Autotrophs and heterotrophs

Living beings can be classified into two major groups: autotrophs and heterotrophs . Autotrophs are those that have the ability to synthesize organic matter from inorganic matter and are thus considered organisms capable of producing their own food.

Autotrophs have this ability through photosynthesis or chemosynthesis. As an example we have plants, algae, some bacteria and archeobacteria.

Heterotrophs are those unable to produce their own food. In the food chain they are known as consumers, as they feed on producers or other consumers.

As an example we have herbivores (rabbit, ox, horse), carnivores (lion, tiger), omnivores (human, bear, bat), hematophagous (animals that feed on blood, such as mosquitoes, lice), detritivores (vultures, vultures), insectivores (frogs), and among others.

Photosynthetic autotrophs

The photosynthesis (autotrophic main process) is performed by chlorophyll being represented by plants, some protists, cyanobacteria and photosynthetic bacteria.

Plants are autotrophic and animals that feed on them or other living beings are heterotrophs (Photo: depositphotos)

With the exception of photosynthetic bacteria , other beings use carbon dioxide (CO 2 ) and water (H 2 0) in photosynthesis , forming carbohydrates and oxygen gas (O 2 ), which is released into the environment. Because it releases oxygen gas, this type of photosynthesis is called oxygen photosynthesis.

The general equation for photosynthesis of eukaryotes and cyanobacteria is:

6 CO 2  + 12 H 2 O      C 6 H 12 O 6  + 6 O 2  + 6 H 2 O

This equation shows that, in the presence of light and chlorophyll, carbon dioxide and water are used in the production of carbohydrate (glucose), oxygen gas and water.

Photosynthetic oxygen beings are essential for the maintenance of life on our planet, because, in addition to being the basis of most food chains , they produce oxygen, a gas kept in the atmosphere in adequate concentrations, thanks mainly to photosynthetic activity.

Photosynthetic light and pigments

Light can only be used in photosynthesis thanks to the presence of specialized pigments that are able to capture light energy. Solar radiation is composed of several wavelengths. Among them, the human eye can only distinguish those that make up visible light or white light.

When passing through a prism, the light is decomposed and the seven colors that make up white light can be perceived. Each color covers a wavelength range.

Pigments have the property to absorb and reflect certain wavelengths. The color of the pigment is given by the reflected wavelength. In photosynthesis, several pigments participate in the absorption of light energy, and in plants these pigments are chlorophylls a and b and carotenoids .

Chlorophylls reflect the wavelength of green light and are therefore green. Carotenoids can be of various colors, from orange, yellowish or reddish, associated with the wavelength they reflect.

How do plants capture carbon?

Carbon dioxide from atmospheric air enters plants through small openings in leaves and green stems . Through these openings, oxygen gas produced in photosynthesis and water released in the form of steam in the transpiration process of the plant also escape .

We will emphasize what happens in the leaves, the main organ of the plant responsible for photosynthesis and transpiration. The exchange of gases and the release of water vapor occur in a controlled manner by means of special structures, the stomata , formed by two stomatal cells that delimit a stoma pore.

The plant can open or close the stomatal pore depending on certain variables. A very important variable is the availability of water. If water is restricted, the stomata close, reducing water loss through transpiration.

Certain tropical plants have adaptations to life in places with high light intensity, high temperatures and low water availability. Under these conditions, stomata can remain closed for a long time during the day, which reduces plant transpiration.

Chemosynthetic autotrophs

Chemosynthesis is a process in which the energy used in the formation of organic compounds comes from the oxidation of inorganic substances, and not from light energy. Chemosynthesis is carried out by some bacteria, which is why they are called chemosynthetic or chemo-autotrophic bacteria.

The main examples of chemosynthetic bacteria are:

  • Ferrobacteria:use chemical energy from the oxidation of iron compounds for the synthesis of organic matter
  • Nitrobacteria:use chemical energy from the oxidation of ammonium ions or nitrite ions for the synthesis of organic matter. Nitrobacteria or nitrifying bacteria (genera Nitrosomonas and Nitrobacter ) exist free in the soil and are of great importance in the nitrogen cycle
  • Sulphobacteria:use energy from the oxidation of hydrogen sulfide (H 2 S), as do chemosynthetic bacteria from underwater thermal sources.

Origin of oxygen and bacterial photosynthesis

The oxygen gas released by photosynthesis carried out by eukaryotes and cyanobacteria comes from water , not carbon dioxide, as was previously thought. The first researcher to propose this was Cornelius van Niel (1897-1985), in the 1930s, when he was studying photosynthetic bacteria.

The researchers found that red sulfur bacteria (or purple sulfobacteria) performed a particular form of photosynthesis in which there was no formation of oxygen gas.

He found that these bacteria use carbon dioxide and hydrogen sulfide (H 2 S) and produce carbohydrate and sulfur. Because it does not produce oxygen, the photosynthesis of these bacteria is called anoxygen.


Heterotrophs are those that do not produce their own food and therefore, beings in this category seek energy by feeding on other living beings. Heterotrophs are consumers who depend directly or indirectly on autotrophs (producers). From Greek heteros = other and trophos = feeder.

When we analyze a food chain, we observe that autotrophs are always at the base of it, while heterotrophs form the other trophic levels, that is, they can be consumers or decomposers .

The primary consumers are herbivorous animals , which feed directly on autotrophs (producers). Secondary consumers feed on primary consumers and so on.

Finally, the decomposers, which feed on decomposing organic matter releasing mineral salts to the environment. This process is of great importance for nutrient cycling and is carried out by fungi and some bacteria.

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