Types of nutrition in animals

Nutrition:The process by which living organisms obtain those substances which are required for their  growth and maintenance and for meeting their energy needs is called nutrition, and the substances are called nutrients of foods.
Organisms have two modes of nutrition :autotrophic and heterotrophic.
In autotrophic nutrition, green plants, some bacteria and protists utilise energy of sunlight for preparing organic food in their own body from simple inorganic materials. These organisms are called phototrophs or autotrophs.Some bacteria use energy released by the oxidation of certain chemical substances for the preparation of food. They are called chemotrophs.
In heterotrophic nutrition animals obtain organic food materials by consuming bodies or products of other plants or animals, such organisms are called heterotrophs. “
Heterotrophic nutrition is of four types-
holozoic, saprozoic, parasitic and symbiotic.

In holozoic nutrition, the solid food is ingested (taken in), digested and then absorbed into the cells.

Holozoic animal may be herbivores (plant eaters,e.g. cow, rabbit), carnivores (flesh eaters, e.g. dog,tiger), omnivores (both plant and animal eaters,e.g. cockroach, man), insectivores (insect eaters,e.g. toads, lizards), frugivores (fruit eaters, e.g.monkeys, birds), sanguivores (feed on blood of vertebrates, e.g. female Anopheles mosquito, leeches), detrivores (feed on decaying organic matter, e.g. earthworms) and fluid feeders (feed on fluids from plants, e.g. butterflies, male mosquitoes).

In saprozoic nutrition, organisms obtain nutrients from decaying organic materials after digesting thesame with the help of enzymes. A few animals (e.g. spider, housefly, etc.) secrete digestive enzymes directly onto their food, which are dead or decaying matters and then suck the digested food,

In parasitic nutrition liquid food material is

obtained by the parasite from the body of host, e.g.Plasmodium, Ascaris.

ln symbiotic nutrition often two organisms live in association and derive nutrition from each other, e.g. Escherichia coli in human intestine synthesizes vit. B12 and in return obtain simpler food from human intestine.

Holozoic nutrition involves four steps. These are - ingestion (feeding of the food), digestion (breaking down of complex organic molecules into simple molecules by hydrolysis), absorption (uptake of simple diffusible molecules by the digestive tract) and egestion (passing out of indigestible material through the anus).

In different animals ingestion occurs by different process on the basis of which they are classified into fluid feeder animals, filter feeder animals and macrophagous animals.

Ingestion occurs in fluid feeder animals by diffusion (eg. parasitic protozoans), pinocytosis(eg. sanguivorous animals like leech, mosquitoes)etc.

Filter feeder animals, also called microphagous animals are those animals which takes small sizedfood particles. Ingestion occur in microphagous animals bymaintaining a water current which bring microscopic organisms. Examples are pseudopodial feeder (Amoeba), ciliary feeder(Paramecium), flagellar feeder (sponges), mucoid feeder (Nereis) and tentacular feeder (Hydra).

Macrophagous animals feed on large sized food particles and have different structure to capture and ingest the food.

Eg. (i) Tentacles with batteries of cnidoblasts in coelenterates. (ii) Eversible and sticky tongue (eg. frog, toad, wall lizard) to capture insects with differently adapted teeth, like well developed incisors in rabbit and well developed canines (tearing teeth) in carnivores (eg. lion, tiger etc.)

Digestion, essentially a hydrolytic process, is carried out by various enzymes using water molecules for cleavage.The enzymes which bring about digestion are called hydrolases.

Digestion may be intracellular (within the cell) or extracellular (in digestive cavity).

The unicellular animals like Amoeba and porifers (Sponges) digest the food within the cells and show intracellular digestion.

The cnidarians (e.g. Hydra, Aurelia etc.) have sac like coelenteron or gastrovascular cavity, which is lined by various type of endoderm cells. The food is partially reduced to small fragments by extracellular digestion by the enzymes secreted by the gland cells of endoderm. The small fragments of the food are ingested by nutritive cells and complete the digestion intracellularly.

The free living Platyhelminthes (e. g. Planarian) also employ both types of digestion i.e. intracellular and extracellular.