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Digestion
The Processes Involved and
Key Terminology
for AS Level
© British Nutrition Foundation 2002
What is Digestion?
Digestion is both the physical and chemical breakdown
of large food particles into smaller ones in order that
they become more soluble and easier to absorb by the
body.
•
Not all of the foods that we eat need to be broken
down e.g. water and simple sugars such as glucose
and many vitamin and mineral salts.
•
All foods that we eat cannot be used by the body until
they have passed into the bloodstream a process
called absorption. Here they are distributed to all the
cells in the body where they sustain the complex
process of metabolism.
Digestion
The process of digestion is a complex one. It
starts in the mouth and continues throughout
the intestinal tract until the actual absorption
of food consumed takes place or until any
waste or by products are passed from the
body in faeces or urine.
The chemical processes involved in
digestion are brought about by enzymes.
These enzymes speed up digestion
meaning that the whole process takes just
a few hours.
Each stage of digestion involves hydrolysis
and is carried out by hydrolysing enzymes
or hydrolase's.
Some less dense food molecules only need
one or two stages of hydrolyses to make
them ready for absorption; where as Protein
requires a large amount of hydrolytic steps
before breakdown is complete.
© British Nutrition Foundation 2002
The Digestive Process
The digestive Process involves a very small number of different enzymes
or hydrolase's which breakdown proteins, carbohydrates and fats –
their names can be seen below.
Name
Food Group
(Substrate)
End Product
Ready for
Absorption.
Amylase
Starch
Maltose
Maltase
Maltose
Glucose
Lipase
Fats
Fatty Acids and
Glycerol
Peptidase
Proteins
Amino Acids
Hydrolase's
Peptides – can be conveniently sub divided into Exopeptidases which
split off amino acids from the ends of protein molecules and
Endopeptidases which breakdown the inside of the protein molecules.
Lipases – only a few lipases are required for the breakdown of fats.
Amylases – that are required for the breakdown of sugars also require
quite a few steps before absorption can take place.
A whole series of hydrolase's are required for the breakdown of
Proteins and carbohydrates as they are so large and complex.
The Stages of Digestion
Mouth
╬
Oesophagus
╬
Stomach
╬
Small Intestine
╬
Large Intestine
The Mouth
The teeth and jaws crush and grind the food
into small particles (physical digestion) and
mix them with salvia. The saliva contains an
enzyme called Salivary Amylase and
sodium chloride which activates the enzyme.
The initial hydrolysis of cooked starchy food
begins in the mouth and continues as the
food travels down the oesophagus and into
the stomach.
The Salivary Amylase is deactivated in the
stomach as it cannot tolerate acid
environments.
Enzyme Action: Salivary Amylase breaks down
starch to sugars
© British Nutrition Foundation 2002
The Oesophagus
The food passes down the tube
which connects the mouth to the
stomach. This tube is called the
oesophagus.
The food is squeezed along the
oesophagus by a process called
peristalsis where the muscles
contract to push the food along.
It takes about 3-6 seconds for
food to go from the mouth to
the stomach.
© British Nutrition Foundation 2002
The Stomach
The stomach is a kind of mixing tank with thick,
muscular walls which churn the food into a liquid
called chyme.
The stomach lining contains cells which produce a
liquid called gastric juice. The gastric juice contains
the enzyme pepsin and an acid called hydrochloric
acid which pepsin needs to work effectively. This acid
also kills most of the bacteria which are present in
food.
Enzyme Action: Pepsin breaks down food - Protein
to Peptides and Amino Acids
The rhythmic churning of the stomach also called
peristalsis mixes the food with the gastric juice.
© British Nutrition Foundation 2002
Digestion in The Stomach cont…
• The acidity of the chyme increases.
• The endopeptidase called pepsin converts the inner part of protein into
simpler molecules called peptones.
• The other enzyme in the gastric juice is rennin which causes milk to
clot.
The production of Gastric Juice is stimulated by the appearance, taste
and smell of food. However it too may be inhibited by excitement,
depression, anxiety and fear.
Food usually stays in the stomach for 1-4 hours, but liquids may pass
through in a few minutes.
The peristaltic action moves the chyme into the lower region of the
stomach where the pyloric valve opens at intervals allowing small
portions of Chyme to leave the stomach and enter the duodenum
(small intestine).
The Small Intestine – The main stage of
digestion!!
• The small intestine is a tube about 6 meters long.
• Chyme is squirted into the duodenum from the stomach. As it enters the
small intestine the chyme is mixed with a digestive juice from the pancreas
called pancreatic juice.
•Another substance called bile, which is made in the liver, is also mixed with the
chyme; and finally intestinal juice which produced in the lining of the
duodenum.
• Pancreatic juice is alkaline which helps to neutralise the acid from the stomach.
It also contains several enzymes which help to break starch, fat and protein.
• Pancreatic Amylase found in the pancreatic juice breaks down starch
converting it to maltose.
© British Nutrition Foundation 2002
The Small Intestine cont..
• Pancreatic Lipase partly hydrolyses or breaks down some fat
molecules converting them to simpler forms for absorption.
• The endopeptidases trypsin and chymotrypsin among others carry
on the breakdown of proteins into peptones.
• Bile contains bile salts which break down fat into tiny droplets. This
process helps lipase split the fat droplets into fatty acids and glycerol.
• Maltase, Lactase and Sucrase break down the double sugars maltose,
lactose and sucrose into absorbable forms.
• Exopepetidases break down the ends of the protein molecules into
dipeptides which contain only two amino acids. These are then broken
down into single absorbable amino acids by the enzyme dipeptidases.
Absorption in The Small Intestine
The chyme is pushed along the small intestine by the action of peristalsis. As
the chyme is digested, vitamins, minerals, amino acids, fatty acids, glycerol,
peptides and sugars are released. These are able to enter the bloodstream
through the intestinal wall. This is called absorption.
The lining of the small intestine is folded and has little finger-like projections
called villi which increase the area for absorption. The surface is only one cell
thick which speeds up the absorption of nutrients.
The amino acids and sugars then pass from the villi wall into the bloodstream
and go to the liver. Most of the fatty acids and glycerol go to the lymphatic system
(lymph glands) from where they are resynthesized and reformed into fat
molecules that are more suitable for use in the body.
The nutrients are then transported in the blood to all the cells of the body. In this
way the cells get the nutrients they need. Some nutrients provide energy and
others are used to repair cells or build new cells. The chyme, which is now less
fluid because some water has been removed, passes into the large intestine.
© British Nutrition Foundation 2002
The Large Intestine
About 7-9hrs after a meal food that has not been digested
in the duodenum passes through the ileocaecal valve
and into the large intestine.
The large intestine is a long tube, inhabited by bacteria.
The remains of chyme contain a mixture of substances
which have not been digested. These include fibre, some
vitamins and minerals and water. Some of the bacteria in
the large intestine break down the fibre to fatty acids and
gas. Other bacteria produce vitamin K and some B
Vitamins.
This bacterial action is not on a large scale but small
molecules formed can be absorbed through the walls of
the large intestine and into the blood.
© British Nutrition Foundation 2002
The Main Function of The Large
Intestine
• The removal of water from the fluid mass as it passes
through the large intestine.
• By the time it reaches the end of the tube known as the
anus it is a semi solid form known as faeces.
• The faeces contain – undigested food material, residues
from digestive juices, large numbers of both living and
dead bacteria and water.
• After having been in the large intestine for about 20hrs
these material are passed out of the body.
This time can be reduced if the diet is high in fibre because of
the increased bulk.
Student Task
1. Using Food Science Nutrition and Health
p.23 complete the diagram summary of
the digestive process.
2. Use Food Science Nutrition and Health
p.22 to complete the handout / diagram of
the digestive system.
3. Past Paper Questions on digestion.
Mrs. C Simpson
Food Science Nutrition and Health.
British Nutrition Foundation.
For further information, access:
www.nutrition.org.uk
© British Nutrition Foundation 2002