59 WJEC Biology AS Level - Marianne Izen - 2nd Edition (1.3 Cell Membranes And Transport)

Question 1

What is co-transport?

Answer 1

Co-transport is a type of facilitated diffusion that brings molecules and ions into cells together on the same transport protein molecule.

Question 2

Give an example of co-transport?
What is its importance?

Answer 2

Sodium-glucose co-transport.

It is significant in absorbing glucose and sodium ions across cell membranes and into the blood in-

a. ileum
b. kidney nephron.

Question 3

What are the steps in sodium-glucose co-transport?

Answer 3

1. A glucose molecule and two sodium ions outside the cell bind to a carrier protein in the cell membrane.
2. The carrier protein changes shape and deposits the glucose molecule and the sodium ions inside the cell. This is facilitated diffusion.
3. The glucose molecule and sodium ions separately diffuse through the cytoplasm to the opposite membrane.
4. The glucose passes into the blood by facilitated diffusion.
5. The sodium ions are carried out of the epithelial cell by active transport, by the same carrier that, simultaneously, moves potassium ions in.
6. Thus, sodium ion concentration remains low in the epithelial cell.
7. Therefore, more sodium ions move in from the intestinal lumen, bringing glucose in on the same carrier molecule (step 1)

Question 4

Co-transport (Key-Term)

Answer 4

A transport mechanism in which facilitated diffusion brings molecules and ions, such as glucose and sodium ions, across the cell membrane together into a cell.

Question 5

Draw a diagram to show sodium-glucose co-transport.

Answer 5