The Human Digestive System — AQA GCSE Biology

SPEC 4.2.2.1 • ORGANISATION — THE HUMAN DIGESTIVE SYSTEM

The Digestive System

Eight named organs, three enzyme classes, two functions of bile — and the one mark-scheme principle that decides every 6-mark digestion question on Paper 1.

The 6-Mark Question That Catches Everyone

A recent AQA Paper 1 included a 6-mark question on cystic fibrosis and digestion. The biology wasn't the problem. The mark-scheme principle was.

Mark scheme — what students wrote that didn't score

"Starch is not absorbed." ❌

"Fats are not absorbed." ❌

"Protein is not absorbed." ❌

The principle

The body does NOT absorb starch, protein, or fats. It absorbs the products of digestion — glucose, amino acids, fatty acids and glycerol. Examiners flagged "lack of precision" as the single biggest reason students lost marks.

In the next 10 minutes, you'll know the eight named organs, the three enzyme classes, the two functions of bile, and the exact mark-scheme wording for products of digestion.

Learning Objectives

By the end of this lesson, you'll be able to:

Objective 1

Identify the eight named organs of the digestive system and state the role of each.

Objective 2

Name the three classes of digestive enzymes — their substrates, products, and sites of production.

Objective 3

State the products of digestion that are absorbed into the blood, and the two functions of bile.

SPEC REF 4.2.2.1 • PAPER 1 • HIGHER TIER

The Digestive System

The digestive system is an example of an organ system — several organs work together to digest food and absorb the products into the blood.

Read the diagram

Food travels through the main tube (mouth → large intestine). The accessory organs on the right — liver, gall bladder, pancreas — never touch food directly. They release secretions (bile, enzymes) into the small intestine.

The Named Organs & Their Roles

The spec names eight organs. The mark scheme accepts these terms verbatim — don't say "the gut" or "tummy."

OrganRole

MouthChewing breaks food into smaller pieces; salivary glands release amylase to start starch digestion

OesophagusConnects mouth to stomach (food tube — no digestion here)

StomachChurns food; produces hydrochloric acid + protease (pepsin) to begin protein digestion

PancreasProduces amylase, protease, and lipase — releases them into the small intestine

LiverProduces bile (an emulsifier — not an enzyme)

Gall bladderStores bile; releases it into the small intestine when fatty food enters

Small intestineCompletes digestion (with pancreatic enzymes + bile); absorbs products of digestion into the blood

Large intestineAbsorbs water from undigested material; forms faeces

The Stomach: Hydrochloric Acid Has TWO Functions

Many students write "stomach acid digests food." That's not what the mark scheme accepts. HCl has two specific functions — both creditable.

Function 1

Kills pathogens in food. The low pH (~pH 2) destroys bacteria and other microbes swallowed with food.

Function 2

Provides the optimum pH for stomach protease (pepsin). Pepsin works best in acidic conditions — HCl creates that environment.

Mark-scheme nuance

HCl itself does not digest food. Protease (pepsin) digests protein; HCl just makes the conditions right. Saying "acid breaks down food" is wrong.

The Three Enzyme Classes

The spec names exactly three classes. Each one breaks down one type of food into one type of product. Memorise all three rows.

Enzyme classSubstrateProduct(s)Sites of production

Amylase (a carbohydrase)Starch (a carbohydrate)Glucose (simple sugars)Salivary glands; pancreas

ProteaseProteinsAmino acidsStomach (as pepsin); pancreas; small intestine

LipaseLipids (fats / oils)Fatty acids AND glycerolPancreas; small intestine

Synoptic forward link — Lesson 2.03

In Lesson 2.03 you'll learn how each enzyme works — the lock-and-key model, optimum pH/temperature, and denaturation. For now, just memorise what each enzyme does.

The Three Word Equations

The spec accepts simple word equations — no chemical symbols required. Each equation must include the enzyme AND the product.

The lipase trap

Lipase produces fatty acids AND glycerol — TWO products. The mark scheme awards a separate mark for each. Writing only "fatty acids" loses a mark on every lipase question.

Bile (1) — What It Is & Where It's Made

Bile is NOT an enzyme. It's a green-yellow alkaline fluid that helps the digestion of fats — but it doesn't digest anything itself.

BILE — DEF

An alkaline emulsifier: it neutralises stomach acid and emulsifies lipids (breaks fats into small droplets, increasing the surface area available for lipase).

Where it's MADE

In the liver.

Where it's STORED

In the gall bladder, until needed.

The classic confusion

Bile is NOT made in the gall bladder — that's just where it's stored. Made in liver. Stored in gall bladder. Released into the small intestine. Memorise the order: liver → gall bladder → small intestine.

Bile (2) — Emulsification & Neutralisation

Bile has two functions in the small intestine. The mark scheme accepts both.

Function 1 — Neutralise

Bile is alkaline. It neutralises acidic food coming from the stomach, providing the optimum pH for enzymes in the small intestine.

Function 2 — Emulsify

Bile emulsifies fats: breaks large fat droplets into small droplets. Same volume of fat, but much larger total surface area — lipase works faster.

Digestion vs Absorption — What Gets Absorbed?

Two different processes. Mark schemes penalise students who confuse them.

Digestion

The breakdown of large, insoluble food molecules into small, soluble molecules.

Where: mouth → stomach → small intestine

Absorption

The movement of small soluble molecules from the gut into the blood.

Where: small intestine (via villi — see Lesson 1.09)

THE most important rule on Paper 1 digestion questions

The body absorbs the PRODUCTS of digestion, NOT the original molecules.

❌ "Starch is absorbed" → wrong

✅ "Starch is broken down to glucose, which is absorbed" → correct

❌ "Protein is absorbed" → wrong

✅ "Protein is broken down to amino acids, which are absorbed" → correct

What Are The Products Used For?

Once absorbed into the blood, the products of digestion are used to build new molecules or release energy.

Glucose

Source: starch digestion (amylase)

Used for: respiration (releases energy); building new carbohydrates

Amino acids

Source: protein digestion (protease)

Used for: protein synthesis — building new tissues, enzymes, hormones

Fatty acids + glycerol

Source: lipid digestion (lipase)

Used for: building new lipids; energy store; cell membranes

Synoptic link — Lessons 1.05 & 1.09

Absorption happens through villi in the small intestine — finger-like projections covered in microvilli. Large surface area, thin walls (one cell thick), good blood supply. You met the surface-area-to-volume principle in Lesson 1.09; villi are the digestive system's version of alveoli.

PEE Worked Example — 6-Mark Applied Question

A person has chronic pancreatitis — long-term inflammation of the pancreas. As a result, their pancreas releases only small amounts of digestive enzymes.

Explain why this person may have difficulty digesting their food AND why they may have difficulty maintaining their body mass. 6 MARKS

Step 1 — Read the command word. "Explain" — give cause-and-effect reasoning. The question has TWO parts — answer both, or get capped at Level 2.

Step 2 — Plan two halves. Half A: digestion failure → which enzymes, which substrates. Half B: absorption failure → less of which products → effect on body mass.

A

Half A — Difficulty digesting food. The pancreas normally produces amylase, protease and lipase. With less of each enzyme: less amylase means starch is not broken down to glucose; less protease means proteins are not broken down to amino acids; less lipase means lipids are not broken down to fatty acids and glycerol.

B

Half B — Difficulty maintaining body mass. Without digestion, fewer products of digestion can be absorbed in the small intestine. Less glucose absorbed → less respiration → less energy for daily activity. Less amino acids absorbed → less protein synthesis → less tissue building and repair. Less fatty acids and glycerol absorbed → less energy storage. So the body mass cannot be maintained.

✓

Mark-scheme tick: ✅ Both halves addressed (Level 3 requirement) · ✅ Named enzymes (amylase, protease, lipase) · ✅ Named products of digestion (glucose, amino acids, fatty acids, glycerol) · ✅ Explicit cause-and-effect links ("less…means…").

Level 3 · 5–6 marks awarded.

Examiner Alerts

Products absorbed, NOT originals

The single most-flagged error on 6-mark digestion questions: writing "starch is absorbed" instead of "starch is broken down to glucose, which is absorbed." The body absorbs the products of digestion — glucose, amino acids, fatty acids, glycerol. Memorise this principle.

Lipase produces BOTH fatty acids AND glycerol

The mark scheme awards a separate mark for each product. Writing only "fatty acids" gives 1 mark instead of 2. Always write both: "fatty acids AND glycerol."

Bile is NOT an enzyme

Bile emulsifies fats (a physical process) and neutralises stomach acid. It does not digest fats — lipase does. Mark schemes explicitly flag students who write "bile breaks down fat."

Two-part questions need two paragraphs

When a question has "and" in it (e.g. "difficulty digesting food and difficulty gaining mass"), write a paragraph for each half. Many students answered only one half and were capped at Level 2.

Common Errors

WrongRight

"the small intestine absorbs starch""the small intestine absorbs glucose — the product of starch digestion"

"stomach acid digests food""HCl kills pathogens AND provides the optimum pH for protease "

"bile is an enzyme""bile is an emulsifier and a neutraliser — NOT an enzyme"

"bile is made in the gall bladder""bile is made in the liver and stored in the gall bladder"

"lipase produces fatty acids""lipase produces fatty acids AND glycerol "

"stomach makes amylase""the pancreas makes amylase; the stomach makes protease (pepsin)"

Action

Screenshot this table. Every single row is a real common error from examiner reports — and every right column is the mark-scheme-approved phrasing.

Practice Question

A patient has had their gall bladder surgically removed (a procedure called cholecystectomy). The patient is advised by their doctor to reduce their intake of fatty foods.

(a) State where bile is produced in the body. 1 MARK

(b) Describe the two functions of bile in the small intestine. 2 MARKS

(c) Explain why this patient now finds it harder to digest fats. 3 MARKS

a

Bile is produced in the liver. (1 mark)

b

Bile is alkaline, so it neutralises the hydrochloric acid coming from the stomach, providing the optimum pH for enzymes in the small intestine. (1 mark)

Bile emulsifies fats — breaks large fat droplets into small droplets, increasing the total surface area for lipase to act on. (1 mark)

c

Without the gall bladder, bile cannot be stored and released in large amounts when fatty food enters the small intestine. (1 mark)

So less bile is available to emulsify fats — fewer small droplets form, and the surface area for lipase is smaller. (1 mark)

This means lipase acts more slowly, less fat is broken down into fatty acids and glycerol, and fat digestion is incomplete. (1 mark)

Total: 6/6. Pattern: location → function → consequence when function is missing.

Recap — Three Takeaways

1. The eight named organs

Mouth · oesophagus · stomach · small intestine · large intestine · pancreas · liver · gall bladder. Food travels through the first five; the last three are accessory organs that release secretions.

2. The three enzyme classes

Amylase (starch → glucose) · Protease (protein → amino acids) · Lipase (lipid → fatty acids AND glycerol). Both lipase products are required for full marks.

3. The principle that decides 6-mark questions

The body absorbs the PRODUCTS of digestion, NOT the originals. Glucose, amino acids, fatty acids, glycerol — these cross into the blood via the villi. Bile emulsifies fats; it is NOT an enzyme.

You're set

Eight organs. Three enzymes. Two bile functions. One absorption rule. You'll handle any digestion question on Paper 1.

Next Up

Lesson 2.03 — Enzymes

You now know the three enzyme classes and what they do. Next: how they work — the lock-and-key model, the active site, and the conditions enzymes need.

Mark-scheme language for denaturation — the term that examiners reward, versus "killed" which they explicitly do not accept.

SEE YOU IN LESSON 2.03.