Digestive System

1. Nutrients
The chemical substance present in the food is called nutrients.Depending upon the quantity or functions nutrients are of two types:
i) Macronutrients or Proximate Principles of food which provide energy,eg. Carbohydrates,Lipids and Proteins.
ii) Micronutrients or Protective Principles of food.They do not provide energy but by their deficiency can cause specific diseases,eg.Minerals,Vitamins and Water.
2. Kinds of Heterotrophic Nutrients
Animals which feed on the high energy organic molecules as food are called Heterotrophs.Their mode of nutrition is known as heterotrophic nutrition.
i)Holozoic Nutrition: It involves taking in of the whole or part of a plant or an animal either in solid or in liquid state,eg.most of free-living Protozoans and animal.
ii)Saprozoic Nutrition: A few animals secrete digestive enzymes directly into their food outside the body.Thus food is digested outside the body is then sucked.This is called Saprozoic Nutrition,eg.Spiders,House fly etc.
iii) Parasitic Nutrition: Parasites take food from the host,eg.Plasmodium,Trypanosoma,Taenia,Ascaris etc.
iv)Symbiotic Nutrition (=Mutualism): Two organism derive food from each other,eg.Escherichia Coli of human intestine synthesises vitamin B12 to be used by man and E. Coli receives food from human intestine.


3. Holozoic Animals
i) Herbivores: Plant eating animals are called Herbivores,eg.Cow,Horse,Rabbit etc.
ii) Carnivores: Flesh eating animals are called Carnivores,eg.Lion,Tiger etc.
iii) Omnivores: These animals feed on plant as well as flesh,eg.Cockroach,Crows,Rats,Bears,Man etc.
Other variations are: i) Insectivores (Insect eaters)-Toads,Lizards etc.
ii)Frugivores(Fruit eaters)-Birds,Bats,Monkeys etc.
iii)Sanguivores(Feeding on blood)-Leeches,Female Mosquitoes,Bedbugs,Vampire Bats etc.
iv)Detritivores(Feeding on decaying organic matters)-Earthworms
v) Fluid Feeders-Butterflies,Male Mosquitoes etc.
vi)Coprophagous(Eat their own faecal matter)-Rabbit
vii) Cannibals (Eat their own fellow or species)-Cockroach,Bedbug etc.


Holozoic nutrition involves four main steps:
i)Ingestion- In take of food
ii)Digestion-Breaking down of complex organic food molecules into simpler and absorbable molecules.Digestion may be Intracellular(in Amoeba) or Extracellular(in man).
iii) Absorption-Simpler organic molecules are absorbed.
iv)Egestion-Elimination of undigested food from the body.
The digestive system of man consists of two main parts: Alimentary Canal and Digestive Glands.