Study behavioral objective 40 and read the pages indicated in the text by E-22. 
 

The Calvin cycle is not the only carbon-fixing pathway used in the light-independent reactions. In some plants, the first product of C02 fixation to be detected is not the three-carbon molecule PGA, but rather the four-carbon molecule oxaloacetate (which is also an intermediate in the Krebs cycle). Plants that employ this pathway (along with the Calvin cycle) are commonly called C4 plants (four-carbon), as distinct from the C3 plants, which use only the Calvin cycle. (The C4 pathway is also referred to as the Hatch-Slack pathway after two Australian plant physiologists who played key roles in its elucidation.) 

The oxaloacetate is formed when carbon dioxide is fixed to phosphoenolpyruvate (PEP) in a reaction catalyzed by the enzyme PEP carboxylase, which is found in the ground substance of the C4 plant cell. The oxaloacetate is then reduced to malate or converted, with the addition of an amino group, to the amino acid aspartate in the chloroplast of the same cell. These steps occur in mesophyll cells. The next step is a surprise: the malate (or aspartate, depending on the species) moves from the mesophyll cells to bundle sheath cells surrounding the vascular bundles of the leaf, where it is decarboxylated to yield CO2 and pyruvate. The CO2 then enters the Calvin cycle and reacts with RuBP to form PGA and other intermediates of the cycle. Meanwhile, the pyruvate returns to the mesophyll cells, where it reacts with ATP to form more molecules of PEP. Hence, the anatomy of the leaves of C4 plants imparts a spatial separation between the C4 pathway and the Calvin cycle. 

The two primary carboxylating enzymes use different forms of the carbon dioxide molecule as a substrate. RuBP carboxylase uses CO2, whereas PEP carboxylase uses the hydrated form of carbon dioxide, the bicarbonate ion (HCO3^-), as its substrate. RuBP carboxylase is found in the chloroplast, whereas PEP carboxylase occurs in the cytoplasmic ground substance. 

Typically, the leaves of C4 plants are characterized by an orderly arrangement of the mesophyll cells around a layer of large bundle-sheath cells, so that together the two form concentric layers around the vascular bundle. This wreathlike arrangement has been termed Kranz anatomy. (Kranz is the German word for "wreath.") In some C4 plants, the chloroplasts of the mesophyll cells have well-developed grana while those of the bundle-sheath cells have either poorly developed grana or none at all. In addition, when photosynthesis is occurring, the bundle-sheath chloroplasts commonly form larger and more numerous starch grains than the mesophyll chloroplasts. 

Fixation of CO2 in C4 plants has a larger energy cost than in C3 plants. For each molecule of CO2 fixed in the C4 pathway, a molecule of PEP must be regenerated at the cost of two phosphate groups of ATP. Also, C4 plants need five ATPs altogether to fix one molecule of CO2, whereas C3 plants need only three. One might well ask why C4 plants should have evolved such a seemingly clumsy and energetically expensive method of providing carbon dioxide to the Calvin cycle. 

Photosynthesis in C3 plants is always accompanied by in the presence of light. Photorespiration is a wasteful process. Unlike mitochondrial respiration, photorespiration is not accompanied by oxidative phosphorylation; hence, it yields no ATP. In addition, photorespiration diverts some of the reducing power generated in the light-dependent reactions from the biosynthesis of glucose into the reduction of oxygen. Under normal atmospheric conditions, as much as 50 percent of the carbon fixed in photosynthesis by a C3 plant may be reoxidized to CO2 during photorespiration. Photorespiration is very active in C3 plants, seriously limiting their efficiency, but it is nearly absent in C4 plants. 

The major substrate oxidized by photorespiration in C3 plants is glycolate. Glycolate is oxidized in the peroxisomes of photosynthetic cells and is formed by the oxidative breakdown of RuBP by RuBP carboxylase, the identical enzyme that fixes CO2 into PGA in the C3 pathway. How is this possible? 

RuBP carboxylase can promote the reaction of RuBP with either CO2 or O2. In fact, the full name of the enzyme is RuBP carboxylase/oxygenase, in recognition of this dual activity. When the CO2 concentration is high and that of O2 is relatively low, RuBP carboxylase fixes CO2 to RuBP to yield PGA. When the CO2 concentration is low and that of O2 is relatively high, the enzyme also acts as an oxygenase, combining RuBP and oxygen to yield phosphoglycolate and PGA, instead of the two PGA molecules normally formed in the carboxylation. The phosphoglycolate is then converted to glycolate-the substrate oxidized during photorespiration. 

High CO2 and low O2 concentrations limit photorespiration. Consequently, C4 plants have a distinct advantage over C3 plants because CO2 fixed by the C4 pathway is essentially "pumped" from the mesophyll cells into the bundle-sheath cells, thus maintaining a high CO2:O2 ratio at the site of the action of RuBP carboxylase. This high CO2:O2 ratio favors the carboxylation of RuBP. In addition, since both the Calvin cycle and photorespiration are localized in the inner, bundle-sheath layer of cells, any CO2 liberated by photorespiration into the outer, mesophyll layer can be refixed by the C4 pathway that operates there. The CO2 liberated by photorespiration can thus be prevented from escaping from the leaf. Moreover, compared with C3 plants, C4 plants are superior utilizers of available CO2, this is in part due to the fact that the enzyme PEP carboxylase is not inhibited by O2. As a result, the net photosynthetic rates (that is, total photosynthetic rate minus photorespiratory loss) of C4 grasses, such as corn (Zea mays), sugarcane (Saccharum officinale), and sorghum (Sorghum vulgare), can be two to three times the rates of C3 grasses, such as wheat (Triticum aestivum), rye (Secale cereals), oats (Avena sativa), and rice (Oryza sativa) under the same environmental conditions. 

Because C4 plants evolved primarily in the tropics, they are especially well adapted to high light intensities, high temperatures, and dryness. The optimal temperature range for C4 photosynthesis is much higher than that for C3 photosynthesis, and C4 plants flourish even at temperatures that would eventually be lethal to many C3 species. Because of their more efficient use of carbon dioxide, C4 plants can attain the same photosynthetic rate as C3 plants but with smaller stomatal openings and, hence, with considerably less water loss. Analyses of the geographic distribution of C4 species in North America have shown that they are generally most abundant in climates with high temperatures. Differences appear to exist, however, between monocots and dicots in the particular type of high temperature favored. For example, C4 grasses are most abundant in regions having the highest temperatures during the growing season; in contrast, C4 dicots are most abundant in regions having the greatest aridity during the growing season. 

A striking illustration of different growth patterns in C4 plants is found in our lawns, which, in the cooler parts of the country at least, consist mainly of C3 grasses, such as Kentucky bluegrass (Poa pratensis) and creeping bent (Agrostis tenuis). Crabgrass (Digitaria sanguinalis), which all too often overwhelms these dark green, fine-leaved grasses with patches of its yellowish green, broader leaves, is a C4 grass that grows much more rapidly in the heat of summer than the temperate C3 grasses mentioned above. 

All of the plants now known, to utilize C4 photosynthesis are flowering plants, including at least 19 families, 3 of monocots and 16 of dicots; however, no family has been found that contains only C4 species. This pathway has undoubtedly arisen independently many times in the course of evolution. 

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