Heterotroph is an organism that is unable to fixed carbon to produce its own food. The nutrients they require are taken as organic carbon from other plants and animals. All of this nutrition obtained is spent as energy for growth and reproduction. Unlike the autotrophs, who have to save some of their energy for photosynthesis. Therefore, it is clear that heterotrophs are not producers of food. But rather they are primary, secondary and tertiary consumers of food. The term heterotroph comes from two Greek words ‘heteros’ and ‘trophe’ to convey the meaning ‘other nutrition. The term was found in 1946 when classifying the microorganisms based on how they get their nutrition.
Respiration in heterotroph is quite different from that of the other organisms. Because it is having a stage known as mineralization. Mineralization is the process of decomposing organic matter into soluble inorganic matter. So that they readily available for the plants to absorb. The organic matter is oxidized and the energy required for the process is gained by respiration of heterotroph. Elemental organic matter like sulfur and nitrogen is converted to soluble inorganic forms H2S and NH4+.
Classification of Heterotrophic Organisms
So, what organisms are classified as heterotrophic organisms? Most opisthokonts and prokaryotes are heterotrophic; for example, prokaryotic bacteria like green non-sulfur bacteria and purple non-sulfur bacteria. Then again all animals in terms of ingestion and fungi in terms of absorption are classified as heterotrophic organisms. Some parasitic plants are also heterotrophic (fully or partially) depending on how they get their nutrition. Fully parasitic plants like broomrape, dodder, Rafflesia and partially parasitic plants like Loranthus and Viscum can consider as examples.
Types of Heterotroph
Heterotroph can be divided into groups depending on their energy source and depending on how they get their electron source.
If chemical energy is the energy source of a heterotroph and carbon source is organic compounds, then they are known as chemoheterotroph. Photoheterotroph are organisms having light energy as their energy source and carbon dioxide as the carbon source. However, chemoheterotrophs gain their energy chemically with a process called chemosynthesis rather than photosynthesis.
Heterotroph can be classified as organotrophs or lithotrophs depending on how they get their electron source. Organoheterotroph gain electrons from reduced carbon compounds (carbohydrates, proteins, fats) from plants and animals while lithoheterotrophs use inorganic compounds to gain electrons like nitrite or sulfur.
Furthermore, the heterotroph can be classified as photoorganoheteretrophs, chemoorganoheterotrophs, chemolithoheterotrophs and mixotrophs. Detailed information about the four sub-types mentioned below to get a clear understanding of heterotroph.
- Photoorganoheterotrophs has light energy as the energy source and organic molecules as the carbon source and the electron source. Some examples are purple non-sulfur bacteria and Rhodobacteria living in polluted water. They do not have the Calvin cycle and therefore, cannot undergo carbon dioxide fixation like the other photoautotrophic organisms.
- Chemoorganoheterotrophs has chemical energy as the energy source, organic compounds like glucose as the carbon source and reduced organic compounds like carbohydrates, fats, proteins as the electron source. An example is the Helicobacter which causes ulcers in humans.
- Chemolithoheterotrophs has chemical energy as the energy source, an organic compound like glucose as the carbon source and inorganic compounds like nitrate, sulfur, ammonium to gain electrons. An example is Oceanithermus profundus which gets energy from the oxidation of inorganic compounds like hydrogen sulfide, thiosulfate and molecular hydrogen.
- Mixotrophs are also known as chemolithomixotrophs or facultative chemolithotrophs. They can thrive in both heterotrophic and autotrophic conditions as the name suggests. They have chemical energy as the energy source and inorganic compound like nitrate, sulfur, ammonium to gain electrons. But they have the ability to use carbon dioxide or organic carbon as the carbon source.
Types of Heterotrophic Nutrition
Heterotrophs are consumers in the food chain and they obtain their nutrients in three ways. The three ways are saprophytic, holozoic and parasitic heterotrophic nutrition.
- Saprophytic nutrition: Organisms that feed on dead organic remains of other plants and animals are known as saprophytes. The digestive mechanism used by these organisms is called extracellular digestion in which digestive enzymes are released into the environment that contains dead organic matter. Then these dead organic matter is broken into simpler compounds and is absorbed by the saprophytes. Carbohydrates are broken into simple sugars, proteins into amino acids and fats are broken into fatty acids. Example- Fungi
- Holozoic nutrition: Organisms that take complex food through their mouth into the digestive system to be broken down into simpler food and then to be absorbed into the blood falls under this category. After the food is absorbed the waste materials or food are excreted. Therefore, holozoic nutrition consists of five steps; ingestion, digestion, absorption, assimilation and defecation. Example- herbivores (cow, deer, goat), carnivores (tiger, lion) and omnivores (humans).
- Parasitic nutrition: Organisms that obtain food from other living organisms falls under this category. The organism that obtains food is known as the parasite and the organisms that give the food is known as the host. Sometimes the parasite can be harmful to the host. Example- tapeworm, plasmodium vivax, head lice and Cuscuta plants.