A1 - co-transport

Question 1

Explain how co-transport
occurs using absorption
of sodium ions and
glucose by cells lining
the mammalian ileum as
an example

Answer 1

-Sodium ions actively transported out of epithelial cells by sodium potassium pump (carrier protein) into the blood
-This maintains a higher concentration of sodium ions in the lumen of the ileum compared with the inside of the epithelial cells
-Sodium ions diffuse into the epithelial cell through a co-transport protein
(carrier protein)
-The movement of sodium ions carries glucose/amino acids through the
same protein into the epithelial cell
-Glucose/amino acids move into the blood by facilitated diffusion (through
another carrier protein)

Question 2

Describe how cells are
adapted for rapid
exchange across
membranes

Answer 2

-Microvilli on epithelial lining > finger-like projections on the cell surface membrane provide a larger surface area for carrier proteins involved in facilitation diffusion,
active transport and co transport
-High density of carrier proteins and protein channels on the membrane

Question 3

Role of diffusion in absorption

Answer 3

-carbohydrates and proteins being digested continuously, and there’s normally a greater conc of glucose and a.a within ileum than in blood
-so there’s conc gradient down which glucose moves by facilitated diffusion from inside ileum into blood
-blood constantly being circulated by heart , glucose absorbed into it is constantly being removed by cells as they use it up during respiration
-maintain conc gradient between inside of ileum and blood
-so rate of movement by facilitated diffusion across epithelial cell-surface membrane is increased