A student studied osmosis using potato. A cube was cut, weighed and left in salt solution of concentration 0.4 mol/dm3. When taken out later it was weighed again and the change in mass worked out. Two other salt concentrations were then tested in the same way.
Judge how good this potato method is, and say what the student should change to make the results more valid.
Our own questions on this topic, marked straight away, so you see what to revise. Free now.
Show model answer for 03.1Hide model answer for 03.1
The method is a reasonable start, but the results would not be fully valid because too many variables are not controlled. Its strengths are that every piece is a cube, the mass is recorded before and after, the change in mass is worked out and salt solutions of different concentrations are used.
To improve validity, cut every cube from the same potato and peel off the skin, because different potatoes and skin change how fast water moves. Cut the cubes to the same size and surface area, because a larger surface area lets water move faster.
Leave each cube in the solution for the same time, keep the temperature steady in a water bath and use the same volume of solution that covers the cube, because time, temperature and volume of solution all change how much osmosis happens.
Blot each cube dry before weighing, because water on the surface adds false mass. Work out the percentage change in mass, because the cubes start with different masses.
Repeat each concentration to spot anomalies, and find a mean, because no mean was worked out. Add a boiled cube as a control, because none was included, and test more than three concentrations, because only three were used.
How the marks are earned
- This is level marked. For 5 to 6 marks, give a judgement that is strongly linked to a good range of correct reasons, and include a strength and improvements with reasons. For 3 to 4 marks, give some reasons that link up, with or without a simple judgement. For 1 to 2 marks, make relevant points that do not link up. An answer with nothing relevant in it earns no marks.
- Strengths worth stating: the same potato used throughout; every piece cut as a cube; weighing at the start and the end; working out the change in mass; testing several salt concentrations.
- Improvements and the reason for each: use one potato for all the cubes, because different potatoes may have been used; peel off the skin, because skin may still be on; blot the cube dry before weighing, because water on the surface adds mass; use a set size or surface area, because the size of the cubes was not controlled.
- More improvements and reasons: repeat the whole investigation, or each concentration, since the test was done once only, and to spot anomalies; find a mean from the repeats, because the student never worked one out; give every cube the same soaking time, since none was set; hold the temperature steady using a water bath, since nothing controlled it; give every cube an equal amount of solution, or make sure every cube is completely covered, because the amount of solution was never fixed.
- More improvements and reasons: calculate the percentage change in mass, because each cube started with a different mass; add a control with a boiled cube, because no control was included; use more than three concentrations, because only three were tested.
Why it scores
Level 3 needs a judgement and a strength. This paragraph gives the judgement first and then the strengths that are really in the method.
Each improvement comes with its reason. Using one potato, removing skin and fixing the size are all controls on the potato itself.
These are controls on the conditions around the potato. The reason is that each one affects the amount of osmosis.
These two points make the measurement fair. The percentage change lets cubes with different starting masses be compared.
Repeats show up anomalies, and a mean reduces the effect of random error. More concentrations give a better picture. Each point is given with its reason.
Revise Cell TransportWhy does a potato cube put in distilled water gain mass? Tick one box.
See how you would score on Cell TransportShow model answer for 03.2Hide model answer for 03.2
The contents of the potato cells are more concentrated than the pure water outside, so water moves into the cells by osmosis and the cube gains mass.
How the marks are earned
- Tick the option saying pure water was more dilute than the contents of the potato cells.
State one difference between active transport and osmosis.
See how you would score on Cell TransportShow model answer for 03.3Hide model answer for 03.3
Active transport needs energy from respiration and can move substances against a concentration gradient, from low to high concentration. Osmosis needs no energy.
How the marks are earned
- Any one: active transport needs energy; it moves substances against or up a concentration gradient, from a low to a high concentration; it moves minerals or ions, or another named substance such as glucose (water need not be mentioned).
- Saying it does not move water is also credited, and so is saying it is not passive.
- Saying only that it is active earns nothing, and neither does saying it goes along or across the concentration gradient.
- Moving down a concentration gradient is not accepted.