Digestive System

GCSE biology

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Food is firstly physically broken down by chewing ­ this chewing allows the stomach to prepare for
digestion by expanding and releasing enzymes. Chewing the food creates a larger surface area for enzymes
to act. Whilst chewing the salivary glands release and enzyme called amylase which mixes with the food
and breaks down the starch in food into sugars (maltose).
The food is then pushed down the oesophagus and into the stomach where an enzyme called pepsin a
protease which breaks down protein into amino acids is released. Pepsin works best at acidic conditions.
The wall of our stomach is muscular and churns our food. The stomach also produces hydrochloric acid to
kill any harmful bacteria that may be present in food.
The content of the stomach then passes into the small intestine. However enzymes such as trypsin in the
small intestine work best in alkali conditions but the substances from the stomach are acidic. This is why the
liver produces bile which is stored in the gall bladder is released into the small intestine to neutralise the acid
from the stomach so the enzymes at the small intestines can work effectively. Bile also emulsifies fats into
smaller droplets which gives them a larger surface area to allow lipase's to break down fats down quicker.
Finally enzymes from the pancreas and glands in the wall of the small intestine are released into the small
intestine to digest remaining substances such as lipase to digest fats into fatty acids and glycerol , proteases
into amino acids and amylase into sugars.
The useful substances such as glucose, amino acids are absorbed into the blood stream at the small
intestine. The small intestine contains bile which allows nutrients to diffuse into the blood stream as they
have a large surface area so digested food is absorbed into the blood quicker, thin membranes to allow
molecules to diffuse out easily and a plentiful supply of blood to keep a high concentration gradient.
From the small intestine the remaining substances which can't be digested or absorbed by the gut passes
into the large intestine where water is absorbed and the remaining waste passes out of the anus as faeces.


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