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The stomach
As food moves through the digestive system it is broken down.
Useful food molecules and water are absorbed into the body.
After food is swallowed, it enters the stomach. food enters
from the gullet
Two things happen to food in the stomach:
 the chemical breakdown of food begins
 microbes are destroyed.
muscle tissue
food leaves
the stomach
From the stomach, food enters the small intestine where
digestion is completed and the small digested food molecules
are absorbed into the bloodstream.
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The intestines
After the small intestine, the remains of the food travels
to the large intestine. All that is left of the food is water
and waste material.
The water is valuable, so it is
absorbed from the large
intestine into the bloodstream.
The waste material cannot be
digested or used by the body.
small
intestine
The undigested waste travels to the
rectum where it is stored until leaving
the body through the anus.
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large
intestine
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Organs of the digestive system
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What is digestion?
The body carries out digestion of food to convert large
insoluble food molecules into smaller soluble ones.
carbohydrate
molecule
protein
molecule
fat
molecule
Large food molecules cannot pass through the walls of the
small intestine. Small food molecules can do this when
dissolved in the bloodstream. Why is this important?
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What are carbohydrates?
Starchy foods contain carbohydrates, which are made
of long chains of identical, small sugar molecules.
one sugar molecule
carbohydrate molecule
The body breaks down long chains of carbohydrates into
the smaller sugar molecules.
These small sugar molecules are used by the body to
release energy and make the body work.
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What are proteins?
Proteins, like carbohydrates, are made of long chains of
small molecules. In proteins, these small molecules are
not identical.
one amino acid
protein molecule
Proteins are made up of chains of small molecules called
amino acids. There are over 20 different kinds of amino acid.
Proteins are used by the body for growth and repair.
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What are fats?
Fats are made up of fat molecules, which contain fatty
acids and glycerol.
fatty acids
glycerol
fat molecule
Fat molecules have to be broken down by the body so
that they can be used for energy storage.
Fats are also used by the body to keep heat in and to
make cell membranes.
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Digestive enzymes
How do digestive enzymes help the process of digestion?
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Different types of digestive enzymes
Digestive enzymes are the chemicals that break large
insoluble food molecules into smaller soluble molecules.
Digestive enzymes are classified by the type of food that
they affect; there are three main types:
 carbohydrase – breaks
carbohydrate into smaller
sugars
 protease – breaks protein
into amino acids
 lipase – breaks fat into
fatty acids and glycerol.
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How do enzymes work?
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Carbohydrate digestion
Carbohydrates are chains of identical sugar molecules.
The digestive enzyme called carbohydrase breaks the
chemical bonds between the individual sugar molecules
in each carbohydrate chain.
carbohydrase
long carbohydrate
molecule
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sugar molecules
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Protein digestion
Proteins are made up of amino acids. There are over 20
different types of amino acids.
Proteins are digested by digestive enzymes called proteases.
These enzymes work in an acidic environment to break
proteins into smaller amino acids.
protease
long protein molecule
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amino acid molecules
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Fat digestion
Fats are digested in two stages.
Firstly, bile (released by the gall bladder) allows the fat to
‘mix’ with water by breaking the fat into smaller droplets.
This is called emulsification.
bile
Secondly, the digestive enzyme lipase breaks each fat
molecule into the smaller glycerol and fatty acid molecules.
lipase
fat molecule
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+
glycerol
fatty acids
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Which enzyme?
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The process of digestion
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Label the digestive system
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Summary
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