B3. Osmosis

Question 1

What is osmosis?

What is the definition of osmosis?
Why can water molecules move through the cell membrane?

What has a water potential of zero?
What does adding solutes do to the water potential?

Answer 1

What is osmosis?
Osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential (i.e. higher concentration of water molecules) to an area of lower water potential (i.e. lower concentration of water molecules). Water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution.
Water molecules are small and can diffuse easily through the cell membrane, but large solute molecules can’t.

Pure water has a water potential of zero. Adding solutes to pure water lowers its water potential-so the water potential of any solution is always negative. The more negative the water potential, the stronger the concentration or solutes in the solution

Question 2

What does it mean if two solutions are isotonic?
How do cells react to isotonic solutions?

What does it mean if a solution is hypotonic and what will happen to a cell and how?

What does it mean if a solution is hypertonic and what will happen to a cell and how?

Answer 2

If two solutions have the same water potential they’re said to be isotonic. Cells in an isotonic solution won’t lose or gain any water-there’s no net movement of water molecules because there’s no difference in water potential between the cell and the surrounding solution.

If a cell is placed in a solution that has a higher water potential it will swell as water moves into it by osmosis. Solutions with a higher water potential compared with the inside of the cell are called hypotonic.

If a cell is placed in a solution that has a lower water potential it may shrink as water moves out of it by osmosis. Solutions with a lower water potential than the cell are called hypertonic.

Question 3

Factors affecting the rate of osmosis
The factors affecting the rate of osmosis are similar to those affecting the rate of diffusion:….

Answer 3

• The water potential gradient - the higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time.
• The thickness of the exchange surface - the thinner the exchange surface, the faster the rate of osmosis.
• The surface area of the exchange surface - the larger the surface area,
  the faster the rate of osmosis.