Osmosis

Question 1

Define osmosis.

Answer 1

The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.

Question 2

What is a selectively permeable membrane?

Answer 2

Cell-surface membranes and other plasma membranes that surrounds the organelles. They are permeable to water molecules and few other small molecules but not large ones.

Question 3

What is a solute?

Answer 3

A solute is any substance that is dissolved in a solvent e.g. water.
Solute + Solvent = Solution.

Question 4

What is the water potential?

Answer 4

It’s the pressure created by water molecules.
Under standard conditions of 25* in temperature and 100kPa in pressure, pure water is said to have a water potential of 0.

Question 5

What are the 4 water potential factors?

Answer 5

• Addition of a solute to pure water will lower its water potential.
• The water potential of a solution must be below 0, with. a negative value.
• More of the solute is added (so the more concentrated), the lower (more negative) its water potential.
• Water will move by osmosis from higher water potential (less negative) to lower water potential (more negative).

Question 6

How can you find the water potential of a cell or tissues?

Answer 6

Place them into a series of solutions of different water potentials, where there’s no net gain or loss of water from the cells or tissues, the water potential on the inside and outside must be the same.

Question 7

Explanation of osmosis.

Answer 7

• Solution on the left has a low concentration of solute molecules while the solution on the right has a high concentration of solute.
• Both solute and water are in random motion due to kinetic energy.
• The selective membrane only allows water molecules to go through, not the solute.
• So the water molecules would diffuse from the left which has the higher water potential to the right which as the lower water potential, this is going down the concentration gradient.
• It will continue until the water potential on either side of the plasma membrane is equal, a dynamic equilibrium is reached and there’s no net movement of water.

Question 8

What is the highest value of water potential?

Answer 8

Pure water - 0 is the highest water potential.

The more negative the value, the lower the water potential.

Question 9

What happens to osmosis in an animal cell?

Answer 9

• Animal cells e.g. RBC contains various solutes in their watery cytoplasm.
• When RBC is placed into pure water, water molecules will be absorbed by osmosis because it has a lower WP.
• Cell surface membrane is very thin, although it’s flexible, it cannot stretch to very far. Therefore the CSM will break, bursting the cell and release its contents (in RBC, it’s called haemolysis).
• To prevent this, cells usually live in a liquid that has the same water potential as them. e.g. blood plasma is the liquid for RBC.
• If RBC is to be placed into a liquid that has a lower water potential than itself, water will leave the cell by osmosis and cause the cell to shrink and become shrivelled.

Question 10

What happens to osmosis in a plant cell?

Answer 10

Cells in plants that might associate with osmosis are:

1) the central vacuole - contains a solution of salts, sugars and organic acids in water.
2) the protoplast - consisting of the outer cell-surface membrane, nucleus, cytoplasm and the inner vacuole membrane. (under normal state, it would stick to the cell wall)
3) the cellulose cell wall - a tough, non-elastic cover that is permeable to large molecules.

Like animal cells, they contain a variety of solutes. When placed in pure water, they also absorb water by osmosis because of their lower water potential but unlike animal cells, they are unable to control where they can locate in a fluid, so they cannot just place themselves in a fluid that has the same water potential as them.
Plant cells are normally situated in pure water by constant absorption from the plant’s root.
When water enters the cell by osmosis, it causes the protoplast to swell and press against the cell wall because the cell wall is limited to how much it can stretch, pressure builds up and that resists the entry of further water. Due to this, the protoplast is pushed against the cell wall and the cell is said to be turgid.
If the plant cell is placed in a solution with a lower water potential, water leaves by osmosis. The volume of the cell decreases and the protoplast will be no longer pressing against the wall, at this point, the cell is said to be at incipient plasmolysis(normal state). Further loss of water will cause the cell to shrink until the protoplast will be pulled away from the cell wall, the condition is that the cell is plasmolysed.