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The Four Stages of Digestion
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1) INGESTION
Ingestion is the consumption of a substance
by an organism. In animals, it normally is
accomplished by taking in the substance
through the mouth into the gastrointestinal
tract, such as through eating or drinking. In
single-celled organisms, ingestion can take
place through taking the substance through
the cell wall.
Besides nutritional items, other substances
which may be ingested include medications,
recreational drugs, and substances considered inedible such as
foreign bodies or excrement. Ingestion is a common route taken by
pathogenic organisms and poisons entering the body.
In humans once food is in our mouth we start to ‘Masticate’ which is
the act of chewing our food. Our teeth and our tongue act to form
the food into a bolus (this is a ground up ball of food).
The Bolus is also combined with saliva. This contains a digestive
enzyme called salivary amylase which aids in Carbohydrate digestion.
This is why we say that digestion begins in the mouth!
2) DIGESTION
When you eat a piece of bread, you don't wake up next day to discover it
growing out of your arm! The food we eat has to be broken down into other
substances that our bodies can use. This is called digestion. Without digestion,
we could not absorb food into our bodies and use it.
Digestion happens in the digestive system, which begins at the mouth and ends
at the anus.
Stages of digestion
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Food is digested in the mouth, stomach and small intestine.
Digested food is absorbed into the bloodstream in the small intestine.
Excess water is absorbed back into the body in the large intestine.
Any undigested food passes out of the anus as faeces
Liver and pancreas
The liver and the pancreas play important part in digestion. The liver produces
bile, which helps the digestion of fat. The pancreas produces chemicals called
digestive enzymes.
Enzymes
Take care - enzymes are not living things. They are just special proteins that
can break large molecules into small molecules. Different types of enzymes can
break down different nutrients:
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carbohydrase or amylase enzymes break down starch into sugar
protease enzymes break down proteins into amino acids
lipase enzymes break down fats into fatty acids and glycerol.
Carbohydrates
Carbohydrates are digested in the mouth, stomach and small intestine.
Carbohydrase enzymes break down starch into sugars.
The saliva in your mouth contains amylase, which is another starch digesting
enzyme. If you chew a piece of bread for long enough, the starch it contains is
digested to sugar, and it begins to taste sweet.
Proteins
Proteins are digested in the stomach and small intestine. Protease enzymes
break down proteins into amino acids. Digestion of proteins in the stomach is
helped by stomach acid, which is strong hydrochloric acid. This also kills
harmful micro-organisms that may be in the food.
Fats
Lipase enzymes break down fat into fatty acids and glycerol. Digestion of fat in
the small intestine is helped by bile, made in the liver. Bile breaks the fat into
small droplets that are easier for the lipase enzymes to work on.
3)ABSORPTION
Digested food molecules are absorbed in the small intestine. This means that
they pass through the wall of the small intestine and into our bloodstream. Once
in the bloodstream, the digested food molecules are carried around the body to
where they are needed.
Only small, soluble substances can pass across the wall of the small intestine.
Large insoluble substances cannot pass through.
Absorption into bloodstream
The inside wall of the small intestine needs to be thin, with a really big surface
area. This allows absorption to happen quickly and efficiently. If the small
intestine had a thick wall and a small surface area, a lot of digested food might
pass out of the body before it had a chance to be absorbed.
To get a big surface area, the inside wall of the small intestine is lined with tiny
villi (one of them is called a villus). These stick out and give a big surface area.
They also contain blood capillaries to carry away the absorbed food molecules.
4)EGESTION
Egestion is the discharge or expulsion of
undigested material (food) from a cell in
case of unicellular organisms, and from the
digestive tract via the anus in case of
multicellular organisms.
It should not be confused with excretion,
which is getting rid of waste formed from the chemical reaction of
the body, such as in urine, sweat.
Egestion:Removal of undigested food or faeces from the gut.
In most animals egestion takes place via the anus,
although the invertebrate flatworms must use the
mouth because their gut has no exit. Egestion refers
solely to indigestible matter which is never absorbed
into the cells - it should not be confused with
excretion of the waste products of metabolism.
Excretion is the transfer of substances out of a living
organism into its environment. At its simplest, for single-cell forms
of life, this involves extrusion across the cell membrane of the
unwanted or potentially toxic by-products of respiration and
metabolism. This is also what is happening continually in the
individual cells of the animal body, but from their immediate
environment substances must move into the blood to be carried away
to the site of their ultimate disposal. (More on this later)