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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. Gastric glands secrete HCL, pepsinogen, mucous.Gastric juice is a secretion of gastric glands located in the lining of the stomach. It is mainly made up of electrolytes, mucus, enzymes, hydrochloric acid, intrinsic factor etc. HCl secreted by parietal cells provides acidic medium for many enzymes to get activated. Neck cells secrete mucus which lubricated the passage of the food. Chief cells secrete pepsinogen which helps in the digestion of proteins after getting activated into pepsin by HCl. Enzymes of the gastric juice bring about digestion of different components of the food. Gastric lipase helps in emulsification of lipids in the stomach. Partially digested food in the stomach is called as chyme and this passes on into small intestine

        •  Partially digested food in the stomach becomes acidic and is known as chyme.

Liver is the largest gland in our body. The liver secretes a yellowish green watery fluid called bile. It is temporarily stored in a sac called the gall bladder. Bile plays an important role in the digestion of fats.   Bile is sent into duodenum through a narrow tube-like structure called the bile duct. Bile breaks the larger fat molecules into tiny droplets, thereby increasing their surface area, which helps in the digestion of fats easily. Bile is a dark green alkaline fluid secreted by liver. It comprises of water, bile salts, bile pigments, fats and inorganic salts. Bile comprises of yellow pigment bilirubin which oxidizes to form green pigment biliverdin only when it enters the intestine.

        •  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.

Pancreas is the mixed gland. It acts as a both endocrine and exocrine gland. The pancreas secretes the pancreatic juice that helps to digest carbohydrates, proteins and fats. The pancreatic juice converts carbohydrates into simple sugars and glucose, proteins into amino acids, and the lipids into fatty acids and glycerol. 

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

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

Small intestine is made up of three regions namely duodenum, jejunum and ileum. Acidic chyme from the stomach is received by the duodenum for further digestion. 

  • Duodenum receives bile form the liver.  Bile provides an alkaline environment for many enzymes to get active. It also reduces the acidity of chyme. Bile plays an important role in the digestion of fats.  
  • Duodenum also receives pancreatic secretions which help in the digestion of food. Pancreas secretes the pancreatic juice that helps to digest carbohydrates, proteins and fats.  
  • Duodenum also secretes some enzymes on its own. Cells lining the inner lining of intestine secrete enzyme rich intestinal juice. Intestinal juice comprises of many enzymes like enterokinase, invertase, maltase and lipase. The intestinal enzymes act upon partially digested proteins, carbohydrates and fats.
  • All these substances bring about digestion of food in the duodenum. The inner walls of small intestine are thrown into many folds which have millions of small finger like projections called villi.
  • Intestinal glands are present in the inner lining of small intestine. These secrete various enzymes which aid in the process of digestion of all the components of food. Maltase, sucrase and lactase bring about digestion of carbohydrates. Peptidases help in digestion of proteins. Enterokinase helps in the activation of other enzymes.
  • 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. Villi increase the surface area for digestion as well as absorption of digested food by eight times.  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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Activities & Simulations

Activity 1
Kitses.com has created an animated user-friendly video with interactive session. Though it is a short and simple interactive video, it describes a lot about the parts of the digestive tract and their functions in the process of digestion. It throws light on different types of food and the way they are digested in our digestive system. The fate of these food items is also explained clearly with their end products. The sites of absorption of these end products is also shown.
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Activity 2
Teachnutrition.org has created an interesting module on 'DIGESTION'. This module has lot of creativity such that digestion process is explained with a sort of game named as Mission control. Every number in the count down is correlated to a part in the digestive system. A single click on the count down number will explain the structure and function of that part in the process of digestion. Each time you want to continue you will be asked few questions on the topic of 'DIGESTION' to make the user well-versed with the topic.
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