Digestion prepares molecules for absorption

By UK A Level Biology · independent study · myAudience

The enzyme routes identify substrates and products rather than forming one universal digestion chain. Amylase and maltase act at different stages, peptidases hydrolyse peptide bonds, and the lipase model yields a monoglyceride plus two fatty acids. Bile disperses droplets without being an enzyme. On the absorption model, sodium and glucose enter together at the apical membrane; the separate basolateral Na⁺/K⁺ pump uses ATP, maintaining the sodium gradient, while glucose exits toward blood. Symbols are not molecular-scale structures or concentration measurements. The dipeptidase label identifies the final dipeptide-to-two-amino-acids hydrolysis arrow; carbohydrate product symbols illustrate identities, not a complete stoichiometric count.

Original myAudience scientific illustration; not a documentary photograph or measured specimen. · Original generated project artwork; no source textbook figures reused. Sources support scientific review, not copied-art licensing.

Digestive enzymes break larger molecules into smaller products. Amylase acts on starch; proteases act on peptide bonds; lipase acts on lipids. Bile salts help disperse lipid droplets and assist the handling of digestion products, but bile is not itself a digestive enzyme.

The ileum offers a large absorptive area. Sodium-linked co-transport helps move glucose or amino acids into epithelial cells, using a sodium gradient maintained by energy-dependent pumping. Distinguish the immediate co-transport mechanism from the pump supplying the gradient.

Endopeptidases split internal peptide bonds, exopeptidases act at chain ends, and membrane-bound dipeptidases split dipeptides. Bile salts help form micelles associated with lipid digestion products, bringing them near the epithelial surface; the micelle is not the same structure as a phospholipid membrane. Sodium–glucose cotransport at the lumen-facing membrane depends on a sodium gradient maintained by pumping at another boundary. Glucose then exits toward blood through an appropriate carrier, so uptake into an epithelial cell is only part of absorption.

Worked example: A sodium gradient is removed from an ileum-cell model. Predict the effect on sodium-linked glucose uptake.

Glucose uptake is coupled to sodium moving down its gradient.

Removing that driving gradient reduces the coupled transport.

Answer: Uptake by that mechanism decreases.

Check your understanding: Does bile hydrolyse triglycerides?

Answer: No; lipase does.

Why: Bile assists dispersion and handling of lipid products rather than catalysing triglyceride hydrolysis.

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