How are large food molecules digested and then absorbed across the small intestine into the blood?
Digestion in mammals: the action of carbohydrases, lipases and proteases (including membrane-bound disaccharidases and dipeptidases); the role of bile salts in lipid digestion; absorption of the products across the ileum epithelium, including co-transport of glucose and amino acids and the absorption of monoglycerides and fatty acids.
An AQA A-Level Biology answer on digestion and absorption. Covers carbohydrase, lipase and protease action, membrane-bound disaccharidases and dipeptidases, bile salt emulsification and micelles, villi adaptations, and sodium-dependent co-transport of glucose and amino acids.
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What this dot point is asking
AQA wants you to describe how the three classes of large biological molecule are digested by specific enzymes, the special roles of bile salts and membrane-bound enzymes, and how the soluble products are absorbed across the ileum epithelium - including the co-transport mechanism for glucose and amino acids.
Why digestion is needed
Large biological molecules (starch, proteins, lipids) are too big and insoluble to cross cell-surface membranes. Hydrolysis by digestive enzymes breaks them into small, soluble molecules that can be absorbed.
Carbohydrate digestion
- Amylase (a carbohydrase, made in the salivary glands and pancreas) hydrolyses starch into maltose.
- Membrane-bound disaccharidases on the epithelial cells of the ileum then complete digestion: maltase hydrolyses maltose into glucose, sucrase hydrolyses sucrose into glucose and fructose, and lactase hydrolyses lactose into glucose and galactose.
Protein digestion
Proteases hydrolyse proteins in stages, which is more efficient because each enzyme is specific:
- Endopeptidases hydrolyse peptide bonds within the polypeptide chain, creating more ends.
- Exopeptidases hydrolyse peptide bonds at the ends of chains, releasing single amino acids and dipeptides.
- Membrane-bound dipeptidases on the ileum epithelium hydrolyse the final dipeptides into individual amino acids.
Breaking the protein internally first (endopeptidases) provides many more terminal ends for exopeptidases to work on, speeding up the whole process.
Lipid digestion and the role of bile salts
Lipids are insoluble in water, which is a problem for water-soluble lipase. The solution is emulsification.
- Bile salts (produced by the liver, stored in the gall bladder) emulsify large lipid droplets into many tiny droplets.
- This greatly increases the surface area of lipid exposed to lipase, speeding up hydrolysis.
- Lipase (from the pancreas) hydrolyses triglycerides into monoglycerides and fatty acids.
- The products associate with bile salts to form tiny structures called micelles, which carry the monoglycerides and fatty acids to the epithelial cell surface.
The ileum is adapted for absorption
The ileum epithelium maximises the rate of absorption (Fick's law again):
- Villi - finger-like folds of the wall that hugely increase surface area.
- Microvilli - the brush border on each epithelial cell, a further large increase in surface area.
- Thin epithelium (one cell thick) and a close capillary network - short diffusion path.
- Rich blood supply and a lacteal - carry away absorbed products, maintaining steep concentration gradients.
- Muscular wall - maintains movement, keeping fresh contents against the surface.
Absorbing the products
Different products are absorbed differently:
Glucose and amino acids - sodium co-transport (indirect active transport).
- A sodium-potassium pump actively pumps sodium ions out of the epithelial cell into the blood, using ATP. This keeps the internal sodium concentration low.
- Sodium ions diffuse from the lumen into the cell down their gradient through a co-transporter protein, dragging glucose (or an amino acid) in against its concentration gradient.
- Glucose and amino acids accumulate in the cell, then leave into the blood by facilitated diffusion down their gradient.
Monoglycerides and fatty acids - diffusion. Being lipid-soluble (non-polar), they diffuse directly across the phospholipid bilayer of the epithelial cell membrane after release from micelles. Inside, they are reformed into triglycerides, packaged into chylomicrons, and enter the lacteal (lymph), not the blood capillary.
Try this
Q1. Explain the advantage of starch being digested by amylase and then a membrane-bound disaccharidase, rather than a single enzyme in the lumen. [2 marks]
- Cue. The disaccharidase is bound to the epithelial membrane, so the final products (e.g. glucose) are made right at the absorption surface, maintaining a steep gradient and reducing loss of product back into the lumen.
Q2. Describe the role of bile salts in the digestion and absorption of lipids. [3 marks]
- Cue. Bile salts emulsify large lipid droplets into smaller ones, increasing surface area for lipase to act; they also form micelles with monoglycerides and fatty acids that carry the products to the epithelial cell membrane for absorption.
Q3. A drug blocks the sodium-potassium pump in ileum epithelial cells. Explain its effect on glucose absorption. [3 marks]
- Cue. The pump no longer removes sodium from the cell, so the sodium gradient into the cell collapses; co-transport of glucose with sodium stops; glucose absorption against its gradient therefore falls greatly because it relies on the sodium gradient.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2017 AQA Paper 25 marksDescribe how the products of starch digestion and protein digestion are absorbed from the lumen of the ileum into the blood.Show worked answer →
A 5-mark answer needs the sodium gradient set up first, then co-transport, then facilitated diffusion into the blood.
- Point 1 (set up the gradient)
- Sodium ions are actively transported (by a sodium-potassium pump) out of the epithelial cell into the blood, using ATP. This keeps the sodium concentration inside the epithelial cell low.
- Point 2 (co-transport into the cell)
- Sodium ions then diffuse from the lumen into the epithelial cell down their concentration gradient through a co-transporter protein, carrying glucose (or amino acids) with them against their own concentration gradient. This is indirect active transport.
- Point 3 (build-up)
- Glucose and amino acid concentration therefore rises inside the epithelial cell.
- Point 4 (into the blood)
- Glucose and amino acids then move out of the cell into the blood by facilitated diffusion down their concentration gradient, through carrier proteins.
- Point 5 (link)
- Both glucose and amino acids use the same sodium co-transport mechanism; the energy ultimately comes from the active transport of sodium, not the sugar itself.
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