What are the parts of the human digestive system, and what does each one do?
The structure and function of the organs of the human digestive system, the food groups in a balanced diet, the difference between mechanical and chemical digestion, and the role of bile.
A focused answer to the WJEC GCSE Biology section 1.3 topic on the digestive system, covering the food groups of a balanced diet, the organs of the alimentary canal and their functions, the difference between mechanical and chemical digestion, and the role of bile.
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What this dot point is asking
WJEC wants you to name the food groups in a balanced diet, describe the organs of the digestive system and their functions, distinguish mechanical from chemical digestion, and explain the role of bile.
A balanced diet
A healthy diet contains the right amounts of several food groups, each with a role.
- Carbohydrates: the main source of energy (for example bread, rice, pasta).
- Proteins: needed for growth and repair (for example meat, fish, beans).
- Lipids (fats): an energy store and insulation (for example butter, oils).
- Vitamins and minerals: needed in small amounts for health (for example vitamin C, iron, calcium).
- Fibre: keeps food moving through the gut and prevents constipation.
- Water: needed for all cell reactions and to make up body fluids.
An unbalanced diet can cause problems such as deficiency diseases (too little of a vitamin or mineral) or obesity and related conditions like type 2 diabetes and heart disease (too much energy-rich food).
Why we digest food
Large molecules like starch and protein cannot pass through the gut wall into the blood, so they must be broken down first. This happens in two ways, which work together.
- Mechanical digestion: physically breaking food into smaller pieces, by the teeth chewing and the muscular stomach churning. This increases the surface area for enzymes.
- Chemical digestion: using enzymes to break the chemical bonds in large molecules, turning them into small soluble ones.
The organs of the digestive system
Food travels along the alimentary canal, with several organs adding enzymes and other juices along the way.
- Mouth: teeth chew the food (mechanical digestion); salivary glands add saliva containing amylase, which begins to digest starch.
- Oesophagus: a muscular tube that carries food from the mouth to the stomach by waves of muscle contraction.
- Stomach: muscular walls churn the food; it produces hydrochloric acid (kills bacteria and gives the right pH for protease) and the enzyme protease, which begins to digest protein.
- Liver: makes bile.
- Gall bladder: stores bile and releases it into the small intestine.
- Pancreas: makes digestive enzymes (amylase, protease and lipase) and releases them into the small intestine.
- Small intestine: digestion is completed by enzymes here, and the small soluble products are absorbed into the blood.
- Large intestine: absorbs water from the remaining material; the leftover waste forms faeces.
The role of bile
Bile is made in the liver, stored in the gall bladder, and released into the small intestine. It is not an enzyme, but it helps digestion in two ways.
- It neutralises the acidic mixture arriving from the stomach, making the small intestine slightly alkaline. This is the right pH for the enzymes that work there.
- It emulsifies fats: it breaks large fat droplets into many tiny droplets. This greatly increases the surface area of fat for the enzyme lipase to act on, so fat is digested faster.
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 function of the stomach and the small intestine in digestion.Show worked answer →
A 4-mark structured question, two marks for each organ.
The stomach has muscular walls that churn the food (mechanical digestion) and produces hydrochloric acid and the enzyme protease (pepsin); the acid gives the right pH for protease and kills many bacteria, and the protease begins to break down protein.
The small intestine is where digestion is completed by enzymes (from the pancreas and the intestine wall) and where the soluble products of digestion are absorbed into the blood.
Markers reward: churning and acid and protease for the stomach; completing digestion and absorbing food for the small intestine. Saying the stomach "stores food" only is not enough for the marks.
WJEC style3 marksExplain two ways that bile helps digestion.Show worked answer →
A 3-mark question on bile.
Bile neutralises the acid that arrives from the stomach, making the small intestine slightly alkaline, which is the right pH for the enzymes there to work. Bile also emulsifies fats, breaking large fat droplets into many small droplets, which increases the surface area for lipase to act on, speeding up the digestion of fat.
Markers reward: neutralises stomach acid to give the right pH for enzymes; emulsifies fats to increase surface area for lipase. A common error is to say bile is an enzyme or that it digests fat itself; bile is not an enzyme.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)