Photosynthesis Mechanism

According to the modern theory regarding the molecular mechanism of photosynthesis, this process is a chain of successive redox reactions, which requires sunlight at early stages (Robin Hill phase), while subsequent steps can occur in the dark (F.F. Blackman phase) (Table 4.3).

Light phase

Dark phase

Different photochemical and photophysical processes, including water photolysis, take place.Enzymatic reactions take place
H2O, chlorophyll, and solar energy are usedCO2, ATP, and NADPH+H+ are used
O2 is formedCarbohydrates (CH2O)n are formed
Solar energy is included in ATP and NADPH+H+The energy of ATP and NADPH+H+ is
included in organic substances
The temperature has no influence on light
Temperature influences enzymatic reactions. A 10 °C increase in temperature leads to a 2–3 times increase in reaction rates
Reactions depend on the amount and
the intensity of light (Emerson effect)
Enzymatic reactions don’t depend on light intensity
Reactions take place in grana thylakoidsReactions take place in the chloroplast
Is common for all speciesThe mechanism differs from one plant
species to another (C3, C4, CAM-

In the light phase of photosynthesis absorption of light occurs by chlorophyll molecules “a” with the participation of auxiliary pigments (chlorophyll “b”, carotenoids, phycobilins) and transformation of solar energy into ATP and NADPH +H+.

All these processes are carried out in photochemically active chloroplasts membranes, and represent a complex system of photophysical, photochemical and chemical reactions.

Photosynthesis Mechanism
Fig: Photosynthesis Mechanism

In the dark phase of photosynthesis carbon fixation by the primary acceptor (ribulose-1,5-diphosphate) happens, involving enzymes located in the chloroplast stroma and with energy consumption in the form of ATP and NADPH+H+ which are the final products of the light phase.

Light Phase of Photosynthesis

The processes occurring during the light phase of photosynthesis can be related to:
(1) Absorption of carbon dioxide
(2) Absorption of solar energy and its transformation into chemical energy.

(1) Absorption of carbon dioxide from the external environment happens through the open osteole (photoactive physiological reaction).

Carbon dioxide enters the sub substomatal cavity, from where it diffuses through the free intercellular spaces to directly contact the cellulose membranes of palisade assimilatory parenchyma, situated on the upper side of the leaf blade, or the cells of the spongy parenchyma from the inferior side.

In the envelopes of assimilatory cells that are continuously irrigated with water absorbed from the soil, the CO2 from the air that circulates in the intercellular spaces, possessing a high hydrosolubility, dissolves and forms carbonic acid (H2CO3), which dissociates in H+, HCO3, CO32. In the ionic form carbon dioxide enters the cytoplasm and reaches chloroplasts.

Consequently, it results that the first condition of photosynthesis is the degree of ostiole opening and the presence of a sufficient amount of water in foliar tissues.

At night, when stomata are closed (photoactive closure) as well as in drought conditions (hydro active closure) when the cellular membranes of the leaf mesophyll cells are dry, photosynthesis is blocked and plant growth stagnates.

Absorption of solar energy and its transformation into chemical energy happens via several successive stages:

  • solar energy absorption and excitation energy migration to the system of pigments;
  • oxidation of the reaction center and stabilization of the separated charges;
  • electron transfer through the electron transport chain (ETC);
  • water photooxidation and molecular oxygen elimination;
  • conjugation of electron transport with proton transfer and the synthesis of ATP.

These processes are carried out in granal and stromal thylakoids with the participation of different molecules that make up two specific structures in superior plants—photosystem I (PS I) and photosystem II (PS II), which differ in their protein components, pigments, and optical properties.

Each photosystem is formed of a reaction center conjugated with electron donors and acceptors together with the “antenna” pigments .

Chromoproteids of the antenna complexes has no photochemical and enzymatic activity. Their role is reduced to the accumulation and transmission of energy quanta to a limited number of molecules, which carry out photochemical reactions.

The Dark Phase of Photosynthesis

There are different methods of reducing carbon dioxide: the Benson-Calvin cycle (C3), the Hatch-Slack-Karpilov cycle (C4), the metabolism of organic acids in Crassulaceae (CAM—“crassulacean acid metabolism”), and photorespiration (Fig).

The Benson-Calvin cycle (pentose-phosphate reduction pathway, photosynthetic type C3) is specific for a group of superior plants and includes a cycle of enzymatic reactions that can be grouped into 3 main stages: carboxylation, reduction and regeneration.

The carboxylation phase, the primary CO2 acceptor is a compound with 5 carbon atoms, ribulose-1,5-diphosphate, which forms as a result of secondary phosphorylation of ribulose-5-phosphate with the participation of ATP and of ribulose phosphate In kinase.

Under the action of ribulose phosphate carboxylase/oxygenase (RUBISCO), ribulose-1,5-diphosphate attaches a molecule of CO2 to the second carbon atom and one molecule of water forming an unstable compound with 6 atoms of carbon, which splits into 2 molecules of 3-phosphoglyceric acid. RUBISCO, the key enzyme of the photosynthesis processes is the most widely spread enzyme on earth.

It is considered that the general quantity of this enzyme is 10 million tonnes or approximately 20 kg per human being.

Fig. General scheme of photosynthetic assimilation of carbon dioxide. Source Link

t is a hydro-soluble, complex enzyme, with a general molecular mass of 500,000 Da, which is formed of eight big and eight small protein subunits (Fig.). Due to the fact that the outcome of this reaction is a molecule with 3 carbon atoms, the cycle is also called C3.

The reduction phase. The 3-phosphoglyceric acid has an energetic level lower than that of carbohydrates and the reduction of this compound to the level of triose phosphates (carbohydrates with 3 atoms of carbon) can happen only when using the energy of ATP and NADPH+H+ (energy that is called assimilation factor).

The phosphoglyceric acid is reduced to phosphoglyceric aldehyde by two reactions. Phosphorylation of 3-phosphoglyceric acid to obtain 1,3 phosphoglyceric acid is performed in the presence of ATP and is carried by phosphoglycerate kinase.

In the second reaction, the latter is reduced to the phosphoglyceric aldehyde in the presence of NADP+H+ and with the participation of phosphoglyceraldehyde dehydrogenase.

The phosphoglyceric aldehyde, under the action of triosephosphate isomerase, is transformed into ph-osphodioxiacetone by isomerization.

This enzymatic phase is the only Calvin cycle reduction step in which the NADP +H+ accumulated in photochemical reactions is used while the ATP was used as an additional energy for producing these reactions.

The regeneration phase. During this phase, the acceptor-ribulose-1,5-diphosphate is regenerated in a cycle of reactions of reciprocal transformation of the carbohydrates with a different number of carbon atoms: trioses, tetroses, hexoses, and sedoheptuloses.

Transketolases and transaldolases participate in such reactions by catalyzing the transfer of fragments made of two carbon atoms (–CO–CH2–OH) and three carbon atoms (–CHOH–CO–CH2–OH) respectively from ketoses to aldoses and isomerases.

When three molecules of CO2 are absorbed, six molecules of reduced phopsphotrioses are formed. Five of these molecules are used in the regeneration of ribulose 5-phosphate and one molecule is set free. During the reaction, three molecules of pentose phosphate are formed out of 5 molecules of triose phosphate.

After a second phosphorylation, in the presence of ATP, these molecules are transformed into ribulose diphosphate and can again perform the function of primary acceptor of carbon dioxide. In order to perform a cycle of reactions the presence of 3 molecules of ATP and 2 molecules of NADPH+H+ is necessary.

The additional phosphoglyceric aldehyde molecule which was not consumed can be used in chloroplasts for the biosynthesis of glucose, fructose, starch, some amino acids, etc. Molecules of phosphoglyceric aldehyde can pass through the membranes of the chloroplast and reach the cytoplasm where hexoses are synthesized .

Fructose-1,6-diphosphate is formed by means of aldolic condensation of phosphodioxyacetone and phosphoglyceric aldehyde. When two molecules of phosphoglyceric aldehyde are condensed glucose-1,6-diphosphate is formed.

Saccharose, which is the main form of carbohydrate transported in plants, consists of a molecule of glucose and one of fructose.

Glucose and fructose represent substrates for the process of respiration, while the resulting intermediary compounds are used to synthesize different organic substances. The Hatch-Slack-Karpilov cycle. At the beginning of the 1960s, it has been found that in certain plants of tropical or subtropical origin (maize, sugar cane, sorghum, millet, etc.) photosynthesis deviates from the basic cycle.

In this species, phosphoenolpyruvic acid serves as a CO2 acceptor. This acid contains a macroergic bond, due to which it has a high reactive capacity. The primary products that are formed as a result of CO2 reduction, consisting of 4 carbon atoms.

Due to this fact, this type of carbon assimilation is called the photosynthetic type C4. This photosynthetic type is common for more than 1000 species originating from tropical areas, which are adapted to conditions of intense illumination and high temperature.

Figure: A-schematic-diagram-of-C3-and-C4-photosynthesis Source : Researchgate

Plants in which photosynthesis takes place according to the C4 cycle, have leaves with a particular anatomic structure (Fig.). The cells of the palisade mesophyll have a small number of chloroplasts with a CO2 fixation role, while the cells of the perivascular sheath are reached in big chloroplasts and have the function of performing photosynthesis of the C4 type.

Fixation of the carbon dioxide takes place in the cytoplasm of mesophyll cells, through a carboxylation reaction of the phosphoenolpyruvate in the presence of phosphoenol-pyruvate carboxylase, resulting in a compound with 4 atoms of carbon—oxaloacetic acid.

In chloroplasts, oxaloacetic acid in the presence of NADP+H+ is formed during the light phase, and NADP-malate dehydrogenase is reduced to malic acid.

In the presence of NH4+ ions, the oxaloacetic acid can be aminated resulting in aspartic acid. The malate (or aspartate) is transported through the plasmodesms from the mesophyll cells to the cells of the perivascular sheath, which are permeable to organic acids and impermeable to CO2.

Here, it is decarboxylated, with the formation of pyruvate and CO2. The pyruvate from the perivascular sheath is transported back to the chloroplasts of mesophyll cells where it undergoes phosphorylation in the presence of ATP and phosphopyruvate synthase, thus regenerating the primary acceptor—the phosphoenolpyruvic acid.

In the chloroplasts of the perivascular sheath, PS II is weakly developed, in comparison to PS I. The ATP necessary to fix CO2 is synthesized as a result of the cyclic transport of electrons while NADP + H+ is formed as a product of the oxidative decarboxylation of the malate concomitantly with CO2.

Due to the fact that two types of cells with two types of chloroplasts participate in this mechanism of photosynthesis, this particular type of photosynthesis is seen as “cooperative photosynthesis” (Karpilov 1970).

Fixation of CO2 via C4 has some advantages:

The phosphoenolpyruvate carboxylase enzyme has a reaction speed higher than that of ribulose 1,5-diphosphate carboxylase and this fact determines the accumulation of carbon dioxide in the cells of the perivascular sheath

Some species can perform the first stages in which the organic acids are formed during the night while, during the day, CO2 is released with subsequent re-assimilation in the Calvin cycle. This fact allows plants to carry some carbon assimilation reactions during the day, even when the stomatal pores are closed (in the absence of exogenous CO2), thus, avoiding strong water elimination.

It is considered that such peculiarities form the basis of higher drought resistance of this group of plants;
Carbon dioxide accumulation in the cells of the perivascular sheath, where the C4 type photosynthetic reaction takes place determines the stimulation of this process and concomitantly blocks the oxidase activity of ribulose 1,5-diphosphate carboxylase and, respectively, photorespiration.

Thus, the unnecessary consumption of organic substances is reduced and the productivity of the plants is increased.

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