Mechanisms for the absorption of the products of digestion by cells lining…

The idea

Villi and microvilli provide a vast surface area for absorption in the ileum. Glucose and amino acids are absorbed via co-transport. Sodium-potassium pumps actively transport sodium ions from epithelial cells into the blood, lowering intracellular sodium concentration. Sodium ions then diffuse back from the lumen through co-transporter proteins, carrying glucose or amino acids against their concentration gradients. These molecules enter the blood by facilitated diffusion. For lipids, bile salts emulsify large droplets into smaller ones. After digestion, bile salts associate with monoglycerides and fatty acids to form micelles. Micelles transport these products to the epithelial membrane, where they diffuse directly across the bilayer.

What earns the marks

• Describe the sodium-potassium pump actively removing sodium ions from the epithelial cell into the blood, using ATP to create a concentration gradient.
• Explain that sodium ions re-enter the cell down their concentration gradient through a co-transporter protein, carrying glucose or an amino acid with them.
• State that glucose is moved against its own concentration gradient into the cell and then leaves the cell into the blood by facilitated diffusion.
• Explain that micelles carry monoglycerides and fatty acids to the epithelial cell membrane, where they break down and release them to diffuse directly through the phospholipid bilayer.
• Link the loss of microvilli, or fewer co-transport, carrier or channel proteins, to a reduced rate of absorption.

Common mistakes and how to put them right

• Saying the micelle itself is absorbed or enters the epithelial cell.

  Put it right: Micelles break down outside the cell; only the monoglycerides and fatty acids diffuse across the membrane.

• Calling co-transport active transport of glucose, with ATP said to act directly on the glucose carrier.

  Put it right: It is indirect active transport; ATP is used by the sodium-potassium pump, not the co-transporter.

• Explaining diarrhoea as water loss without a water potential argument.

  Put it right: State that the water potential of the lumen falls, or that of the cells rises, so water leaves the cells by osmosis.

Exam tips

• Start every co-transport answer at the sodium-potassium pump; the sequence only makes sense in that order.
• Damage to the ileum lining requires two ideas: fewer co-transport proteins and reduced surface area, followed by the water potential consequence.
• Clearly distinguish between the emulsification of lipid droplets by bile salts and the subsequent formation of micelles from digested monoglycerides and fatty acids.