Heterotrophic nutrition
Heterotrophic nutrition is the mode of nutrition exhibited by heterotrophs. Heterotrophs are the organisms that depend on plants or other organisms for their food.  Heterotrophic mode of nutrition is of different types – saprophytic as in fungi, parasitic as in leeches and symbiotic as in hermit crab.
        •  Unicellular organisms exhibit holozoic type of nutrition. e.g. Amoeba and Paramecium. This type of nutrition involves ingestion of liquid or solid organic material, digestion, absorption and assimilation to utilise it. e.g.Food in the food vacuoles of the amoeba and paramecium is digested by lytic enzymes.
        •  Multicellular organisms exhibit a complex process in obtaining their nourishment.

Digestion:  The process of breaking down complex food substances into simple molecules is called as digestion.

Digestive system in human beings consists of alimentary canal and digestive glands.
        •  Alimentary canal is made up of mouth, buccal cavity, pharynx, oesophagus, stomach, intestine, rectum and anus.
        •  The digestive glands are the salivary glands, the gastric glands, the liver, the pancreas and the intestinal glands.

Digestion in buccal cavity is brought about by three pairs of salivary glands opening into the oral cavity.
        •  Salivary glands include submaxillary, sublingual and parotid glands.
        •  Amylase, a digestive enzyme in saliva, breaks down the starch in food into simpler sugar.
        •  Saliva also prevents tooth decay due to the presence of amylase, lysozyme and minerals.

Peristalsis includes a series of muscular contractions in the oesophagus that push the food forward to the stomach.

The stomach is divided into three compartments namely cardiac, fundus and pylorus.  The junction of oesophagus and stomach is guarded by valve which does not allow the food to travel in backward direction.
        •  In the stomach, food is mixed with the gastric juices secreted by the gastric glands.
        •  Gastric juice is a combination of hydrochloric acid, enzymes like pepsin, lipase and mucous.
        •  Partially digested food in the stomach becomes acidic and is known as chyme.

The liver is the largest gland in the body. It secretes bile juice
        •  Bile makes the food alkaline for the action of pancreatic and intestinal enzymes in the small intestine.
        •  Bile brings about emulsification of fats which are later digested by intestinal lipases.

The exocrine part of pancreas secretes pancreatic juice which includes trypsin and lipase that help break down proteins and fats.

Liver and pancreas open into intestine through hepato-pancreatic duct to throw their secretions for further digestion of food.

The small intestine is divided into duodenum, ileum and jejunum.
        •  The intestinal enzymes such as invertase, maltase and lipase act upon partially digested proteins, carbohydrates and fats.
        •  Proteins, carbohydrates and fats are simplified into amino acids, glucose, fatty acids and glycerol in a liquid medium known as chyle.
        •  Specific structures called as villi in the small intestine increase its surface area to ensure efficient and rapid absorption of nutrients. Blood capillaries in the villi absorb nutrients and transport the food to all the cells in the body.

The large intestine comprises caecum, appendix, colon, rectum and anus. The large intestine absorbs water from undigested food and forms solid waste.
        •  The rectum stores the solid excreta until it is ready to be excreted from the digestive system through anus.
        •  The appendix is a small, hollow, finger-like pouch, which hangs at the end of the cecum. It does not have any function in the digestive system of humans. However, it is functional in herbivores such as cows.


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