearing in mind that we have uricil instead of thymine in RNA. So if you have an adinine base then the complimentary base pair for that is uricil. The enzyme RNA polymerase is the enzyme responsible for catalyzing the joining of those mRNA nucleotides. The text is cut off a little bit here, but it does say condensation reaction. So that joining together of those adjacent nucleotides is catalyzed by RNA pymerase. It's a condensation reaction and it creates phosphodiester bonds. So that is how we create our mRNA molecule. And once it's copied, the mRNA is modified and then it can leave the nucleus through the nuclear envelope pores.

Second stage of protein synthesis which is translation. So that modified mRNA has left the nucleus through the nuclear pore and then it attaches to the small subunit of the ribosome at the start codon. The tRNA molecule with the complimementary antic-codon to the start codon aligns opposite the mRNA and that is held in place by the ribosome and as we can see here the ribosome can hold two tRNA molecules at a time and once the peptide bond is formed the previous tRNA molecule is detached and the ribosome then moves along. So that's what we can see in that image. And forming that peptide bond requires an enzyme and also ATP. So the ribosome continues to move along the tRNA enabling the next complimentary tRNA antic-codon to align to the next complimentary codon on mRNA and it continues until the ribosome reaches a stop codon which causes the ribosome to detach and it ends translation. That polyeptide chain that is now created then enters the GGI body for folding and modifications.

The next part of this topic is enzymes. Enzymes are biological catalysts made up of globular proteins. The active site which we can see here is a specific and unique shape due to the folding and bonding in the tertiary structure of a protein. Due to that specific shape active site, enzymes can only attach to substrates that are complimentary in shape. So we describe them as being specific. Enzymes catalyze both intracellular and extracellular examples. And intracellular means inside of a cell. Extracellular means outside of a cell. So for example, catalase is an intracellular enzyme inside liver cells that breaks down hydrogen peroxide into oxygen and water. Trison is an extracellular enzyme in the small intestines that hydrayes proteins. And the way that enzymes are catalyzing reactions is they are lowering the activation energy. So all reactions require a certain amount of energy before they occur and that's what the activation energy is taking us into chemistry. Now when the enzymes attach to the substrate they lower that activation that is needed and that is why the reaction speeds up. So what you need to be able to do is explain how enzymes lower the activation energy. And there's two different hypotheses that went about explaining this. The lock and key is the older model and this model suggests that the enzyme is like a lock and that the substrate is like a key and that key is perfectly complimentary in shape. So it fits into the lock. In this analogy due to the enzyme specific tertiary structure, it's completely complimentary. So it can slot that substrate into the active site. And the theory here is stating that when we then get these enzyme substrate complexes, the charged groups within the active site were thought to distort the substrate and therefore it lowered the amount of energy required to break the bonds in the substrate. Now since we've started to learn more about the molecular structure of proteins and understanding that they are actually slightly flexible and can move that hypothesis was updated. So the induced fit hypothesis is the current accepted model and this one suggests that an enzyme is like a glove and the substrate is like your hand. And by that we mean the glove and the hand are not perfectly complimentary in shape until you put your hand inside the glove. So the induced fit model is when the enzyme active site is induced, meaning it's caused to change shape around the substrate. So initially the substrate and the active site aren't perfectly complimementaryary in shape but when the substrate collides into it it induces that active site to mold around the substrate and then it does become perfectly complimentary. Now when that happens that enzyme substrate complex occurring puts strains on the bonds in the substrate and therefore it lowers the activation energy required to catalyze that reaction.

Now because enzymes are globular proteins they are very sensitive to certain conditions and the following conditions can affect the rate of enzyme controlled reactions. temperature, pH, enzyme concentration, and substrate concentration. One really important edit just to be aware of as I go through all of these variables now, such as temperature that you're about to see here, but also to do with substrate enzyme concentration, you need to say the frequency of successful collisions increases. And frequency is key for the mark scheme. It's not going to say that on all of the slides. I've edited this in right now for the 2025 video, but just be aware that you have to say frequency of successful collisions increases to get the marking points.

So if we start with temperature, we can see this particular shaped curve. There is a increase in rate with temperature. We reach our optimum and then there is a sudden decrease in rate at higher temperatures. And the explanation for this is at lower temperatures there is less kinetic energy and therefore you're less likely to have a successful collision between the enzyme and the substrate. However, at higher temperatures there is now so much kinetic energy it can start to cause lots of additional movement and those high temperatures will start to break the bonds in the tertiary structure. So for example, the hydrogen bonds are going to be breaking that causes 3D shape to unfold and you lose that unique shape active site. Now the Q10 temperature coefficient is a measure of the rate of change of an enzyme controlled reaction as a result of increasing the temperature by 10° C. And the formula to work this out is R2 / R1. R1 is the rate of reaction at a temperature of and your X temperature is whatever your first temperature is. R2 is the rate of reaction at a temperature 10° higher than the one you're comparing it to.

Next, then we have a look at the pH and too high or too low a pH will interfere with the charges in the amino acids in the active site. that will cause the ionic and the hydrogen bonds to break and therefore that unique 3D shape of the tertiary structure unfolds that changes the shape of the active site and the enzyme denatures. Now as indicated in this graph here enzymes do have different optimal phes though depending on where they work. So for example any of the proteases digesting proteins that are found in the stomach they actually have an optimum of around one or two. So pH1 or two because they are working in acidic conditions whereas some enzymes actually work better in slightly alkaline conditions for example trison and amalayise because in the small intestines it is slightly alkaline.

We then move on to looking at the impact of enzyme and substrate concentration. Now these do not cause the enzymes to denature or cause any change in shape to the enzyme. These have an effect based on the idea of saturation. So if there is a low concentration of substrate, the reaction will be lower as there will be fewer collisions possible between the enzyme and the substrate because there are just fewer molecules there to potentially collide. If you were to increase the substrate concentration because there are now more molecules present, you're more likely to have a collision, therefore more likely for enzyme substrate complexes to occur and the rate would increase. However, the rate would eventually plateau. So, it would level off at a maximum rate of reaction and that's because it would reach the point when all the enzyme active sites are in use. Or in other words, the enzymes are saturated and you would now have to add more enzymes as well as more substrate to increase the rate.

Moving on to the enzyme concentration effect. At a low enzyme concentration, there will be a low rate of reaction. And that's because if there's fewer enzymes, there's fewer active sites for the substrate to bind to and therefore there'd be fewer enzyme substrate complexes. Increasing the enzyme concentration will increase the rate of reaction because there'll now be more enzyme substrate complexes. But at high enzyme concentrations, unless unlimited substrate is added, the rate of reaction will plateau because there will be insufficient substrate to bind to the large number of enzymes and you'd end up with lots of empty enzymes not being used.

Competitive inhibitors also affect the rate of enzyme controlled reactions. And there's two types of inhibitors we're going to look at. Competitive with the first type 2025 edit that I'm adding in here to do with mark scheme specificity. For OCR, you do have to say that the competitive inhibitors are similar in shape, not the same. So they are similar in shape to the substrate. So make sure you are using the word similar in your answer. A competitive inhibitor is the same shape or very similar in shape to the substrate and that means it's complimentary in shape to the active site and it can actually bind to the active site and that's what we're seeing here. This is the competitive inhibitor. It's binding to the active site that forms an enzyme inhibitor complex instead of an enzyme substrate complex. It prevents the substrate from binding and therefore it lowers the rate of reaction. Now most competitive inhibitors are reversible and reversible means that they can be removed from the enzyme whereas if it was non-reversible that means it is permanently bound to that enzyme. Now the importance or the relevance of that is if you were to add in a much much higher concentration of substrate they would actually start colliding and banging into the inhibitor knocking the inhibitor out and therefore the substrate would then be able to bind. So with competitive inhibitors at high concentration of substrates they actually don't have um any impact anymore. And that's what we can see here in this graph. At low substrate concentrations, the competitive inhibitor has lowered the rate of reaction compared to the enzyme reaction with no inhibitor. But at a higher concentration, they both plateau at the same maximum rate.

The non-competitive inhibitors though, these bind onto the enzyme at a position other than the active site and we call that the alossteric site. Because the inhibitor is bound to the enzyme, it causes the protein to change shape and therefore it changes the shape of the active site and the substrate can no longer bind regardless of how much is added. And that's why we see there is a lower rate and even plateaus at a lower rate. And it doesn't matter if you continue to add more and more substrate. Just interrupting here to add in another 2025 edit to make sure your answers are really mark scheme specific. For this bit here, you'd need to say that enzyme substrate complexes form less frequently. So that is the phrase that they'd be looking for. They're forming less frequently and therefore the rate of reaction is much lower because the active site is now a different shape. We get fewer enzyme substrate complexes.

Now some inhibitors are what we call end product inhibitors and this is when the product of the reaction is a reversible inhibitor for the enzymes involved in controlling that reaction and this is really useful because it enables reactions to be controlled. So essentially it can turn reactions on and off. So if there is a lot of the product already present from this reaction, that product binds to the enzyme and inhibits it and it prevents any more of that reaction happening. But when the product starts to run low, the inhibitor is no longer going to be able to inhibit the enzyme and the reaction starts up again. So that prevents resources from being wasted.

That brings us on to the idea of co-enzymes, co-actors and prosthetic groups. Some enzyme controlled reactions require an additional nonproin molecule such as a co-enzyme co-actor or a prosthetic group to catalyze the reaction. So if we have a look at co-enzymes and co-actors first, some reactions require atoms to be carried from one reaction to the next in a multi-step pathway of reactions and that is actually the case in both respiration and photosynthesis. Some enzymes also require a nonproin molecule to bind to the active site to make it complimentary to the substrate and that is what a co-actor and a co-enzyme is. The difference between the two is that co-enzymes are organic molecules and co-actors are inorganic molecules meaning they don't contain carbon. A prosthetic group on an enzyme is a type of co-actor but they differ in that they are permanently attached to the enzyme by a covealent or a non-coovvealent force.

Precursor activation is when enzymes often occur in an inactive form and they require to be activated by a co-actor so that they can actually work. And this prevents enzymes from causing damage within cells and ensures they are only used when they are needed. An enzyme is activated by the binding of a co-actor as this causes a change in the shape to the tertiary structure so that the active site now becomes complimentary enough in shape to its substrate for it to bind.

We then move on to biological membranes. So biological membranes all cells and all organal membranes are composed of a phospholipid billayer and that's what we mean by a biological membrane. Plasma membranes provide this partially permeable membrane and they're the site of chemical reactions and they have a role in cell communication as well. And the plasma membrane has this model to represent the different components and properties known as the fluid mosaic model. So the plasma membrane is described as a fluid mosaic model due to the movement of the phosphoipids, proteins, glyoproteins and glyolippids but also the arrangement or the pattern of the proteins within the phosphoipids. The phosphoipids align as a blayer which we can see here. We've got two layers of that phospholipid and that's due to the hydrophilic heads being attracted to water which are therefore on the outside of the membrane and the hydrophobic tails being repelled by water which is why they face inwards to each other.

Proteins within the cell surface membrane can be extrinsic which means on the outside or intrinsic which means going all the way through. The exttrinsic or peripheral proteins provide mechanical support or they can make glyoproteins and glyolippids and the functions of those are in cell recognition as receptors. The intrinsic or integral proteins are the carriers or channel proteins which are involved in the transport of molecules across the membrane. Protein channels like we can see here form tubes that fill with water to enable water soluble ions to diffuse. Whereas the carrier proteins will bind with ions and larger molecules such as glucose and amino acids. They then change shape to transport the molecule to the other side of the membrane. Finally, there's cholesterol which we can see just here. That's present in some membranes and this restricts the lateral movement of other molecules in the membrane. And this is useful as it makes the membranes less fluid at high temperatures and this will prevent water and dissolved ions from leaking out of the cell or out of an organel.

So these membranes are affected by certain factors. Temperature does have an effect on the structure and the permeability of that phosphoipid billayer. And there's two reasons why. Firstly, at high temperatures, it increases the kinetic energy of those phospholipids. So they're going to have even more movement and that increase in the fluidity of the membrane means that there's going to be larger gaps periodically between the phospholipids and therefore the permeability increases and it makes it easier for particles to cross the membrane. The second reason is high temperatures could dene the carrier and the channel proteins in the membrane. And if that happens and the protein channels and carriers become wider, then even more molecules are going to be able to move across that cell membrane that weren't initially able to. So therefore, it becomes more permeable. Solvents can also damage the cell membrane structure. Organic solvents like alcohol, for example, ethanol dissolve lipids. So they will dissolve the phospholipid billayer in the membranes and that damage causes the fluidity of the membrane to increase and become more permeable.

Next we move on to movement across the membranes and there are six key modes of transport in and out of cells that we're going to have a look at and it's this six just here. So starting with simple diffusion, this is the net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached. This process does not require ATP. For molecules to diffuse across the membrane, they must be lipid soluble and small. And that's what we can see here. We've got a high concentration of one side of the membrane compared to the other. And these lipids soluble molecules then dissolve in that phosphoid billayer and move down their concentration gradient. And this will continue until there is no longer a concentration gradient present.

Facilitated diffusion is still a passive process with the movement going down the concentration gradients. However, it's for molecules that are either too large to diffuse through the phospholipid blayer or for molecules that are not soluble in lipids. And so instead they have to move through the proteins that are embedded within the phospholipid billayer. So that movement of ions and those polar molecules would be through either protein channels or the protein carriers.

Osmosis is the movement of water from an area of a higher water potential to an area of lower water potential or more negative. And it happens across a partially permeable membrane. And the more negative a water potential, that means the more concentrated the solution. You have less water but more solutes dissolved in it. And there's different types of solutions that occur that have an impact on osmosis. An isotonic solution is when the water potential of the solution is the same in the solution and the cell. And because there is no water potential gradient, there'd be no net movement of water in or out of the cell. And in an animal cell, we can see here's a red blood cell as an example, that has no impact on the overall size or shape of the cell.

A hypotonic solution is when the water potential of the solution is more positive. So it's closer to zero, which basically means there is more water compared to the solute. So as a result, water will move from the solution into the cell causing it to swell and it could even eventually cause it to burst if enough water has moved in. In a hypertonic solution, that means the solution is more negative than the water potential in the cell. So as a result, water within the cell will move out by osmosis into the solution and that causes the cell to shrivel up. But the term we use for that in animal cells is cration. And we say in plant cells they are plasmalized. In plant cells as well they will not burst as readily because they have that cellulose cell wall. So instead they become really swollen and we describe it as turgid.

The next type of transport is active transport. And this is the movement of molecules and ions from an area of lower concentration to an area of higher concentration which means it's going against the concentration gradient and for this reason it requires energy in the form of ATP from respiration and it involves carrier proteins. is a selective process as only certain molecules are able to bind to a receptor site on the carrier proteins. When they do that, ATP will bind to the protein on the inside of the membrane. It then gets hydraized into ADP and PI. That causes the carrier protein to change shape and open towards the inside of the membrane. And as a result, the molecule that was attached is released to the other side of the membrane. The phosphate group or that PI molecule is then released from the protein and the protein reverts to its original shape and the process can continue to happen as long as ATP is present.

The next one is endoccytosis and this is a type of active transport but it is the bulk transport of molecules into a cell. The cell surface membrane bends inwards around the molecules surrounding it to form a vicle. And that's what we can start to see happening here. It's bending inwards and eventually folds all the way around to form a vicle. The vicle pinches off and moves within that cytoplasm. And endocytosis can be classed as either fagocytosis as we've got on the left or pinocytosis that we have on the right. And when it's a solid particle being taken in, it's called fagocytosis. When it's a liquid being taken in, that is when it's punocytosis. This requires energy from ATP because to change the shape of the membrane around that material does require energy.

Exocytosis is another type of bulk transport, but this time it's the movement of molecules out of a cell. So the molecules are contained with a vicle. Vicles move towards the cell surface membrane fuse with the membrane and the contents of that vicle is then released to the outside of the cell. This

The process requires energy because ATP is needed to move the vesicle along the cytoskeleton towards the cell surface membrane.

Cell division, cell diversity, and cellular organization. So, cell division first of all. In eukaryotic cells, they enter the cell cycle and then divide by mitosis or meiosis. Prokaryotic cells replicate by binary fission. And then viruses do not undergo cell division, as they are non-living.

The cell cycle comprises three key stages. We have interphase, split into G1, S phase, and G2. Then there's the nuclear division, which is either mitosis or meiosis. And then finally, cytokinesis.

So, let's begin with interphase. And this is the longest stage of the cell cycle. G1, which is the first part of interphase, is when protein synthesis occurs to make proteins involved in synthesizing organelles. The organelles then replicate. The cell is checked that it is the correct size, has the correct nutrients, growth factors, and that there is no damaged DNA. If a cell doesn't pass these checks, then replication does not continue.

Then we have S phase, and this is when DNA is replicated. G2 is when the cell continues to grow, energy stores increase, and the newly replicated DNA is checked for any potential copying errors. I've edited this slide for 2025 to make sure that the descriptions of what's happening in G1 and G2 are really mark scheme specific. So make sure you have a look at the level of detail on this slide when you're making your notes or flashcards, because this is the level of detail required for the mark scheme.

So, then move on to the first option for nuclear division, which is mitosis. One quick edit I've made here is to make sure it's really mark scheme specific. For mitosis, you have to say it creates two genetically identical diploid cells. So you would have to say "genetically identical" to get the mark. And it's used for growth, tissue repair, and asexual reproduction in plants, animals, and fungi.

There are four key stages: prophase, metaphase, anaphase, and telophase, or PMAT as a way to try and remember it. Prophase is when the chromosomes condense and become visible. And in animal cells, the centrioles, which are shown here in yellow, separate and move towards the opposite poles of the cell. The centrioles create spindle fibers, which are released from both poles to create a spindle apparatus. And these will attach to the centromere and the chromatids on the chromosomes in late prophase, early metaphase. Plants have spindle apparatus, but they don't actually have the centrioles.

Metaphase is the next stage, and this is when those spindle fibers have attached to the centromere, and it causes the chromosomes to align along the equator of the cell. The spindle assembly checkpoint also occurs in this stage, and this is where there is a check to make sure that every chromosome has a spindle fiber attached to its centromere before mitosis carries on to the next stage, which is anaphase.

So, in anaphase, this is when the spindle fibers start to shorten and move towards the centrioles and pull the sister chromatids they are bound to towards those opposite poles as well. This causes the centromere to divide into two, and the individual chromatids are then pulled to the opposite pole. This stage requires energy in the form of ATP, which is provided by respiration in the mitochondria.

Next, we have telophase, or telophase, and the chromosomes are now at each pole of the cell and become longer and thinner again. So, therefore, they start to no longer be visible. The spindle fibers will disintegrate, and the nuclear membrane starts to reform around those chromosomes.

Cytokinesis is then the final stage of the cell cycle, and this is when the cytoplasm splits. So we get two genetically identical cells. And in animals, the way this happens is a cleavage furrow forms in the middle of the cell, and the cytoskeleton causes the cell membrane to draw inwards until the cell eventually splits in two. In plant cells, the cell membrane splits into two new cells due to the fusing of vesicles from the Golgi apparatus. The cell will form new sections around the membrane to complete the division into two cells.

Just interrupting here to point out a change to this slide to make sure your answer is really mark scheme specific. You have to say that it causes the cell surface membrane to draw inwards, and the cell surface membrane splits, not cell membrane. It's specifically the cell surface membrane. So you have to say that word "surface."

Now, you can actually observe mitosis, and this is one of the required practicals. And the stages of mitosis can be viewed using a light microscope in onion or garlic root tips. So you would need to take a thin slice of the root tip from either an onion or a garlic and place it on top of a microscope slide. You then break it down a little bit with a mounted needle. A stain is then added, and the purpose of this is to make the chromosomes visible when you put your slide under the microscope. You would then sometimes add acid to help to break down the cellulose connections between the cell walls. Then you would place a coverslip on top and push down. The reason we push down is to squash the tip to achieve a single layer of cells to ensure light can pass through, and therefore you can see the individual cells and the chromosomes inside of them.

And that's what we can see here: that single layer of cells, light's passing through them. Most of the cells here are in interphase, and we can tell that because chromosomes are not visible. But we do have two where the chromosomes are visible. So we could use this to calculate the mitotic index. So this is calculated by counting how many cells are visible in the field of view, meaning the section that you're looking at, and by counting the number of cells that are currently in mitosis. And it's essentially a percentage, this. So the number of cells in mitosis divided by the total number of cells, and then you'd multiply it by 100 to give that answer as a percentage.

Now, when they have questions like this, they usually tell you the total number of cells, so that you don't end up spending lots of time in an exam just counting lots of cells, and instead, you just have to work out the number of cells in mitosis. And like I said, in this example, we've got two that are in metaphase.

So, the other type of nuclear division is meiosis, and this is when we have two nuclear divisions, and that results in four genetically different haploid daughter cells. The two rounds of division are referred to as meiosis one for the first round and meiosis two for the second. Both stages include prophase, metaphase, anaphase, and telophase, and cytokinesis at the end. But interphase only happens at the very beginning, before meiosis one.

Now, we said it makes a haploid cell, and what we mean by haploid and diploid is a haploid cell, represented by a single letter N, is when you have one copy of each chromosome. A diploid cell, or 2N, is when you have two copies of each chromosome.

The genetic differences are introduced in meiosis by two key processes: independent assortment of homologous chromosomes and crossing over. So let's have a look at crossing over first. During prophase one, the homologous chromosomes pair to form bivalents. And by bivalent, we mean two homologous chromosomes next to each other. And that's what we can see here. We've got our homologous chromosomes, represented as one in red, one in green. And they look like this X structure because the DNA is replicated. So we have two sister chromatids attached by a centromere to make a chromosome, and here is the homologous chromosome. Crossing over, genetic material can occur. So these non-sister chromatids, meaning a chromatid from the different chromosomes, crossing over and form what we call a chiasma, and that is where the crossing over happens.

Now, the tension that that creates can result in breaks occurring, and we then get this exchange in material of those chromatids. And as a result, the alleles that were on this chromatid are now going to be part of this chromosome. So we have this new combination of alleles in the resulting gametes.

Independent assortment also increases the genetic diversity, and this happens during metaphase one, where the homologous pairs of chromosomes line up opposite each other on either side of the equator. But it is random for each homologous pair which side the maternal and the paternal chromosomes align. And that's what we can see here. We've only been shown an example with three homologous pairs, but it shows you all the possible combinations of how they could align at the equator. And for humans, we have 23 homologous pairs. So that means there would be two to the power of 23 possible combinations of how those homologous pairs could align. And that works out to over 8 million different combinations. And that's what we can see here. And you can use that formula, 2 to the power of n, where n is the number of homologous pairs, to work out the number of possible combinations for any different organism.

Now, in metaphase two, the sister chromatids within one chromosome also are lining up at the equator, and their orientation on each side of the equator introduces another chance for increased genetic variation. And as a result, each gamete receives different combinations of the maternal and paternal chromosomes.

We then move on to this idea of organization, looking at specialized cells as well. Now, multicellular organisms are organized in the following way. Cells are the smallest structure. Then we get tissues, organs, organ systems, and those will work together to create the entire organism.

Now, you do need to be aware of a selection of specialized cells, and we've got here the summary of all of the ones that you need to know about. So I recommend that you take a screenshot of this, make your own table, make your own notes, or even turn this into a flashcard. So you've got the five specialized cells, and for each one, you've got your description here of the structure of the cell and how that links to the function.

Now, I've added this into the 2025 video because you do actually also need to know about root hair cells. So, these are added in here. So, these are cells on the surface of the roots, and they have long projections to increase the surface area for osmosis of water and also the active transport of mineral ions from the soil. They have a thin cell wall to reduce the diffusion distance.

We then move on to the different tissues. Same thing again. Screenshot it, print it, have it in your notes, or copy it out, or even better, turn it into flashcards. Just to point out a key edit here. These are tissues. So, I've edited the slides that you're going to see coming up, which said epithelial cells and ciliated epithelial cells. However, these are the tissues. So, at this point, you would just say squamous epithelia tissue and ciliated epithelia tissue. But those are made up of ciliated epithelial cells. But it's really important that when you're describing the tissue, you don't put "cell" at the end. These are the different tissues that you need to be aware of the structure of each and the function of them.

We then move on to stem cells, and these are undifferentiated cells that can self-renew, meaning continually divide and become specialized. Different types of stem cells have differentiation abilities, and the types that you need to know about are totipotent, pluripotent, multipotent, and unipotent.

Totipotent cells can divide and produce any type of body cell. During development, totipotent cells transcribe only part of their DNA, resulting in cell specialization. Totipotent cells occur only for a limited time in early mammalian embryos. They then develop into pluripotent stem cells, and these are found in embryos and can become almost any type of cell. The only cell that they can't form is the placenta. So, for this reason, they are used in research with the prospect of using them to treat human disorders.

There are issues with this, though, as sometimes this treatment doesn't work, or the stem cells can continually divide, and that results in tumors. On top of those practical issues, there are ethical issues as well, because there's a debate as to whether it's right to make a therapeutic clone of a patient in order to create an embryo genetically identical to them to get the stem cells to cure a disease and then after that, destroying the embryo as well.

So, the other two types of stem cells are multipotent and unipotent, and these occur in mature mammals, and they can only divide into a limited number of types of cells. Multipotent cells, such as the ones found in bone marrow, can differentiate into a limited number of cells, and in bone marrow, that is into the different types of blood cells. Unipotent cells can only differentiate into one other type of cell.

So, the potential uses of stem cells are in both research and medicine, and these include potentially being used to repair damaged tissues, or in the treatment of neurological conditions such as Alzheimer's and Parkinson's, or it could be research into developmental biology.

So that takes us to the end of all of topic two. I hope you found it helpful, and if you made it to the end, congratulations. [Music]

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Module 2 ENTIRE TOPIC OCR A: cells, biological molecules, enzymes, membranes cell division.

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