0610 · 3.2Osmosis

Water, solutions and diffusion

0:00Put lettuce leaves in a bowl of plain water and they stay crisp. Sprinkle salt over them, and within minutes they go limp and floppy. Nothing has been taken away from the leaves except water. So why does salt make water leave the leaves? The answer is osmosis, the way water moves into and out of cells, and it explains everything from how plants stand upright to why a drink of seawater makes you more thirsty.

0:20By the end of this video you will be able to describe osmosis, explain why water matters so much to living things, describe the classic experiments with dialysis tubing and potato strips, and explain what happens to plant cells in different solutions, using the terms turgid, turgor pressure, plasmolysis and flaccid. First, three words you need. When you dissolve sugar in water, the sugar is the solute, the substance that dissolves.

0:44The water is the solvent, the liquid it dissolves in. And the mixture is a solution. A dilute solution has only a little solute dissolved in it. A concentrated solution has a lot of solute dissolved in it. You also need the idea of diffusion from the last video. Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration, as a result of their random movement.

1:08Osmosis is a special case of diffusion, so keep that idea in mind. Let us start with why water matters so much. Water is an excellent solvent. Many different substances dissolve in it, and the syllabus wants you to describe this role with reference to digestion, excretion and transport. In digestion, large food molecules are broken down into small ones, and the small molecules dissolve in water. Only dissolved substances can be absorbed into the blood, so without water as a solvent, the products of digestion could not be absorbed.

1:39In excretion, waste substances are removed from the body dissolved in water. The kidneys remove urea, a waste made from excess amino acids, dissolved in water as urine. And in transport, substances are carried around dissolved in water. Blood plasma is mostly water, and it carries dissolved glucose, mineral ions, urea and carbon dioxide to wherever they need to go. In plants, water carries dissolved mineral ions up from the roots, and dissolved sugars around the plant.

Osmosis and water potential

2:06Now to osmosis itself. Cells are surrounded by a cell membrane, and the cell membrane is partially permeable. Partially permeable means it lets some molecules through, but not others. Small molecules, like water, can pass through it. Larger molecules, like sugar, cannot pass through it easily. The syllabus wants you to state that water diffuses through partially permeable membranes by osmosis, and that water moves into and out of cells by osmosis through the cell membrane.

2:32So osmosis is the diffusion of water, through a partially permeable membrane. Here is how it works. Imagine a partially permeable membrane with pure water on one side and a sugar solution on the other. On the pure water side, every particle is a water molecule. On the sugar side, some of the space is taken up by sugar molecules, so there are fewer free water molecules there. All the water molecules are moving randomly.

2:56Some hit the membrane and pass through it, in both directions. But there are more free water molecules on the pure water side, so more of them pass from the pure water into the sugar solution than pass back the other way. The sugar molecules are too big to cross. So there is a net movement of water from the pure water into the sugar solution. So the rule is simple.

3:16In osmosis, water moves from a dilute solution into a more concentrated solution. Notice which way that is. The water moves towards the side with more solute. The solute itself does not move across the membrane. For the extended syllabus, you need to describe osmosis using a term called water potential. Water potential is a way of saying how much free water there is, and so how likely water is to move out of a solution.

3:40Pure water has the highest water potential of all. A dilute solution has a high water potential. A concentrated solution has a low water potential. The syllabus describes osmosis as the net movement of water molecules from a region of higher water potential, a dilute solution, to a region of lower water potential, a concentrated solution, through a partially permeable membrane. Notice that this is the same idea as before.

4:04Water moves from where there is more free water to where there is less. This wording matters in the exam. Do not write that water moves from a high concentration to a low concentration, because it is not clear whether you mean the concentration of water or the concentration of solute. Water potential avoids that confusion. Water always moves from higher water potential to lower water potential.

Investigating osmosis in tissues

4:25Now let us see how you can investigate osmosis. A classic experiment uses dialysis tubing, a thin, partially permeable plastic tubing that behaves like a cell membrane. You tie one end of the tubing, fill it with a concentrated sugar solution, and fix a narrow glass tube into the top. Then you stand it in a beaker of pure water. Over the next half an hour, the liquid rises up the glass tube.

4:46The water outside has a higher water potential than the sugar solution inside, so water moves into the tubing by osmosis. The extra water pushes the liquid up the glass tube. The sugar cannot get out, because its molecules are too big to pass through the tubing. You can also run it without the glass tube, by weighing the tubing before and after. If the tubing is filled with sugar solution and put in water, it gains mass.

5:08If you swap them, filling the tubing with water and putting it into a concentrated sugar solution, it loses mass. In each case, water has moved from the dilute side to the concentrated side. The syllabus also asks you to investigate the effects on plant tissue of putting it into solutions of different concentrations. The usual tissue is potato. You cut several potato strips of the same size, blot them dry, and weigh each one.

5:32Then you put each strip into a sugar solution of a different concentration, from pure water up to a concentrated sugar solution, and leave them for about half an hour. Afterwards you take the strips out, blot them dry again, and reweigh them. In pure water and in very dilute solutions, the strips gain mass and feel firm. In concentrated solutions, they lose mass and become soft and bendy. Because the strips do not all start at exactly the same mass, you compare them using the percentage change in mass.

5:58To work it out, take the change in mass, divide it by the starting mass, and multiply by 100. Here is an example. A potato strip in pure water starts at 2.50 grams and ends at 2.80 grams. The change in mass is 2.80 minus 2.50, which is 0.30 grams. Then 0.30 divided by 2.50 is 0.12, and multiplied by 100 that is a 12 percent increase. A strip that loses mass gives a negative percentage change.

6:30If you plot percentage change in mass against the concentration of the solution, you get a line that slopes downwards. In these example results, the strip in pure water gained 12 percent, and the strip in the most concentrated solution lost about 18 percent. At one concentration in the middle, the line crosses zero. There, the potato neither gained nor lost mass. At that point, there was no net movement of water.

6:52The solution outside had the same water potential as the cell sap inside the potato cells. So you can use the graph to estimate the concentration of the potato cells themselves.

Osmosis in plant cells

7:02So what is happening inside the plant cells? Let us look at one cell in each kind of solution. Start with a plant cell in pure water. The water outside has a higher water potential than the cell sap, so water moves into the cell by osmosis, into the cytoplasm and on into the vacuole. The vacuole swells and pushes the cytoplasm outwards against the cell wall. The pressure of the water inside the cell pushing outwards on the cell wall is called turgor pressure.

7:27A cell in this state, full of water and firm, is described as turgid. And the cell does not burst, because the strong cell wall resists the pressure. This is what the syllabus means when it says that plants are supported by the pressure of water inside the cells pressing outwards on the cell wall. Turgid cells press against each other, and that holds the stems and leaves up. Now put a plant cell into a concentrated solution.

7:49This time the cell sap has a higher water potential than the solution outside, so water moves out of the cell by osmosis. The vacuole shrinks, and the cytoplasm no longer presses against the cell wall. There is no turgor pressure, and the cell becomes soft. A cell in this state is described as flaccid. If even more water leaves, the cytoplasm and the cell membrane shrink so much that they pull away from the cell wall.

8:12This is called plasmolysis, and the cell is said to be plasmolysed. The space between the cell wall and the membrane fills with the outside solution, because the cell wall lets it straight through. This explains the potato strips. Strips in pure water gained mass, because their cells took in water and became turgid, so the strips felt firm. Strips in concentrated solutions lost mass, because their cells lost water and became flaccid, and some became plasmolysed, so the strips were floppy.

8:37It also explains the lettuce. The salt makes a concentrated solution on the leaves, water leaves the cells by osmosis, the cells become flaccid, and the leaves go limp. A plant that wilts on a hot day is doing the same thing. Its cells have lost turgor pressure.

Osmosis in plants and animals

8:51Finally, the syllabus asks you to explain why water potential and osmosis matter for the uptake and loss of water by organisms. Take a plant first. The water in the soil is a very dilute solution, so it has a high water potential. The cell sap in a root hair cell is more concentrated, so it has a lower water potential. So water moves from the soil into the root hair cell by osmosis.

9:11This is how plants take in water. That water then moves from cell to cell across the root by osmosis, each time from a cell with a higher water potential to a neighbouring cell with a lower water potential, until it reaches the tubes that carry it up the plant. If the soil becomes too concentrated, for example if too much fertiliser or salt is added, the soil water can end up with a lower water potential than the root cells.

9:34Then water moves out of the roots, and the plant wilts and can die. Animal cells take in and lose water by osmosis too, but they have no cell wall to protect them. Put a red blood cell into pure water, and water moves in by osmosis until the cell swells and bursts. Put it into a concentrated solution, and water moves out until it shrinks and becomes wrinkled. That is why your body keeps the water potential of your blood plasma steady.

9:57If it changed too much, your cells would gain or lose too much water. It is also why drinking seawater is dangerous. It is so concentrated that it draws water out of your cells, rather than giving them water. Let us collect the common mistakes. The first is saying that water moves from a high concentration to a low concentration without saying high concentration of what. Use water potential, or say from a dilute solution to a concentrated solution.

10:22The second is saying that sugar or salt moves by osmosis. Osmosis is only the movement of water. The solute stays where it is. The third is forgetting the partially permeable membrane. Without it, it is just diffusion, not osmosis. The fourth is saying that a plant cell bursts in pure water. It does not, because the cell wall resists the turgor pressure. Animal cells can burst, because they have no cell wall.

10:46And the fifth is mixing up the terms. Turgid describes a firm cell, turgor pressure is the pressure of the water pushing on the wall, flaccid describes a soft cell, and plasmolysis is when the membrane pulls away from the wall.

Osmosis recap

10:58Let us recap. Water is a solvent, so it carries dissolved substances in digestion, excretion and transport. Osmosis is the net movement of water molecules from a region of higher water potential, a dilute solution, to a region of lower water potential, a concentrated solution, through a partially permeable membrane. Water moves into and out of cells by osmosis through the cell membrane. In dialysis tubing and in potato strips, water always moves towards the more concentrated solution.

11:26In a dilute solution, plant cells take in water and become turgid, with turgor pressure pushing on the cell wall, and that pressure is what supports a plant. In a concentrated solution, plant cells lose water, become flaccid, and can become plasmolysed. And osmosis is how root hair cells take in water from the soil, and why animal cells must be kept in a solution with the right water potential.

11:48In the next video, we will see how cells move substances the other way, against a concentration gradient, by active transport.