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 cell
|Digestion occurs outside the cell in the cavity of alimentary canal
|Digestive enzymes are secreted by surrounding cytoplasm into the food vacuole
|Digestive enzymes are secreted by special cells into the cavity of alimentary canal
|Digestive products are diffused into cytoplasm
|Digestive products diffuse across the intestinal wall into various parts of body
|Less efficient method
|More 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.