Osmosis

Question 1

What is 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

Solutions and water potential

Answer 2

the addition of a solute to pure water will lower its water potential
• the water potential of a solution (water + solute) must always be less than zero, that is, a negative value
• the more solute that is added (i.e., the more concentrated a solution), the lower (more negative) its water potential
• water will move by osmosis from a region of higher (less negative)
water potential (e.g., -20 kPa) to one of lower (more negative) water potential (e.g., -30 kPa).

Question 3

Explanation of osmosis

Answer 3

• The solution on the left has a low concentration of solute molecules while the solution on the right has a high concentration of solute molecules.
• Both the solute and water molecules are in random motion due to their kinetic energy.
• The selectively permeable plasma membrane, however, only allows water molecules across it and not solute molecules.
• The water molecules diffuse from the left-hand side, which has the higher water potential, to the right-hand side, which has the lower water potential, that is, down a water potential gradient
• At the point where the water potentials on either side of the plasma membrane are equal, a dynamic equilibrium is established and there is no net movement of water.

Question 4

understanding water potential

Answer 4

The highest value of water potential, that of pure water, is zero, and so all other values are negative. The more negative the value, the lower the water potential.

Question 5

Osmosis and animal cells

Answer 5

Animal cells, such as red blood cells, contain a variety of solutes dissolved in their watery cytoplasm. If a red blood cell is placed in pure water it will absorb water by osmosis because it has a lower water potential. Cell surface membranes are very thins and although they are flexible, they cannot stretch to any great extent. The cell-surface membrane will therefore break, bursting the cell and releasing its contents (in red blood cells this is called haemolysis). To prevent this happening, animal cells normally live in a liquid which has the same water potential as the cells. In our example, the liquid is the blood plasma. This and red blood cells have the same water potential.
If a red blood cell is placed in a solution with a water potential lower than its own, water leaves by osmosis and the cell shrinks and becomes shrivelled (see Table 1).