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Digestive system


Kids health digestive system

( from http://toplowridersites.com/related-pictures-kids-health-digestive-system-video/ )

The digestive process

( from http://www.kidsdiscover.com/infographics/digestive-system-for-kids/ )



Breaking larger molecules down into smaller molecules (General idea!)

from http://www.bbc.co.uk/bitesize/ks3/science/organisms_behaviour_health/diet_drugs/revision/5/

Digestion and enzymes

Our teeth break food down into small pieces when we chew. This is only a start to the process of digestion, as chewed pieces of food are still too large to be absorbed by the body. Food has to be broken down chemically into really small particles before it can be absorbed. Enzymes are needed so that this break-down happens quickly enough to be useful.


Take care – enzymes are not living things. They are just special proteins that can break large molecules into small molecules. Different types of enzymes can break down different nutrients:

  • carbohydrase or amylase enzymes break down starch into sugar
  • protease enzymes break down proteins into amino acids
  • lipase enzymes break down fats into fatty acids and glycerol.


Carbohydrates are digested in the mouth, stomach and small intestine. Carbohydrase enzymes break down starch into sugars.

The saliva in your mouth contains amylase, which is another starch digesting enzyme. If you chew a piece of bread for long enough, the starch it contains is digested to sugar, and it begins to taste sweet.

Diagram shows how carbohydrase molecules break down starch into sugar during digestion


Proteins are digested in the stomach and small intestine. Protease enzymes break down proteins into amino acids. Digestion of proteins in the stomach is helped by stomach acid, which is strong hydrochloric acid. This also kills harmful micro-organisms that may be in the food.

Diagram shows protease molecules breaking down proteins into amino acids


Lipase enzymes break down fat into fatty acids and glycerol. Digestion of fat in the small intestine is helped by bile, made in the liver. Bile breaks the fat into small droplets that are easier for the lipase enzymes to work on.

Digestion breaks down fat molecules into fatty acid molecules and glycerol molecules

Things that are not digested

Minerals, vitamins and water are already small enough to be absorbed by the body without being broken down, so they are not digested.

Digestive enzymes cannot break down fibre, which is why it cannot be absorbed by the body.

How does the digestive system break larger molecules down into smaller molecules?

Hydrolysis (adding water molecules as part of a chemical reaction.)



Digestion of Carbohydrates

Starch is a carbohydrate made of a large number of glucose units, bonded together. Has to be broken down into single sugar units to be absorbed.

1. Mouth (chewing, and enzymes in saliva)

2. Esophagus (tube connects mouth to stomach)

3. Stomach (acid kills bacteria, enzymes do more digestion)

4. Small intestine (more enzymes totally digest food)

5. In the intestine, broken-down molecules are so tiny, that they pass through the intestine wall, into the bloodstream

6. Once in blood, molecules travel to all cells in the body, where they are used.



Absorption and egestion


These are the processes that happen in the digestive system:

ingestion (eating) → digestion (breaking down) → absorptionegestion

Digested food molecules are absorbed in the small intestine. This means that they pass through the wall of the small intestine and into ourbloodstream.
Once in the bloodstream, the digested food molecules are carried around the body to where they are needed.

Only small, soluble substances can pass across the wall of the small intestine.
Large insoluble substances cannot pass through.

Absorption into bloodstream

Diagram shows food in the small intestine, and the wall of the small intestine which divides it from the blood stream.

Shows carbohydrase combining with the food in the small intestine.

Shows the food cells passing through the wall of the small intestine into the blood stream

The inside wall of the small intestine needs to be thin, with a really big surface area.

This allows absorption to happen quickly and efficiently. If the small intestine had a thick wall and a small surface area, a lot of digested food might pass out of the body before it had a chance to be absorbed.

To get a big surface area, the inside wall of the small intestine is lined with tiny villi (one of them is called a villus).
These stick out and give a big surface area. They also contain blood capillaries to carry away the absorbed food molecules.

Diagram of villli, showing the walls which are just 1 cell thick, and the network of capillaries, and the blood vessels


Excess water is absorbed back into the body in the large intestine. What is left then is undigested food. This is stored in the rectum, the lower part of the large intestine, until we are ready to go to the toilet. It then comes out of the rectum through the anus as feces. This process is called excretion.

Sample questions

Secretions from the pancreas contain compounds called lipases. Lipases increase the rate of digestion of lipids. Lipases are an example of which of the following?

A. enzymes
B. hormones
C. nucleic acids
D. simple sugars


Each part of the human digestive system has a specific function. Which of the
following activities best models the role of the esophagus?

A. shaking a small piece of chalk in a plastic bottle
B. cutting a clay cube into smaller and smaller cubes
C. squeezing a small greased ball through plastic tubing
D. placing a small piece of egg in dilute hydrochloric acid


Here are some problems associated with four organs of the human digestive system.

1 Acid inside this organ begins to dissolve some of its tissue lining.

2 An infection prevents the movement of nutrients through this organ’s walls into the bloodstream.

3 Inflamed tissue prevents the smooth passage of food through this organ
after swallowing.

4 Swelled-up veins make eliminating wastes from this organ difficult.

Which organ is most likely the stomach?
A. organ 1 .   B. organ 2     C. organ 3     D. organ 4


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