5.5 - Osmosis

Question 1

Define solute

Answer 1

Substance dissolved in a solvent (e.g. water)

Question 2

Define concentration

Answer 2

Amount of solute in a certain volume of aqueous solution

Question 3

Define water potential

Answer 3

Pressure exerted by molecules as they collide with membrane or barrier

Question 4

What is the water potential of PURE water?

Answer 4

• Has the highest w.p. of 0
• Every solution has a -ve water potential
• Higher concentration = lower water potential
• Pure water at 25 degrees and 100kPa is 0 w.p.

Question 5

What happens when 2 solutions of different concentrations are separated by a partially permeable membrane?

Answer 5

• Water moves between the solutions
• Water moves from an area of high w.p. (low solute concentration) to an area of low w.p. (high solute concentration) until equilibrium is reached.

Question 6

What is hydrostatic pressure?

Answer 6

Pressure exerted by water

- If water diffuses into a solution in a closed system the volume increases, increasing hydrostatic pressure.

Question 7

What is cytolysis?

Answer 7

When a cell is paced in a solution of higher w.p. of its cytoplasm, water diffuses in, increasing hydrostatic pressure causing cell to burst.

Question 8

What is crenation?

Answer 8

Cell placed in solution of lower w.p. than the cytoplasm, water leaves via osmosis (cell shrivels)

Question 9

How is cytolysis and crenation prevented?

Answer 9

Multicellular organisms have control mechanisms to ensure they remain isotonic (equal water potential).

Question 10

How do plant cells become turgid?

Answer 10

• They cannot control the water potential of surrounding fluids as roots are surrounded by pure water
• Water enters plant cell - turgor pressure increases as water pushes against walls
• Cells resist entry of water meaning the cell is turgid

Question 11

What is plasmolysis?

Answer 11

When water leaves the plant cell, volume of cytoplasm falls pulling plasma membrane from cell wall leading to cell collapsing