Heterotrophic Nutrition

Heterotrophic mode of nutrition is most commonly seen in humans. Nutrition is broadly classified into two types- Autotrophic nutrition and Heterotrophic nutrition. In heterotrophic nutrition, the organism obtains and uses organic nutrients from other species. All the organisms belonging to Kingdom Fungi and Kingdom Animalia show completely heterotrophic nutrition. Organisms of Kingdom Plantae show a completely autotrophic type of nutrition, except for some parasitic, symbiotic, and partially heterotrophic plants.

Heterotrophic Nutrition Definition

Heterotrophic nutrition is a type of nutrition in which organisms rely on other organisms for nutrition, either directly or indirectly.

The organisms that follow heterotrophic nutrition are known as heterotrophs. They are incapable of preparing their own organic food. They obtain from other organisms. All heterotrophs (except blood and gut parasites) convert complex nutrients into simple compounds via Digestion that can be absorbed and utilized by the cells. The soluble products of digestion are then broken down for energy release (Respiration) and other developmental processes (Metabolism).  

Types of Heterotrophic Nutrition 

The heterotrophic mode of nutrition can be categorised into three types based on their technique of intaking nutrition :

• Saprophytic Nutrition
• Parasitic Nutrition
• Holozoic Nutrition

Saprophytic Nutrition

In saprophytic mode of nutrition, organisms which obtain nutrients from dead and decaying organic matter. These organisms are called saprophytes. In saprophytes, the digestive process entails dissolving complex organic materials into less complex nutrients outside their body by releasing their digestive juices. The organism's cell membranes are used to absorb the digested nutrients. During the process of saprophytic nutrition, proteins, lipids, and carbohydrates are broken down into less complex molecules that can be easily absorbed by the organism.

Fungi (e.g. yeast) and certain types of bacteria are examples of saprophytes. These organisms are also responsible for the deterioration of bread and other similar food products and even cause Fermentation.

Parasitic Nutrition

Parasites are organisms that feeds on simple nutrients from the body of another organism called the host and cause harm to it. These organisms can be external or internal. Few examples of paraites are Ascaris (roundworms) which live in the human gut causing Ascariasis, Cymothoa exigua, a rare parasite that is also known as tongue-eating louse because it lives in the mouth of the marine fish Lithognathus, etc. Parasites are completely dependent on their host organisms for their survival. There are two types of parasites:

• Ectoparasites: The parasites that infect the host's body externally to obtain their nutrition are called Ectoparasites. These include parasites like ringworm, bedbugs, lice, etc.
• Endoparasites: The parasites that infect the host body internally to obtain their nutrition are called Endoparasites. These include parasites like Tapeworms, Ascaris, etc.

Holozoic Nutrition

It is a type of heterotrophic nutrition in which complex food is consumed by the body and processed internally. Simple nutrients are absorbed by the body after digestion, whereas undigested substances are expelled from the body. Most vertebrate species as well as single-celled creatures like Paramecium and Amoeba exhibit this style of nourishment.

Types of Holozoic Organisms

Holozoic nutrition is observed in these three types of organisms:-

• Herbivores: The animals that are directly dependent on plants for their nutrition are called Herbivores. E.g. Zooplankton, Deer, Cow, etc.
• Carnivores: The animals that feed on other animals are called Carnivores. E.g. Lion, Tiger, Shark, etc.
• Omnivores: The animals that feed on both plants and animals are called Omnivores. E.g. Humans, Rats, etc.

Examples of Heterotrophic Nutrition

Nutrition in Amoeba

Amoeba displays a holozoic mode of nutrition in the which the organsim is ingested as a whole and then absorbed by the body.

Following steps are involved in the process:-

1. Ingestion: The amoeba encircles the food with its pseudopodia. The food is then swallowed up by the process of phagocytosis.
2. Digestion: The food vacuoles are rich in digestive enzymes, which assist in the breakdown of food into simpler organic nutrients.
3. Absorption: The digested nutrients are then absorbed into the cytoplasm, leaving undigested materials behind. This absorbed food is used to generate energy for cell growth and development.
4. Egestion: The undigested food is ejected from the cell by rupturing the cell membrane.