PHOTOSYNTHESIS

From Big Medical Encyclopedia

FOTOSYNTEZ — process of transformation of light energy to energy of chemical bonds of the organic matters necessary for life activity of both photosynthesizing organisms, and other organisms not capable to independent synthesis of organic matters.

T. it is carried out by various g of p at 11 pases of m and and at t about tr about f and ykh a shouting of hectare of N and zm about in (see) — the higher green plants, multicellular green, red and brown seaweed, one-celled — evglenovy, di-yoflagellyatami and diatomic seaweed, and also blue-green algae, green and purple bacteria. Peculiar F. proceeds in the galophilic bacteriums containing a pigment a bacteriorhodopsin (see Rhodopsin).

T. — it is the only process in the nature, to-ry goes with storage of energy (with increase in thermodynamic potential) and provides with available chemical energy all organisms existing in the nature (except for chemosynthetic bacteriums).

Biol. role F. it is huge. Within a year as a result of F. on Earth it is acquired apprx. 200 billion t of carbon dioxide gas, formed ok, 100 billion t of organic matter also are allocated apprx. 145 billion t of molecular oxygen. T. provides with energy all live organisms. T., carried out by photosynthesizing organisms, the first on Earth, caused emergence of oxygen and the ozone screen in the thickness of the atmosphere of Earth, created conditions for biological evolution (with - m. Life).

At all photosynthesizing organisms, except for bacteria, process F. represents the synthesis of organic matters from carbon dioxide gas and water by means of energy of light which is followed by release of oxygen. Photosynthesizing bacteria also synthesize on light organic matters from - carbon dioxide gas, however instead of water they use other hydrogen donators — inorganic (e.g., thiosulphate, hydrogen sulfide, gaseous hydrogen) or organic (e.g., isopropyl alcohol or milk to - that) connections.

In all cases at F. transformation of light energy to chemical includes oxidation-reduction electron transfer and the formation of ATP interfaced to it, i.e. phosphorylation (see). The main part of energy at the same time is stabilized in the form of the recovered connections (in particular, NADFN), and a smaller part — in the form of ATP (see biological oxidation).

Overall equation F. in a habit view it is possible to write down as follows:


where hv — a light quantum, DN2 — hydrogen donator, D — the oxidized condition of the donor, and CH2C — a fragment of a molecule of carbohydrate. The specific nature of hydrogen donator depends on a type of a photosynthesizing organism and therefore the overall equation of photosynthesis of the higher plants and seaweed can be written down in such look:

where p it is equal 6 that corresponds to education as an end product F. molecules of hexose

(C6H1206). As hydrogen donator in this case serves water whereas the recovered final acceptor, i.e. NADFN, directly joins in recovery of carbon dioxide gas to carbohydrates. At recovery of carbon dioxide gas there is an increase in free energy approximately on 120 kcal counting on 1 Ghat of carbon.

Experiments, in to-rykh used marked isotones 180.2, carbon dioxide and water, allowed to prove that the molecular oxygen which is formed at F., comes from water molecules.

For photosynthesizing bacteria, hydrogen donator in to-rykh slu7-zhit hydrogen sulfide (e.g., green sulfuric bacteria), the general equation F. it is possible to present in such form:


and for the purple bacteria using isopropyl alcohol —


Photosynthetic processes at the higher plants and nek-ry seaweed are carried out in special cellular structures — chlorolayers (see the Cell), the inner membrane to-rykh is in a special way packed. In these photosynthesizing membranes all pigments, and also enzymes are localized, to-rye are necessary for primary photoreactions. Photosynthesizing bacteria contain other specialized educations — chromatophores, in membranes to-rykh pigments and specific enzymes are also localized. Blue-green algae do not support specialized photosynthesizing structures, and the membranes containing pigmental and fermental systems are formed directly of a cellular membrane. Thus, all cell of such organism reminds chloroplast of the higher plants.

T. includes so-called light (going with use of light energy) and dark (proceeding for lack of light) processes, or stages.

The light photochemical stage, on an extent a cut occurs stabilization and storage of the light energy transformed then in chemical, begins fotosin-tetichesky pigments of plants and seaweed with absorption of light quantums, from to-rykh the major role plays a chlorophyll (see), and at bacterial F. — backteriochlorophyll (see Bacteria, Pigments).

The light stage at kislorodvyde-lposhchy photosynthesizing


nizm of an org includes two light reactions happening consistently. Each of these light reactions is carried out in different photochemical systems F., differing with the spatial relationship in photosynthesizing membranes, a ratio of the pigments included in a pigment in structure - it is white kovy complexes, reactive centers (actinic fractions of a pigment) and carriers of electrons. Energy of light quantums is absorbed by light gathering pigments of both photosystems — the ground mass of a chlorophyll, carotinoids (see), fikobilinam (see Pigments) and in time about 10-11 — 10 ~ 8 sec. migrate to the actinic pigment of reactive centers of these systems capable at obtaining excitation energy to be oxidized and give an electron to the next acceptor, to-ry transfers him then on a chain of electronic carriers. The pigment of a reactive center is recovered from the next to it donors of an electron (see Redoxreactions).

Thus, energy of the absorbed light causes a cathode rays from donors to acceptors, and finally electrons are transferred from water to NADF. The energy of light used for «raising» of electrons «up» (against thermodynamic potential), provides recovery of NADF and oxidation of water, i.e. process, to-ry for lack of energy sources cannot spontaneously go.

The recovered final acceptor (NADFN) then joins in a «dark» stage F. (recovery of carbon dioxide gas to carbohydrate), proceeding for lack of light with the participation of enzymatic systems. In a cycle of transformations of carbohydrates primary acceptor of carbon dioxide gas is ribulezo-1,5-diphosphate. The oxidation of water leading to release of free oxygen happens with the participation of the enzymatic system containing manganese in quality of one of components (assume that it is a cofactor of enzyme).

Studying of problem F., in addition to all-biological, has practical value. In particular, problems of food, creation of life support systems at space researches, uses of photosynthesizing organisms for creation of various biotechnical devices are directly connected with F.

Bibliography: Biophysics of photosynthesis,

under the editorship of A. B. Rubin, M., 1975; In about l -

kenstein M. V. Biofizika, page 429, M., 1981; Lenindzher A. Biokhi


a miya, the lane with English, page 533, M., 1976; Physiology of photosynthesis, under the editorship of. A. A. Ni-chiporovich, M., 1982. B. A. Gulyaev.

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