5.5 osmosis (e)

Question 1

What is osmosis?

Answer 1

The diffusion of water through a partially permeable membrane down a water potential gradient. The net movement of water is from a high water potential to a low water potential until it reaches a water potential equilibrium, on both sides of the membrane.

Question 2

What is water potential and its units?

Answer 2

The pressure exerted by water molecules as they collide with a membrane or container. Measured in Pa, kPa. Symbol: psi: Ψ

Question 3

Describe pure water potential

Answer 3

• Pure water has a water potential of 0kPa at standard temperature and pressure - 25°C and 100kPa
• highest/least negative value

Question 4

Relationship between concentration and water potential

Answer 4

The more concentrated a solution, the lower its water potential and the more negative its value is

Question 5

What happens when an animal cell is placed in a hypotonic solution? (higher Ψ)

Answer 5

• water moves into the cell by osmosis, increasing hydrostatic pressure, cell can burst due to no cell walls - cytolysis
• The contents of the cell are released

Question 6

What happens when an animal cell is placed in a hypertonic solution? (lower Ψ)

Answer 6

• water moves out of the cell, decreasing hydrostatic pressure, the cell crenates
• cell shrinks and becomes shrivelled, more concentrated cell contents

Question 7

How do animals stop cytolysis and crenation from occurring?

Answer 7

• animals usually have control mechanisms to make sure their cells are continuously surrounded by aqueous solutions with an equal water potential (isotonic)
• in blood this is plasma

Question 8

What happens when a plant cell is placed in a hypotonic solution? (higher Ψ)

Answer 8

• water enters
• hydrostatic pressure pushed the membrane against the rigid cell walls, creating turgor pressure
• as turgor pressure increases, it resists further entry of water, and is turgid
• protoplast pushed against the cell wall

Question 9

What happens when a plant cell is placed in a hypertonic solution? (lower Ψ)

Answer 9

• water leaves
• leads to a reduction in the volume of the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall
• becomes plasmolysed
• protoplast completely pulled away from the cell wall
• space filled with external solution of lower water potential

Question 10

Osmosis investigation for plant cells

Answer 10

• pieces of potato or onion can be placed into sugar or salt solutions with different concentrations thus different water potentials
• as the plant tissues loses or gains water, it will increase or decrease in mass and size

Question 11

Osmosis investigation for animal cells

Answer 11

• eggs can be used - chicken eggs with the shell removed has a single-membrane bound structure remaining and they behave in the same way as a cell when placed in solutions of varying water potentials
• place in different concentrations of sugar syrup
• osmosis will occur based on the concentration
• if egg is hard-boiled the membrane will be damaged