The digestive system

The food we eat is mostly made of big molecules that cannot get into our blood. Digestion breaks them down into small, soluble molecules that can be absorbed and carried to every cell. It starts with your teeth and ends about 30 hours later, after a 9-metre journey through the digestive system.

Key facts

  • Food goes: mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus. The liver, gall bladder and pancreas help along the way.
  • Teeth: incisors cut and bite, canines tear and grip, molars and premolars crush and grind. Children have 20 milk teeth; adults have 32 teeth.
  • The stomach churns food with hydrochloric acid (which kills bacteria) and a protease enzyme.
  • Digestion finishes in the small intestine, and the small molecules are absorbed into the blood through millions of villi. The large intestine absorbs water.
  • Enzymes speed up digestion: amylase breaks starch into sugars, protease breaks proteins into amino acids, and lipase breaks lipids (fats and oils) into fatty acids and glycerol.
  • Bile is made in the liver and stored in the gall bladder. It neutralises stomach acid and breaks fat into small droplets, so lipase works faster.
  • Villi have a large surface area, a wall only one cell thick and a good blood supply, so food molecules are absorbed quickly.

Worked examples

Example 1

Put these in the order food passes through them: stomach, mouth, large intestine, oesophagus, small intestine.

  1. Food is chewed in the mouth and swallowed down the oesophagus (gullet).
  2. The stomach churns it with acid and enzymes.
  3. In the small intestine digestion finishes and the food molecules are absorbed.
  4. The large intestine takes back water from what is left. mouth → oesophagus → stomach → small intestine → large intestine

Answer: mouth → oesophagus → stomach → small intestine → large intestine

Example 2

A fox has long, pointed canine teeth. What does this tell you about its food, and what are canines for?

  1. Incisors are flat and sharp at the front for cutting and biting.
  2. Canines are long and pointed: they grip and tear food, like meat.
  3. Molars at the back crush and grind. A fox has big canines, so it is a carnivore: it eats meat.

Answer: it eats meat; canines grip and tear

Example 3

Name the enzyme that digests starch, and say what it makes.

  1. Starch is a carbohydrate made of long chains of sugar molecules.
  2. Amylase breaks the chains into sugars. starch → sugars
  3. Amylase is made in the salivary glands and the pancreas, so starch digestion starts in the mouth.

Answer: amylase; it makes sugars

Example 4

Explain how bile helps us digest fat.

  1. Bile is made in the liver, stored in the gall bladder and released into the small intestine.
  2. It emulsifies fat: it breaks big drops into many tiny droplets, which have a much larger surface area for lipase to work on.
  3. It is alkaline, so it neutralises the acid from the stomach. The enzymes in the small intestine work best in these conditions.

Answer: it breaks fat into small droplets (bigger surface area for lipase) and neutralises stomach acid