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Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially using solar energy. This chlorophyll function converts inorganic matter into organic matter thanks to the energy provided by light.

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 perform photosynthesis, since "photoheterotrophs" use organic compounds, and 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 of crucial importance for life on Earth, since in addition to maintaining the normal level of oxygen in the atmosphere, almost all forms of life depend directly as an energy source, or indirectly as the ultimate source of energy in their food.

The amount of energy captured by photosynthesis is immense, about 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 unclear 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 Earth's atmosphere, a symptom of photosynthesis, they appeared roughly 3.5 billion years ago in Archeano.

What influence does photosynthesis have on climate change?

Photosynthesis allows to reduce the amount of carbon dioxide in the atmosphere atmosphere in a natural way. Carbon dioxide is a greenhouse gas. The presence of too high a concentration of this type of gas in the atmosphere prevents heat from escaping to the outside.

When the sun's rays enter the atmosphere, part of them heat the planet and part of them bounces back into space. Some of this bounced radiation bounces back against the greenhouse gases and cannot get out. Some of these gases occur naturally, like clouds; but others are artificially generated. The burning of fossil fuels, for example generate gases of this type.

On the other hand, especially the large forest areas, the plants do not stop absorbing this extra carbon dioxide. For this reason, photosynthesis is a natural process that contributes to not aggravating the problem of climate change thanks to solar energy.

What is the chemical reaction of photosynthesis?

During photosynthesis, with chlorophyll mediation, solar radiation will convert six molecules of CO 2 and six molecules H 2 O into one molecule of glucose (C 6 H 12 O 6 ), a sugar essential for the life of the plant. As a by-product of the reaction, six oxygen molecules are produced, which the plant releases into the atmosphere through stomata in the leaf.

6 CO2 + 6 H2O → C6H12O6 + 6 O2

PhotosynthesisChlorophyll photosynthesis is the primary process of producing organic compounds from clearly dominant inorganic substances on Earth. Furthermore, photosynthesis is the only biologically important process capable of collecting solar energy, on which, basically, life on Earth depends.

What are the phases of photosynthesis?

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

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

Bright phase

The luminous phase or light dependent reaction is the step of photosynthesis in which solar energy is converted into chemical energy. Light is absorbed by chlorophyll and other photosynthetic pigments such as carotene and is used to fragment water, so oxygen is produced as waste.

The photosynthetic process takes place within the chloroplasts. Within these there is a system of membranes that form piles of flattened bags (thylakoids), called grain, and of the grains of the connecting strips (intergraniche lamellae). Within these membranes we find chlorophyll molecules. Chlorophyll molecules are added together to form so-called photosystems. Photosystem I and photosystem II can be distinguished.

Photosystems are a set of pigment molecules arranged to surround a special chlorophyll "trap" molecule. 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 is formed by an LHC (complex that captures light) is made up of approximately 70 chlorophyll molecules a and b, and 13 different types of polypeptide chains, and a reaction center that includes approximately 130 chlorophyll molecules to said P 700 , a particular type of chlorophyll that has the maximum light absorption at 700 nm.

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

All of these molecules are capable of capturing the energy of solar radiation. However, only those with chlorophyll are able to move to an excited state that activates the photosynthetic reaction. Molecules that only have the uptake function are called antenna molecules; those that activate the photosynthetic process are called reaction centers.

The "light phase" is dominated by chlorophyll a. Chlorophyll 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 enables the promotion of electrons from lower-energy atomic orbitals to higher-energy orbitals.

These are immediately replaced by cleavage of water molecules (which, by H 2 O, is divided into two protons, two electrons and one oxygen by photolysis, operated by the OEC oxygenic 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 to a lower energy level. The lost energy is used to pump protons from the stroma into the thylakoid space, creating a proton gradient.

Eventually, the electrons reach photosystem I. Photosystem I, in turn, has lost other electrons due to light. The electrons lost by photosystem I are transferred to ferredoxin, which reduces NADP + to NADPH. Through the ATP synthase membrane protein located in the thylakoid membrane, the H + ions released by the passage of hydrolysis water from space to the stromal thylakoids, that is, towards a gradient, synthesize ATP from groups free 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 diffused solar radiation than with direct light, with the same power of incoming light. A 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 actually leads to an increase in temperature that causes more water to evaporate from the plant.

Carbon fixation phase or Calvin cycle

The carbon fixation phase or Calvin cycle (also called the dark phase or light independent phase) involves the organization of CO 2 . Its incorporation into organic compounds and the reduction of the compound obtained thanks to the 'ATP derived from the light phase.

The Calvin cycle uses the energy of short-lived electronically excited carriers to convert carbon dioxide and water into organic compounds that can be used by the body. This set of reactions is also called carbon fixation. The key enzyme in the cycle is called RuBisCO.

The enzymes in the Calvin cycle are functionally equivalent to most of the enzymes used in other metabolic pathways such as gluconeogenesis and the pentose phosphate pathway. However, enzymes in the Calvin cycle are found in the stroma of the chloroplast rather than the cell cytosol, separating the reactions.

These enzymes are activated in light, and also by products of the light-dependent reaction. These regulatory functions prevent the Calvin cycle from breathing into carbon dioxide. Energy (in the form of ATP) would be wasted in carrying out these reactions that have no net productivity.

What factors influence the process?

The most important external factors involved in the performance of photosynthesis are:

  • Temperature: Each plant species has a temperature range in which it feels most comfortable. Within this range, the efficiency of the process varies as a consequence of an increase in the mobility of the molecules.
  • Carbon dioxide concentration: Photosynthetic performance increases proportionally with the concentration of carbon dioxide in the air under constant light radiation conditions.
  • Oxygen concentration: the higher the oxygen concentration in the air, the lower the photosynthetic performance. This variation is due to photorespiration processes.
  • Luminous intensity: the higher the luminous intensity, the higher the performance, up to exceeding certain limits. Once these limits are exceeded, irreversible photooxidation of photosynthetic pigments occurs.
  • The lighting time: there are species that have a higher photosynthetic production the greater the number of hours of light.
  • Water shortage: in the absence of water on the ground and water vapor in the air, photosynthetic performance decreases. If the plant detects lack of water, it closes the stomata to avoid drying it out. The counterpart is that this self-protection system hinders the entry of carbon dioxide. Furthermore, increasing the concentration of internal oxygen triggers photorespiration.
  • The color of light: depending on the chlorine of the light and the characteristics of the species, the photosynthetic conversion is different.

Published: August 24, 2018
Last review: March 15, 2020