Biology IGCSE Edexcel - Human Nutrition

What is a balanced diet?

A diet containing the correct proportions of all required nutrients: carbohydrates, proteins, lipids (fats/oils), vitamins, minerals, dietary fibre, and water.

Why do humans need carbohydrates, proteins and lipids (fats)?

Carbohydrates: source of energy

Proteins: for growth and repair of tissues, enzymes, body structure.

Lipids (fats/oils): energy storage, insulation, making cell membranes, and other body functions.

What is digestion?

Digestion is the chemical and mechanical breakdown of food. 

It converts large insoluble molecules into small soluble molecules, which can be absorbed into the blood. 

What is mechanical digestion?

Breaking food into smaller pieces without changing its chemical structure (e.g. chewing in the mouth, churning in the stomach)

What is ingestion?

Taking food or drink into the body through the mouth.

What is egestion?

Removal of undigested food material from the body as faeces through the anus.

What happens in the mouth during digestion?

Chemical digestion occurs when the enzyme amylase in the saliva breaks down starch into maltose. 

Mechanical digestion occurs as the teeth break the food down into smaller chunks. This gives a larger surface area for the enzymes to work.

What happens in the stomach during digestion?

Mechanical digestion occurs as the muscular stomach wall churns up the food.

The stomach wall secretes hydrochloric acid which kills bacteria to protect us from food poisoning.

An enzyme called pepsin breaks down protein in the stomach into amino acids.

A sphincter (ring of muscle) holds the food in the stomach until it is ready to be released into the first part of the small intestine called the duodenum.

What is the duodenum?

The first part of the small intestine; it receives partially digested food from the stomach.

What happens in the duodenum during digestion?

Several enzymes are added into the duodenum by the pancreas in the form of a liquid called pancreatic juice.

These include: amylase, trypsin and lipase.

How does food move along the gut?

Food moves along the gut by a process called peristalsis.

The gut has two layers of muscle, one is circular and the other is longitudinal.

They work together to push the food along like a squeezing wave.

What is the ileum?

The ileum is the last (final) part of the small intestine. It ends at a valve that leads into the large intestine.

What happens in the ileum during digestion?

By this stage the food has been broken down fully into small enough molecules to be absorbed into the blood.

The ileum is adapted for this process.

It has a very large surface area due to villi and microvilli.

What happens in the large intestine during digestion?

It absorbs water (and some salts / minerals) from the undigested food remains, turning the liquid waste from the small intestine into semi‑solid faeces.