How does the digestive system break down food into small molecules the body can absorb and use?
Describe the human digestive system, the role of digestive enzymes in breaking down food, and the absorption of digested food in the small intestine
A focused answer to the O-Level Combined Science outcome on digestion. The parts of the alimentary canal, physical and chemical digestion, the action of carbohydrase, protease and lipase, and absorption in the small intestine.
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What this dot point is asking
SEAB wants you to outline the human digestive system, explain how enzymes chemically break large food molecules into small ones, and describe how the small intestine absorbs the products. The enzyme-substrate-product table and the adaptations of the small intestine for absorption are reliable exam marks.
The answer
The alimentary canal
Food passes through a long tube, the alimentary canal, in this order:
- mouth (teeth break food up; saliva contains amylase),
- oesophagus (carries food to the stomach),
- stomach (churns food; gastric juice with protease and hydrochloric acid),
- small intestine (digestion completed; products absorbed),
- large intestine (water absorbed),
- rectum and anus (faeces stored and removed).
The liver makes bile and the pancreas makes digestive enzymes that act in the small intestine.
Physical and chemical digestion
- Physical (mechanical) digestion breaks food into smaller pieces (chewing by teeth, churning by the stomach), increasing the surface area for enzymes.
- Chemical digestion uses enzymes to break the chemical bonds in large food molecules, turning them into small, soluble molecules that can be absorbed.
The digestive enzymes
Three main classes of enzyme break down the three main food groups:
- Carbohydrases (e.g. amylase) break starch into simple sugars (glucose),
- Proteases (e.g. pepsin) break proteins into amino acids,
- Lipases break fats into fatty acids and glycerol.
Each enzyme works best at a particular pH: pepsin in the acidic stomach, the others in the slightly alkaline small intestine.
The role of bile
Bile, made by the liver and stored in the gall bladder, is not an enzyme. It emulsifies fats, breaking large fat droplets into many small ones. This greatly increases the surface area for lipase to act on, speeding up fat digestion. Bile is also alkaline and neutralises the acid from the stomach.
Absorption in the small intestine
The small molecules are absorbed through the wall of the small intestine into the blood. The small intestine is well adapted:
- a very large surface area from folds, villi and microvilli,
- thin walls (one cell thick) for a short diffusion distance,
- a rich blood supply that carries absorbed food away, keeping a steep concentration gradient,
- a lacteal in each villus to absorb fats.
Examples in context
Example 1. Why babies are weaned slowly. A baby's digestive enzymes mature gradually, so solid foods rich in starch and protein are introduced slowly as the gut becomes able to digest and absorb them. This reflects the need for the right enzymes to break each food group into absorbable molecules.
Example 2. Lactose intolerance. People who lack the enzyme lactase cannot break down the sugar lactose in milk, so it passes undigested to the large intestine and causes discomfort. This shows what happens when a specific carbohydrase is missing and a food cannot be digested into an absorbable form.
Try this
Q1. State the products of the complete digestion of (a) starch, (b) protein, (c) fat. [3 marks]
- Cue. (a) Simple sugars (glucose); (b) amino acids; (c) fatty acids and glycerol.
Q2. Explain the role of bile in the digestion of fats. [2 marks]
- Cue. Bile emulsifies fats into small droplets, increasing the surface area for lipase to act, which speeds up fat digestion (and it neutralises stomach acid).
Q3. Describe one way the small intestine is adapted for absorption and explain its effect. [2 marks]
- Cue. It has villi that give a large surface area, so more digested food can be absorbed at once (faster absorption).
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksName the three main classes of digestive enzyme, state the food each one breaks down, and name the small molecule produced in each case.Show worked answer →
Carbohydrases (for example amylase) break down carbohydrates (starch) into simple sugars (such as glucose / maltose).
Proteases (for example pepsin) break down proteins into amino acids.
Lipases break down fats (lipids) into fatty acids and glycerol.
Markers reward correctly pairing each enzyme class with its substrate (carbohydrate, protein, fat) and the correct small product (simple sugars, amino acids, fatty acids and glycerol).
Original4 marksThe small intestine is adapted for the efficient absorption of digested food. Describe two adaptations of the small intestine and explain how each one increases the rate of absorption.Show worked answer →
Adaptation 1: the inner surface is folded and covered in tiny finger-like villi (and microvilli). This gives a very large surface area, so more digested food can be absorbed at once, increasing the rate of absorption.
Adaptation 2: each villus has a thin wall (one cell thick) and a rich blood supply (a network of capillaries). The short diffusion distance and the steep concentration gradient (because absorbed food is carried away quickly) speed up absorption.
(Other valid points: a lacteal for absorbing fats; the small intestine is long.)
Markers reward two correct adaptations, each linked to faster absorption (large surface area, thin wall and short diffusion distance, good blood supply maintaining the gradient).
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