3.3.3 Digestion and absorption

Digestion is the work of breaking macromolecules into pieces small enough to cross a membrane. AQA expects you to know which enzymes hydrolyse which bonds, where each step happens, and how each kind of product gets absorbed at the ileum — because the absorption route is not the same for every product.

Carbohydrases hydrolyse glycosidic bonds in stages from mouth to brush border.

Starch is hydrolysed in three locations, and the bond broken at every stage is the glycosidic bond. The first enzyme is salivary amylase, secreted into the mouth, and the work is finished at the brush border of the ileum by membrane-bound disaccharidases.

1. Salivary amylase in the mouth hydrolyses starch to the disaccharide maltose. Activity continues into the stomach until the acidic environment inactivates the enzyme.
2. Pancreatic amylase, secreted into the duodenum, completes the hydrolysis of residual starch and intermediate dextrins to maltose.
3. Brush-border disaccharidases (maltase, sucrase, lactase) hydrolyse the disaccharides to monosaccharides at the same membrane where the products are absorbed.

Write hydrolyse (or hydrolysis) explicitly. "Break down", "split", and "cut" alone do not earn the digestion mark. The credited verb is hydrolyse.

Name the specific bond. Glycosidic bond for starch and disaccharides. "Bond" alone is not credited. The bond name is the mark; the act of hydrolysis is a separate mark.

Because the disaccharidases are embedded in the same membrane as the monosaccharide transporters, the final hydrolysis and the absorption step happen at the same surface. The monosaccharide produced by a brush-border enzyme has no diffusion distance to cover before it is taken up by the cell.

Three peptidase classes hydrolyse peptide bonds in coordinated sequence.

Polypeptides arriving in the small intestine are reduced to free amino acids by three peptidase classes, classified by the site of action on the chain. Endopeptidases and exopeptidases are secreted into the duodenum by the exocrine pancreas; dipeptidases are membrane-bound at the brush border of the ileal epithelium. The bond broken in every case is the peptide bond.

Write hydrolyse somewhere in the answer. The peptidase synergy mark is conditional on the word. "Cleave", "cut", and "break" are not accepted as substitutes for hydrolyse here.

The synergy between endopeptidases and exopeptidases is what makes protein digestion fast. Each internal cleavage by an endopeptidase produces two new free chain ends; across the protein pool, the total number of free ends multiplies with every pass. Exopeptidases then have more sites to attack simultaneously, and total digestion proceeds faster than either class could achieve alone.

Pitfall — Peptidase synergy is a compound mark

The peptidase synergy compound mark needs both halves in one sentence.

Endopeptidases hydrolyse internal peptide bonds AND exopeptidases hydrolyse terminal peptide bonds. Stating only one half earns nothing for the synergy mark. The consequence — more free ends, more surface area for exopeptidase action, faster total digestion — is the follow-up mark, conditional on naming both peptidase classes and the position each cleaves.

Lipid digestion runs as four ordered stages from emulsification to membrane crossing.

Lipids are hydrophobic and coalesce into large droplets in the intestinal lumen, exposing only a small surface area to the surrounding water. Lipase is water-soluble and acts only at the lipid–water interface, so unaided hydrolysis would be too slow to be physiologically useful. The four-stage sequence solves the surface-area problem and delivers the products to the membrane.

1. Emulsification. Bile salts, made in the liver and stored in the gall bladder, are released into the duodenum and break large fat droplets into many small droplets. The process is mechanical; no bonds are broken. The consequence is a large increase in surface area for lipase to act on.
2. Hydrolysis by lipase. Pancreatic lipase hydrolyses the ester bonds in the triglycerides of the small droplets, releasing fatty acids and monoglycerides.
3. Micelle formation. Bile salts associate with the released fatty acids and monoglycerides to form micelles — small spherical structures that transport the lipid products through the aqueous intestinal contents to the brush border surface of the ileal epithelial cells.
4. Diffusion across the membrane. At the membrane, the micelle breaks down close to the cell surface. The fatty acids and monoglycerides are non-polar and diffuse directly across the phospholipid bilayer of the cell-surface membrane. The micelle itself does not cross.

Emulsification is mechanical. Don't write "emulsification produces micelles" or "emulsification causes hydrolysis". Emulsification breaks large droplets into small ones; that is all it does. Lipase, not emulsification, breaks the ester bonds.

Write micelles transport or micelles carry. Don't write "micelles diffuse across the membrane" or "micelles are absorbed intact". Only the fatty acids and monoglycerides released from the micelle cross the bilayer.

Write simple diffusion (or just diffusion). Don't write "facilitated diffusion" for fat, fatty acid, monoglyceride, or fat-soluble vitamin absorption. This is a stable reject across years; no carrier protein is involved.

Microvilli are folds of the cell-surface membrane on every absorptive cell.

The ileum is folded at three levels: gut wall, villi, microvilli. Villi are finger-like projections of the mucosa, each made of many cells. Microvilli are folds of one cell's own cell-surface membrane, on the apical (luminal) face of each absorptive epithelial cell.

Write folded cell-surface membrane. Don't write "hairs", "cilia", or "villi" for microvilli. All three are explicitly rejected on mark schemes that test microvillus structure.

Pitfall — Microvilli versus villi

Microvilli versus villi — keep them separate.

Microvilli are folds of a single cell's apical membrane. Villi are multicellular projections of the gut wall, each one covered by many absorptive epithelial cells and supplied internally by a capillary network and a lacteal. Confusing the two collapses the structural mark, because microvillus structure is then described at the wrong scale entirely.

The brush-border enzymes — disaccharidases and dipeptidases — are embedded in the microvillus membrane itself. The final hydrolysis of carbohydrates and peptides therefore occurs at the same surface where the resulting monosaccharides and amino acids are absorbed.

Glucose and amino acids cross by co-transport; fats and fat-soluble vitamins cross by simple diffusion.

Two absorption routes operate at the same membrane, one for each chemical class of digestion product. Glucose and amino acids cross by co-transport with sodium. Fatty acids, monoglycerides, and fat-soluble vitamins cross by simple diffusion through the phospholipid bilayer.

1. Sodium-potassium ATPase on the basolateral membrane of the epithelial cell uses ATP to expel sodium ions from the cytoplasm into the tissue fluid, keeping intracellular sodium concentration persistently low.
2. A co-transporter protein on the luminal cell-surface membrane binds one sodium ion and one glucose molecule (or one amino acid) at the same time.
3. Sodium flows into the cell down its concentration gradient through the co-transporter; the glucose or amino acid is carried in with it, against its own concentration gradient. No ATP is hydrolysed at the co-transporter itself; the energy was paid earlier by the sodium-potassium pump.

Name the sodium-potassium pump and the basolateral location explicitly. Writing only "active transport" loses the load-bearing marks. The pump's identity and its membrane location are both credited points.

Co-transport with sodium is for glucose and amino acids only. Don't write "co-transport with sodium" for fat, fatty acid, monoglyceride, or fat-soluble vitamin absorption. The lipid route is simple diffusion.

Key terms

• hydrolysis
• micelle
• fatty acids
• lipase
• bile salt
• surface area
• cell-surface membrane
• folded cell-surface membrane
• ileum
• diffusion
• concentration gradient
• co-transport