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3.3.4Exchange and Transport

Mechanisms of Absorption — AQA A Level Biology 7402 Revision Notes & Topical Questions

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📚 AQA A Level Biology 7402 Hub / Topical Revision
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3.3.4 Mechanisms of Absorption — AQA A Level Biology 7402 notes: Explain how the products of digestion are absorbed across the ileum epithelium by diffusion, facilitated diffusion, co-transport and micelle-mediated uptake.The wall of the ileum is folded into thousands of finger-l…

  • co-transporter protein
  • chylomicrons
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Microvilli

Section 1 of 8The ileum as an absorption surface

The wall of the ileum is folded into thousands of finger-like villi, and each epithelial cell has microvilli on its surface (the brush border).

1
These features greatly increase the surface area.
2
The epithelium is a single cell thick,
  • giving a short diffusion distance, and each villus has a rich blood capillary network and a lacteal.
    • (a lymph capillary)
3
The blood in the capillaries carries absorbed molecules away, maintaining the concentration gradient.
4
Villi have muscle that contracts to mix the contents and maintain the gradient.
5
Epithelial cells have many mitochondria to provide ATP for active transport.

Section 2 of 8Absorption of glucose and amino acids: co-transport

Glucose is absorbed against its concentration gradient when levels in the lumen fall, so it requires active transport by co-transport with sodium ions:

The Na+/K+ pump in the basal membrane of the epithelial cell uses ATP to pump sodium ions out into the blood, and potassium ions in.
This lowers the sodium ion concentration inside the cell, producing a concentration gradient of sodium between the lumen and the cell.
Co-transporter protein
Sodium ions diffuse into the cell from the lumen through a co-transporter protein, which carries a glucose molecule (or an amino acid) in with them.
Glucose accumulates in the cell, and then diffuses into the blood through a facilitated diffusion carrier in the basal membrane.

The process is called indirect active transport because ATP is used directly to maintain the sodium gradient, but not to move glucose itself.

Section 3 of 8Facilitated diffusion and other products

When the glucose concentration in the lumen is high, some glucose also enters by facilitated diffusion via channel or carrier proteins, without ATP. Fructose is absorbed by facilitated diffusion only. Amino acids are absorbed by co-transport with sodium ions, using different carrier proteins from glucose. Both enter the capillaries and are carried by the hepatic portal vein to the liver.

Lipids are handled differently.

Micelles
Monoglycerides and fatty acids associate with bile salts to form small micelles.
The micelles bring the lipid products into contact with the epithelial cell membrane.
Because they are lipid-soluble, the monoglycerides and fatty acids diffuse across the phospholipid bilayer; the bile salts stay in the lumen and are reused.
Chylomicrons
Inside the cell they are reassembled into triglycerides in the smooth endoplasmic reticulum, combined with proteins into chylomicrons by the Golgi apparatus,
  • and released by exocytosis.
Chylomicrons enter the lacteal and travel in the lymph before joining the blood.

Section 4 of 8Link to other topics

This topic brings together cell membranes (diffusion, facilitated diffusion, active transport and exocytosis), enzyme action (membrane-bound disaccharidases) and gas exchange ideas (surface area and concentration gradient). Absorption is also the basis of oral rehydration therapy: a solution containing glucose and salts is given to people with cholera or severe diarrhoea, because glucose and sodium absorbed by co-transport draw water in by osmosis, restoring hydration.

Section 5 of 8Adaptations of an epithelial cell

A cell in the ileum epithelium is a good example of structure matching function.

1
The microvilli increase surface area for the carrier proteins and membrane-bound enzymes.
2
Many mitochondria supply ATP for the sodium-potassium pump.
3
Tight junctions between the cells stop the contents of the lumen leaking between cells,
  • so everything must go through the cell membrane, where it can be controlled.
4
The numerous carrier and channel proteins determine which substances are absorbed.
5
Rough endoplasmic reticulum and Golgi apparatus are involved in packaging chylomicrons.

Rate of absorption is increased by high surface area, short diffusion distance, steep gradients (maintained by blood flow and the removal of sugar to the liver) and the number of carrier proteins. In the disease coeliac disease, villi are damaged by an immune reaction to gluten, so the surface area is reduced and absorption falls.

Cholera toxin makes epithelial cells release chloride ions, so water leaves the blood by osmosis, causing severe diarrhoea.

Section 6 of 8Exam-style questions

  1. Explain how glucose is absorbed from the lumen of the ileum into the epithelial cell when its concentration in the lumen is lower than in the cell. [4]4 marks

  2. Describe how the products of lipid digestion are absorbed and transported. [4]4 marks

  3. Explain why oral rehydration solutions contain glucose and sodium chloride. [3]3 marks

Section 7 of 8Key terms

TermMeaning
VillusFinger-like projection of the ileum wall that increases surface area.
MicrovilliFolds in the cell-surface membrane of epithelial cells.
Co-transportMovement of two substances across a membrane by the same carrier protein.
MicelleTiny droplet formed by bile salts with fatty acids and monoglycerides.
ChylomicronLipoprotein particle that carries lipid from the intestine into the lymph.
LactealLymph capillary in a villus.
Facilitated diffusionPassive movement through carrier or channel proteins.

Section 8 of 8Exam technique

1
Stating that glucose is directly pumped by ATP; the energy is used indirectly to maintain the sodium gradient.
2
Saying that fats are absorbed through the blood capillaries.
3
Forgetting that the sodium gradient is produced by the Na+/K+ pump in the other (basal) membrane.
4
Confusing micelles with chylomicrons.
5
Writing that bile salts are absorbed with the lipid products and travel in the lacteal.

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