Digestion: Physical and Chemical Breakdown of Food

Physical digestion

Physical (mechanical) digestion breaks food into smaller pieces without changing its chemical structure. In the mouth, the teeth cut and grind food while the tongue mixes it with saliva, a process called mastication.

In the stomach, the muscular wall contracts and relaxes to churn food into a semi-liquid mass called chyme. Physical digestion does not produce new substances, it only increases the surface area of food, so that enzymes in chemical digestion can act on it faster.

Chemical digestion

Chemical digestion uses enzymes to break large, insoluble food molecules into small, soluble molecules that the body can absorb. Each digestive enzyme is specific: it acts on only one type of substrate, breaking a particular chemical bond.

Chemical digestion converts starch into simple sugars, proteins into amino acids, and fats into fatty acids and glycerol, changing the food into new, absorbable substances.

Enzymes at each stage

How it is examined

Exam questions on this standard often ask you to classify a given process as physical or chemical digestion, to name the enzyme and product for a stated substrate, or to complete a table linking each organ to the type of digestion occurring there. A common trick question asks about bile: bile is not an enzyme and does not chemically digest fat, it physically emulsifies fat into smaller droplets so that lipase can work faster.

Why food must be digested

The food we eat is made of large, insoluble molecules, starch, proteins and fats, that are far too big to pass through the wall of the gut into the blood. Digestion solves this by breaking them into small, soluble molecules that can be absorbed: glucose from carbohydrates, amino acids from proteins, and fatty acids with glycerol from fats.

Physical and chemical digestion work as a team. Chewing and churning break food into smaller pieces, which increases the surface area exposed to enzymes; the enzymes then break chemical bonds far faster on that larger surface.

Without the physical stage, chemical digestion would be too slow to supply the body with nutrients in time.

Digestion is only the first step. Once the food molecules are small and soluble, they are absorbed through the wall of the small intestine into the blood and lymph, and are then carried to every cell.

There they are assimilated, used to release energy, build new tissue and repair worn-out parts, which is why efficient digestion matters for growth and health.

Common mistakes to avoid

Worked exam-style question

Question. A student chews a piece of bread for a minute and it begins to taste sweet. (a) Name the enzyme responsible and the region where it acts.

(b) State the substrate and the product of this reaction. (c) Explain why chewing the bread first makes this digestion faster.

Model answer. (a) The enzyme is salivary amylase, produced by the salivary glands and acting in the mouth. (b) The substrate is starch and the product is maltose, a sugar, which is why the bread tastes sweet.

(c) Chewing is physical digestion that breaks the bread into smaller pieces and increases its surface area; more starch is then exposed to the amylase, so the enzyme acts on more substrate at once and the reaction is faster.

Practice question

Try it. A meal contains rice (rich in starch), egg (rich in protein) and butter (rich in fat). For each food, name the main product formed when it is fully digested, and name one enzyme that acts on it.

Exam tip

Key terms

• Digestion, the breakdown of large food molecules into small, absorbable ones.
• Enzyme, a biological catalyst that speeds up the digestion of one substrate.
• Substrate, the food molecule an enzyme acts on.
• Carbohydrate, starch and sugars, digested to glucose.
• Amino acid, the small, soluble product of protein digestion.

Source:SRC-DSKP-EN

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