Photosynthesis – Mechanism,History,Importance

Photosynthesis is the process of solar energy absorption by chlorophyll molecules and its conversion to the energy of chemical bonds by the synthesis of organic substances from carbon dioxide and water.The main photosynthetic organ is the leaf.

It has acquired several adaptations during evolution, for instance a large surface area for absorbing CO2, a multitude of stomata for water but also for gas exchange, the presence of photosynthesizing cells organized in a bilayer structure to form a palisade parenchyma and a spongy parenchyma adapted for gas exchange, the synthesis of photoprotective pigments such as anthocyanins and carotenoids.

Chloroplasts are the main photosynthesizing organelles, with a double membrane of which the internal one forms folds rich in chlorophyll and which either have the form of overlaid disks (granal thylakoids) or traverse the chloroplast from one edge to another (thylakoids of the stroma).

The synthesis of the chloroplast components is regulated by both the chloroplast and the nuclear genome. The process of photosynthesis is possible due to a series of pigments for which the presence of a
chromophore and a system of conjugated bonds are characteristic. These are represented by chlorop-hylls (a, b, c, d characteristic for different taxonomic units), carotenoids (carotenes, xantophylls, carot-enoidic acid) and phycobilins (phycoerythrin, phycocyanin) all of these having specific light absorption patterns along the radiation spectrum.

Consequently some of these molecules act as auxiliary pigments absorbing and transferring energy to the “main” pigments that carry the most important reactions of photosynthesis. Photosynthesis occurs in two stages: the light phase (Hill phase), which happens in the granal thylakoids and the dark phase
(Blackman phase), progressing in the chloroplast stroma.

The light phase is characterized by energy absorption by the light-harvesting complexes containing
chlorophyll molecules “a” and auxiliary pigments (chlorophyll “b”, carotenoids, phycobilins), the transfer of the electrons from the reaction center by the Electron Transport Chain (ETC) coupled with the transport of protons, transformation of the proton gradient into ATP and NADPH+H+, water photolysis and O2 release.

In the dark phase, fixation of CO2 by ribulose-1,5-diphosphate happens mediated by the enzyme RUBISCO, carbohydrate synthesis, with consumption of the ATP and NADPH+H+ formed during the light phase. In parallel with photosynthesis a process called photorespiration occurs characterized by CO2 elimination and O2 absorption. It is known to intensify during intense illumination, high temperatures or low CO2 concentration.

Historical Background

• 1771—J. Priestley has demonstrated that O2 is consumed by animals and is produced by plants.
• 1779—J. Ingenhousz has shown that light is necessary for green plants to produce oxygen.
• 1818—J. Pelletier and J. Caventou extracted a pigment from green leaves and called it chlorophyll.
• 1840—J.B. Bousingault proposed a global reaction for photosynthesis.
• 1845—J.R. Mayer showed that solar energy is transformed in the energy of chemical bonds.
• 1875—K.A. Timireazev formulated the idea of the global role of green plants.
• 1877—W. Pfeffer introduced the notion of photosynthesis, from gr. photos —“light” and synthesis—“to put together”.
• 1922—O. Warburg realized the first measurements of the quantum outcome of photosynthesis.
• 1936—H. Gaffron and K. Woll formulated the concept of the presence of the reaction’s photochemical centers.
• 1938—R. Hill demonstrated elimination of oxygen during the light phase of photosynthesis.
• 1943—R. Emerson and collaborators investigated the phenomenon of photosynthesis relaunching—“the Emerson effect”.
• 1951—L. Duysens investigated the absorption and migration of the excitation energy in auxiliary pigment complexes.
• 1954—D. Arnon and collaborators discovered photosynthetic phosphorylation.
• 1956—M. Calvin and collaborators identified the reactions of CO2 reduction to carbohydrates.
• 1960—R. Hill and F. Bendall proposed the Z-scheme of electron transport during the light phase.
• 1960—S.E. Karpilov, M.D. Hatch and C.R. Slack (1966) discovered the C4 cycle of carbon assimilation.
• 1961—P. Mithcell elaborated a chemiosmotic hypothesis of the oxidative and photosynthetic phosphorylation mechanism.

Brief Details

Solar energy accumulation during photosynthesis depends to a big extent on the level of cell protection against oxidative degradation. Multiple antioxidant components (omega-3 fatty acids, vitamin E, carotenoids, etc.)—substances, which act to neutralize free radicals—protect both vegetal and animal cells preventing apoptosis, because the fundamental cellular signaling processes and the mechanisms of protection and adaptation are very conservative throughout the entire living world.

Multiple extracts of algae and higher plants, products of photosynthesis, are capable to manipulate signaling processes in human cells and as a result, gene expression. For example, phytoestrogens (a group of flavones that play the role of messengers in plant—microbe interaction) mimic the activity of the human hormone estrogen.

Carotenoids, such as zeaxanthin and lutein protect the photosynthetic systems from the destructive action of ultraviolet light, but can also be found in the human retina (Lutein is apparently employed by animals as an anti-oxidant and for blue light absorption while zeaxanthin may serve as a photopro-
tectant for retina from the damaging effects of free radicals produced by blue light).

An important role in the regulation of plant life is played by the solar light spectrum, which, besides photosynthesis, participates in the regulation of certain physiological processes (like growth and development).

Importance of Photosynthesis and the Global Role of Green Plants

Photosynthesis is the process through which the energy of light is absorbed by chlorophyll molecules and converted to potential chemical energy of organic substances, composed of carbon dioxide and water. This process includes a large number of reactions, however, for autotrophic organisms producing oxygen, it can be summarized by the following equation:

According to this equation, the outcome of photosynthesis is light-dependent CO2 fixation with its reduction to carbohydrates and the oxidation of H2O to O2. However, oxygen formation is not characteristic for all photosynthetic organisms. Some photosynthetic bacteria use as a hydrogen donor other inorganic substances (hydrogen sulfide, thiosulfite, etc.) or organic compounds (lactic acid, isopropanol). For instance, green sulfur bacteria use hydrogen sulfide. In this case the summary
equation of photosynthesis is as follows:

In the process of photosynthesis, besides carbon dioxide that is the main acceptor of hydrogen ions in photosynthetic autotrophic organisms, sulfate or nitrogen can be reduced, forming hydrogen. Thus, the process of photosynthesis in different photosynthetic organisms can take place with the participation of different donors and acceptors of hydrogen ions.

The appearance of green plants capable of photosynthesis (2 billion years ago) marked a very important step in the evolution of life on Earth. Photosynthesizing organisms and, consequently, all living organisms have gained access to a limitless and renewable source of electrons, participating in all bioenergetic processes—water.

This fact has determined the extent of photosynthesis, assuring energy flow and transformation in the biosphere. Eukaryotic organisms (superior green plants and eukaryotic algae) and prokaryotes (photosy-nthetic bacteria and cyanophytic algae) capture solar energy, and convert it into potential chemical energy.

This is why photosynthesis can be considered a phenomenon of cosmic nature, and the only means by which energy from a celestial body is fixed and stored on Earth in the form of biomass used later in all
life processes of vegetable and animal organisms. The cosmic role of green plants has been widely described and argued by the Russian scientist K.A. Timireazev.

Photosynthesis ensures the continuous existence of life by purifying the atmosphere from carbon dioxide. The oxygen released within this physiological process, replenishes its amounts in atmosphere, keeping it within limits optimal for respiration. Annually, green plants release 460 billion tons of oxygen in the external environment, representing the only natural source of oxygen.

Photosynthesis represents also the primary source of all organic substances as well: carbohydrates, lipids, vitamins, proteins, hormones, glycosides, tannins, etc. These compounds synthesized by plants for their own needs serve later as the main nutrition source for the multitude of trophic chains. Photosynthesis constitutes a most crucial step for energy and matter circulation in nature.

Fig. The cycle of photosynthesis products in nature

The unique physiological importance of this process can be generalized by the following contribution of photosynthesis in:
• transforming the nature of the atmosphere from a reducing one into an oxidant
one which lead to the advent and spread of aerobic organisms;
• conversion of solar radiation into metabolic energy;
• purifying the atmosphere through gas exchange;
• formation of organic substances from inorganic ones;
• formation of the reserves of mineral resources;
• ensuring the circuit of carbon in nature.

Photosynthesis as the most complicated fundamental biological process, is a research object for biologists, physicists, chemists, mathematicians, etc. Knowledge of the molecular mechanisms of photosynthesis is very important in solving many industrial-economic problems related to the use of ecologically pure unlimited energy sources (for example, obtaining oxygen and molecular hydrogen through water photooxidation), in increasing the photosynthetic productivity of plants, ensuring long-term cosmic expeditions with organic matter and molecular oxygen.

The Leaf as a Specialized Photosynthesis Organ

Photosynthesis takes place in all plant cells that contain green pigments (leaves, branches, young stems, sepals, unripe fruits), but the organ specialized in fulfilling this function is the leaf, which shows some features formed during a long process of adaptation and improvement:

• a large, flat area, adapted for absorption of large amounts of solar energy and CO2 from the atmosphere;
• an epidermis provided with stomata through which gas exchange and transpiration occurs. Depending on the positioning of stomata in plants, they can be divided in 2 groups: amphistomatic (with stomata present on both sides of the leaf) and epistomatic (with stomata located only on one side of the leaf);
• the presence of organelles specialized for photosynthesis—chloroplasts;
• a bilayer structure, the assimilatory parenchyma being differentiated in palisade parenchyma that plays the main photosynthetic role and spongy parenchyma with a pronounced role in gas exchange. In young branches, seeds and unripe fruits, assimilatory cells with chloroplasts are located in the parenchymal layers under the epidermis. Intercellular spaces are very small which causes reduced CO2 absorption from the external environment (in comparison with green leaves);
• the presence of conducting channels (phloem and xylem) which deliver mineral compounds and water to mesophyll cells and transport the elaborated sap with synthesized organic compounds to all plant organs.

Fig. Structure of the leaf blade in plants with C3 and C4 types of photosynthesis

During evolution leaves have changed generating a great diversity, determined by the structural changes adopted for carbon assimilation . Some plants originating from tropical and subtropical zones (corn, sugar cane, etc.) have leaves with a particular anatomical structure that differs from the leaves of plants growing in temperate climate (300.000 species of plants), adapted to carry out photosynthesis in certain environmental conditions.

The leaves of these species are well vascularized, the mesophyll is homogenous containing granal chloroplasts while conducting vessels are surrounded by a compact layer of parenchymal cells, forming a sheath of perivascular assimilatory tissue with big agranal chloroplasts Perivascular sheath cells are separated from the mesophyll and from the air of intercellular spaces by a film, resistant to carbon dioxide diffusion.

Adaptive changes developed for the fulfillment of photosynthetic functions have been directed both towards ensuring the optimal conditions for intense absorption of solar radiation and to protect cells from photooxidation caused by visible spectrum radiation and UV rays. Depending on environmental conditions, the cell size, the morphology of the assimilatory tissue, the content and ratio of basic pigm-entsn (chlorophylls and carotenoids) change, allowing photosynthesis to proceed in conditions of both strong radiation (for some desert species), and low light (for tropical species living in the shade).

In some species, the cells of the superior epidermis of the leaves, can focus light due to their shape increasing its intensity by 15–20 times. Leaves of the plants from sunny zones, have a small area, are thick, have a larger number of stomata and long palisade cells with chloroplasts containing less chlo-rophyll, but assimilating carbon more efficiently.

Another measure to protect the cellular structures from optical radiation consists in the synthesis of auxiliary pigments with photoprotective properties. Such substances are anthocyanins, present in higher concentrations in young and senile plants; they are often formed as a result of plant response to a high intensity of the visible light, to ultraviolet radiation, to low or high temperatures and to other stress factors.

These red pigments are located in the cells of the superior epidermis and provide an effective screening in the green region of the spectrum in which the leaves are mostly “transparent”. On the action of UV radiation, the synthesis of several phenolic compounds is induced. They are accumulating in the cuticle and epidermal cells, ensuring UV absorption and tissue protection from its damaging effect.

Other compounds playing the role of photoprotection in foliar tissues are carotenoids, which ensure, at low concentrations, a strong absorption in the indigo-blue region of the spectrum blocking photo-destructive processes. Their synthesis is activated before the period of vegetative pause in deciduous trees and before drought in tropical species—a period associated with the destruction of the ph-otosynthetic apparatus and exposure to photooxidative stress determined mostly by the high intensity of the solar light.