Photosynthesis, Carbon Fixation and
Photosynthesis, Carbon Fixation and
Photosynthesis takes place in subcellular membrane-bound compartments called chloroplasts . As radiotracers such as carbon-14 became available to researchers following World War II (1939-45), one application was to define the biochemistry of photosynthetic CO2 fixation. Major class divisions in the plant kingdom are based on how CO2 is fixed.
Many important biological processes are sustained by cycles that continuously consume and renew one or more key intermediates while producing some other major product. Photosynthesis is sustained by the Calvin-Benson cycle.
The C3 photosynthetic mechanism is so named because the carbon atom of a molecule of CO2 taken up by an illuminated leaf is first detected in the three-carbon compound 3-phosphoglyceric acid (PGA). The vast majority of higher plants and algae are C3 species. PGA is formed when CO2 combines with a 5-carbon sugar, ribulose biphosphate (RuBP). The reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, an abundant protein in all green tissues. This multifunctional enzyme has come to be called rubisco.
During each turn of the Calvin-Benson cycle, two molecules of PGA (a total of six carbon atoms) undergo a complex series of enzyme-catalyzed transformations in which the carbon atoms pass through metabolite pools consisting of three-, four-, five-, six-, and seven-carbon sugar phosphates. These reactions regenerate RuBP, which then combines with CO2 to form two PGAs and complete the cycle. So, of the six (2 H 3) original carbon atoms in PGA, five give rise to RuBP and the one remaining appears as one of the six carbon atoms in the sugar glucose-6-phosphate (G6P). Therefore, for every six CO2 molecules fixed, one G6P leaves the Calvin-Benson cycle for synthesis of starch, sucrose, cellulose, and ultimately all of the organic constituents of the plant.
In terms of pure chemistry, the conversion of CO2 to carbohydrate is an example of reduction, in which a source of energy-rich electrons is required. As the term photosynthesis suggests, the energy for the reductive reactions of the Calvin-Benson cycle comes from visible light. An extensive membrane system in the chloroplast harbors the pigments (chlorophylls and carotenoids) that transfer light packets (quanta) to specialized pigment-protein sites where they energize individual electrons extracted from molecules of water (H2 O). The oxygen atoms in the water are released as O2. Each high-energy electron consumes the energy of two quanta. Two electrons are used to convert a compound called nicotinamide adenosine dinucleotide phosphate from its oxidized form (NADP+) to its reduced form (NADPH). The sequence of electron transport from H2 O to NADP+ also fuels the phosphorylation of adenosine diphosphate (ADP) to high-energy adenosine triphosphate (ATP ). Both NADPH and ATP interact directly with the enzymes of the Calvin-Benson cycle during fixation of CO2. Two molecules of NADPH and three molecules of ATP are required to fix each molecule of CO2 during C3 photosynthesis.
C3 plants also engage in an active CO2-releasing process called photorespiration that operates concurrently with normal photosyn-thesis in the light. Photorespiration drains away useful energy, and is thus a wasteful process. Since the CO2 formed by photorespiration is rapidly re-fixed by photosynthesis it is difficult to measure directly, and its existence was not suspected until the 1950s. Since then, biochemists and physiologists have elucidated the mechanism, but have not come to agreement on its purpose, if any. It is important to note that photorespiration is not the same as the ubiquitous respiratory CO2 released from mitochondria in all eukaryotic cells, including animal and plant tissues.
Photorespiration starts with the formation of a two-carbon phosphoglycolic acid molecule during photosynthesis. Since this is a potent inhibitor of the Calvin-Benson cycle, its metabolism to nontoxic derivatives is essential. First, the phosphate group is removed (by action of an enzyme called a phosphatase) to yield glycolate. The following series of conversions: 2 glycolate → 2 glyoxylate → 2 glycine → serine → hydroxypyruvate → PGA results in the formation of a Calvin-Benson cycle intermediate (PGA) that is used to make RuBP. Notice that the four glycolate carbon atoms ultimately appear as one molecule of PGA (three carbon atoms). The fourth atom of carbon is released as CO2 during the glycine to serine conversion, and this is the source of CO2 released in photorespiration. Additional photosynthetic energy (i.e., NADPH and ATP) is consumed during metabolism of photorespiratory PGA, refixation of CO2, and reassimilation of ammonia released during the glycine → serine step. Hence, photorespiration drains energy away from productive photosynthesis.
Photorespiration can be observed by a number of means. When a stream of CO2-free air is passed over a C3 leaf, release of CO2 by the leaf results in an elevated concentration of this gas in the downstream flow. This release rate is highly dependent upon illumination of the leaf and will be depressed severalfold by darkening. Another method relies on the fact that the rate of photosynthetic fixation of CO2 is directly dependent on the concentration of CO2 at low levels of this component. Hence, sealing a leaf in a small transparent vessel under illumination will cause the concentration of CO2 inside to fall until the rate of uptake equals the rate of evolution due to photorespiration. The final equilibrium concentration of CO2 (called the CO2 compensation point) is highly dependent on the concentration of O2 in the gas and is commonly employed as a robust, although indirect, measure of photorespiration. But the most direct indicator of photorespiration is based on comparison of rates of CO2 uptake at high and low levels of O2 in the surrounding atmosphere. Lowering the O2 concentration from the normal 21 percent to 1 to 2 percent can result in an instantaneous 30 percent increase in photosynthetic rate (see below). This response of photosynthesis to O2 is attributed to photorespiration and is called the Warburg Effect for its discoverer Otto Warburg.
Although the source of phosphoglycolic acid for photorespiration was for some time a controversial subject, it is now widely accepted that it originates at the site of CO2 fixation. Specifically, when RuBP binds to rubisco its structure is perturbed, rendering it vulnerable to attack by either CO2 or O2. Reaction of RuBP with CO2 yields two PGAs while reaction with O2 results in formation of one PGA molecule and one phosphoglycolic acid molecule. The probability that a bound RuBP will react with either CO2 or O2 is governed by the relative concentrations of these gases in the aqueous environment of the chloroplast. Hence, CO2 and O2 are considered to compete for the bound RuBP. This competition accounts for the increase in photorespiration at high O2 concentration, and the fact that photorespiration can be almost completely suppressed by high concentrations of CO2 even in the presence of O2. Measurements with purified rubisco in the laboratory indicate that the rate of photorespiratory release of CO2 is about 20 percent of the total rate of CO2 uptake for a healthy C3 leaf in air at 25°C. Photorespiration increases considerably with temperature, however. Photorespiration is most significant when temperatures are high and plants must close stomata to prevent water loss. Without access to fresh CO2 from the atmosphere, photorespiration becomes the major reaction catalyzed by rubisco.
The role of photorespiration in plant metabolism is the subject of debate. It has been suggested to be a means of disposal of excess photosynthetic energy. Also, it may provide a way to protect the leaf from damaging effects of light that could occur if CO2 levels inside the leaf were to fall below some critical threshold. Still, there may be no essential role for photorespiration. It is probably an anomaly of the rubisco mechanism that appeared on this planet before O2 was present in the atmosphere. Later, as O2 levels in the atmosphere rose due to photosynthesis, this vulnerability to O2 affected photosynthesis and growth. Interestingly, some plants have evolved means to suppress photorespiration while retaining rubisco and the Calvin-Benson cycle.
Familiar species possessing the C4 photosynthesis mechanism are maize, sorghum, sugarcane, and several common weeds. The defining feature of CO2 fixation in this case is involvement of two distinct cell types that shuttle metabolites back and forth to complete a modified photosynthetic cycle. Microscopic examination of leaf sections reveals two chloroplast-containing cell types in an arrangement termed Kranz anatomy. Bundle sheath cells form a cylindrical layer one cell deep around each leaf vein. These cells are typically enlarged, thick walled, and densely packed with chloroplasts. At least two layers of loosely packed mesophyll cells separate adjacent bundle sheath strands. Although mesophyll cells resemble those observed in C3 leaves, they function much differently.
When CO2 enters the leaf it is first fixed in the mesophyll cells by the enzyme phosphoenolpyruvate (PEP) carboxylase. The carbon atom from CO2 is first detected in the four-carbon organic acid oxaloacetic acid (OAA), hence the name C4 photosynthesis. The OAA is then reduced to malic acid or converted to the amino acid aspartic acid depending on species. Malate and aspartate are transported to bundle sheath cells where they are decarboxylated, thereby releasing CO2. This newly formed CO2 is refixed by rubisco and metabolized by the Calvin-Benson cycle present in the bundle sheath chloroplasts. The remaining three carbon atoms derived from the malate and aspartate are transported back to the mesophyll cells as pyruvic acid to regenerate the three-carbon PEP.
The characteristic carboxylation/decarboxylation sequence of C4 photosynthesis pumps CO2 from mesophyll to bundle sheath cells, thereby accomplishing one desirable end. The concentration of CO2 in bundle sheath cells of C4 plants is severalfold higher than in leaf cells of C3 species. Hence, photorespiration is virtually absent in C4 leaves. Since none of the other enzyme-catalyzed reactions is sensitive to O2, the Warburg effect is not observed and the CO2 compensation point (a reliable indicator of photorespiratory capacity, see above) is very low for C4 leaves. Somewhat more light energy is required to fix each molecule of CO2 using the C4 pathway since PEP regeneration requires ATP. Although 2 NADPH are consumed as in C3 plants, the ATP requirement for C4 photosynthesis is four to five per CO2 fixed.
Crassulacean Acid Metabolism (CAM).
The crassulacean acid metabolism (CAM) mode of photosynthesis was discovered first in plants of the family Crassulaceae but familiar species include pineapple and cacti. It is considered an adaptation to life in arid environments. CAM photosynthesis resembles C4 photosynthesis in terms of the pathway of fixation of carbon. The prominent difference, however, is that CAM plants take up CO2 from the atmosphere at night and synthesize malic acid via PEP carboxylase. During the daytime the leaf pores (stomata) that admit CO2 close to conserve water. Malic acid is decarboxylated and the CO2 is refixed by the Calvin-Benson cycle. Some of the starch accumulated during daytime is converted to PEP at night to support CO2 fixation. Also, unlike C4 photosyn-thesis, all of the CAM reactions take place in each leaf cell.
Significance of Carbon Fixation Reactions
The choice of CO2 fixation pathway has profound implications for how a plant responds to the innumerable combinations of light intensity, leaf internal CO2 concentration, temperature, and water status in the natural environment. As discussed above, at normal atmospheric CO2 levels photosynthesis is lower by at least 25 percent in C3 plants than it would be if photorespiration were absent. The generally higher rates of photosynthesis in C4 plants are attributable to both suppression of photorespiration in these species and the superior ability of PEP carboxylase to fix CO2 at the very low concentrations of this gas that can occur inside leaf tissue. These differences are most pronounced at high light intensity. Photosynthesis in C3 leaves attain maximal rates at light levels of about 50 percent of full sunlight. However, C4 photosynthesis continues to increase with light intensity even in full sunlight. It is little wonder that the highest yielding crop species use the C4 mechanism. Conversely, C3 plants are capable of more efficient use of light quanta when light levels are low, as would be the case for shaded conditions. Also, high temperatures favor C4 plants because the number of molecules of H2 O lost to evaporation via the stomata (transpi-ration) per CO2 fixed is much lower for these species compared to C3 species. However, C3 plants tend to be more competitive in cool environments. Finally, although projected increases in global atmospheric CO2 levels during the twenty-first century should enhance photosynthesis in all species, associated changes in distribution of temperature and rainfall will also exert great influence on the composition and characteristics of Earth's flora.
see also Atmosphere and Plants; Cactus; Calvin, Melvin; Chloroplast; de Saussure, Nicholas; Ingenhousz, Jan; Photosynthesis, Light Reactions and.
Richard B. Peterson
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