Photosynthesis is the general process that uses light energy to synthesize organic compounds from carbon dioxide . The organisms capable of carrying out this process are plants, algae and some prokaryotes, such as cyanobacteria.

All photosynthetic organisms are photoautotrophic, this means that they can obtain their sustenance from light . The products derived from photosynthesis maintain most of the autotrophic organisms (producers in the food chain), as well as heterotrophic consumers.

Process of photosynthesis

Photosynthesis is a biological oxidation-reduction process , that is, an electron transfer from a donor to an acceptor. In most photoautotrophic beings, carbon dioxide is the electron acceptor and water is the donor:

bold CO subscript bold 2 bold more bold 2 bold H subscript bold 2 bold O bold arrow with bold light above open parentheses bold CH subscript bold 2 bold parenthesis bold more bold subscript 2 bold more bold H subscript bold 2 bold O

In this case, the water H 2 O acts as a reductant, so oxidized, and the released electrons energized are transferred to CO 2 , producing oxygen O 2 and carbohydrates . Plants, algae and cyanobacteria are able to conduct this endergonic reaction driven by light , which has a free energy change (Δ Gº ‘) of +2840 kJ / mol for the synthesis of a hexose (a six-carbon carbohydrate, like glucose).

Machinery of photosynthesis: photosystems

Photosystems are two protein complexes associated with pigments, that is, they are a set of several proteins that bind to molecules that absorb the energy of light. Two photosystems are known: the photosystem I (PSI) and the photosystem II (PSII).

In each system, two components are distinguished:

  • the antenna : constituted by photosynthetic pigments that can only capture light energy and transfer it to the reaction center;
  • the reaction center : here are the target pigments that are capable of transferring electrons and initiating the chain of chemical reactions of photosynthesis.
Light absorption range Less than or equal to 700 nm less than or equal to 680 nm
Pigment antenna Chlorophyll alfa and chlorophyll beta Chlorophyll alpha, beta and xanthophyll
Pigments from the reaction center Chlorophyll P700 Chlorophyll P680

Phases of photosynthesis

In photosynthesis, two stages can be distinguished: a phase dependent on light and another phase independent of light. The two phases occur simultaneously in different parts of the chloroplast.

Phase dependent on light energy

The phase of light-dependent photosynthesis begins with the arrival of photons (units of light energy ) to photosystem II or PSII. This causes the excitation of the P680 chlorophyll electrons, which jump to energy orbitals further away from the atomic nucleus. These electrons are captured by the primary electron acceptor and then the proteins of the electron transport chain until they reach the active center of the photosystem I or PSI.

The light energy is also absorbed by the antenna pigments of the PSI and transferred to the chlorophyll P700 from the reaction center, which results in the excitation of electrons leaping towards a primary acceptor of the PSI. The electrons lost by the P700 chlorophyll are replaced by electrons from the PSII.

From the primary acceptor, the electrons go through a conveyor chain, that is, a series of compounds and proteins that pass the electrons from one to the other. Finally, the electrons are transferred to the NADP + molecule , which together with the protons stored in the stroma, form the NADPH, by the action of the enzyme ferredoxin-NADP + reductase FNR.

Energy is also harnessed by the enzyme ATP synthetase to form ATP from ADP and phosphate, in a process called photophosphorylation . Consequently, the energy obtained from light in this phase of photosynthesis is contained in ATPand NADPH .

Animation of how photons activate PSII and PSI photosystems to form ATP and NADPH.

Independent phase of light energy

NADPH and ATP produced in the light-dependent phase are used in this phase or phase of carbon fixation. These reactions occur in the chloroplast stroma.

Calvin-Benson cycle

The Calvin-Benson cycle comprises a set of reactions that lead to the synthesis of organic molecules. Each CO 2 that incorporates two and three ATP NADPH needed. It consists of three phases:

  1. CO 2 fixation : atmospheric carbon dioxide binds to a five carbon molecule, ribulose diphosphate or RuDP, forming an unstable six-carbon compound that is then broken into two phosphoglycerate molecules of three carbons each. This phase is carried out by the Rubisco enzyme .
  2. Reduction of the fixed CO 2 : the phosphoglycerate is reduced to glyceraldehyde phosphate (G3P), that is, it receives the electrons of the NADPH produced in the light-dependent phase. The glyceraldehyde phosphate can serve to regenerate the RuDP or be used for the biosynthesis of carbohydrates, amino acids and lipids.
  3. Regeneration of RuDP : to close the cycle it is necessary to regenerate the initial ribulose diphosphate. For that, five of the six molecules of glyceraldehyde phosphate produced in the previous phase are used.

The general chemical reaction for the Calvin-Benson cycle is:

bold 3 bold CO subscript bold 2 bold more bold 6 bold NADPH bold more bold 5 bold H subscript bold 2 bold bold bold 9 bold ATP bold right arrow bold bold bold bold bold bold bold bold bold bold bold bold darker bold more bold 6 bold NADP raised to bold more bold more bold 9 bold ADP bold more bold 8 bold P subscript bold i

Types of photosynthesis

There are several types of photosynthesis depending on the mechanisms of fixation of carbon dioxide:

  • Photosynthesis C3: refers to the fixation of carbon dioxide in a three-carbon molecule, such as 3-phosphoglycerate 3-PGA acid. It occurs in most terrestrial plants.
  • Photosynthesis C4: refers to the fixation of carbon dioxide in a four-carbon molecule, such as oxalacetic acid. It occurs in the species of corn ( Zea mays ) and sugar cane ( Saccharum sp. ).
  • Photosynthesis CAM: receives this name by the abbreviations in English of acid metabolism of the crasuláceas, that includes to the succulent plants (cactus, pineapple and agave).

Importance of photosynthesis

Thanks to photosynthesis it was possible to expand life on earth millions of years ago in evolution and perpetuate itself through the centuries to the present by:

  • Provide food for heterotrophic organisms.
  • Provide biomass.
  • Provide fossil fuels.
  • Generate the oxygen required for the respiratory activity of all multicellular organisms and many unicellular organisms.

Factors that affect photosynthesis

Of the environmental factors that have the greatest incidence on photosynthesis, we have:

  • The intensity of light : the speed of photosynthesis increases as the light intensity increases to 600 watts, a value from which it remains constant.
  • The temperature : as the temperature increases, the photosynthetic rate increases, up to 30ºC, from which it decreases.
  • The concentration of CO 2 : as the concentration of CO 2 increases, the photosynthetic rate increases depending on the ambient temperature.

The chloroplast: organelle of photosynthesis

In eukaryotes, the reactions of photosynthesis take place in a specialized plastid : the chloroplast. The plastids are unique organelles of the plants, formed by two membranes, external and internal.

The chloroplast is hemispherical or lens-shaped in vascular plants, measuring between 5-8 μm long and 3-4 μm thick. The photosynthetic equipment of the chloroplast is contained within the membranous system of the thylakoids.

See also Cell

Pigments of photosynthesis

The molecules that absorb light are called pigments . When a pigment absorbs a photon, it goes from its lower energy state to an excited state. The excited state is nothing more than the jump of an electron from a position close to the nucleus to a higher energy level in the case of atoms. In molecules, there are two types of excited states: the singlet state and the triplet state.

All photoautotrophic organisms contain some form of pigment known as chlorophyll. The chlorophyll molecules have a porphyrin ring that binds to a Mg magnesium atom in the center. The different chlorophylls vary in parts of their chemical structure that affect the absorption properties of light.

The thylakoid membranes of plants and algae contain two types of chlorophylls. Chlorophyll alfa is found in all the reaction centers and in the antennas, while chlorophyll beta is found in the antennae. Each reaction center can have up to 250 chlorophyll molecules.

Chlorophyll is green because it absorbs light in the range of blue (430 nm) and red (680 nm) of the visible spectrum. Green light is not absorbed and rather reflected.

Other pigments that the plants possess are the carotenoids (responsible for the yellow-orange color) and the anthocyanins (responsible for the red and purple colors).

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