Explain why the digestion of starch is necessary before it can be absorbed into the blood.

AQA A-Level Biology (7402) — 3.3.3 Digestion and Absorption · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Starch is a large, insoluble polysaccharide and therefore cannot cross the intestinal wall.

It must be hydrolysed by amylase into glucose molecules.

Glucose is a small, soluble monosaccharide.

Glucose can then diffuse (or be actively transported) across the gut wall into the bloodstream.

Once in the blood, glucose is delivered to cells where it is utilised for cellular respiration.

Examiner tips

Use the exact terms ‘starch’, ‘amylase’, ‘glucose’, ‘diffusion/active transport’, ‘blood’ and ‘cells’. Show the logical sequence: large → broken down → small → absorbed → used. Keep the answer concise, 5 points, no extra wording.

Common mistakes

Confusing starch with glucose in the first point. Forgetting to mention that starch is insoluble or that glucose is soluble. Using vague verbs such as ‘taken up’ instead of ‘diffusion/active transport’.

Mark scheme (5 marks)

Starch is a large / insoluble molecule (that cannot pass through the wall of the small intestine / gut wall)
Starch must be broken down / digested into glucose (by amylase / enzymes)
Glucose is a small / soluble molecule
Glucose can pass through the wall of the small intestine into the blood (by diffusion / active transport)
Glucose is transported in the blood to cells / used for respiration

Key terms in this question

digestion · starch