Photosynthesis of plants is a complex physico-biochemical process, due to which plants convert electromagnetic energy in the sun into chemical energy used in organic compounds. This process is based on a chain of redox chemical reactions, as a result of which electrons are transferred from reducing donors, which are hydrogen and water, to acceptors, which are oxidizing agents. In this case, carbohydrates are formed and O2 is released during the oxidation of water.
Photosynthesis of plants has two successive stages. The first stage is called light (photochemical). At this stage, quantum light energy is converted into chemical energy for bonds of high-energy compounds, as well as into a universal reducing agent. In the second stage, called dark (metabolic), the obtained chemical energy and universal reducing agent go through a cycle to fix and restore carbon dioxide, resulting in the formation of carbohydrates. The mechanism of photosynthesis separates the light and dark stages not only in time but also in space. The light stage takes place in special thylakoid energy-transforming membranes, while dark reactions occur either in the stroma of the chloroplast or in the cytoplasm.
Photosynthesis and plant respiration is based on the absorption of light quanta, where the main role is played by chlorophylls, the absorption spectrum of which includes the visible region, as well as parts of the infrared and ultraviolet regions adjacent to it. The main pigment for all photosynthetic plants is chlorophyll a. Green algae, mosses and vascular plants also have chlorophyll b, which expands the absorbed light spectrum. Some algae species also contain chlorophylls c and d. In addition to chlorophylls, carotenoids and phycobilins also participate in the absorption of light.
After light absorption, the photochemical stage begins, in which two photosystems of types I and II (FS1 and FS2) take part. Each of the photosystems consists of a reaction center, where charge separation takes place, an electric transport circuit, where electrons are oxidized, and a set of components that perform processes for water photooxidation and regeneration of the reaction center. In reaction centers, quantum light energy is converted into chemical energy, and then the electrons move according to the gradient of the electrochemical potential, which is an electron-transport chain of photosynthesis.
A type II photosystem performs reactions to photooxidize water, resulting in the formation of oxygen and proton H +. In parallel with photosynthetic electron transport, the process of proton transfer from the chloroplast to the intrathylakoid region occurs. As a result of reactions, NADPH and ATP are formed, which are the primary products of photosynthesis. Further, plant photosynthesis forms enzymatic reactions in which proteins, carbohydrates and fats are obtained from carbon dioxide. If the dark metabolism has a non-carbohydrate orientation, then amino acids, organic compounds and proteins are formed.
Metabolic processes according to the type of CO2 fixation are subdivided into C3, C4, and CAM photosynthesis. At the same time, carbohydrates that are formed at the dark stage of photosynthesis can be deposited in chloroplasts in the form of starch compounds, leave the chloroplasts to form new cells, and act as an energy source for metabolic reactions.
Photosynthesis of plants uses only 1-2 percent of the absorbed light energy. The intensity of the photosynthesis process is affected by the spectral composition and intensity of light, temperature, the water regime of the plant and its mineral nutrition, the concentration of CO2 and O2, as well as other environmental factors.