The human digestive system

What you'll learn

Food must be broken down into small, soluble molecules before your body can absorb and use it — and that is the job of the digestive system. For AQA GCSE Biology you need to know the organs of the digestive system, the enzymes that break down carbohydrates, proteins and fats, where each enzyme is made, and the role of bile. This guide covers the structure of the digestive system, digestion by enzymes, the products of digestion, the importance of bile, and how the small intestine is adapted for absorption. By the end you should be able to describe the pathway of food, match each enzyme to its substrate and product, and explain how food is absorbed.

Key terms and definitions

Digestion — The breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed.

Enzyme — A biological catalyst that speeds up reactions, including the breakdown of food.

Carbohydrase — An enzyme that breaks down carbohydrates; amylase is an example.

Protease — An enzyme that breaks down proteins into amino acids.

Lipase — An enzyme that breaks down lipids (fats) into fatty acids and glycerol.

Bile — An alkaline liquid, made in the liver, that emulsifies fats and neutralises stomach acid.

Emulsify — To break large fat droplets into small droplets, increasing the surface area.

Villi — Tiny finger-like projections in the small intestine that increase the surface area for absorption.

Core concepts

Why food must be digested

The food we eat contains large, insoluble molecules — carbohydrates, proteins and fats — that are too big to be absorbed into the blood. Digestion breaks these into small, soluble molecules that can pass through the wall of the small intestine into the bloodstream. Digestion is carried out mainly by enzymes.

The organs of the digestive system

Food passes through the digestive system in order:

• Mouth — food is chewed and mixed with saliva containing amylase.
• Stomach — food is mixed with hydrochloric acid and protease enzymes.
• Small intestine — digestion is completed and soluble products are absorbed.
• Large intestine — water is absorbed from the remaining material.

The liver, pancreas and salivary glands are also part of the system: the liver makes bile, the pancreas makes enzymes, and the salivary glands make amylase.

Digestive enzymes

There are three main types of digestive enzyme, each acting on a different food group:

Each enzyme has a specific shape that matches its substrate, which is why amylase only breaks down starch and protease only breaks down protein.

The products of digestion and their use

The small, soluble products of digestion are used by the body:

• Glucose is used in respiration to release energy.
• Amino acids are used to build new proteins.
• Fatty acids and glycerol are used to build cell membranes and can be used as an energy store.

The role of bile

Bile is made in the liver and stored in the gall bladder before being released into the small intestine. It has two important jobs:

1. It is alkaline, so it neutralises the acid that arrives from the stomach. This is important because the enzymes in the small intestine work best in alkaline conditions, and stomach acid would slow them down.
2. It emulsifies fats, breaking large fat droplets into many small droplets. This increases the surface area of the fat, so lipase can break it down faster.

Note that bile is not an enzyme — it does not chemically digest fat, it only physically breaks it into smaller droplets so lipase can work more quickly.

Absorption in the small intestine

Once food is fully digested, the small, soluble molecules are absorbed through the wall of the small intestine into the blood. The small intestine is well adapted for this because it is lined with millions of tiny finger-like projections called villi. Villi:

• Give a very large surface area for absorption.
• Have a thin wall (one cell thick), so molecules pass through quickly.
• Have a good blood supply to carry absorbed molecules away and maintain a concentration gradient.

These adaptations make absorption fast and efficient.

The role of each organ in more detail

It helps to see what each organ contributes. In the mouth, teeth break food into smaller pieces (physical digestion, increasing surface area) and saliva adds amylase to start digesting starch. The stomach produces hydrochloric acid, which kills bacteria and provides the acidic conditions that stomach protease needs, and churns the food into a liquid. The pancreas makes all three types of enzyme and releases them into the small intestine. The small intestine completes digestion and absorbs the products. The large intestine absorbs water from the remaining undigested material, forming faeces. Understanding this division of labour makes it easier to explain where each stage of digestion happens.

Physical and chemical digestion

Digestion has two parts that work together. Physical (mechanical) digestion breaks food into smaller pieces without changing it chemically — for example, chewing in the mouth and churning in the stomach. This increases the surface area of the food so that enzymes can act on it more quickly. Chemical digestion uses enzymes to break the large molecules into small soluble ones. Bile's emulsification of fats is another example of physical digestion helping chemical digestion, because breaking fat into small droplets increases the surface area for lipase. The two processes together make digestion far more efficient than either could alone.

Worked examples

Example 1: Matching enzyme to product

Which enzyme breaks down protein, and what is the product? Protease breaks down protein into amino acids.

Example 2: Explaining the two roles of bile

Explain the two functions of bile in digestion. Bile neutralises the acid from the stomach, providing the alkaline conditions that intestinal enzymes need. It also emulsifies fats, breaking large droplets into small ones to increase the surface area for lipase to act on.

Example 3: Why emulsification speeds up digestion

Explain why emulsifying fat helps lipase to work faster. Emulsification breaks large fat droplets into many small droplets, which greatly increases the total surface area of the fat. Lipase acts on the surface of the droplets, so a larger surface area means the fat is broken down more quickly.

Example 4: Villi adaptations

Give two ways the small intestine is adapted for absorption. It has villi that provide a large surface area, and the wall is only one cell thick, so molecules pass through quickly. A good blood supply also maintains a concentration gradient.

Common mistakes and how to avoid them

A very common error is saying bile is an enzyme or that it "digests" fat. Bile is not an enzyme — it emulsifies fat physically and neutralises acid. The chemical breakdown of fat is done by lipase.

Students often mix up the products. Learn them: carbohydrates → simple sugars; proteins → amino acids; lipids → fatty acids and glycerol. Forgetting glycerol is a frequent slip.

Another mistake is saying enzymes are "used up" during digestion. Enzymes are catalysts and are not used up — one enzyme molecule can break down many substrate molecules.

When describing villi, do not just say "large surface area". Give the reason it helps — more area means more absorption — and add the thin wall and good blood supply for full marks.

Finally, remember the stomach is acidic while the small intestine is alkaline; bile is what changes the conditions between them.

Exam technique for "The human digestive system"

A table linking each enzyme to its substrate, its product and where it is made is the single most useful thing to memorise — questions frequently test these directly.

For bile questions, always give both functions: neutralising stomach acid and emulsifying fats. Explaining why each matters (enzymes need alkaline conditions; emulsification increases surface area) earns the higher marks.

Absorption questions expect you to name villi and explain their adaptations in terms of surface area, thin walls and blood supply. Use precise terms throughout — substrate, product, emulsify, villi — rather than everyday language.

Quick revision summary

• Digestion breaks large, insoluble food molecules into small, soluble ones that can be absorbed.
• Carbohydrase (amylase) → sugars; protease → amino acids; lipase → fatty acids and glycerol.
• Enzymes are made in the salivary glands, stomach, pancreas and small intestine.
• Bile (from the liver) is alkaline — it neutralises stomach acid and emulsifies fats to increase surface area for lipase. It is not an enzyme.
• The small intestine absorbs the products; villi give a large surface area, thin walls and a good blood supply.
• Products are used for respiration (glucose), building proteins (amino acids) and membranes/energy store (fatty acids and glycerol).