All living things need food and nutrients. According to the method of assimilation of carbon and the method of formation of organic substances, all cells (and living organisms) are divided into two large groups: autotrophs and heterotrophs.
Autotrophic organisms form organic matter on their own using only carbon dioxide (CO2), water (H2O) and mineral salts.
Autotrophs are divided into two groups: photosynthetics (phototrophs) and chemosynthetics (chemotrophs).
For photosynthetics, sunlight is a source of energy for biosynthetic reactions. Phototrophs include cells of green plants containing chlorophyll and bacteria capable of photosynthesis (for example, cyanobacteria).
Chemosynthetics use the energy released during chemical transformations of inorganic compounds to synthesize organic substances.
Chemosynthesis is the formation of organic compounds from inorganic ones due to the energy of redox reactions of nitrogen, iron, and sulfur compounds.
Chemosynthetics are the only organisms on Earth that do not depend on the energy of sunlight. These include some types of bacteria:
iron bacteria oxidize ferrous iron to ferric:
Fe2 + → Fe3 + + E;
sulfur bacteria oxidize hydrogen sulfide to molecular sulfur or to sulfuric acid salts:
H2S + O2 = 2H2O + 2S + E,
H2S + O2 = 2H2SO4 + E;
nitrifying bacteria oxidize ammonia to nitrous and nitric acids, which interact with soil minerals to form nitrites and nitrates:
NH3 → HNO2 → HNO3 + E.
The energy released in the oxidation reactions of inorganic compounds is converted into the energy of high-energy ATP bonds and only then is spent on the synthesis of organic compounds.
The role of chemosynthetics is great, since they are an indispensable link in the natural cycles of the most important elements: sulfur, nitrogen, iron, etc. They destroy rocks, participate in the formation of minerals, and are used in wastewater treatment (sulfur bacteria). Nitrifying bacteria enrich the soil with nitrites and nitrates, in the form of which nitrogen is assimilated by plants.
Heterotrophic organisms cannot independently synthesize organic substances from inorganic compounds and need their constant absorption from the outside. Eating food of plant and animal origin, they use the energy stored in organic compounds and build their own proteins, lipids, carbohydrates and other biopolymers from the obtained substances.
Heterotrophs include animals, fungi, and many bacteria.
Depending on where the heterotrophic organisms get their nutrients from, they are divided into groups: saprophytes, parasites, holozoa.
Saprophytes (saprotrophs) feed on dead organic residues (bacteria of putrefaction, fermentation, lactic acid bacteria, many fungi).
Parasites exist only on living organisms, causing harm to them (pathogenic bacteria, fungi-parasites of plants, animals and humans; parasitic animals and plants).
The third group of heterotrophs is holozoa. Holozoan nutrition includes three stages: eating, digesting and absorbing digested substances. It is more often observed in multicellular animals with a digestive system. Holozoic feeding animals can be subdivided into carnivores, herbivores, and omnivores.
There are also organisms capable of using both autotrophic and heterotrophic feeding methods. Such organisms are called mixotrophs. This is, for example, green euglena, which is a phototroph in the light and a heterotroph in the dark.
Some plants, such as the Venus flytrap or sundew, are able to replenish nitrogen deficiencies by catching and digesting insects.
Other plants have partially switched to a parasitic way of life and can receive organic substances from the host’s body with the help of special modifications of the roots (mistletoe, Peter’s cross, dodder).
The organic substances obtained by auto- or heterotrophic means cannot directly provide energy to the processes occurring in the cell. Due to the energy of the chemical bonds of these substances, a universal source of energy, ATP, is necessarily synthesized.