2.3 Transport across cell membranes

Question 1

The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the same. Describe it.

Answer 1

The arrangement of phospholipids, proteins, glycoproteins and glycolipids is known as the fluid-mosaic model of membrane structure. Fluid because each component can move around the others, mosaic because there are several types of component.

Question 2

Cholesterol may be present in cell membranes. What does it do?

Answer 2

Cholesterol restricts the movement of other molecules making up the membrane, reducing fluidity and permeability.

Question 3

There are 5 ways substances can move across membranes. What are they?

Answer 3

1. simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer)
2. facilitated diffusion (involving the roles of carrier proteins and channel proteins)
3. osmosis (explained in terms of water potential)
4. active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP)
5. co-transport (e.g. the absorption of sodium ions and glucose by cells lining the mammalian ileum).

Question 4

How might cells be adapted for rapid transport across their internal or external membranes?

Answer 4

Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes.