3.2.3 Transport across cell membranes

Question 1

Give three molecules that are present in animal cell membranes.

Answer 1

Any of these three:

• Phospholipids
• Cholesterol
• Proteins
  
  • Channel/Carrier proteins
  • Receptor proteins
• Glycoprotein
• Glycolipids

Question 2

What effect does cholesterol have on the cell membrane?

Answer 2

Cholesterol restricts the movement of the phospholipids, making the membrane less fluid and more rigid.

Question 3

Explain why the plasma membrane can be described as having a fluid mosaic structure.
[2 marks]

Answer 3

• The membrane is described as fluid because the phospholipids are constantly moving.
• It is described as a mosaic because the proteins are scattered throughout the membrane like tiles in a mosaic.

Question 4

Diffusion is a passive process. What does this mean?

Answer 4

No energy is needed for it to happen.

Question 5

How do the microvilli increase the rate of diffusion?

Answer 5

• Microvilli give the cell a larger surface area.

- Hence more particles can be exchanged in the same amount of time.

Question 6

Chloride ions are transported into a cell across its cell-surface membrane by facilitated diffusion.
What type of molecule must be present in a cell membrane for the facilitated diffusion of chloride ions to take place?
[1 mark]

Answer 6

channel proteins
(Channel proteins transport charged particles, such as ions)

Question 7

Chloride ions are transported into a cell across its cell-surface membrane by facilitated diffusion.
Explain why the simple diffusion of chloride ions across a cell-surface membrane would be extremely slow.
[2 marks]

Answer 7

E.g.

• Ions are water soluble…
• …and the centre of the phospholipid bilayer is hydrophobic.

Question 8

Chloride ions are transported into a cell across its cell-surface membrane by facilitated diffusion.
The chloride ions in the cell are not immediately used up. Describe and explain what will happen to the rate of facilitated diffusion of the chloride ions into the cell over time.
[2 marks]

Answer 8

• The rate of facilitated diffusion will slow down/level off.
• As diffusion progresses,…
  
  • …the concentration gradient of the chloride ions will decrease.
    or
  • …the concentration of chloride ions inside and outside of the cell will become the same (an equilibrium will be reached).

Question 9

The student produced the sucrose solutions with different concentrations from a concentrated sucrose solution.
Name the method she would have used to produce these sucrose solutions.
[1 mark]

Answer 9

Dilution series