Cells: Transport Across Membranes - Osmosis

Question 1

What is osmosis?

Answer 1

• Osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential

Question 2

What is water potential and how does the concentration of water molecules affect it?

Answer 2

• Water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution
• Water of a higher water potential has a higher concentration of water molecules
• Water of a lower water potential has a lower concentration of water molecules

Question 3

What symbol represents water potential?

Answer 3

• Represented by “psi” Ψ

Question 4

What is the water potential of pure water?

Answer 4

• Pure water has a water potential of 0

Question 5

How does solute affect water potential?

Answer 5

• Water molecules are small and can diffuse easily through the cell membrane, but large solute molecules can’t
• When a solute is in water, the water molecules bind to the solute and are prevented from fitting through the gaps in a membrane
• So a solute reduces the ability of water to move
• The more solute there is dissolved in water, the more negative the water potential is. i.e the lower the water potential

Question 6

What does isotonic mean?

Answer 6

• An isotonic solution is when the water potential is the same in the solution and the cell within the solution
• No net movement of water molecules because there’s no difference in water potential

Question 7

What does hypotonic mean?

Answer 7

• Hypotonic is when the water potential of a solution is more positive (closer to zero) than the inside of the cell
• Cell will swell as water moves into it by osmosis

Question 8

What does hypertonic mean?

Answer 8

• Hypertonic is when the water potential of a solution is more negative than the cell
• Cell may shrink (flaccid in plants) as water moves out of it by osmosis

Question 9

Describe what happens to animal cells in hypertonic solutions

Answer 9

• When placed in a hypertonic solution, the cells lose water by osmosis and shrink
• They are described as being crenated

Question 10

Describe what happens to red blood cells in hypotonic solutions

Answer 10

• When placed in a hypotonic solution, the cells gain water by osmosis and burst
• The bursting of red blood cells when subjected to a hypotonic
  environment is called haemolysis
• Animal cells don’t have cell walls so the pressure causes them to burst

Question 11

Describe an animal cell in an isotonic solution

Answer 11

• When placed in an isotonic solution, there is no net loss or gain of water and the cell’s water content remains steady

Question 12

Describe a plant cell in a hypotonic solution

Answer 12

• Plant cell becomes fully turgid

- The net movement of water by osmosis is into the cell and the protoplast swells and presses against the cell wall

Question 13

Describe a plant cell in a hypertonic solution

Answer 13

• The net movement of water by osmosis is out of the cell and the protoplast shrinks
• The protoplast is pulled completely away from the cell wall and the cell is fully plasmolysed (flaccid)
• Plasmolysis - contraction of the protoplast of a plant cell as a result of loss of water from the cell

Question 14

Describe a plant cell in an isotonic solution

Answer 14

• There is no net movement of water in this case and no pressure potential as the protoplast ceases to press against the cell wall
• The cell displays incipient plasmolysis where the membrane is just beginning to pull away from the cell wall

Question 15

Factors Affecting the Rate of Osmosis

Answer 15

• 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

• Thickness of exchange surface
- The thinner the exchange surface, the faster the rate of osmosis

• Surface area of the exchange surface
- The larger the surface area, the faster the rate of osmosis