How does the body break down food and absorb the useful parts?
Describe the parts of the human digestive system, the process of physical and chemical digestion, and the absorption of digested food
A scaffolded answer to the N(A)-Level Biology outcome on human digestion. The main parts of the digestive system, physical and chemical digestion by enzymes, and how digested food is absorbed in the small intestine.
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What this dot point is asking
This outcome wants you to name the main parts of the human digestive system, describe how food is broken down by physical and chemical digestion, and explain how the digested food is absorbed. You should be able to follow food along the gut, say what happens at each main part, and explain how the small intestine is adapted for absorption. The marks reward clear, ordered description and linking structure to function.
The answer
The main parts of the digestive system
Food passes through a long tube called the gut (alimentary canal):
- Mouth. Teeth chew the food (physical digestion). Saliva, containing the enzyme amylase, begins to break down starch into sugar.
- Gullet (oesophagus). A tube that pushes food down to the stomach by waves of muscle action.
- Stomach. A muscular bag that churns the food and adds acid and the enzyme protease, which begins to break down protein. The acid also kills many bacteria.
- Small intestine. Where most digestion is completed and where digested food is absorbed into the blood. Enzymes from the pancreas and the intestine wall finish breaking down the food.
- Large intestine. Absorbs water from the leftover material, forming faeces.
- Anus. Where faeces leave the body (egestion).
Physical and chemical digestion
There are two kinds of digestion. Physical digestion breaks food into smaller pieces without changing the molecules, mainly by the teeth chewing and the stomach churning. This gives a larger surface area for enzymes to act on. Chemical digestion uses enzymes to break large, insoluble food molecules into small, soluble ones that can be absorbed. For example, amylase breaks starch into sugar, protease breaks protein into amino acids, and lipase breaks fats into fatty acids and glycerol.
Absorption
Digested food is absorbed mainly in the small intestine. Its inner wall is folded and covered in millions of tiny finger-like structures called villi. Each villus is thin-walled and has a good blood supply. Small soluble molecules such as glucose and amino acids pass across the villus wall into the blood, mostly by diffusion (and some by active transport). The blood carries them to the rest of the body.
Examples in context
Example 1. Why doctors advise chewing food well. Chewing increases the surface area of the food, so enzymes can act on it faster and digestion is more efficient. It also makes food easier to swallow. This is the physical digestion step setting up the chemical digestion that follows.
Example 2. Why the stomach can handle acid. The stomach makes strong acid that helps protease work and kills bacteria in food. The stomach wall is protected by a layer of mucus, so the acid digests the food but not the stomach itself, an example of structure suiting a harsh job.
Try this
Q1. State where most absorption of digested food takes place. [1 mark]
- Cue. The small intestine.
Q2. Name the enzyme that breaks down starch and state what it produces. [2 marks]
- Cue. Amylase; it breaks starch down into sugar (simple sugars such as glucose).
Q3. Explain how the villi help with the absorption of food. [2 marks]
- Cue. The villi give a large surface area and have thin walls and a good blood supply, so food is absorbed quickly across a short distance into the blood.
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 marksExplain the difference between physical digestion and chemical digestion, giving one example of where each happens in the body.Show worked answer →
Physical digestion is the breaking of food into smaller pieces without changing it chemically. An example is the teeth chewing food in the mouth (the stomach churning food is also accepted).
Chemical digestion is the breaking down of large food molecules into smaller, soluble ones using enzymes. An example is amylase breaking starch into sugar in the mouth (or protease breaking protein in the stomach).
What markers reward: the idea that physical digestion only breaks food into smaller pieces (no change to the molecules), while chemical digestion uses enzymes to change large molecules into small soluble ones, plus a correct example of each. Saying chewing is chemical digestion is a common error.
Original4 marksThe small intestine is adapted for the absorption of digested food. Describe two of these adaptations and explain how each helps.Show worked answer →
Adaptation 1: the small intestine is very long and its inner wall is folded and covered in tiny finger-like villi. This gives a very large surface area, so more digested food can be absorbed at the same time.
Adaptation 2: each villus has a thin wall (one cell thick) and a rich blood supply. The thin wall means a short distance for food to diffuse across, and the blood carries the food away to keep the concentration gradient steep, so absorption is fast.
What markers reward: two adaptations each linked to how they speed up absorption (large surface area; short diffusion distance; good blood supply keeping the gradient steep). A named feature with no explanation scores only half.
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