Transcription
Imagine you are stranded on a raft in the middle of the ocean. You have no emergency food or water, but you also know that humans cannot survive for about 3 days without fluids. Do you drink the seawater or not? Cells have a very limited range of conditions under which they can survive. If the conditions are too hot, too cold, too watery, too salty, too acidic, or too basic, they cannot function properly. Therefore, we will explore what happens when you place a cell in a saltier (hypertonic), more watery (hypotonic), or same salt concentration (isotonic) solution. Remember that cell membranes are selectively permeable; they allow some particles to pass, but not others. Usually, larger molecules cannot fit through the membrane without special channels. The cell membrane is equipped with special protein channels called Aquaporins, which allow water molecules to pass without expending energy. However, how easily water molecules can cross will depend on something called concentration. Concentration is a measure of how much solute there is per volume of solvent. In other words, whether the liquid is saltier or more watery. We would normally expect molecules to follow a process called diffusion. They will flow from areas of higher to lower concentration. Eventually, they will reach equilibrium, where there will be equal concentrations on both sides of the membrane. However, in the case of a liquid like saltwater, the salt molecules are too large to fit through the cell membrane. So the only particle that can move is water. Let's look a little closer. Water molecules are strongly attracted to salt molecules. They cluster around the salt, really, really not wanting to move. The positive parts of a water molecule attach to the negative parts of sodium chloride, and vice versa. The interactions between oxygen and sodium, and between hydrogen and chlorine are dipole-ionic interactions, which you may remember from chemistry. Water molecules not attached to a salt molecule are more likely to move. Today, we will immerse a cell in three types of liquids: a very solute-rich liquid, a very watery liquid, and a liquid that is in between. Then we will see what happens to the water molecules. In this first scenario, the liquid surrounding the cell contains many solute molecules within it. This liquid is hypertonic compared to the cell. It has a much higher concentration of solute particles, and a much lower concentration of water. Unfortunately, the solute molecules cannot pass through the membrane to reach equilibrium, but water molecules can. Water molecules on the outer surface of the cell will be hindered from passing through by the many solvent molecules. However, the molecules on the inside have far fewer solvents in the way. Water will begin to rush out of the cell. Some water will come in, but the net movement, the overall movement of water molecules, will be outward, and the cell will shrink. We call this crenation, the shrinking of the cell. This is also how pickles are made. Now let's immerse a cell in a much less concentrated solution; a hypotonic solution. Compared to the cell, it is much less salty. Some water molecules will leave the cell, but more will rush in because they are not blocked by dissolved particles. The cell grows in size, and it may also undergo lysis and burst! Placing a cell in an isotonic solution, where the concentrations inside and outside the cell are equal, is more fun. Equal amounts of water molecules continue to enter and exit the cell. The system is in a state of dynamic equilibrium. Particles continue to move, but the net movement is zero. The cell neither grows nor shrinks, and it is more likely to survive. Let's now return to the question of seawater. Seawater is incredibly salty and much saltier than our body cells. Because it is a hypertonic solution, if you were to immerse your cells in it, they would shrivel up and die. However, the real danger lies in how your kidneys interact with seawater. Your kidneys will try to remove the toxic salt levels using water drawn from your own cells. Your body will actually use more water to remove the salt than was originally contained in the seawater itself. In short, drinking seawater is a bad idea, even in extreme circumstances. You're better off drinking your own urine! So, there we have it! Hypertonic, hypotonic, and isotonic solutions, plus some practical examples! Thanks again for watching! Take care.