Absorption in the Ileum Revision Notes | A Level AQA Biology

1. Why Absorption in the Ileum Deserves Your Full Attention

Digestion breaks macromolecules down into their monomers — but that is only half the story. Getting those monomers from the gut lumen into the bloodstream is a precise, energy-driven operation, and the ileum is where most of it happens. The mechanisms here are a favourite target for AQA examiners precisely because students often confuse the routes taken by different molecules.

Two distinct processes govern absorption: co-transport for amino acids and monosaccharides, and micelle-mediated diffusion for lipid products. Each exploits the physical chemistry of the molecule being absorbed. Understanding why each route exists — not just what happens — is what separates a grade 7 answer from a grade 9 one.

2. Co-Transport of Monosaccharides and Amino Acids

Picture the epithelial cell lining the ileum. It has two faces: the apical membrane (facing the gut lumen, covered in microvilli) and the basolateral membrane (facing the blood capillary). These two faces do very different jobs, and understanding both is essential.

Glucose and amino acids are polar, water-soluble molecules. They cannot cross the phospholipid bilayer by simple diffusion — they need carrier proteins. The clever part is how the cell powers their entry without spending ATP directly at the apical membrane.

The sodium gradient is the engine. Sodium ion concentration is kept low inside the epithelial cell by the sodium-potassium pump (Na⁺/K⁺-ATPase) on the basolateral membrane, which continuously pumps three Na⁺ out for every two K⁺ in, using ATP. This creates a steep electrochemical gradient — sodium ions are strongly attracted back into the cell. The co-transporter at the apical membrane harnesses that attraction to drag glucose or an amino acid in alongside the sodium ion.

Once inside the cell, glucose and amino acids leave via facilitated diffusion through carrier proteins (e.g. GLUT2 for glucose) on the basolateral membrane, moving down their concentration gradient into the blood capillary of the villus. From there, they enter the hepatic portal vein and travel to the liver.

Worked example: Tracing Co-Transport Step by Step

Describe, in sequence, how a molecule of glucose moves from the gut lumen into the blood capillary of a villus.

1. Na⁺/K⁺-ATPase on the basolateral membrane pumps 3 Na⁺ out of the cell and 2 K⁺ in, using 1 ATP. This keeps intracellular [Na⁺] low. — This is primary active transport — ATP is used here. The resulting low [Na⁺] inside the cell is the energy store that powers the next step.
2. At the apical membrane, the SGLT1 carrier protein binds one Na⁺ ion (from the lumen, where [Na⁺] is high) and one glucose molecule simultaneously. — Both binding sites must be occupied before the protein changes conformation. This is why the process is called co-transport — two substances move together.
3. Na⁺ moves down its electrochemical gradient into the cell; the energy released drives glucose into the cell against its own concentration gradient. — This is secondary active transport — no ATP is used at this step, but the process depends on the ATP spent in step 1 to maintain the gradient.
4. Glucose concentration inside the cell rises. GLUT2 carrier proteins on the basolateral membrane allow glucose to diffuse down its concentration gradient into the blood capillary. — This step is passive — no energy required. The concentration gradient is maintained because blood flow continuously removes glucose from the capillary.
5. The Na⁺ that entered in step 3 is pumped back out by the Na⁺/K⁺-ATPase, ready to drive another cycle of co-transport. — The pump runs continuously, so the sodium gradient — and therefore the driving force for co-transport — is never depleted.

Answer: Glucose moves from lumen → epithelial cell (via SGLT1 co-transport, powered by Na⁺ gradient) → blood capillary (via GLUT2 facilitated diffusion). The Na⁺ gradient is maintained by the Na⁺/K⁺-ATPase using ATP.

The same logic applies to amino acid absorption. Different co-transporter proteins handle different amino acid classes, but the principle is identical: sodium ions enter alongside the amino acid, the gradient is maintained by the basolateral pump, and the amino acid exits by facilitated diffusion into the blood capillary.

Lipid digestion produces fatty acids and monoglycerides — molecules that are largely non-polar and therefore insoluble in the watery environment of the gut lumen. This creates a transport problem: how do non-polar molecules travel through an aqueous medium to reach the epithelial surface?

3. Micelle Formation and Lipid Absorption

The answer is micelles. Bile salts, secreted by the liver and stored in the gall bladder, are amphipathic molecules — they have a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. In the gut lumen, bile salts arrange themselves around clusters of fatty acids and monoglycerides, with their hydrophobic tails pointing inward (towards the lipid) and their hydrophilic heads pointing outward (towards the water). The result is a micelle: a tiny, water-soluble sphere with a lipid core.

Micelles are not permanent structures. As they reach the apical membrane of an epithelial cell, the fatty acids and monoglycerides are released and diffuse directly through the phospholipid bilayer — no carrier protein needed, because they are non-polar and the membrane is non-polar. The bile salts remain in the lumen and are recycled, travelling to the ileum where they are reabsorbed and returned to the liver via the enterohepatic circulation.

Inside the epithelial cell, fatty acids and monoglycerides are transported to the smooth endoplasmic reticulum, where they are reassembled into triglycerides. These triglycerides, along with cholesterol and phospholipids, are then packaged with proteins in the Golgi apparatus to form chylomicrons. Chylomicrons are too large to enter the narrow blood capillaries directly, so they are released by exocytosis into the lacteals — the blind-ended lymphatic vessels at the centre of each villus.

From the lacteals, chylomicrons travel through the lymphatic system, eventually entering the bloodstream via the thoracic duct at the subclavian vein. This is a fundamentally different route from glucose and amino acids, which go straight into the blood capillaries and the hepatic portal vein. That distinction is a reliable exam question target.

4. Comparing the Two Absorption Routes

Now that both mechanisms are clear, it is worth placing them side by side. AQA examiners frequently ask you to 'compare' or 'contrast' the absorption of lipids with that of amino acids or monosaccharides — a command word that demands both similarities and differences.

One similarity worth noting: both routes ultimately deliver nutrients to the bloodstream. The difference is the path taken and the degree of processing required inside the cell. Lipid products require far more intracellular handling before they can leave the epithelium.

5. Structural Adaptations of the Ileum That Support These Mechanisms

The mechanisms above only work efficiently because the ileum is structurally specialised to maximise the rate of absorption. These adaptations increase surface area and reduce diffusion distances — both of which increase the rate of transport according to Fick's law.

• Villi: finger-like projections of the mucosa that increase surface area and bring epithelial cells close to the lumen contents.
• Microvilli (brush border): microscopic projections on the apical surface of each epithelial cell, further amplifying surface area and housing the co-transporter proteins and digestive enzymes.
• Lacteals: lymphatic capillaries at the centre of each villus, providing the route for chylomicron absorption.
• Dense capillary network: blood capillaries within each villus maintain a low concentration of absorbed glucose and amino acids, sustaining the concentration gradient for facilitated diffusion out of the cell.
• Thin epithelium: a single layer of epithelial cells minimises the diffusion distance between lumen and blood.

These structural features are not separate from the mechanisms — they are prerequisites for them. Without microvilli housing SGLT1 proteins, co-transport could not occur at the rate needed. Without lacteals, chylomicrons would have nowhere to go. Structure and function are inseparable here.

6. Exam Technique: Securing Full Marks on Absorption Questions

Absorption questions are predictable in structure but unforgiving in precision. The marks are awarded for specific mechanistic detail, not general statements. Here is how to approach the most common question types.

When asked to 'explain' co-transport, the mark scheme will expect you to state: (1) the Na⁺/K⁺-ATPase creates a low intracellular [Na⁺] by active transport; (2) Na⁺ moves into the cell down its electrochemical gradient via the co-transporter; (3) this movement drives glucose or amino acid into the cell against its concentration gradient; (4) the molecule exits by facilitated diffusion on the basolateral side. Missing any of these steps costs marks.

For micelle questions, examiners want to see that you understand why micelles are necessary (lipids are non-polar and insoluble in the aqueous lumen) and what happens to the bile salts after the lipids are released (they remain in the lumen and are recycled — they do not enter the epithelial cell with the fatty acids).