What are the organs of the digestive system, and what does each one do?
The organs of the human digestive system and their functions, the role of mechanical and chemical digestion, and the action of bile.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on the digestive system, covering the digestive organs and their functions, mechanical and chemical digestion, and the role of bile in emulsifying fats and neutralising stomach acid.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to name the organs of the human digestive system and their functions, distinguish mechanical from chemical digestion, and describe the role of bile.
The organs of the digestive system
The digestive system is one long tube (the gut) with several organs adding juices along the way:
- Mouth: teeth chew the food (mechanical digestion) and saliva adds amylase to start digesting starch.
- Oesophagus: a muscular tube that pushes food down to the stomach by waves of muscle contraction (peristalsis).
- Stomach: churns food and adds hydrochloric acid (which kills microbes and gives the right pH) and the enzyme protease.
- Small intestine: where most digestion is completed and where soluble food is absorbed into the blood.
- Large intestine: absorbs water from the remaining material, forming faeces.
Three organs add juices but food does not pass through them:
- Liver: makes bile.
- Gall bladder: stores bile and releases it into the small intestine.
- Pancreas: makes digestive enzymes (amylase, protease and lipase) that act in the small intestine.
Mechanical and chemical digestion
Mechanical digestion matters because breaking food into smaller pieces gives a larger surface area for enzymes to act on, speeding up chemical digestion. The two processes work together.
The role of bile
By increasing the surface area of fats and providing the right pH, bile speeds up chemical digestion even though it does not break any bonds itself.
Why each region has its own pH
Enzymes only work well at the right pH, and different parts of the gut have very different conditions. The stomach is strongly acidic because it makes hydrochloric acid, which kills most microbes in food and gives the stomach enzyme protease the low pH it needs. The small intestine is slightly alkaline, because the acidic mixture from the stomach is neutralised by bile and by alkaline juices from the pancreas. This matters because the enzymes that finish digestion in the small intestine would be denatured by acid, so the pH must be changed for them to work.
The journey of a meal
It helps to trace a meal through the gut. Food is chewed in the mouth and mixed with saliva, then swallowed and pushed down the oesophagus by muscle contractions (peristalsis). In the stomach it is churned with acid and protease for a few hours. It then enters the small intestine, where bile and pancreatic juice complete digestion and the soluble products are absorbed. The remaining material passes into the large intestine, where water is absorbed, leaving faeces to be removed. Knowing this order lets you place any organ or process in context.
Try this
Q1. Name the organ that makes bile. [1 mark]
- Cue. The liver.
Q2. Give one reason food is chewed before being swallowed. [1 mark]
- Cue. Chewing increases the surface area so enzymes can digest the food faster (or it makes food easier to swallow).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe the role of bile in digestion. Refer to where it is made, where it is stored, and two of its functions.Show worked answer →
A Unit 1 structured question worth 4 marks. Reward: bile is made in the liver (1) and stored in the gall bladder (1); it emulsifies fats, breaking large fat droplets into smaller ones to give a larger surface area for lipase to work on (1); and it is alkaline, so it neutralises the acid from the stomach, providing the right pH for enzymes in the small intestine (1). Markers credit the liver, the gall bladder and the two functions. A common error is to say bile is an enzyme; it is not, it speeds up digestion physically and by changing pH.
WJEC style3 marksExplain the difference between mechanical and chemical digestion, giving one example of each.Show worked answer →
A Unit 1 explain question. Mechanical digestion physically breaks food into smaller pieces without changing it chemically, for example chewing with the teeth or churning in the stomach (1 mark for definition, 1 for example). Chemical digestion uses enzymes to break large molecules into small soluble ones, for example amylase breaking starch into sugars (1 mark). Markers reward the contrast (physical breaking versus enzyme breakdown) and one valid example of each. A common error is to call chewing chemical digestion.
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