It is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into watery blood plasma.

Types of food:

  • Diffusible food: includes water, minerals, amino acids, vitamins, etc. which are micro-biomolecules and need no digestion
  • Non-diffusible food: includes proteins, fats, nucleic acids, etc. which are macro-biomolecules and need digestion.

Types of digestion:

Digestion of food occurs within the cellDigestion occurs outside the cell in the cavity of alimentary canal
Digestive enzymes are secreted by surrounding cytoplasm into the food vacuoleDigestive enzymes are secreted by special cells into the cavity of alimentary canal
Digestive products are diffused into cytoplasmDigestive products diffuse across the intestinal wall into various parts of body
Less efficient methodMore efficient method
Occurs in unicellular or lower form of organisms
Pinocytosis: liquid enters
Phagocytosis or Endocytosis:  solid enters.
Occurs in multicellular or higher form of organism
Ingressive zone: intake and mastication of food
Progressive zone:  conduction and early digestion
Degressive zone:  final digestion and absorption of food Egressive zone:  temporary store the faeces.