Heterotrophic Nutrition

Heterotrophic nutrition is known to be the mode of nutrition in which certain organisms are dependent on other organisms in order to survive. Organisms that cannot prepare their own food and have to depend on other organisms are known as heterotrophs.

Common examples of heterotrophs include animals, fungi and bacteria.

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Heterotrophic nutrition is the mode of gaining complex but pre-made food. Organisms that are heterotrophic have to acquire and consume organic substances they come across. All non-green plants and animals are heterotrophic in nature and depend on autotrophs for food. From a more ecological point of view, the secondary or tertiary consumers in a food chain are always heterotrophs.

Types of Heterotrophic Nutrition

Organisms have various kinds of heterotrophic nutrition which can be classified into the following:

  • Holozoic Nutrition- This is a kind of heterotrophic nutrition where organisms intake solid food; a method that is also called ingestive nutrition. The food consumed may be either another organism or a plant. There are three different types of holozoic organisms and are classified on the basis of the food they consume. They are- carnivores, herbivores and omnivores.

  • Saprophytic Nutrition- This is a kind of heterotrophic nutrition where organisms obtain their food sources from remains of decaying organic substances, including dead organisms, decomposed leaves and plant remains, excreta, food articles and more.

  • Parasitic Nutrition- This is the third kind of heterotrophic nutrition where an organism derives its food from another living organism. The organism that derives its food and shelter from the other organism is known as a parasite, while the organism that lends food and shelter is known as a host.

Below is a detailed explanation of each of the kinds of heterotrophic modes of nutrition and the organisms that classify under each.

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Holozoic Nutrition

The internal processing and ingesting of both solid and liquid food in an organism is known as holozoic nutrition. This is a step by step process that involves ingestion, digestion, absorption, assimilation and excretion.

During this process, food that has been consumed is slowly broken down by the process of digestion, into smaller organic particles. After the useful particles and food have been absorbed, the unimportant and ingested particles are eliminated from the body using the process of excretion.

All vertebrates classify under animals that exhibit holozoic nutrition. Apart from them, other animals exhibiting holozoic nutrition are certain unicellular organisms like the amoeba.

Holozoic Nutrition in Amoeba

Amoeba is a unicellular organism that shows holozoic nutrition, and below is a breakdown of the process in which it happens.

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  • First, the amoeba encircles the food by projecting its pseudopodia, after which it uses the process of phagocytosis to engulf the food.

  • Next is the process of digestion which occurs when the food vacuoles of amoeba that are rich in digestive enzymes, then help break the food down into smaller and easily- digestible substances.

  • After which, the cytoplasm absorbs the digested food while leaving the undigested substances behind. The food already absorbed, on the other hand, is further utilised in the production of energy that eventually helps in the development and growth of the cell.

  • Lastly, the process of holozoic nutrition in amoeba ends when the cell membrane ruptures to excrete the unwanted and undigested food substances.

Types of Holozoic Organisms

Holozoic organisms can be classified into the following three types:

  • Herbivores- They are animals that depend upon plants for their source of food and nutrition. Examples of herbivores include deers, elephants, cows among others.

  • Carnivores- They are animals that feed on, consume or depend on other animals for their source of food. Examples of carnivores include tigers, wolves, lions among others.

  • Omnivores- They are animals that can survive on other animals or plants for their source of food. Examples of omnivores are pigs, ants, raccoons, and cockroaches among others.

Saprophytic Nutrition
Saprophytic nutrition is the process of animals feeding on dead and decomposed substances or organisms for energy, food and nutrition. Organisms that follow saprophytic nutrition are called saprophytes. Saprophytes hold a highly important position in the ecosystem since they help to keep the environment and surroundings clean, free of unwanted matter and also help in the process of recycling nutrients.
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Common examples of saprophytes include fungi and a couple of types of bacteria. These organisms release specific enzymes that act on complex organic substances and help to break them down into smaller and simpler particles that are easily consumable.

The Process of Digestion in Saprophytes

Originating from a Greek word, Saprophytes are usually referred to as “plant”- the word “phyte” means plants. These are mostly recognised for the use of a certain kind of digestive mechanism called extra-cellular digestion, that is classic of saprophytes.  In this process, certain digestive substances are secreted into the surroundings which help in breaking down organic substances into simpler matter. The remnant nutrients then go through the process of metabolism by directly getting absorbed through the organism’s cell membranes.
Proteins, fats and starch are broken down into simpler substances during the process of saprophytic nutrition; while proteins are digested into amino acids, fats are broken down into fatty acids. Starch is digested into simple forms of sugar, all of which in the end are transported through the membranes of the cell.
Parasitic Nutrition
Parasitic nutrition is a form of heterotrophic nutrition where an organism lives on or feeds off another living organism. Organisms living in and sourcing food from the host organism are called parasites. Parasites often derive their nutrients and energy directly from the host, something that can be harmful to the host organism’s health, and may also end up in killing the host.  Parasites are completely dependent on their hosts in order to survive, since the host organism provides nutrition, energy, food and shelter.
A host organism may either be a plant or an animal. Common examples of parasites are lice on human heads, tapeworms, Cuscuta plants, barnacles among others.
Parasites can be of two types, which are:
These parasites remain outside the body of the host and obtain their food and energy from the organism. Examples of ectoparasites are bed bugs, lice, ticks and mosquitoes.
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These parasites remain inside the body of the host organism and obtain their food and energy from the host. Examples of endoparasites are tapeworm, ascaris, plasmodium vivax and others.
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An example of a highly unusual parasite can be the Cymothoa exigua, also known as the tongue-eating poise. This one is mostly found on the mouth of the fish Lithognathus; the parasite cuts off blood supply to the tongue, as a result completely severing it off and causing it to fall off. The parasite then goes on to attach itself to the remaining tongue of the fish and starts acting as it’s new tongue.

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