Essentially photosynthesis - This is a chemical reaction, as a result of which six CO2 molecules are combined with six water molecules and one glucose molecule is formed - the building block of our organic matter. Formed during photosynthesis, molecular oxygen is only a by-product. However, this very "by-product" is one of the main sources of atmospheric oxygen, so necessary for higher organisms.
It would seem that everything is very simple: the cell of the photosynthetic organism is a kind of “cone” for the chemical reaction of the two components. But in actual fact, the reaction mechanism is much more complicated. It turns out that the process consists of two reactions: "light" and "dark". The first is related to the splitting of a water molecule into hydrogen and oxygen using light energy. Sunlight is absorbed by the cell's special light-absorbing pigment chlorophyll (colored green). Next, energy is transferred into ATP molecules, which release the received energy in the second stage of photosynthesis - the “dark” reaction. The “dark” reaction is a direct reaction between carbon dioxide and hydrogen to form glucose.
Photosynthesis can be carried out by plants, algae, and certain types of microorganisms. Thanks to their livelihoods, it becomes possible to exist, for example, animals, whose diet consists of organic substances. But is photosynthesis the only form of converting carbon dioxide to organic matter? Not. It turns out that nature provides another alternative route for the formation of organic substances from CO2 - chemosynthesis.
The difference between chemosynthesis and photosynthesis is the absence of a “light” reaction. As a source of energy, cells of chemosynthetic organisms use the energy not of sunlight, but the energy of chemical reactions. Which ones? Oxidation reactions of hydrogen, carbon monoxide, reduction of sulfur, iron, ammonia, nitrite, antimony.
Of course, each chemosynthetic organism uses its own chemical reaction as an energy source. For example, hydrogen bacteria oxidize hydrogen, nitrifying bacteria convert ammonia to the nitrate form, etc. However, they all accumulate the energy released during the chemical reaction in the form of ATP molecules. Then the process proceeds according to the type of reactions of the dark stage of photosynthesis.
Only some types of bacteria possess the ability to chemosynthesis. Their role in nature is enormous. They do not "produce" atmospheric oxygen, do not accumulate large quantities of organic matter. However, the chemical reactions that they use in the course of their livelihoods play a key role in biogeochemistry, providing, among other things, the circulation of nitrogen, sulfur and other elements in nature.