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Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially, using the energy of solar radiation.

Photosynthesis occurs in plants, algae, and some groups of bacteria, but not in archaea. Photosynthetic organisms are called "photoautotrophs," but not all organisms that use light as an energy source effect photosynthesis, since "photoheterotrophs" use organic compounds, not carbon dioxide, as a carbon source. In plants, algae and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. photosynthesis is crucial for life on Earth, because in addition to maintaining the normal level of oxygen in the atmosphere, almost all life forms directly depend as a source of energy, or indirectly as the ultimate source of energy in their food .

The amount of energy captured by photosynthesis is immense, of approximately 100 terawatts: this is about six times the energy consumed annually by human civilization. In total, photosynthetic organisms convert about 100 billion tons of carbon into biomass each year.

It is not clear when the first organisms capable of implementing photosynthesis appeared on Earth, but the presence of striated formations in some rocks due to the presence of oxide suggests that seasonal cycles of oxygen in the Earth's atmosphere, a symptom of photosynthesis , they appeared roughly Three thousand five hundred million years ago in Archeano.

Chemical reaction of photosynthesis

During photosynthesis, with the mediation of chlorophyll, the solar radiation will convert six CO2 molecules and six H2O molecules into a glucose molecule (C6H12O6), a fundamental sugar for the life of the plant. As a byproduct of the reaction, six oxygen molecules are produced, which the plant releases into the atmosphere through the stomata found in the leaf.

6 CO2 + 6 H2O → C6H12O6 + 6 O2

The photosynthesis of chlorophyll is the primary process of production of organic compounds of inorganic substances clearly dominant on Earth. In addition, photosynthesis is the only biologically important process capable of collecting solar energy, on which, basically, life on Earth depends.

Phases of photosynthesis

The photosynthesis of chlorophyll, also called oxygen photosynthesis due to the production of oxygen in molecular form, is carried out in stages in two phases:

  • The phase dependent on light (or light phase), dependent on light;
  • The carbon fixation phase of which the Calvin cycle is part.

The second phase of photosynthesis is also called phase in the dark; The term, however, can be misleading, since it does not refer to the absence of light, since some enzymes involved in this phase are activated directly by their own light, so that the phase of light is carried out simultaneously and not at night. In fact, in the absence of light there is a shortage of ATP and NADPH, which are formed during the period of light and the stomata are closed, so there is no access to CO2; Even inactivity of certain enzymes that are dependent on light is also produced (Rubisco, 3-PGA dehydrogenase, phosphatase and ribulose kinase 1,5 bis-phosphate).

The photosynthetic process takes place inside the chloroplasts. Within these there is a system of membranes that form piles of flattened bags (thylakoids), said grain (from the Latin, the singular 'granum'), and the grains of the connecting laths (intergraniche lamellae). Within these membranes, we find chlorophyll molecules, added to form the so-called photosystems. Photosystem I and photosystem II can be distinguished. Photosystems are a set of pigment molecules arranged to surround a special "trap" molecule of chlorophyll. The energy of the photon passes from molecule to molecule until the special chlorophyll is reached. In photosystem I, the trap molecule is excited by a wavelength of 700 nm, in photosystem II of 680 nm.

Photosystem I consists of an LHC (complex that captures light) is composed of approximately 70 molecules of chlorophyll a and b, and 13 different types of polypeptide chains, and a center of reaction that includes approximately 130 chlorophyll molecules to said P 700, a particular type of chlorophyll that has maximum light absorption at 700 nm.

Photosystem II is also composed of an LHC, consisting of approximately 200 chlorophyll molecules in and b, as well as different polypeptide chains, and a reaction center that is formed from approximately 50 chlorophyll molecules in said P 680, which has the maximum absorption of sunlight at 680 nm.

All these molecules are capable of capturing the energy of solar radiation, but only those of chlorophyll are able to move to an excited state that activates the photosynthetic reaction. Molecules that only have the function of uptake are called antenna molecules; those that activate the photosynthetic process are called reaction centers. The "light phase" is dominated by chlorophyll a, whose molecules selectively absorb light in the red and blue-violet parts of the visible spectrum, through a series of other adjuvant pigments. The energy captured by the chlorophyll molecules allows the promotion of electrons from lower energy atomic orbitals to higher energy orbitals. These are immediately replaced by the cleavage of water molecules (which, by H2O, is divided into two protons, two electrons and one oxygen thanks to photolysis, operated by the OEC photosynthetic photosynthesis associated with photosystem II).

The electrons released by the photosynthesis of chlorophyll II are fed to a transport chain consisting of cytochrome B6f, during which they lose energy and move at a lower energy level. The lost energy is used to pump protons from the stroma into the thylakoid space, creating a proton gradient. Finally, the electrons reach photosystem I, which in turn, due to light, has lost other electrons. The electrons lost by photosystem I are transferred to ferredoxin, which reduces NADP + to NADPH. Through the membrane protein ATP-synthase located in the thylakoid membrane (membranous layers internal to the chloroplast or, in the case of autotrophic bacteria, distributed in the cytoplasm), the H + ions released by the hydrolysis water passage from the space to the stroma thylakoids, that is, to gradient, synthesize ATP from free groups of phosphate and ADP. One molecule of ATP can be formed every two electrons lost by photosystems.

Several studies have shown that the plant grows more with diffuse solar radiation than with direct light, with the same power of incoming light. One study emphasizes, however, the relevance of other conditions that modify the growth of plants that vary with light, such as humidity and temperature; direct light, in fact, leads to an increase in temperature that causes more water to evaporate in the plant.

The phase of carbon fixation or the Calvin cycle (also called phase in the dark or light independent phase) involves the organication of CO2, namely its incorporation into organic compounds and the reduction of the compound obtained thanks to the 'ATP derived from the light phase.

In this cycle there is a fixed organic compound, ribulose-bisphosphate, or RuBP, which is transformed during the reaction until it returns to its initial state. The 12 molecules of ribulose bisphosphate present in the Calvin cycle react with water and carbon dioxide to undergo a series of transformations by the carboxylase enzyme ribulose-bisphosphate, or rubisco. At the end of the process, in addition to the newly synthesized 12 RuBP, 2 molecules of glyceraldehyde 3-phosphate arise, which are expelled from the cycle as the product of the net fixation.

To be activated, the Calvin cycle requires chemical energy and support through the hydrolysis of 18 ATP in ADP and oxidation of 12 NADPH in NADP + and free hydrogen ions H + (which are protons). The ATP and NADPH consumed during the Calvin cycle are taken from those produced during the light phase and once they are oxidized, they become part of the group available for reduction. In general, in the Calvin cycle we are consumed 6 molecules of CO2, 6 of water, 18 of ATP and NADPH 12 to form 2 glyceraldehyde 3-phosphate (abbreviated as G3P), 18 free phosphate groups, 18 ADP, 12 protons, 12 NADP +.

The two molecules of glyceraldehyde 3-phosphate formed during the Calvin cycle are used to synthesize glucose, in a process perfectly inverse to glycolysis, or for the formation of lipids such as fatty acids or amino acids (by the addition of an amino group in the structure). The final products of photosynthesis, therefore, play a critical role in the anabolic processes of autotrophic organisms.

In addition to a synthetic photosynthetic cycle (only during the day and during the growing season) and glucose derived polysaccharides, the plants also an opposing oxidative cycle (cellular respiration) (day and night throughout the year) photosynthetic products used precisely as food of the plants themselves.

The general equilibrium of oxygen and CO 2 flows, however, from and to the external environment is in favor of photosynthesis or the plant behaves as a 'carbon well accumulation' (absorbent) rather than as a 'source' ( emitter) to the external environment and vice versa a carbon 'source' of oxygen instead of a 'well' of oxygen. This is because part of the carbon absorbed and not used by the oxidative cycle of the plant remains fixed in the form of cellulose and lignin in the cell walls of the "dead" cells that make up the internal wood of the plant. The oxidation phase of the plants is what makes the plant a living being like the others. The same oxidative cycle causes the internal temperature of the plant, in turn thermoregulated by physiological processes, to be generally different from that of the external environment.

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Last review: August 24, 2018