Digestion and a Balanced Diet

Section: Biology | Syllabus: Cambridge Primary Stage 5 Science (0097)

The Human Digestive System

The digestive system breaks food down into pieces small enough for the body to absorb and use.
Food passes through a long tube of connected organs in a fixed order.

mouth → oesophagus → stomach → small intestine → large intestine → anus

Organ	Function
Mouth	Teeth break food into smaller pieces; saliva moistens it and swallowing begins
Oesophagus	A muscular tube that moves food from the mouth to the stomach
Stomach	Churns food and mixes it with acid and digestive juices to break it down further
Small intestine	Digestion is completed here, and digested food is absorbed into the blood
Large intestine	Absorbs water from the undigested material that is left
Anus	Where the remaining waste (faeces) leaves the body

Many vertebrates - animals with a backbone, such as mammals, birds, reptiles and fish - have a digestive system with a similar overall plan to humans.
The exact length and shape of each organ can differ between species, but food still moves through a one-way tube from mouth to anus.

An adequate diet supplies enough food and energy for a person's needs.
A balanced diet supplies the right amounts of different types of nutrients, not just enough food overall.
Too little or too much of a nutrient can harm health, so variety and the right proportions both matter.

Nutrient	Main job in the body	Example foods
Carbohydrate	Supplies energy	Bread, rice, pasta, potatoes
Protein	Supports growth and repair	Meat, fish, eggs, beans
Fat	Stores energy; protects organs	Oils, butter, nuts
Vitamins and minerals	Support many body functions and health	Fruit and vegetables
Fibre	Helps food move through the gut	Wholegrains, vegetables
Water	Needed by every cell in the body	Drinking water, many foods

A balanced diet includes the right proportions of each nutrient group

Worked Example: Using a Food Label

Question: A cereal contains 4.5 g protein per 100 g. A learner eats 200 g. Calculate the protein eaten and explain why protein is needed. [2]

200 g is twice 100 g, so multiply 4.5 g by 2. 4.5 g × 2 = 9.0 g. Answer

The learner eats 9.0 g of protein. Protein is needed for growth and repair.

MistakeDigestion means food simply travels through the body.

FixDigestion breaks food down; movement, digestion, absorption and egestion are different processes.

MistakeThe large intestine absorbs most of the digested food.

FixMost digested food is absorbed in the small intestine; the large intestine mainly absorbs water.

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