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There are two types of digestion:
mechanical and chemical.
Mechanical digestion is when food
is ground up using the teeth.
Another example of mechanical
digestion is when the food is
churned by the movements in the
alimentary canal.
Chemical digestion is the breaking
down of food using enzymes.
Enzymes break large food
molecules into smaller ones.
As food is broken down, it needs
to be transported through each
part of the digestive system.
To get from the mouth to the
stomach, the food has to travel
down the oesophagus. It is not
gravity that pulls the food down, it
is waves of peristalsis. Peristalsis
is the contraction and relaxation of
circular muscles in the wall of the
gullet.
Peristalsis is also used in the
stomach to break up the chunks of
food into a mush as part of
mechanical digestion.
The point of digestion is to break down large, insoluble food
molecules into smaller, soluble ones that can be easily absorbed into
the bloodstream. Enzymes help to do this. There are different types
of enzymes which each break down different food substances.
Carbohydrates are broken down by the enzyme carbohydrase. It is
found in saliva in the mouth and in the pancreatic juice that is poured
into the small intestine from the pancreas. It breaks down starch
molecules into glucose molecules.
Proteins are broken down by the enzyme, protease. This is found in
the pancreatic juice and also in gastric juice which is found in the
stomach. This enzyme breaks protein molecules into amino acid
molecules.
Fats and oils, or lipids, are broken down by the enzyme lipase. This is
found in the pancreatic juice. Lipase breaks fat molecules into fatty
acid and glycerol molecules.
The juices that are found in the digestive
system contain enzymes to break down
food molecules. However, different
enzymes work more efficiently when they
are at their different optimum pHs.
In the gastric juice found in the stomach,
there is an enzyme called protease. The
juice also contains hydrochloric acid to kill
bacteria and to make the juice acidic; the
optimum pH for protease.
The enzymes that break down the food in
the small intestine, however, work more
efficiently in an alkaline. Therefore, a juice
called bile is added. It is made by the liver
and is stored in the gall bladder. It also
emulsifies fats.
Once food has been digested, it needs to be absorbed into the blood
and then taken to the cells. It is absorbed into the blood by diffusion.
Diffusion is the movement of molecules from a high concentration to a
low concentration. The part of the small intestine where this takes
place is the ileum. It is specially adapted to allow rapid diffusion to
take place:
It is very long to allow time for digestion and absorption.
It has a very large surface area, giving a bigger surface through
which diffusion can take place. This is because it has villi and
microvilli. These are finger like projections.
The walls of the villi are only one cell thick. This means that the food
molecules can easily travel through it.
Different food types pass into different places in the villi. Digested
carbohydrates and proteins pass into the blood vessel in the villi, and
are then transported in the circulatory system to all the cells of the
body. Digestion fats (or lipids), however, pass into the lymph vessel
to be stored under the skin.
Food that cannot be digested is
passed into the large intestine.
The first part of the large intestine is
the called the colon. This is where
the water in the large intestine is
absorbed back into the blood,
making the remaining food in the
large intestine into semi solid
faeces.
This is stored in the part of the large
intestine called the rectum.
Finally, the faeces are passed out of
the body altogether through the last
part of the digestive system: the
anus.
Mouth
Oesophagus
Small intestine (ileum)
Stomach
Small intestine (duodenum)
Digested food
Indigestible
food
Fats
Carbohydrates and proteins
Lymph vessel
Large intestine
Blood vessel
Rectum
Anus
(C.H.O.)
carbohydrase
Glucose
and
simple
sugars
WATER
PROTEINS
CARBOHYDRATES
FATS + OILS
(C.H.O.)
lipase
Glycerol
and fatty
acids
(C.H.O.N.)
protease
VITAMINS + SALTS
Amino
acids
Keeping
the body
in good
health
(H2O)
(various elements)
excess
Many
uses in
the body
Excess broken
down in liver
Excess
stored in
liver as
glycogen
Energy
produced
in cell
respiration
Stored
under the
skin
Form
proteins in
the body
urea
glucose
Passed to the
kidneys by
blood for
removal as
urine
Some out in
sweat