Digestive System (Part-I) 

Introduction: The collective processes by which a living organism takes food which are necessary for their growth, maintenance and energy needs is called nutrition. The chemical substances present in the food are called nutrients.

DIFFERENT MODE OF NUTRITION IN ORGANISMS: It is important to know the different modes of nutrition in all living organisms in order to understand energy flow within the ecosystem. Plant produces high energy organic food from inorganic raw materials. They are called autotroph and the mode of nutrition is known as autotrophic nutrition. Animals feed on those high energy organic food, are called as heterotrophs and their mode of nutrition is known as heterotrophic nutrition
Heterotrophic nutrition further sub-categorise in holozoic, parasitic, and saprophytic mode of nutrition based on the pattern and class of food that is taken inside.

Holozoic Nutrition: It involves taking entire organic food and this can be in the form of whole part of plant or animal. Most of the free living protozoans, humans and other animals fall under this category.

Saprophytic Nutrition: The organism fulfils the requirement of food from the rotten parts of dead organisms and decaying matter. The organisms secrete digestive enzymes outside the body on their food and then take in digested food. It is a kind of extra-cellular digestion. Examples: Housefly, Spiders etc.

Parasitic Nutrition: The organism fulfils the requirement of food from the body of another organism. The parasites are of two distinct types, one which lives inside the host and the other which lives outside. The internal parasites usually multiply inside the body cavity of host and most of the times are life threatening while the other lives outside and can play the role of vectors in spreading diseases. Example of internal parasites are plasmodium, tapeworms etc. while the example of external parasites may include mostly fleas and insects.