How does the human body break down food into molecules it can use?
Describe the human digestive system and the role of mechanical and chemical digestion
A focused answer to the O-Level Biology outcome on human digestion. The path of food through the gut, the difference between mechanical and chemical digestion, and the main digestive enzymes and what they break down.
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What this dot point is asking
SEAB wants you to describe the human digestive system: the organs food passes through, the difference between mechanical and chemical digestion, and the main enzymes that break down carbohydrates, proteins and fats. You should be able to explain why digestion is necessary and the role of bile.
The answer
The path of food
Food passes along the alimentary canal in order: mouth, gullet (oesophagus), stomach, small intestine, large intestine, and out through the rectum and anus. Along the way, organs such as the liver and pancreas add digestive juices.
- Mouth: teeth chew the food (mechanical digestion); saliva contains amylase, which starts to digest starch.
- Gullet: muscular waves (peristalsis) push food down to the stomach.
- Stomach: muscular walls churn the food (mechanical), and it makes protease (pepsin) and acid; the acid kills bacteria and gives pepsin its low optimum pH.
- Small intestine: most chemical digestion and absorption happen here, using enzymes from the pancreas and the intestine, plus bile from the liver.
- Large intestine: water is reabsorbed from the undigested remains, forming faeces.
Mechanical versus chemical digestion
Mechanical digestion is the physical breakdown of food into smaller pieces (by the teeth and the churning stomach) without changing it chemically. It increases the surface area for enzymes to act on.
Chemical digestion is the breakdown of large, insoluble food molecules into small, soluble molecules by enzymes.
The main digestive enzymes
- Amylase (a carbohydrase) breaks down starch into maltose (a sugar). Made in the salivary glands and pancreas.
- Protease (such as pepsin in the stomach and trypsin from the pancreas) breaks down protein into amino acids.
- Lipase breaks down fats into fatty acids and glycerol. Made in the pancreas.
The role of bile
Bile is made by the liver and stored in the gall bladder. It is not an enzyme. It emulsifies fats, breaking large fat droplets into many small ones, which increases the surface area for lipase to work on, speeding fat digestion. Bile is also alkaline, helping neutralise the acid from the stomach.
Why digestion is needed
Large food molecules are too big and insoluble to pass through the gut wall into the blood. Digestion turns them into small, soluble molecules that can be absorbed and used by cells.
Examples in context
Example 1. Why we chew thoroughly. Chewing breaks food into smaller pieces, increasing the surface area for enzymes such as amylase to act on. Well-chewed food is therefore digested faster, showing mechanical and chemical digestion working together.
Example 2. The acidic stomach. The stomach's acid gives pepsin its low optimum pH so it works well, and it also kills many bacteria in food. This shows how conditions in the gut are matched to the enzymes that work there.
Try this
Q1. State the difference between mechanical and chemical digestion. [2 marks]
- Cue. Mechanical digestion physically breaks food into smaller pieces; chemical digestion breaks large molecules into small soluble ones using enzymes.
Q2. Name the enzyme that digests protein and state its products. [2 marks]
- Cue. Protease (pepsin or trypsin) digests protein into amino acids.
Q3. Explain the role of bile in fat digestion. [2 marks]
- Cue. Bile emulsifies fat into small droplets, increasing the surface area for lipase, so fat is digested faster (bile is not an enzyme).
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.
Original6 marks(a) State the difference between mechanical digestion and chemical digestion. (b) Name the enzyme that digests starch, the enzyme that digests protein, and the enzyme that digests fat, and state the products of each.Show worked answer →
(a) Mechanical digestion is the physical breakdown of food into smaller pieces (for example by chewing teeth or churning by the stomach) without changing it chemically. Chemical digestion is the breakdown of large food molecules into smaller, soluble molecules by enzymes.
(b) Amylase (carbohydrase) digests starch into maltose (a sugar). Protease (such as pepsin or trypsin) digests protein into amino acids. Lipase digests fats into fatty acids and glycerol.
Markers reward the contrast between physical breakdown (mechanical) and enzyme breakdown into soluble molecules (chemical), and the three enzymes correctly paired with their substrates and products.
Original4 marksExplain why food must be digested before it can be used by the body, and explain the role of bile in the digestion of fats.Show worked answer →
Large food molecules such as starch, protein and fat are too big and insoluble to be absorbed through the wall of the small intestine into the blood. Digestion breaks them into small, soluble molecules (sugars, amino acids, fatty acids and glycerol) that can be absorbed and used by cells.
Bile is made by the liver and is not an enzyme. It emulsifies fats, breaking large fat droplets into many small droplets. This increases the surface area of the fat for lipase to act on, so fat is digested faster.
Markers reward the idea that molecules must be made small and soluble to be absorbed, and that bile emulsifies fat to increase surface area for lipase (not that bile digests fat itself).
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