How does the digestive system break down food and absorb it into the blood?
The organs of the digestive system and their functions, the digestive enzymes amylase, protease and lipase with their substrates and products, the role of bile, and absorption in the villi of the small intestine.
A focused CCEA GCSE Biology answer on digestion, covering the organs of the digestive system, the enzymes amylase, protease and lipase with their substrates and products, the role of bile, and absorption by the villi.
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What this dot point is asking
CCEA wants you to name the organs of the digestive system and their roles, describe the digestive enzymes amylase, protease and lipase with their substrates and products, explain the role of bile, and explain how the villi in the small intestine are adapted for absorption.
The organs of the digestive system
The digestive enzymes
| Enzyme | Substrate | Products | Made in |
|---|---|---|---|
| Amylase | Starch | Maltose | Salivary glands, pancreas |
| Protease | Protein | Amino acids | Stomach, pancreas |
| Lipase | Fats (lipids) | Fatty acids and glycerol | Pancreas |
The small, soluble products (sugars, amino acids, fatty acids and glycerol) can pass through the gut wall into the blood; the large molecules they came from could not.
The role of bile
Absorption in the villi
The small intestine is lined with millions of tiny finger-like projections called villi. They are adapted for fast absorption by a large surface area, a thin wall (one cell thick), and a good blood supply that keeps a steep concentration gradient.
Examples in context
- Example 1. Why emulsifying fat speeds up digestion
- A large drop of fat has a small surface area, so lipase can only act on the outside. Bile emulsifies the fat into many tiny droplets, hugely increasing the total surface area exposed to lipase. The lipase can then break down the fat much faster. This is the same surface-area-to-volume idea that appears throughout biology, applied to digestion.
- Example 2. The liver loses a job in coeliac disease
- When villi are damaged (as in coeliac disease), their surface area falls sharply, so less food is absorbed and the person can become malnourished even on a good diet. This shows just how important the villi adaptations are: remove the large surface area and absorption drops, no matter how well the food has been digested.
- Example 3. Why digestion is both physical and chemical
- Before any enzyme acts, the teeth chew food into smaller pieces and the stomach churns it. This physical breakdown increases the surface area of the food, giving the enzymes more area to work on, so chemical digestion is faster. Bile then emulsifies fat in the same way, increasing surface area for lipase. Chemical digestion is the breaking of bonds by enzymes (amylase, protease and lipase) to make small soluble molecules. The two work together: physical breakdown speeds up the chemical breakdown that actually produces molecules small enough to be absorbed. Keeping the two ideas separate, while seeing how they support each other, is a common stumbling block in CCEA digestion questions.
Try this
Q1. State the products of the digestion of fats by lipase. [1 mark]
- Cue. Fatty acids and glycerol.
Q2. Give two ways a villus is adapted for absorption. [2 marks]
- Cue. Large surface area (and thin wall, and good blood supply) - any two.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20204 marksName three digestive enzymes and state the substrate and product of each.Show worked answer →
Four marks: three enzymes, each with its job, plus accuracy.
Amylase breaks down starch into maltose (a sugar). It is made in the salivary glands and pancreas.
Protease breaks down proteins into amino acids. It is made in the stomach and pancreas.
Lipase breaks down lipids (fats) into fatty acids and glycerol. It is made in the pancreas.
Markers reward each enzyme matched with the correct substrate and product. A common slip is saying amylase makes glucose; it makes maltose, which is later broken into glucose.
CCEA 20183 marksExplain how the small intestine is adapted for the absorption of digested food.Show worked answer →
Three marks for three linked adaptations of the villi.
The inner surface is folded into millions of villi, and each villus has microvilli, giving a very large surface area for absorption.
Each villus has a thin wall, only one cell thick, so digested food has a short distance to diffuse into the blood.
Each villus has a good blood supply (a network of capillaries) that carries absorbed food away quickly, keeping a steep concentration gradient.
Markers reward large surface area, thin wall and good blood supply, each linked to faster absorption.
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Sources & how we know this
- CCEA GCSE Biology specification — CCEA (2017)