Coursework: Investigating the effects of intracellular ions in model plant cells

Question 1

What is osmosis?

Answer 1

The net movement of water by diffusion across a partially permeable membrane from an area of higher water potential to an area of lower water potential down a water potential gradient.

Question 2

What results are expected for different experimental setups?

Answer 2

1. High ion concentration in bag, low ion concentration in water bath: The mass of the bag should increase over time as water moves into the bag by osmosis.
2. High ion concentration in water bath, low ion concentration in bag: The mass of the bag should decrease over time as water moves out of the bag by osmosis.
3. Same ion concentration in bag as in water bath: No change in mass over time.

Question 3

What do these results tell us about the effects of intracellular ion concentration in plants?

Answer 3

If intracellular ion concentration is kept higher than surroundings, then water is able to move into the plant cell quite easily by osmosis due to steep water potential gradient and the plant cell is able to obtain enough water. The protoplasm expands and pushes against the cell walls, meaning turgor pressure is maintained.
If intracellular ion concentration is lower than surroundings, then water would move out of the plant cells and into the surroundings by osmosis. This means that protoplasm shrinks and the plasma membranes shrivel and eventually come away from the cell walls and the plant cells become plasmolysed. The plant would then begin to wilt.

Question 4

What are the potential sources of student errors in the experiment?

Answer 4

1. The dialysis bag could have not been tied properly and the contents may have leaked, affecting the concentration of contents and the mass change.
2. If hands were not washed before handling the bags, substances on hands such as oils may have blocked some pores, decreasing overall rate of osmosis.
3. The student could have not wiped the bag before putting on scale, so surface water would have added to mass.
4. The bag may not have been fully submerged.

Question 5

What are the limitations of the experiment?

Answer 5

1. Temperatures may not have been kept constant for each experiment which would affect rates of osmosis.
2. The surface area of the bag may not be exactly the same across each experiment.

Question 6

How valid is the experiment?

Answer 6

1. Plant cell plasma membranes have carrier proteins which may use active transport to load ions in/out of the cell, changing the concentration of ions dynamically.
2. Plant cell plasma membranes have channel proteins that allow for exchange of ions between cells and external medium.
3. Plant cells photosynthesise. This uses up water as well as produces more solutes to alter water potentials.
4. In plant cells, the cell walls are rigid ad will resist the expansion of the protoplasm when a certain size is reached, preventing more uptake of water.

Question 7

What may affect rate of osmosis?

Answer 7

1. Temperature.
2. Surface area.
3. Water potential gradient.
4. Stirring mixture.

Question 8

What variables need to be kept the same throughout all experiments?

Answer 8

1. Surface area of partially permeable membrane.
2. Temperature.
3. Volume of water in the partially permeable membrane.
4. Time the experiment is carried out for.
5. Type of ions.