
264 BIOLOGY
No significant digestive activity occurs in the large intestine. The
functions of large intestine are:
(i) absorption of some water, minerals and certain drugs;
(ii) secretion of mucus which helps in adhering the waste (undigested)
particles together and lubricating it for an easy passage.
The undigested, unabsorbed substances called faeces enters into the caecum
of the large intestine through ileo-caecal valve, which prevents the back flow of
the faecal matter. It is temporarily stored in the rectum till defaecation.
The activities of the gastro-intestinal tract are under neural and
hormonal control for proper coordination of different parts. The sight, smell
and/or the presence of food in the oral cavity can stimulate the secretion of
saliva. Gastric and intestinal secretions are also, similarly, stimulated by
neural signals. The muscular activities of different parts of the alimentary
canal can also be moderated by neural mechanisms, both local and through
CNS. Hormonal control of the secretion of digestive juices is carried out by
local hormones produced by the gastric and intestinal mucosa.
CALORIFIC VALUE OF PROTEIN, CARBOHYDRATE AND FAT
(Boxed item – Not for evaluation)
The energy requirements of animals, and the energy content of food, are
expressed in terms of measure of heat energy because heat is the ultimate form
of all energies. This is often measured to as calorie (cal) or joule (J), which is the
amount of heat energy required to raise the temperature of 1 g of water by 1 °C.
Since this value is tiny amount of energy, physiologists commonly use kilocalorie
(kcal) or kilo joule (kJ). One kilo calorie is the amount of energy required to raise
the temperature of 1 kg of water by 1 °C. Nutritionists, traditionally refer to kcal
as the Calorie or Joule (always capitalised). The amount of heat liberated from
complete combustion of 1 g food in a bomb calorimeter (a closed metal chamber
filled with O
2
) is its gross calorific or gross energy value. The actual amount of
energy combustion of 1 g of food is the physiologic value of food. Gross calorific
values of carbohydrates, proteins and fats are 4.1 kcal/g, 5.65 kcal/g and 9.45
kcal/g, respectively, whereas their physiologic values are 4.0 kcal/g, 4.0 kcal/g
and 9.0 kcal/g, respectively.
16.3 ABSORPTION OF DIGESTED PRODUCTS
Absorption is the process by which the end products of digestion pass
through the intestinal mucosa into the blood or lymph. It is carried out by
passive, active or facilitated transport mechanisms. Small amounts of
monosaccharides like glucose, amino acids and some electrolytes like
chloride ions are generally absorbed by simple diffusion. The passage of
these substances into the blood depends upon the concentration gradients.
However, some substances like glucose and amino acids are absorbed with
the help of carrier proteins. This mechanism is called the facilitated transport.
Transport of water depends upon the osmotic gradient. Active
transport occurs against the concentration gradient and hence requires
energy. Various nutrients like amino acids, monosaccharides like glucose,
electrolytes like Na
+
are absorbed into the blood by this mechanism.
Fatty acids and glycerol being insoluble, cannot be absorbed into the