Chemical digestion of food

Where it happens

Chemical digestion happens along the alimentary canal, wherever a digestive enzyme meets its substrate: in the mouth, the stomach and the small intestine. Enzymes from the salivary glands, gastric glands, pancreas and small intestine wall each act on a specific food type.

Each site provides its own conditions. The mouth is roughly neutral, which suits salivary amylase.

The stomach lining secretes hydrochloric acid, giving an acidic medium that activates pepsin and kills bacteria. The duodenum receives bile and pancreatic juice, both alkaline, which neutralise the acidic chyme so that pancreatic amylase, trypsin and lipase can work in a slightly alkaline medium.

The final enzymes, maltase and peptidase, are held in the wall of the small intestine itself. Chemical digestion is aided by physical digestion, the chewing and churning that break food into smaller pieces and increase the surface area on which enzymes act.

Inputs and outputs

• Input: starch from rice, bread and noodles, a polysaccharide too large to be absorbed.
• Input: protein from meat, fish, eggs and legumes, a chain of amino acids.
• Input: lipids from oils and fats, mainly triglycerides made of glycerol and 3 fatty acids.
• Input: enzymes from the salivary glands, gastric glands, pancreas and intestinal wall, plus bile from the liver and hydrochloric acid from the stomach.
• Output: glucose from starch, amino acids from protein, and fatty acids and glycerol from lipids.
• Output: water is used up in every hydrolysis reaction, because each bond broken takes in one molecule of water.

The steps

1. Mouth: salivary amylase begins breaking starch into maltose while chewing increases the surface area; protein and lipid are not digested here.
2. Stomach: hydrochloric acid stops salivary amylase and creates an acidic medium; pepsin breaks protein into shorter polypeptides.
3. Duodenum: bile from the gall bladder emulsifies lipids into small droplets and, with pancreatic juice, neutralises the acid.
4. Duodenum: pancreatic amylase continues starch breakdown to maltose, and trypsin continues protein breakdown to polypeptides.
5. Duodenum: pancreatic lipase breaks the emulsified lipid droplets into fatty acids and glycerol.
6. Ileum: maltase in the intestinal wall breaks maltose into glucose, and peptidase breaks polypeptides into amino acids.
7. Ileum: the finished products, glucose, amino acids, fatty acids and glycerol, are absorbed through the villi into the blood and lymph.

Why it matters and how it is controlled

Large food molecules such as starch, protein and fat are insoluble and too large to cross the wall of the small intestine. Chemical digestion breaks them down into small, soluble molecules that can be absorbed into the blood and used by the body for energy and growth.

Control of chemical digestion is achieved by matching each enzyme to its own site and medium. Every enzyme is specific to one substrate and works fastest at its optimum pH and at body temperature.

Pepsin has an acidic optimum and is released only in the stomach; trypsin and lipase have an alkaline optimum and are released into the duodenum, where bile and pancreatic juice have already raised the pH. Moving food along at the right rate, by peristalsis, gives each enzyme enough time to act before the food reaches the next region.

The sequence also protects the body. Protein-digesting enzymes are released in an inactive form and switched on only inside the gut lumen, so they cannot digest the cells that make them.

The mucus lining of the stomach shields the stomach wall from both pepsin and hydrochloric acid.

How it is examined

You may be asked to name the enzyme that acts on a given food type, to state where each enzyme is produced and where it acts, to complete a table linking substrate, enzyme and product, or to explain why bile is not an enzyme but still helps lipid digestion.

A standard structured item shows a table with columns for region, enzyme, substrate and product and leaves blanks in each column. Fill it using the site rule: amylase acts in mouth and duodenum, pepsin only in the stomach, trypsin and lipase only in the duodenum, maltase and peptidase only in the ileum.

Experiment-style items compare test tubes containing starch with amylase at different pH values or temperatures and ask you to explain the results using the idea of optimum conditions and enzyme denaturation. Essay items ask for the complete digestion of one food class from mouth to ileum, and marks are given for naming the enzyme, its source, its site of action and its product at every stage.

Common misconceptions

Worked exam-style question

Question. A student ate a meal of fried noodles with egg. (a) Name the enzyme that begins the digestion of the noodles in the mouth and state its product.

(b) Explain why digestion of the egg protein starts only in the stomach and not in the mouth. (c) The oil in the fried noodles is digested in the duodenum.

Describe the roles of bile and lipase in this digestion. (d) Explain why the products named in (a) cannot be absorbed until the food reaches the ileum.

Model answer. (a) Salivary amylase digests starch into maltose. (b) Saliva contains no protease.

Protein digestion needs pepsin, which is secreted by the gastric glands and works only in the acidic medium created by hydrochloric acid in the stomach, so the enzyme and conditions for protein digestion are absent in the mouth. (c) Bile, produced by the liver and stored in the gall bladder, emulsifies the oil into small droplets, which increases the surface area for enzyme action; it also neutralises the acidic chyme.

Lipase from the pancreas then hydrolyses the lipid into fatty acids and glycerol. (d) Maltose is a disaccharide that is too large to cross the intestinal wall; it must first be broken down by maltase in the wall of the ileum into glucose, a monosaccharide small enough to be absorbed.

Source:SRC-DSKP-EN

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