DIGESTIVE SYSTEM AND FUNCTIONS At the end of this article, students should be able to:
Describe with appropriate illustration different types of alimentary tracts
Explain the feeding mechanism of some animal
Explain how some insectivorous plants feed

The digestive system is a complex and vital physiological system in the human body responsible for the breakdown of food into smaller, absorbable nutrients that provide energy and support various bodily functions.

It consists of a series of organs and structures that work together to achieve digestion, absorption, and elimination. Let’s delve into the details of the digestive system and its functions:

Organs of the Digestive System:

  1. Mouth: The digestive process begins in the mouth, where teeth break down food mechanically, and salivary glands release saliva-containing enzymes like amylase to initiate the chemical breakdown of carbohydrates.
  2. Pharynx: The pharynx serves as a common passage for both food and air, directing food into the oesophagus while preventing it from entering the windpipe.
  3. Esophagus: The oesophagus is a muscular tube that transports food from the mouth to the stomach through a series of coordinated contractions called peristalsis.
  4. Stomach: In the stomach, gastric glands release gastric juices containing hydrochloric acid and pepsin, which help digest proteins. The stomach also churns and mixes food into a semi-liquid substance called chyme.
  5. Small Intestine: The small intestine is where most digestion and nutrient absorption occur. It is divided into three parts: the duodenum, jejunum, and ileum. Enzymes from the pancreas (pancreatic enzymes) and bile from the liver help digest carbohydrates, fats, and proteins. Villi and microvilli in the small intestine increase the surface area for absorption.
  6. Liver: The liver produces bile, which is stored in the gallbladder and released into the small intestine to emulsify fats, making them easier to digest and absorb.
  7. Pancreas: The pancreas releases digestive enzymes into the small intestine to further break down carbohydrates, proteins, and fats. It also regulates blood sugar levels by producing insulin and glucagon.
  8. Large Intestine (Colon): The large intestine absorbs water and electrolytes from undigested food, forming faeces. Beneficial bacteria in the colon aid in the fermentation of undigested carbohydrates and the production of certain vitamins (e.g., vitamin K and some B vitamins).

Functions of the Digestive System:

  1. Ingestion: The process of taking food into the mouth, where it is broken down into smaller pieces through chewing and mixed with saliva.
  2. Digestion: Digestion is the process of breaking down complex food molecules into simpler forms that can be absorbed and used by the body. This includes mechanical digestion (chewing, churning) and chemical digestion (enzymes breaking down nutrients).
  3. Absorption: In the small intestine, nutrients, water, vitamins, and minerals are absorbed into the bloodstream and transported to cells throughout the body for energy and various functions.
  4. Transportation: Food is moved through the digestive tract by muscular contractions, such as peristalsis in the oesophagus and small intestine, to ensure efficient digestion and absorption.
  5. Storage and Elimination: The stomach stores food and releases it slowly into the small intestine. The large intestine absorbs water and stores and compacts undigested food, forming faeces. When ready, faeces are eliminated through the rectum and anus in a process known as defecation.
  6. Secretion: Glands throughout the digestive system release digestive juices and enzymes that aid in breaking down food.
  7. Protection: The digestive system has protective mechanisms to prevent harmful substances from entering the bloodstream, including mucus production, stomach acid, and the immune system.
  8. Regulation: Hormones and neural signals regulate digestive processes, ensuring coordination between different parts of the system and maintaining overall homeostasis.

The digestive system plays a crucial role in providing the body with the nutrients needed for growth, energy production, and overall health.

It exemplifies the complexity and efficiency of the human body’s physiological processes in converting food into usable resources. Proper nutrition, hydration, and a balanced diet are essential for maintaining the health and functionality of the digestive system.


The digestive system includes the alimentary tracts or canal and all the glands and organs associated with the digestion and assimilation of food in animals.  

Digestion is the breaking down of large molecules of food into simple and absorbable forms for use by animals.

The digestive system including the alimentary tracts varies from one animal to another.

Organisms from simple to complex have different types of digestive systems or alimentary tracts. However, there is no definite alimentary canal in plants.


  1. The tongue: (i) the tongue rolls the food into a bolus. (ii) it aids movement of food in the mouth (iii) it allows mixing of food with saliva or ptyalin (iv) it aids swallowing of food into the gullet or oesophagus.
  2. The salivary gland: (i) the salivary gland secretes saliva which contains an enzyme called ptyalin. The ptyalin breaks down starch into maltose which is later swallowed into the gullet in the form of a bolus. (ii) It allows easy chewing or movement of food in the mouth for swallowing. (iii) it also serves as a solvent for food. The saliva is slightly alkaline.
  3. Oesophagus or Gullet: the oesophagus connects the mouth to the stomach. The food swallowed is passed down through the oesophagus by a peristaltic movement into the stomach.
  4. Stomach: in the stomach, the food is temporarily stored for a few hours and it is released at regular intervals by the opening of the pyloric sphincter.
  5. In the stomach, the gastric gland secretes gastric juice which contains two enzymes—renin and pepsin.
    The renin acts on milk (or it helps to curdle the milk) while the pepsin breaks down proteins to peptones.
  1. The gastric gland also secretes hydrochloric acid (Hcl) which creates an acid medium for two enzymes to act. The Hcl also helps to kill some bacteria in the stomach. The food is churned by muscular contraction of the stomach wall (churning movement) which enables the mixing of food with digestive juice. The churning movement then converts the food into a semi-liquid state called chyme.
functions of the Duodenum

digestion of food also takes place in this region of the alimentary canal. The duodenum contains the pancreas which secrets pancreatic juice that contains three enzymes

These enzymes are:

the conversion of food by enzymes

i. Amylase: this converts starch to maltose
ii. Lipase: lipase converts fats and oil to fatty acids and glycerol.
iii. Trypsin: it converts protein and peptones to polypeptides.
The pancreatic juice is alkaline and provides a medium for enzymes.

The digestion of fat and oil is aided by a green alkaline liquid called bile which is secreted by the liver and stored in the gall bladder. The bile helps in the emulsification of fats, i.e. breaking down fats into tiny droplets.

At the end of digestion in the duodenum the food now in liquid form called chyle passes into the ileum or small intestine.

digestion of food in the Small intestine

: the small intestine or ileum is found between the duodenum and the large intestine. Two major events take place in the small intestine. These events are (a) digestion and (b) absorption of the digested food.

  1. Digestion: the digestion of food also takes place in the small intestine or ileum. The intestinal wall secures intestinal juice which contains the following enzymes: lipase, pepsin, maltase, sucrase and lactase. Lipase converts fats and oil to fatty acids and glycerol, erepsin converts polypeptides to amino acids, maltase converts maltose to two units of glucose, sucrase converts sucrose to glucose and fructose while lactase converts lactose to glucose and galactose.

In man, the final digestion of food ends in the small intestine. The end product is the digestion of protein is amino acids, fats and oil are fatty acids and glycerol, while that of starch is glucose.

  1. Absorption of digested food: the end products of digestion of food (amino acids, glucose, fatty acids and glycerol) are absorbed in the small intestine by tiny finger-like structures called villi (singular villus).

The folding of the small intestine combined with the presence of numerous villi creates a large surface area for the absorption of digested food.
The inner surface layer or epithelium of each villus is thin. This allows the absorption of the end product by either diffusion or active transport through it.

The glucose and amino acids are easily absorbed by blood capillaries through the epithelium while the fatty acids and glycerol enter the lacteal where they are carried through the lymph vessels which eventually empty their contents into the blood vessels near the heart. The blood then carries the fats and other food materials to various parts of the body where they are needed.

  1. Caecum and appendix: in man, the functions of the caecum and appendix are not well known but the caecum usually contains some bacteria which aid minor digestion of cellulose. Some vitamins such as K and B-complex are partially synthesized in this region.
  2. Large intestine: the undigested food passes into the colon or large intestine. Here, water is absorbed. This absorption of water concentrates the waste products and turns them into faeces.

The faeces is passed into the rectum and finally out of the body through the anus. For a summary, see the table below showing enzyme, source, location, substrate and effects HERE YOU WILL FIND EVERY Reason why an athlete in a race would be given a glucose drink rather than a piece of bread Glucose is the simplest form of carbohydrate/soluble form of carbohydrate, energy giving food substance which does not require any form of digestion. Hence it is absorbed in the ileum/small intestine where it diffuses/enters into the bloodstream/circulatory system. It is then assimilated and oxidized within the shortest time/quickly/immediately to give energy to the athlete.

what is bread? Bread is a complex carbohydrate/polysaccharide/starch which will require a series of digestive processes/hydrolysis which starts in the mouth and ends in the small intestine/ileum. This takes a longer time to produce energy for use and cannot give immediate energy.


There are five modifications and mechanisms of feeding associated with some organisms. These feeding mechanisms include:

Absorbing mechanisms, e.g. tapeworm:

the tapeworm is an endoparasite which carries out parasitic feeding on its host i.e. man. It has no mouth but absorbs digested food from the intestine of its host. The body of the tapeworm is modified and adapted for parasitic feeding as follows:

i. The alimentary canal is absent, hence food is absorbed through its entire body surface.
ii. The tapeworm has hooks and suckers which are used for attachment to the intestine of the host to avoid dislodgement.
iii. The body has a thick cuticle which resists the digestive enzymes of the host.
iv. The flat body surface of the tapeworm provides a large surface area for the absorption of already digested food.
v. The entire body is used also for the absorption of food.

Biting and chewing mechanism, e.g. grasshopper or cockroach:

the grasshopper or cockroach has mouthparts adapted for biting and chewing. These insects have four different mouth parts which are modified and adapted for biting or chewing food. These mouth parts are:

i. Labrum or upper lip: the grasshopper has a labrum or upper lip which prevents the food from falling off the mouth.
ii. Mandibles: it possesses a pair of mandibles which are heavy, toothed and jaw-like structures used for cutting and chewing food materials.

iii. Maxillae: the grasshopper also has a pair of maxillae which is also a biting blade. This breaks down the food which the mandible has chewed into smaller particles.
iv. Labium: the labium (lower lip) prevents the wastage of food from the mouth.

  1. Sucking mechanism: there are three popular organisms which exhibit sucking mechanism. These are mosquitoes, butterflies and houseflies. These insects have different modifications of mouth parts adapted for feeding on food through the mechanism of sucking.


Grinding mechanism:

grinding mechanism is common among mammals, e.g. man, cattle, sheep, and goats. These animals are capable of grinding the food before swallowing. This grinding is aided by the presence of hard and strong teeth made of enamel and dentine. The animals are adapted to the grinding mechanism by the following features:

i. They possess different sets of teeth to grind food.

ii. The teeth are hard and strong enough to withstand biting, chewing grinding and cracking of food.

iii. They possess incisors which are sharp with flat edges used for cutting off bits of food.

iv. Animals have pointed canine teeth which are used for tearing food.

v. There is the presence of premolars and molars with undulating and wide surfaces used for grinding food.

vi. The absence of front teeth [incisors] in sheep [a herbivore] is a special adaption as it helps to grip the grasses during feeding by the animal.

Trapping and absorbing mechanism:

the trapping and absorbing mechanisms are common among insectivorous or carnivorous plants such as bladderwort and sundew. Bladderwort and sundew have structures which enable them to adapt to this mode of feeding.

I. Sundew, for example, traps insects by undergoing nastic movements in response to touch from the body of the insects.
II. The sundew leaf has long hairs which carry digestive glands.
III. Insects on landing on these hairs cause other hairs to curl over the insect and cover it.

IV. The sundew is capable of secreting a fluid rich in enzymes to digest the insect externally.
V. The protein synthesized is easily absorbed by the carnivorous plant into its body.


Organisms exhibit different feeding habits.
These are:

Filter feeding

Filter feeders which are also called microphagous feeders feed on very tiny organisms which cannot be easily picked to the satisfaction of the feeder.

Filter feeders are mainly aquatic animals and they have to wallow in water through a sieve-like structure into their body in order to collect a reasonable quantity of their prey or food. Examples of filter feeders are mosquito larvae, mussels, ducks and prawns.

filter feeding in mosquito larva

The mouth of mosquito larva, for example, has horny jaws and two small ciliated appendages or mouth brushes. These are used to create a current of water. The small colloidal particles or plankton, small plants and animals are filtered from water and ingested into the mouth as food. fluid feeding

Animals which feed on any fluid materials are classified as fluid feeders. These are two major groups of fluid feeders in relation to the digestive system. These are:
i. Wallowers: these organisms rest within or wallow in their food, e.g. the tapeworm in the intestine of man. The tapeworm lives within the digested food of its host and absorbs the already digested food of the host.
ii. Suckers: suckers are organisms, mainly insects, which feed by sucking fluid from plants and animals. Examples of suckers are bugs, mosquitoes, butterflies, aphids, tsetse fly and houseflies. The fluid-feeding mechanism of mosquitoes, houseflies and butterflies has been discussed earlier. saprophytic feeding

Saprophytes are mainly non-green plants which do not have chloroplasts and therefore cannot manufacture their own food but they have a very complex digestive system. They then feed on dead and decaying organic matter from which they derive their food.

Examples of saprophytes are Rhizopus, mushroom and mucor. Their body is adapted in the following ways:
i. They have hyphae instead of roots through which they pour out enzymes for digestion.
ii. They are capable of carrying out extracellular digestion, i.e., digestion of food outside the body cells of the plant.
iii. The digested portion of the organic matter is later reabsorbed into the body.

parasitic feeding

Parasitic feeding is found in both plants and animals. Animal parasites are tapeworm, roundworm, liver fluke, louse, tick and guinea worm while plant parasites are Cassytha, dodder and mistletoe.

Parasites are structurally modified organisms that depend wholly or partially on other living organisms for their food and survival. The structural adaptation of tapeworms, for example, demonstrating parasitic feeding has been discussed earlier in this chapter


feeding in protozoa

Protozoa like amoeba exhibits a holozoic mode of nutrition. Amoeba feeds on micro-organisms like diatoms, demids or other organic particles.

It engulfs the food particles by p

cutting out its pseudopodia which surround the food particle by forming a cup-shape when the two ends of the pseudopodia touch. The pseudopodia on meeting or touching, fuse and enclose the food into the body with a little drop of water; this forms a food vacuole. The food is digested inside the food vacuole.

  1. scale of preference
  2. digestive system

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