Print Email Download Reference This Send to Kindle Reddit This
submit to reddit

Free Biology Essay | Autotrophs and Heterotrophs

Bacteria are unicellular micro-organisms that are found everywhere on earth. They are classified under prokaryotes. Prokaryotes are organisms which do not have a membrane bound nucleus unlike eukaryotes which possess a well formed nucleus. Though the bacteria do not possess the nucleus, they have genetic material in their DNA. The bacterial DNA is found in their cytoplasm.

Though bacteria can cause diseases, only a small percentage of bacteria do so. The rest of the bacteria are harmless. The pathogenic bacteria are called parasitic bacteria and they thrive on the nutrition of other living matter for their survival. On the other hand, the saprophytic bacteria lead a symbiotic relationship with other living matter and do not cause any harm.

Bacteria can be classified under various categories. They can be classified according to shape, nutritional and oxygen requirements, staining characteristics, motility, presence of cell wall and spore formation. Based on the nutritional requirements, they are classified in to -

Heterotrophic bacteria

Heterotrophic bacteria grow on organic substances formed by animals and plants. They break down complex organic substances from these sources and derive carbon. Heterotrophic bacteria are divided in to -

The heterotrophic bacteria secrete enzymes which act on the complex organic substances and derive energy. Most of the heterotrophic bacteria are aerobic organisms and they need oxygen for their survival. Based on their relation with the energy sources they are again divided in to -

Heterotrophs exhibit two basic strategies for deriving energy from the organic substrates -


Autotrophic bacteria are organisms whose sole source of nutrition is carbon dioxide. They need only water, inorganic salts and carbon dioxide for growth. They are divided in to -

Soil biology

Plants are terrestrial organisms and they obtain most of their nutrients from the soil. Animals derive energy indirectly from plants and other animals. This requires utilization of many elements such as inorganic compounds from the environment which are converted in to organic substances before they can be used as energy. The cycling of these elements is catalyzed by the bacteria present in the soil.

Soil is the upper most layer of earth which varies in depth from a few inches to over twenty feet. The type of the soil depends on the four factors including the parent rock, the climate, the age and the biological factors like plants, bacteria and organic substances. Most of the biological activity occurs at the surface. Most of the organic substances are found in the upper layer or the upper horizon. Majority of the bacteria are found in the upper six to twelve inches of soil. Both autotrophic and heterotrophic bacteria are found in the soil. Among the two heterotrophic bacteria are more commonly found in the soil. The heterotrophic bacteria in soil belong to the order Eubacteriales and Actinomycetales. The Actinomycetales are represented by the genera Streptomyces, Nocardia and Micromonospora. These bacteria have an earthy odour.

Maximum bacterial growth and activity in the soil are found around the roots of the plants. This region is called 'rhizosphere'. Almost all the ecological interactions occur in this region. These interactions could be favorable, unfavorable, indispensable and sometimes lethal. The most important function of the soil microorganisms is to decompose various kinds of organic matter.

As said earlier, cycling of elements is done by bacteria. This is done to make the elements available for reuse. The elements are usually found in the reduced state inside the cell. When they are mineralized, they are in an oxidized state. They serve three functions -

Bacteria are involved in three types of cycles in the cycling of the elements necessary for energy -

Carbon cycle

About 50% of the dry weight of all living organisms is composed of carbon. The ultimate source of this organic carbon is the atmospheric carbon dioxide.

Role of heterotrophs

Role of autotrophs

Nitrogen Cycle

Nitrogen is the most important structural element of all living organisms. Though it is abundant in nature, it cannot be utilized by the plants and animals. The nitrogen cycle is concerned with incorporation of atmospheric nitrogen and organic nitrogen of dead plants and animals on to forms that are usable by higher organisms. Bacteria, both heterotrophs and autotrophs are involved in this cycle.

There are five processes of nitrogen cycle -

Role of heterotrophs in nitrogen cycle

Role of autotrophs

Sulphur cycle

Sulphur is essential for all living organisms as sulphur containing amino acids are present in all proteins. It occurs in both organic and inorganic combinations. Cycling of sulphur is similar to nitrogen cycle. Transformation between organic and elemental states and between oxidized and reduced state are carried out by bacteria.

Role of heterotrophs

Heterotrophs in the soil degrade the proteins in the soil and liberate sulphur.

Role of autotrophs

Autotrophs oxidize various forms sulphur like hydrogen sulphide. The phototrophic autotrophs oxidize hydrogen sulphide in to elemental sulphur. The chemosynthetic autotrophs oxidize sulphur in to sulphates. Sulphate is the most suitable source of sulphur for plants. This sulphate is assimilated and converted in to proteins.

Fresh water biology

Fresh water is defined as water containing less than 1% of salt. The different fresh water zones include - ponds and lakes and streams and rivers.

Ponds and lakes

Ponds and lakes consists of three zones-

Streams and rivers

These are water bodies which move in one direction. The characters of these water bodies change during their course. There are three parts of these water bodies - source, mid stream and mouth. The place where they reach and join another water source including ocean is called the' mouth'. The source of these water bodies contains high oxygen levels and hence the heterotrophs thrive here very well. Near the mouth, the light penetration and the oxygen content are very less and autotrophic bacteria are found in this region.

Both heterotrophs and autotrophs exist and survive in the fresh water bodies. The processes are very similar to those that occur in the soil. In the fresh water bodies, these bacteria take care of the inorganic requirements of the algae which is similar to the plants on the soil. These organisms get in to these water bodies from air, soil, sewage, organic wastes, dead plants and animals. Climatic, geographical and biological conditions bring about great variation in the bacterial population. Rivers and stream show their highest count during rainy season. Dust blowing in to the rivers and streams also contributes to many bacteria. Animals contribute to the bacterial population by bathing and dropping their excreta.

Among the various cycling of elements, the nitrogen cycle is most common one occurring in the water bodies.

Role of heterotrophs

Role of autotrophs