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).
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
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 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 osteole 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 (hydroactive 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 centre 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 (Fig. 4.20).
Chromoproteids of the antenna-complexes have 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 in 3 main stages: carboxylation, reduction and
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 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 instable 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 tones or approximately 20 kg per human being.Fig. General scheme of photosynthetic assimilation of carbon dioxide.
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 triosephosphates (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 phosphoglyceric aldehyde in the presence of NADP+H+ and with participation of phosphoglyceraldehyde dehydrogenase.Fig. Identiﬁcation of the large (a) and small (b) subunits of the enzyme ribulose-1,5- diphosphate carboxylase with the indication of the amino acid sequence. M Protein markers with known molecular weights; [1–3] proteins extracted from leaves of different genotypes of sunﬂower; [4, 5] proteins extracted from calathidia of different genotypes of sunﬂower.
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 atom: trioses, tetroses, hexoses, 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, of some amino acids etc. Molecules of phosphoglyceric aldehyde can pass through the membranes of the chloroplast and reach the cytoplasm where hexoses are synthesized (Fig.).
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 out 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 areFig. General scheme of type C3 photosynthesis
Fig. C4 leaf anatomy and C4 photosynthetic mechanism.
formed as a result of CO2 reduction, consist 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 the tropical areas, which are adapted to conditions of intense illumination and high temperature.
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 CO2 ﬁxation role, while the cells of the perivascular sheath are reach 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 of, 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+ formed during the light phase and of 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 phos-
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, uring 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 a 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 unnec-ssary consumption of organic substances is reduced and the productivity of the plants is increased.