Page 49 - Biology Form Two

Biology for Secondary Schools

the food from flowing back into the The stomach walls consist of circular
oesophagus. and longitudinal muscles. These
muscles contract and relax causing
Digestion in the stomach movement and thorough mixing of
The stomach has elastic muscular food with the gastric juice, which
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walls that can stretch to accommodate form a creamy semi-liquid substance
about two litres of food and drinks. called chyme. Thus, food is converted
The food is stored temporarily in the into a digestible form or chyme in the
stomach before further digestion. stomach. The gastric juice contains the
That is why people do not need to eat following substances:
food constantly. Figure 2.5 shows the
structure of a human stomach. Hydrochloric acid (HCl)
This provides an acidic medium in
Cardiac sphincter
the stomach for enzymes to work
properly, kills disease-causing germs
and converts pepsinogen into pepsin.
The acidic environment facilitate the
Stomach absorption of certain nutrients such as
iron and calcium(minerals) and vitamin
B
12
Pepsinogen
Pyloric sphincter
This is an inactive form of pepsin
enzyme. This form cannot attack the
Figure 2.5: Structure of a human stomach
walls of the stomach which are protein

The walls of the stomach have gastric in nature.
glands that secrete gastric juice, a Pepsin
chemical substance which contains This is an active form of pepsinogen. It
hydrochloric acid and enzymes. The is an enzyme responsible for breaking
contraction and relaxation of the down proteins into smaller molecules
muscles of the stomach wall causes a called peptides and amino acids.
mechanical breakdown of the food into Rennin
smaller particles. This process is called This is an enzyme that coagulates milk.
churning. The process supplements This allows milk proteins to stay in the
the mechanical breakdown of food stomach long enough to be broken
that occurred in the mouth through down into peptides.
chewing by the teeth. Churning allows
food to mix well with the gastric juice.

Student’s Book - Form Two 43

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