Life Processes: Heterotrophic Mode of Nutrition
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Life Processes: Heterotrophic Mode of Nutrition |
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Heterotrophic Nutrition
The mode of nutrition in which an organism takes food from another organism.
Organism, other than green plants and blue green algae follow this mode of nutrition.
Divided into three parts:
Saprophytic Nutrition
Holozoic Nutrition
Parasitic Nutrition
6.2.4 Nutrition in Human Beings
The alimentary canal is basically a long tube extending from the mouth
to the anus. In Fig. 6.6, we can see that the tube has different parts.
Various regions are specialised to perform different functions. We eat various types of food which has to pass through the same
digestive tract. Naturally the food has to be processed to generate
particles which are small and of the same texture. This is achieved by
crushing the food with our teeth. Since the lining of the canal is soft, the
food is also wetted to make its passage smooth. When we eat something
we like, our mouth ‘waters’. This is actually not only water, but a fluid
called saliva secreted by the salivary glands. Another aspect of the food
we ingest is its complex nature. If it is to be absorbed from the alimentary
canal, it has to be broken into smaller molecules. This is done with the
help of biological catalysts called
enzymes. The saliva contains an
enzyme called salivary amylase that
breaks down starch which is a
complex molecule to give sugar. The
food is mixed thoroughly with saliva
and moved around the mouth while
chewing by the muscular tongue.
It is necessary to move the food in
a regulated manner along the digestive
tube so that it can be processed
properly in each part. The lining of
canal has muscles that contract
rhythmically in order to push the food
forward. These peristaltic movements
occur all along the gut.
From the mouth, the food is taken
to the stomach through the food-pipe
or oesophagus. The stomach is a large
organ which expands when food
enters it. The muscular walls of the
stomach help in mixing the food
thoroughly with more digestive juices.
These digestion functions are
taken care of by the gastric glands present in the wall of the stomach.
These release hydrochloric acid, a protein digesting enzyme called
pepsin, and mucus. The hydrochloric acid creates an acidic medium
which facilitates the action of the enzyme pepsin. What other function
do you think is served by the acid? The mucus protects the inner lining
of the stomach from the action of the acid under normal conditions. We have often heard adults complaining about ‘acidity’. Can this be related
to what has been discussed above?
The exit of food from the stomach is regulated by a sphincter muscle
which releases it in small amounts into the small intestine. From the
stomach, the food now enters the small intestine. This is the longest part
of the alimentary canal which is fitted into a compact space because of
extensive coiling. The length of the small intestine differs in various
animals depending on the food they eat. Herbivores eating grass need a
longer small intestine to allow the cellulose to be digested. Meat is easier
to digest, hence carnivores like tigers have a shorter small intestine.
The small intestine is the site of the complete digestion of
carbohydrates, proteins and fats. It receives the secretions of the liver
and pancreas for this purpose. The food coming from the stomach is
acidic and has to be made alkaline for the pancreatic enzymes to act.
Bile juice from the liver accomplishes this in addition to acting on fats.
Fats are present in the intestine in the form of large globules which makes
it difficult for enzymes to act on them. Bile salts break them down into
smaller globules increasing the efficiency of enzyme action. This is similar
to the emulsifying action of soaps on dirt that we have learnt about in
Chapter 4. The pancreas secretes pancreatic juice which contains
enzymes like trypsin for digesting proteins and lipase for breaking down
emulsified fats. The walls of the small intestine contain glands which
secrete intestinal juice. The enzymes present in it finally convert the
proteins to amino acids, complex carbohydrates into glucose and fats
into fatty acids and glycerol.
The digested food is taken up by the walls of the intestine. The inner
lining of the small intestine has numerous finger-like projections called
villi which increase the surface area for absorption. The villi are richly
supplied with blood vessels which take the absorbed food to each and
every cell of the body, where it is utilised for obtaining energy, building
up new tissues and the repair of old tissues. |
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