5.5- Osmosis

Question 1

What is osmosis?

Answer 1

The diffusion of water through a partially permeable membrane from a region of high water potential to a region of low water potential.

• passive process.
• no energy required.

Question 2

Explain water potential.

Answer 2

The pressure exerted by water molecules as they collide with a membrane/container.

• measured in Pa or kPa
• pure water at RTP = 0kPa (highest possible value for water potential).

The more concentrated a solution, the more negative the water potential.

Diffusion of water causes increase in volume and in a closed sys’,tem, this can increase pressure (hydrostatic pressure).

Question 3

Osmosis in animal cells.

Answer 3

In a solution with higher water potential than the cytoplasm:
-water moves into the cell. Hydrostatic pressure increases. Cell surface membrane can’t stretch so breaks and the cell bursts (cytolysis).

If placed in solution with lower water potential than the cytoplasm:
- lose water by osmosis. Reduction in volume of the cell. Cell surface membrane will crenate. (Becomes shrivelled)

Isotonic:
-water constantly enters and leaves but at an equal rate.

Multicellular animals have control mechanisms to ensure cells are surrounded by solutions with equal water potential. (Eg. Blood plasma)

Question 4

Osmosis in plant cells.

Answer 4

-plant cells have strong cellulose walls surrounding the cell surface membrane.

When water enters by osmosis:

• increased hydrostatic pressure pushes membrane against rigid walls (turgor).
• turgor pressure increases and resists entry of more water. Therefore the cell=rigid.

When placed in a solution of lower water potential:

• water is lost.
• reduction in volume of cytoplasm.
• cell is plasmolysed as membrane is pulled away from the cell wall.