The term autotroph has been derived from two Greek wards-auto means self and troph means nutrition. In this mode of nutrition, the organisms prepare their own food from simple raw materials like water, carbon dioxide and mineral salts in the presence of sunlight. Chlorophylls present in the chloroplast or green plants are the site of food production. Accordingly all green plants are the examples of this category. The process by which they synthesize food is known as photosynthesis. Some nongreen becteria like sulphur bacteria can use energy which they derive from some chemical reactions occurring in them. With this energy they manufacture their food. This process is called chemosynthesis. Thus the autotrophs include both the photosynthetic and che4mosynthetic organisms.
The word heterotrophy has been derived from two Greek words-hetero means different and troph refers to nutrition of food. The organisms which derive their food from others are known as heterotrophic organisms. They depend for their food on other organisms, hence they are called consumers. All animals, human beings and non-green plans like fungi come under this category. They consume complex organic food prepared by autotrophs or producers and break it into simple from to derive nourishment. Thus the difference between heterotrophy and autotrophs is basically in the mode of production of food. Due to lack of chlorophyll, heterotrophs cannot synthesize their food while autotrophs can perform photosynthesis.
Heterotrophs may be parasitic, saprophytic and holozoic.
The term has been derived from two Greek works: Para means feeding and sites means grains. Parasitic organisms are those which live on or inside other living organisms to derive their food. Such a mode of nutrition is known as parasitic nutrition. A parasite derives its food (nutrition) from the host in different ways the mode of feeding depends upon its habit, habitat, and modifications. An ectoparasite, which lives on the outer surface of the host, may have certain devices to obtain blood from the host. A mosquito gets a blood meal by inserting its mouth parts into the skin, but a leech has rasping jaws which lacerate the skin of cattle and man. The blood which oozes out is sucked by the leech. A hook-work living in the intestine of man sucks the digested food of the host through its mouth. But a tapeworm which lives in the intestine of man does not even have a mouth of its own. Neither does it have an alimentary canal. A tape-worm thus obtains nutrition through its body surface. An intracellular parasite such as Plasmadium lives on the protoplasm of the cell it has invaded, viruses, fungi and some non-green plants have parasitic mode of nutrition.
In plants, the fungus puccinia is a parasite on wheat and barberry plants; Cuscuta or dodder plant (amar bel) is a parasite on many plants and obtains food from them. Parasites often lack well-developed organ systems but their body parts are modified for attachment to the host or for sucking food from the host’s body.
The word saprophyte has been derived from the Greek words sapro meaning rotten and phyto meaning plants. Saprophytic organisms derive their food from decomposing dead organisms. The complex organic compounds become simpler in dead organisms when the decomposition sets in. they feed on substances which were once part of living organisms such as stored food, wood, leather and rotten plant products. The common examples of saprophtyes are fungi (moulds, mushrooms, yeasts) and many bacteria. Saprophytes secrete enzymes which are released on he substrate (i.e., the place on which they grow). These enzymes digest and break down the complex food material like starch into simpler ones. The soluble end product like sugar is then absorbed back by the saprophyte. This is called extracellular digestion. The saprophytic mode of nutrition can best be shown by the common bread mould, Rhizopus mucor (pin mould), Neurospora (pink bread mould), Morchella (sponge fungus) and Agaricus (mushrrom) also represent similar mode3 of nutrition
The word holozoic has been derived from Greek words: holos means whole and zoon means animal. Holozoic nutrition involves ingestion of complex organic substances. The food of most animals contains large organic substances. In this mode, small or large particles of food are consumed through an opening called mouth (Ingestion). Then these are hydrolyzed into simpler and soluble forms (digestion). Simplified products are absorbed into the body and the undigested product is removed from the body (Egestion).