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1.1GENERAL DESCRIPTION OF PHOTOSYNTHESIS
Photosynthesis is the most vital bioenergy-generating process, without which life on earth and the existence of our biosphere would be impossible. Sunlight enables photosynthesizing organisms to transform CO2 and water and produce sugars and other organic compounds. Photosynthesis occurs in plants, algae, and some bacteria, including cyanobacteria, formerly called blue-green algae (for background information, see Rabinowitch and Govindjee 1969). In plants and algae, this process takes place in chloroplasts, which are filled with thylakoids. Thylakoid membranes are where the light phase of photosynthesis occurs, while the stroma contains enzymes essential for the dark phase of photosynthesis (the Calvin–Benson cycle) (Figure 1.1).
This multi-stage and extremely complicated process leads to the conversion of the energy of absorbed photons into stable chemical energy of organic compounds. In the most general terms, photosynthesis can be described with the following simplified formula:
The so-called “light phase” of photosynthesis consists of three major subphases:
1.Absorption of light and transfer of excitation energy within the pigment antenna, followed by its trapping at the reaction centers (excitation of the reaction center chlorophyll).
2.Transport of electrons: The primary event in the reaction centers involves the transfer of an excited electron in a chlorophyll molecule to an intermediate acceptor, that is, pheophytin (Pheo) (in Photosystem II [PSII]) or chlorophyll (in Photosystem I [PSI]).
3.Stabilization of the energy of electrons during oxidation–reduction reactions (the photosynthetic transport of electrons) during the generation of ATP and the formation of the reducing power in the form of NADPH.
The absorbed photosynthetically active radiation (PAR) is used for the synthesis of adenosine triphosphate (ATP) and Nicotinamide adenine dinucleotide phosphate (NADPH), as noted in Subphase 3. These two products, obtained in the light phase, are used (for the synthesis of sugars from CO2 and H2O) during the dark phase, also known as the biochemical phase, which includes the Calvin–Benson cycle (see Govindjee 2010 for Benson’s contributions).
As implied above, during the biochemical phase, CO2 is bound and reduced to the level of sugars with the involvement of ATP and NADPH generated during the previous light phase. The key reaction during CO2 binding to a five-carbon compound is catalyzed by the Rubisco enzyme (ribulose-1,5-biphosphate carboxylase/oxygenase) located in the stroma. Rubisco acts very slowly; it takes almost 1 s for one molecule of the enzyme to bind three substrate molecules, which is why plants need large amounts of this enzyme. The total amount of Rubisco makes up around 50% of all proteins present in the chloroplast (see Blankenship 2014). In the carboxylation process, a CO2 molecule is bound to ribulose-1,5-biphosphate (a five-carbon molecule) to form a transient six-carbon compound, but this is then quickly hydrolyzed to produce two three-carbon molecules of 3-phosphoglyceric acid (PGA). Next, PGA is reduced to the level of three-carbon sugar(s), 3-phosphoglyceraldehyde and dihydroxyacetone phosphate, and it is at this stage that NADPH and ATP are used. An accompanying process involves the regeneration of ribulose-1,5-biphosphate (as a preparatory stage for the binding of another CO2 molecule). During these three steps (carboxylation, reduction, and regeneration), three carbon dioxide molecules and three molecules of ribulose-1,5-biphosphate produce a net yield of one molecule of 3-phosphoglyceraldehyde (a triose), while three molecules of ribulose-1,5-biphosphate are regenerated. The entire cycle is known as the Calvin–Benson cycle (named after its discoverers, Melvin Calvin and Andrew A. Benson; see Govindjee 2010). Sugars produced during this cycle are used in the synthesis of sucrose, starch, and many other organic compounds. In plants, the first products of the CO2 incorporation in the Calvin–Benson cycle are three-carbon compounds (PGA), which is why the cycle is also called the C-3 pathway, and plants in which it takes place are called C3 plants (Berg et al. 2005). Most plants grown in the moderate climate zone belong to this group.
FIGURE 1.1Two phases of photosynthesis and their mutual dependence in a chloroplast. The light-dependent phase (light phase) occurs in thylakoids and generates assimilatory power (NADPH and ATP), which is used in the dark phase (Calvin–Benson cycle). The dark phase occurs in the stroma, where CO2 is bound and reduced to sugars (organic compounds). (Adapted from Kalaji, M.H. and Łoboda, T., Chlorophyll Fluorescence to Study the Physiological Status of Plants [fluorescencja chlorofilu w badaniach stanu fizjologicznego roślin], Wydawnictwo SGGW [in Polish], Warsaw, 2009.)
Rubisco acts in two different ways in C3 plants: as carboxylase or as oxygenase, the latter catalyzing the binding of an oxygen molecule to ribulose-1,5-biphosphate. In the second case (oxygenation), molecules of phosphoglycolate and PGA are produced, and because one oxygen molecule is also bound during the process, the reaction is known as photorespiration (see Ogren 2005). The reaction causes depletion of net O2 and release of CO2. At 25˚C, and at an O2 concentration of 21% and a CO2 concentration of 0.037%, the Rubisco activity is nearly three times higher than its oxygenase activity (Berg et al. 2005).
Some warm climate plants have developed mechanisms that enable them to enhance CO2 concentration at the site where Rubisco acts, which markedly decreases photorespiration. In such plants, carbon dioxide, with the participation of phosphoenolpyruvate carboxylase (PEPC), becomes fixed to molecules of phosphoenol pyruvate (PEP) in mesophyll cells. This process yields a four-carbon compound, oxaloacetate, which is reduced to malate or aminated to aspartate, depending on the plant. The above process has been named the C4 pathway, and plants in which it occurs are called C4 plants (this pathway is also called the Hatch–Slack pathway after its main proponents; see Hatch 2005). C4 plants include maize, sorghum, sugarcane, and numerous weeds. Malate or aspartate molecules in C4 plants are transported to bundle sheath cells, where they are subsequently decarboxylated to three-carbon compounds (pyruvate or PEP), which involves CO2 release. Depending on the decarboxylating enzyme, C4 plants form malate or aspartate in bundle sheaths. Plants can be divided into three subtypes: NADP-ME, NAD-ME, and PEPCK (possessing NADP-dependent malate enzyme (ME), NAD-dependent malate enzyme (ME), or PEP carboxylase (CK), respectively).
The carbon dioxide released in bundle sheath cells is immediately fixed in the Calvin–Benson cycle inside these cells. In cell surroundings, CO2 concentration is low, because mesophyll cells effectively protect bundle sheath cells against air access. In this way, Rubisco of C4 plants attains high carboxylase activity with very low oxygenase activity. The elimination of photorespiration enhances the efficiency of the whole process of photosynthesis, which enables C4 plants to successfully compete with C3 plants (Edwards and Walker 1983).
To avoid high water loss, some plants growing in warm and dry climates, known as Crassulacean acid metabolism plants (CAM type; see Black and Osmond 2005), close the stomata during the day and open them at night. They fix carbon dioxide at night using PEP carboxylase and reduce the produced pyruvate to malate, which is then transported to and accumulated in vacuoles. Similarly to C4 plants, during the day, they decarboxylate malate and use the produced CO2 in the Calvin–Benson cycle (Ferreyra et al. 2003).
Chloroplasts are bounded by a double lipoprotein membrane (outer and inner membrane). Chloroplasts contain stroma, a protein matrix with embedded disc-shaped sacs known as thylakoids. Thylakoids are gathered in stacks referred to as grana. Chloroplasts...
