Mechanical and Chemical Digestion

Digestion is how your body breaks the food you eat into pieces small enough to use for energy and growth. It happens in two ways at the same time: mechanical digestion, which breaks food apart physically, and chemical digestion, which breaks food apart with special liquids. Together they turn a bite of food into tiny nutrients your body can absorb.

Mechanical digestion is the physical breaking of food into smaller pieces. It starts when your teeth chew food in your mouth, and continues when your stomach muscles squeeze and churn the food. Mechanical digestion does not change what the food is made of — it just makes the pieces smaller so chemical digestion can work faster.

Chemical digestion breaks food into tiny molecules using special liquids. Saliva in your mouth begins breaking down starches; strong acid in your stomach breaks down food further; and enzymes in your small intestine finish the job. Unlike mechanical digestion, chemical digestion actually changes the food into simpler substances the body can absorb.

• Mouth: teeth chew (mechanical) and saliva starts breaking down starch (chemical).
• Stomach: muscles churn the food (mechanical) and acid breaks it down (chemical).
• Small intestine: enzymes finish chemical digestion, and the nutrients are taken into the blood — the next step, nutrient absorption.

The easy way to tell them apart: mechanical digestion changes the size of the food (smaller pieces), while chemical digestion changes what the food is made of (into molecules). Both happen together in the mouth and the stomach, which is why they are such a good team.

Related lessons

• Grade 5 Science