KS3 Biology
8A Food and
Digestion
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Contents
8A Food and Digestion
A balanced diet
The digestive system
Digestive enzymes
Summary activities
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Why do we need food?
Humans need to consume a balanced diet
which contains a variety of different types
of food.
The main nutrients the body needs are:
carbohydrates for energy;
proteins for growth and repair;
fats to store energy;
vitamins and minerals to keep the body healthy.
The amount of each food type needed is related to the
proportions in the food pyramid.
Which food type does the body need the most of?
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Nutrients in food
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What is a balanced diet?
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What are carbohydrates?
Starchy foods contain carbohydrates which are made of
long chains of identical small sugar molecules.
carbohydrate
one sugar
molecule
molecule
The long chains of carbohydrates are broken down into the
smaller sugar molecules by the body.
The small molecules from carbohydrates 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.
protein one
molecule amino
acid
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.
fat fatty
molecule acids
glycerol
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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Contents
8A Food and Digestion
A balanced diet
The digestive system
Digestive enzymes
Summary activities
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What is digestion?
The body carries out digestion of food to convert large
insoluble food molecules into smaller soluble ones.
fat
carbohydrate molecule
molecule protein
molecule
Small food molecules can pass through the walls of the
small intestine and then dissolve into the blood stream.
Large food molecules cannot do this.
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Mechanical digestion
Digestion begins in the mouth where food is broken down
by the teeth. This is called mechanical breakdown. The
small parts of food are mixed with saliva and swallowed.
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The stomach
After food is swallowed it enters the stomach, which is
basically a muscular bag filled with hydrochloric acid.
food enters
from the gullet
muscle
food leaves tissue
the stomach
Two things happen to food in the stomach:
the chemical breakdown of food begins;
microbes are destroyed.
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The small intestine
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 large intestine
After the small intestine, the
remains of any undigested food
travel to the large intestine.
All that is left of the food is
water and waste material.
The water is valuable and so is absorbed in the large intestine
into the blood stream.
The waste material cannot be digested or used by the body.
This undigested waste travels to the rectum where it is stored
until leaving the body through the anus.
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Digestion summary
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Contents
8A Food and Digestion
A balanced diet
The digestive system
Digestive enzymes
Summary activities
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Digestive enzymes
How do digestive enzymes help the process of digestion?
Digestive enzymes are the chemicals that break large
insoluble food molecules into smaller soluble molecules.
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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, so 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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Digestive enzyme animation
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Enzymes and carbohydrate digestion
Carbohydrates are chains of identical sugar molecules.
The digestive enzymes called carbohydrases break the
chemical bonds between the individual sugar molecules in
each carbohydrate chain.
carbohydrase
long
carbohydrate sugar
molecule molecules
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Enzymes and protein digestion
Proteins are made up of amino acids. There are 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 amino acid
protein molecule molecules
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Enzymes and 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 fatty acid molecules .
lipase +
fat molecule glycerol fatty
acids
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Digestive enzymes activity
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Contents
8A Food and Digestion
A balanced diet
The digestive system
Digestive enzymes
Summary activities
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Glossary
carbohydrate – A nutrient in food that provides energy.
enzyme – A chemical that helps digestion by breaking large
molecules into smaller ones.
fat – A nutrient in food that provides a store of energy.
large intestine – The organ where water is removed from
undigested food.
minerals – Compounds in food that provide the elements
needed in small amounts for a healthy diet.
protein – A nutrient in food needed for growth and repair.
small intestine – The organ where digestion is completed
and digested food molecules are absorbed.
stomach – The organ where food is mixed with acid and
enzymes.
vitamins – Substances found in food that are needed in
small amounts for health.
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Anagrams
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Multiple-choice quiz
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