A striking feature of this experiment is how thin the layer is. It shows how slowly molecules move by diffusion.
Q2 The rate of diffusion depends on the kinetic energy of the molecules in the liquid. The higher the temperature, the faster the molecules move and the quicker the rate of diffusion. Using warm water speeds up diffusion. (Avoid really hot water as it may melt the bag.)
Q3 Peeled grapes in strong sugar solution shrink as water moves out of the cells by osmosis. The concentration of the sugar solution is greater than the solute concentration inside the grape cells. As a result there are more free water molecules inside the cells. These water molecules move out by osmosis across the partially permeable cell membrane to where there are fewer free water molecules.
Peeled grapes swell up in water. There is a higher solute concentration inside the grape cells so there are fewer free water molecules compared with the water surrounding the grape. As a result water moves into the grape by osmosis.
There is very little change in the unpeeled grapes, either in size or texture. They are there for comparison; without them it is difficult to recognise changes in volume of the peeled grapes.
Q4 a In the unpeeled grapes the change in size is limited by the outer peel (skin). Grape peel seems to be impressively waterproof, preventing uptake or loss of water from the grape.
b In the peeled grapes the change in size is limited by the cell walls. In sugar solution once all of the cells are fully plasmolysed and the inside of the grape is at osmotic equilibrium with the sugar solution there will be no further shrinking. Once the cells of the grape in water are fully turgid there will, again, be no further change.
Q5 After soaking in the salt solution, the tissues of both potato and cucumber are extremely soft and pliable. This is because many of the cells in the tissue have been plasmolysed – the water has moved out of the cell by osmosis, leaving only a small amount of concentrated cytoplasm and reduced vacuole. Without the water pushing out on them, the cell walls are no longer rigid so the tissue becomes pliable. The cucumber slices are more flexible than the potatoes, which might be related to the closeness of packing of cells within the tissue or the difference in concentration across the membrane. Potatoes appear to have a higher solute concentration in their cells compared with cucumber, so the difference in free water molecules between the cytoplasm and the external salt solution is smaller in potatoes. Hence less water is removed by osmosis.
In water, the cells of both the potatoes and the cucumber have absorbed water by osmosis, making them fully turgid. In each cell the cell membrane is pressing against the cell wall, making the cell wall rigid.
Q6 The cells in water should have lysed. Students may observe the ‘ghosts’ (empty red blood cell membranes). The haemoglobin is free to mix with the water, giving a clear red solution. The blood cells in the isotonic solution should remain the same and those in the hypertonic solution should have shrivelled up. (Technically, they are crenated, but students do not need to know this term.)
Q7 The only change in plant cells would be that they become turgid or plasmolysed. You would be able to see the cytoplasm pulling away from the cell wall in the plasmolysed cells if you used red onions or a similar plant with pigment in the vacuole, but there would be little visible effect in the turgid ones. There should, of course, be no change in an isotonic solution.
To demonstrate some methods of transport within and between cells.
To develop scientific explanations using ideas about transport across membranes and membrane structure.
These experiments demonstrate diffusion and osmosis. Teachers/lecturers may choose to do all of them as demonstrations or to have students doing some themselves.
Experiment 1 – Diffusion through a plastic bag
This experiment demonstrates diffusion through a differentially permeable membrane (a plastic bag). Requirements per student or
group of students Notes
Medium-sized (200–300 cm3) beaker
About 20 cm3 thick starch solution A tablespoon (about 15 g) of starch in 300 cm3 of water works well. Heat starch to dissolve and boil for a minute or so. The final texture is like custard. This makes it less runny and easier to keep in the bag.
Warm water To fill the beaker, leaving sufficient space to allow for submersion of the starch-filled bag. The temperature is not critical. Hand-hot (about 40 °C) works well.
Iodine (dissolved in KI) Starch indicator solution, 0.01 M (see Hazcard 54B for further safety information).
Clear plastic bag (and elastic band if needed)
The thin ones used for vegetables at the supermarket work well, as do cheap sandwich bags. If small bags are used and knotting the bag is difficult, elastic bands may be needed to seal them. Labels for beakers