The digestion process
© Food – a fact of life 2009
Extension
Learning objectives
• To recognise the organs involved in digestion;
• To understand the major functions of each body
part in the digestive process;
• To understand the four major phases of digestion;
• To understand the functions of different enzymes.
© Food – a fact of life 2009
Organs involved in digestion
What is the route food takes through the body?
© Food – a fact of life 2009
Organs involved in digestion
Mouth
The gastrointestinal
(GI) tract involves
the stomach, small
intestine and colon.
Oesophagus
Stomach
Small intestine
Colon
Anus
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Rectum
Mouth
Mastication is the action of the teeth and the jaws
working together to break food down. Food needs to
be chewed to be broken down into pieces small
enough to swallow.
Breaking the food down also gives it a larger surface
area for the digestive enzymes to work on.
There are two different types of teeth in the mouth
- Incisors to tear food e.g. meat
- Molars to grind the food.
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Saliva
Saliva contains the enzyme amylase which breaks
down starch into simple sugars.
It also moistens the food allowing easier passage
through the gastrointestinal tract.
Saliva is secreted from glands found under the tongue
and at the back of the mouth.
The sight, smell, taste or even the thought of food will
start to increase the amount of saliva secreted.
© Food – a fact of life 2009
Moving on from the mouth
Food is masticated and mixed with saliva in the mouth.
The tongue and cheeks help to push the food into the
teeth and also shape the food into a ball or bolus
before being swallowed.
The bolus is passed through to the oesophagus.
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Oesophagus
The oesophagus is similar to a conveyor belt as it
transfers the food from the mouth to the stomach in 3-6
seconds.
Circular muscles in the wall of the oesophagus relax in
front of the bolus whilst circular muscles behind the
food contract, pushing the bolus onward.
This is called peristalsis.
People do not have conscious control over the
muscles in the oesophagus. Even if someone is upside
down, the food will be passed on to the stomach.
© Food – a fact of life 2009
Stomach
The stomach is an expandable sack made up of three
different layers of muscles where the bolus will be
churned for a few minutes or up to 2 or 3 hours.
The bolus is mixed with hydrochloric acid which helps
to kill any bacteria present.
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Stomach
The enzyme pepsin is also active in the stomach. It
starts to break down protein to form peptides and
amino acids.
Alcohol and a little water is absorbed by the stomach.
Alcohol is absorbed through the stomach wall and
taken to the liver where it is metabolised.
When the food has been churned into a creamy
mixture known as chyme, the pyloric sphincter (a ring
of muscles) opens and chyme is released gradually
into the small intestine.
© Food – a fact of life 2009
Small intestine
Chyme passes out of the stomach through the pyloric
sphincter into the small intestine.
The small intestine is a tube about 6 metres long.
The small intestine is divided into three sections, the
duodenum, jejunum and the ileum.
The first section of the small intestine is the duodenum.
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Duodenum
The duodenum receives about 12 grams of chyme
each time the pyloric sphincter opens.
The duodenum is about 25 centimetres long and in the
shape of a horse shoe.
In the duodenum, chyme is diluted with bile salts (from
the gall bladder) and pancreatic juices (from the
pancreas).
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Bile
Bile is produced in the liver and stored in the gall
bladder. This contains bile salts which emulsify fat.
Fat is normally insoluble in water. The bile salts are
released into the duodenum making it easier to
break down fat.
The bile allows the fats to mix in with the watery
digestive juices, and allows the enzyme lipase to
digest the fats efficiently.
Fat can take from 3 to 5 hours to be broken down
and absorbed.
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Pancreatic juices
The pancreas provides alkaline pancreatic juices.
These juices contain sodium bicarbonate to
neutralise the hydrochloric acid mixed into the
chyme from the stomach.
Pancreatic juices also contain digestive enzymes
such as:
●Trypsin and chymotrypsin – break down protein to
peptides and amino acids.;
●Pancreatic amylase – breaks down starch and
glycogen to maltose;
●Lipase – breaks down fat to fatty acids and
glycerol.
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Peristalsis
Peristalsis is the action of waves of muscular
contractions which moves food along the digestive
system.
Dietary fibre aids peristalsis because it increases the
bulk of the bolus or chyme being moved along.
© Food – a fact of life 2009
Wall of the small intestine
The inner surface of the small intestine is folded into
finger-like structures called villi, which greatly increase
the surface area available for absorption.
The villi have a surface area of about 30m2, this is
equivalent to the size of a tennis court!
© Food – a fact of life 2009
Digestion in the wall of the
small intestine
• Protease breaks down peptides to amino acids.
• Maltase breaks down maltose to glucose.
• Sucrase breaks down sucrose to glucose and
fructose.
• Lactase breaks down lactose to glucose and
galactose.
• Lipase breaks down fats to fatty acids and glycerol.
© Food – a fact of life 2009
Substances absorbed in the
small intestine
•
•
•
•
•
•
•
Water;
Alcohol;
Sugars;
Minerals;
Water soluble vitamins;
Peptides and amino acids;
Fatty acids, glycerol and fat soluble vitamins.
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Absorption
The villi in the small intestine have a high blood supply.
There are two types of absorption which occur here.
Passive – through the process of osmosis the nutrients
pass through the wall of the small intestine and into the
blood supply.
Active – a carrier transports the nutrient through the
wall of the small intestine into the blood supply. This
type of absorption requires energy.
Once in the blood the nutrients are transported to the
liver via the hepatic portal vein. The liver filters, converts
the nutrients into substances that can be used by the
body’s cells for energy and growth.
© Food – a fact of life 2009
Absorption
Most fatty acids and glycerol pass into the lymphatic
system, and then the bloodstream.
Once in the blood, nutrients are carried to all the cells
of the body. Some are oxidised to produce energy
and other are used to repair the cell or to build new
cells.
Once the nutrients have been absorbed, the
remaining chyme passes into the large intestine or the
colon.
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Colon
The colon is a tube just over one meter long, which is
inhabited by bacteria.
The main function of the colon is to absorb water into
the bloodstream.
Bacteria in the colon ferment dietary fibre (NSP) and
produce fatty acids and gas.
Other bacteria produce vitamin K, which is also
absorbed.
The products of bacterial digestion, along with water
and any remaining minerals are absorbed leaving a
residue behind.
© Food – a fact of life 2009
Colon
The watery residue moves along the colon, and the
faeces are formed and stored in the rectum before
being excreted through the anus.
Young children gradually learn to control this action.
It may take 12-24 hours for the faeces to pass through
the colon. This time can be reduced if the diet is high in
fibre.
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Gut Bacteria
The gut contains bacteria
Stomach 101 – 103 cfu/ml
• 400 – 500 different
species
• includes potentially
pathogenic (e.g.
Clostridia) &
potentially beneficial
(e.g. Bifidobacteria &
Lactobacilli) bacteria.
•These are measured
in colony forming units
per millilitres (cfu/ml).
© Food – a fact of life 2009
Duodenum & jejunum
102 – 105 cfu/ml
Colon 1010 – 1012 cfu/ml
Summary of the phases of digestion
Ingestion - this is the physical intake of foodstuffs into
the gastrointestinal tract.
Digestion - a series of physical and chemical
processes which begin in the mouth, but take place
mainly in the stomach and small intestine.
Absorption - the passage of the digested food
substances across the gastro-intestinal lining, or
mucosa, into the blood and lymph.
Elimination - the excretion, or elimination, of those food
substances that cannot be digested (such as cellulose)
or without any nutritional value in the faeces.
© Food – a fact of life 2009
Describe the digestion of …
Fish and chips
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Hamburger
Review of the learning objectives
• To recognise the organs involved in digestion;
• To understand the major functions of each body
part in the digestive process;
• To understand the four major phases of digestion;
• To understand the functions of different enzymes.
© Food – a fact of life 2009
For more information visit
www.foodafactoflife.org.uk
© Food – a fact of life 2009