Composition of Food
Humans are adapted for feeding and
digestion so that food molecules can
reach the body cells.
The main groups of food we eat are:
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Composition of Food
Carbohydrates contain the elements
carbon, hydrogen & oxygen
Fats contain the elements carbon,
hydrogen & oxygen
Proteins contain the elements carbon,
hydrogen, oxygen & nitrogen
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Carbohydrates provide us with energy
for our normal activities.
They consist of long chains of glucose
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Fats release some energy
for our use if required
while the rest is stored in
adipose tissue beneath the Fatty acid
This is used as insulation.
Each fat is made up of a Fatty acid
glycerol molecule joined to
3 fatty acid molecules.
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Our body requires protein to build it up
and for growth and repairing damaged
Proteins are built up from long chains of
There are only about 20 different amino
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Vitamins are required for good health.
They do not provide energy.
They act as coenzymes which are
required in the various biochemical
activities within cells.
Some examples of vitamins, their source
and the deficiency disease are given in
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Vitamin Source Deficiency Disease
A Milk, fresh veg. Night Blindness
B1 (Thiamine) Cereal grains Beri beri
B3 (Niacin) Cereal, lean Pellagra
C (Ascorbic Citrus fruits Scurvy
D Liver, fish, milk Rickets
E Wheat germ Reproductive
K Green veg. Blood doesn’t clot
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Minerals are chemical elements which
are required in very small quantities.
They have a whole variety of functions,
some of which are shown in the
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Mineral Required For:
Iron Haemoglobin in Red Blood Cells
Sodium All cells
Calcium Healthy teeth & bones
Potassium Contraction of muscles
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The food we eat passes from the mouth to
the anus through the “alimentary canal” or
As food passes through this canal it is
processed in a number of ways and by a
number of organs.
Other organs, such as the salivary glands,
liver and gall bladder are attached to the
alimentary canal by ducts, and these are
called associated organs.
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Mouth Salivary gland
Gall bladder Pancreas
Large intestine Small intestine
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The food is physically broken down into
manageable sizes by the teeth.
It is mixed with a fluid called saliva,
which contains the enzyme salivary
amylase, to digest starch.
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Saliva also contains mucus which makes
the food easier to swallow.
The food is then moved through the
gullet (oesophagus) to the stomach by a
process called peristalsis.
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here moves this
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The stomach is a muscular bag with a
valve at either end.
The CARDIAC sphincter at the top
allows food in.
The PYLORIC sphincter at the bottom
allows food to leave.
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The stomach wall produces digestive
juices from gastric glands which
secrete mucus, acid and enzymes.
For digestion to work properly the food
must be well mixed with these juices.
The stomach muscles contract and relax
to mix up the juices and food.
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Mucus and Gastric Gland
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The mucus-secreting cells release a
slimy mucus which sticks to the wall of
the stomach and protects it from
damage by the strong digestive
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The acid-secreting cells release
hydrochloric acid which has 2
It creates conditions needed to produce
It provides the optimum pH for the
enzymes to work.
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The enzyme-secreting cells release an
inactive enzyme called pepsinogen.
Once the pepsinogen comes in contact
with the acid, it is converted to an
active enzyme called pepsin.
The pepsin can now break down proteins
in the stomach to peptides, which will
later be broken down further into amino
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The longitudinal muscles contract to
make the stomach shorter and fatter.
The circular muscles contract to
squeeze the stomach.
Once the food has been in the stomach
for a while, it is released slowly, small
drops at a time into the small intestine.
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The food enters the small intestine
from the stomach and is moved along by
A number of digestive enzymes break
down the large insoluble food molecules
to small soluble food molecules before
they are absorbed out of the intestine.
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The small intestine’s function is to absorb
soluble food molecules and it is well suited to
this job in a number of ways.
It is very long (About 6m). The inner surface is
folded into thousands of finger-like villi.
The lining of each villus is very thin. (Only one cell
Each villus has a network of vessels for absorbing
the food, both blood capillaries and lymphatic
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The glucose and the amino acids are
absorbed into the blood capillaries.
These tiny blood capillaries all join up to
form the hepatic portal vein and
transport the absorbed food to the
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The liver stores most of the glucose as
glycogen until it is required, but some
glucose remains in the blood.
The amino acids are used in the body
for growth and repair, but if there are
any excess amino acids, these are
broken down by the liver into urea.
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This is later removed by the kidneys
and released in the urine.
Vitamins and minerals are transported
to cells where they have special
functions which help to keep the body
Any excess vitamins and minerals are
either stored or excreted.
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Fats and fatty acids are absorbed by
the lacteals which contain a fluid called
This lymph is transported in the lymph
vessels which will eventually drain into
the blood system.
The products of fat digestion can be
used for energy, as insulation or simply
stored until required.
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When the food eventually reaches the
large intestine (Colon), it is mainly water
and indigestible waste materials.
The function of the large intestine is to
absorb water back into the bloodstream
leaving a semi-solid called faeces.
The faeces is passed into the rectum
(bowel) where it is stored until it is
eventually eliminated through the anus.
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