The Digestive System

The digestive system is an organ system responsible for breaking down food into small, soluble molecules that can be absorbed into the blood and transported to cells throughout the body. This lesson covers the structure and function of each organ in the digestive system, as required by the AQA GCSE Combined Science Trilogy specification (8464).

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Overview of the Digestive System

The digestive system is essentially a long tube (the alimentary canal or gut) running from the mouth to the anus, with several associated organs that assist in digestion.

graph TD
    A["Mouth"] --> B["Oesophagus"]
    B --> C["Stomach"]
    C --> D["Small Intestine"]
    D --> D1["Duodenum"]
    D --> D2["Ileum"]
    D2 --> E["Large Intestine"]
    E --> F["Rectum"]
    F --> G["Anus"]
    H["Salivary Glands"] -.-> A
    I["Liver"] -.-> D1
    J["Gall Bladder"] -.-> D1
    K["Pancreas"] -.-> D1

Exam Tip: You must be able to identify each organ on a diagram of the digestive system. Practise labelling diagrams until you can do it from memory.

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Organs of the Digestive System

The Mouth

• Food is broken down mechanically by the teeth (chewing increases the surface area for enzymes to act on)
• Salivary glands produce saliva, which contains the enzyme salivary amylase
• Salivary amylase begins the chemical digestion of starch into maltose (a sugar)
• The tongue shapes food into a ball called a bolus for swallowing

The Oesophagus

• A muscular tube connecting the mouth to the stomach
• Food is moved along by waves of muscular contraction called peristalsis
• No chemical digestion takes place here — it is purely a transport tube

graph LR
    A["Circular muscles contract behind the bolus"] --> B["Food is squeezed forward"]
    B --> C["Muscles ahead relax to allow food to pass"]
    C --> D["Wave repeats along the length of the tube"]

The Stomach

• A muscular sac that churns and mixes food with gastric juice
• Glandular tissue in the stomach wall produces:
  • Protease enzymes (e.g. pepsin) that begin protein digestion
  • Hydrochloric acid (HCl) — creates pH 1–2 which kills bacteria and provides the optimum pH for pepsin
  • Mucus — protects the stomach lining from being digested by the acid and enzymes
• The stomach stores food temporarily and releases it gradually into the small intestine

The Liver

• The liver is the largest internal organ
• It produces bile, which is stored in the gall bladder and released into the duodenum via the bile duct
• Bile has two important roles:
  1. Emulsifies fats — breaks large fat droplets into smaller droplets, increasing the surface area for lipase to act on
  2. Neutralises stomach acid — bile is alkaline, so it raises the pH in the duodenum to provide the optimum conditions for intestinal enzymes

Exam Tip: Bile is not an enzyme. It does not chemically digest food — it emulsifies fats (a physical process) and neutralises acid. This is a common mistake in exams.

The Pancreas

• A gland located behind the stomach
• Produces pancreatic juice which contains:
  • Amylase (digests starch to sugars)
  • Protease (digests proteins to amino acids)
  • Lipase (digests lipids/fats to glycerol and fatty acids)
• Pancreatic juice is released into the duodenum (first part of the small intestine)

The Small Intestine

The small intestine has two main sections:

Section	Function
Duodenum	Receives bile and pancreatic juice; most chemical digestion occurs here
Ileum	Main site of absorption of digested food molecules into the blood

The ileum is specially adapted for absorption:

Adaptation	How It Helps
Very long (about 6–7 metres)	Provides a large surface area for absorption
Lined with villi (finger-like projections)	Further increases surface area
Each villus has microvilli on its surface	Even more surface area for absorption
Villi have a thin epithelium (one cell thick)	Short diffusion distance for molecules
Villi have an extensive network of blood capillaries	Maintains a steep concentration gradient by carrying absorbed molecules away quickly
Villi contain lacteals (part of the lymphatic system)	Absorb fatty acids and glycerol

Exam Tip: When explaining why villi are efficient at absorption, state at least three adaptations and for each one explain how it increases the rate of absorption. This is a common 6-mark question.

The Large Intestine

• Absorbs water from the undigested food, forming faeces
• Contains beneficial bacteria that produce some vitamins (e.g. vitamin K)

The Rectum and Anus

• The rectum stores faeces before they are expelled
• The anus is the opening through which faeces leave the body (egestion)

Exam Tip: Egestion (removing undigested food through the anus) is not the same as excretion (removing metabolic waste products such as urea or carbon dioxide). Never confuse the two in an exam.

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Summary of Digestion Along the Alimentary Canal

Organ	Mechanical Digestion?	Chemical Digestion?	Key Substances
Mouth	Yes (teeth chew)	Yes (salivary amylase)	Saliva
Oesophagus	Yes (peristalsis)	No	—
Stomach	Yes (churning)	Yes (pepsin)	HCl, pepsin, mucus
Small intestine (duodenum)	No	Yes (amylase, protease, lipase)	Bile, pancreatic juice
Small intestine (ileum)	No	Completion of digestion	Absorption of nutrients
Large intestine	No	No	Absorption of water

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Common Mistakes