Digestion: turning food into usable nutrients — Biology, 11–13 years
Food cannot help the body while it is still in large pieces. Digestion breaks it into small substances that can cross into the blood and be used by cells.
What digestion does
Digestion is the body’s way of taking apart food. Teeth and muscles break it up, while juices change some large molecules into smaller ones, such as glucose and amino acids. These can cross the wall of the small intestine and enter the blood.
Why break food apart?
The problem is size: a piece of bread cannot pass through the intestine wall, but tiny glucose molecules can. Digestion solves this by using movement, acids and enzymes. Scientists worked this out by observing organs, testing digestive juices and tracing what the body absorbs.
A bite of bread
Imagine eating a bite of bread. First, teeth crush it and saliva begins changing starch; then the stomach churns it with acid. In the small intestine, enzymes finish breaking starch into glucose. The intestine absorbs the glucose into the blood, while most fibre continues towards the large intestine.
The stomach is not the whole system
A common mistake is to say that the stomach digests all food and sends it straight into the body. This feels reasonable because the stomach is famous and food visibly enters it. In fact, digestion begins in the mouth, continues through several organs, and most nutrient absorption happens in the small intestine.
Digestion in everyday choices
Digestion helps explain why eating a varied diet matters: the body needs different small nutrients, not just one kind of food. It also explains why fibre is useful even though most fibre is not absorbed. Food labels, medical advice and treatment for digestive problems all rely on this process.
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