Animals carry out heterotrophic nutrition. These organisms have organs and structures which are specialised for breakdown of the complex food molecules taken in into simple absorbable and usable forms.

Holozoic Nutrition

In holozoic nutrition solid complex food substances are ingested, digested and assimilated in the bodies of animals.
The ingested food is broken down mechanically by teeth. The type, arrangement and number of teeth in a mammal is refered to as dentition.
The teeth are arranged in groups to occupy specific positions in the jaw.
The dentition determines the mode of feeding that is whether herbivorous, carnivorous or omnivorous.
Breakdown of large complex molecules of food materials into simple usable forms is known as digestion. Physical breakdown is done by teeth while enzymes chemically breakdown the food.

You have learnt about the functions of enzymes and teeth respectively. You learned that enzymes speed up the rate of chemical reactions in the body, for example, break down of large complex molecules into simple, usable molecules.

Much of the food a human being takes in (ingests) is made up of large complex molecules. It is first broken down physically by the teeth and then chemically by enzymes into simple and soluble molecules before it can be absorbed. Along the alimentary canal, enzymes chemically digest carbohydrates, proteins and lipids.

Digestion in the Mouth

Mastication is carried out in the mouth whereby food is physically broken in to small pieces using teeth.These small pieces can then be chemically digested by the help of enzymes in the mouth and along the digestive canal.


During swallowing the epiglottis closes over the glottis thereby covering the air passage. The tongue then forces the bolus into the oesophagus.

Peristalsis during swallowing of food

Digestion in the Stomach
The bolus of food enters the stomach through a muscular valve called cardiac sphincter. The arrival of food into the stomach stimulates the stomach wall to secrete the hormone gastrin.Gastrin stimulates gastric glands to secrete gastric juice. Gastric juice contains:

Enzyme pepsin which is produced in an inactive form pepsinogen. Pepsin breaks down proteins into peptides.

Digestion in the Duodenum

Chyme enters the duodenum and stimulates secretion of the hormone cholecystokinnin. This hormone in turn stimulates the pancreas to secret enzymes lipase, amylase and trypsin contained in pancreatic juice. The hormone also stimulates production of bile.
The duodenum is the first part of the small intestine. Chyme is let into the duodenum in small amounts through the pyloric sphincter.

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