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CLASS: SS2                                                                                                    



(1) Introduction to digestion and digestive system

       Types of Alimentary tracts

(2) Description and functions of parts of the alimentary canal

(3) Modification of alimentary canal



Digestive System, series of connected organs whose purpose is to break down, or digest, the food we eat. Food is made up of large, complex molecules, which the digestive system breaks down into smaller, simple molecules that can be absorbed into the bloodstream. The simple molecules travel through the bloodstream to all of the body’s cells, which use them for growth, repair, and energy.

All animals have a digestive system, a feature that distinguishes them from plants. Plants produce their own food in a process called photosynthesis, during which they use sunlight to convert water and carbon dioxide into simple sugars. But animals, including humans, must take in food in the form of organic matter, such as plants or other animals.


Digestion is the breaking down of complex food substances into simple, soluble and diffusible form by mechanical and chemical means.

Digestion generally involves two phases: a mechanical phase and a chemical phase. In the mechanical phase, teeth or other structures physically break down large pieces of food into smaller pieces. In the chemical phase, digestive chemicals called enzymes break apart individual molecules of food to yield molecules that can be absorbed and distributed throughout the body. These enzymes are secreted (produced and released) by glands in the body.


This is a tubular passage between the mouth and the anus, including the organs through which food passes for digestion and elimination as waste. The alimentary canal is also called the digestive tract or gut.

In most holozoic animals, digestion and absorption of food take place in the alimentary canal or gut. A simple unicellular animal does not have an alimentary canal.


There are different types of alimentary tracts in animals. However, there is no definite alimentary canal in plant. This includes:

  1. Intracellular alimentary tract: this is a digestion that takes place inside the cell, within a food vacuole.g., unicellular organism like amoeba
  2. Extracellular alimentary tract: this is a type of digestion that occur in multi-cellular organisms. The mouth opens into a sac-like gut cavity where food is digested. E.g. Hydra.
  3. Simple alimentary tract for simple digestion of food. E.g. Birds.
  4. Compartmentalised alimentary tract by ruminants for digestion of cellulose. E.g. ruminants like cow.
  5. Complex Human alimentary tract; a long tube stretching from the mouth to the anus. Most of it is coiled up in the abdominal cavity. It is divided into several regions: mouth, oesophagus, stomach, and intestines. Each region plays a role in the digestion and /or absorption of food.

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