To trace a food component, follow it organ by organ and say what happens to it in each one: mechanical change, chemical change, or movement. Exam questions often pick one nutrient, such as starch or protein, and ask you to describe its route.
This skill opens human nutrition and digestion. Every later lesson zooms into one stage of the route you build here.
What happens to food in each organ?
Digestion has two parts. Mechanical digestion breaks food into smaller pieces without changing the molecules, for example chewing or churning. Chemical digestion uses enzymes to break large insoluble molecules into small soluble ones.
| Organ | Main role for food | Chemical digestion? |
|---|---|---|
| Mouth | Chewing; saliva with amylase | Starch to maltose begins |
| Oesophagus | Peristalsis moves food down | No |
| Stomach | Churning; acid; pepsin | Protein digestion begins |
| Liver and gall bladder | Bile made and stored, then released into the small intestine | Bile emulsifies fat but is not an enzyme |
| Pancreas | Releases amylase, protease and lipase into the small intestine | Yes, the enzymes come from here |
| Small intestine | Final digestion; absorption of products | Yes |
| Large intestine | Absorbs water | No |
| Rectum and anus | Stores and removes faeces (egestion) | No |
Worked example
The data are invented and the example is a model of the route. A student eats a piece of cheese. Trace the protein in it.
Step 1, mouth: teeth cut and grind the cheese. This is mechanical only. Saliva has no enzyme that digests protein.
Step 2, oesophagus: peristalsis pushes the food to the stomach. No digestion.
Step 3, stomach: muscles churn the food. Pepsin, a protease, starts breaking protein into shorter chains. Stomach acid gives the low pH that pepsin needs.
Step 4, small intestine: proteases from the pancreas and from the intestine wall finish the job. The end products are amino acids.
Step 5, absorption: amino acids are small and soluble. They pass through the wall of the small intestine into the blood.
A complete answer names the organ, the enzyme, and the product at each stage where chemical digestion happens.
The mistake to watch for
A common slip is to say that starch is digested in the stomach because “the stomach digests food”.
Mistaken answer: “Starch is digested by amylase in the stomach and then absorbed.”
Amylase in saliva starts starch digestion in the mouth. It stops working in strong stomach acid. Digestion of starch is completed in the small intestine by pancreatic amylase and then enzymes in the gut wall.
The correction is to learn which enzymes work in each organ.
Ask “which enzymes are in this organ, and which food do they act on?” Stomach: protein. Mouth and small intestine: starch. Small intestine: all three food groups.
Check yourself
Try these without notes, then open each answer.
1. State where starch digestion begins and name the enzyme.
Show answer
It begins in the mouth. The enzyme is amylase in saliva, which breaks starch into maltose.
2. A student says, “The large intestine digests fibre and absorbs digested food.” Correct this in one sentence.
Show answer
Digestion and absorption of digested food are mainly done in the small intestine. The large intestine mainly absorbs water, and the remaining undigested material is removed as faeces.
3. Fat in a meal is broken into two products. Name the enzyme and the products, and say which fluid helps first.
Show answer
Bile emulsifies the fat into small droplets. Lipase then digests it into fatty acids and glycerol.
Where this leads next
Next, fix the three-part pattern with linking an enzyme with its substrate and products. Then test the route in the human nutrition and digestion practice set.
If you can recite the route but lose marks when a question asks about one nutrient, a teacher in online one-to-one Biology tuition can help. They drill starch, protein and fat separately until each route is clear.