While there are several factors that affect photosynthetic rate, the three main factors are light intensity, carbon dioxide level, and temperature. The net efficiency of photosynthesis is estimated by the net growth of biosynthesis and the amount used for respiration. The requirements for achieving high energy conversion are optimal temperature, light, nutri­tion, leaf canopy, absence of photorespiration, and so forth. Many plant species can be distinguished by the type of photosynthetic pathway they utilize. Most plants utilize the C3 photosynthesis route. C3 determines the mass of carbon present in the plant material. Poplar, willow, wheat, and most cereals are C3 plant species. Plants such as perennial grass, Miscanthus, sweet sorghum, maize, and artichoke all use the C4 route of photosynthesis and accumulate significantly greater dry mass of carbon than the C3 plants. Advances in crop production, agricultural techniques, and so forth have led to potential applications in low-cost bio­mass production with high conversion efficiencies. Further, introduction of alternative nonfood crops on surplus land and the use of biomass as a sustainable and environmentally safe alternative make biomass an attractive renewable energy resource. The potential of biomass energy derived from forest and agricultural residues worldwide is estimated at about 30 EJ/yr. For the adoption of biomass as a renewable energy source, the cultivation of energy crops using fallow and marginal land and efficient processing methods are vital [3].

C3 metabolism in plants and the pentose phosphate pathway. In C3 plants, the pathway for reduction of carbon dioxide to sugar involves the reduc­tive pentose phosphate cycle. This involves addition of CO2 to the pentose bisphosphate, ribulose-1,5-bisphosphate (RuBP). The enzyme-bound carboxylation product is hydrolytically split, through an internal oxidation — reduction process, into two identical molecules of 3-PGA. An acyl phos­phate of this acid is formed by reaction with ATP. This is further reduced with NADPH. Five molecules of the resulting triose phosphate are con­verted into three molecules of the pentose phosphate, ribulose 5- phosphate. Three molecules of ribulose 5-phosphate are converted with ATP to give the carbon dioxide acceptor, RuBP, thereby completing the cycle. When these three RuBP molecules are carboxylated and split into six PGA molecules and these are reduced to triose phosphate, there is a net gain of one triose phosphate molecule over the five needed to regenerate the carbon dioxide acceptor. Triose phosphate is formed in this cycle and can either be converted into starch for storage of energy inside the chloroplast, or it can serve its primary function by being transported out of the chloroplast for subsequent biosynthetic reactions. In a mature leaf, sucrose is synthesized and exported to the rest of the plant, thus providing energy and reduced carbon for growth [4]. Wheat, potato, rice, and barley are examples of C3 plants. A representative C3 cycle is shown in Fig. 2.3.

C4 metabolism in plants. In air that contains low carbon dioxide in rela­tion to oxygen, oxygen competes for the carbon dioxide binding site of the ribulose bisphosphate carboxylase. This is known to set off a process of photorespiration in plants, and it is believed that the C4 plants have evolved from such a mechanism. Such plants possess a specialized leaf morphology called “Krantz anatomy” and a special additional CO2 trans­port mechanism. This typically overcomes the problem of photorespi­ration. Such avoidance of photorespiration is known to result in higher growth rates. The Krantz anatomy is characterized by the fact that the vascular system of the leaves is surrounded by a vascular bundle, or bundle-sheath cells, which contain enzymes of the reductive pentose phosphate cycle. The reduction of CO2 is similar to that of C3 plants, except that the CO2 for carboxylation of CO2 is derived not from the stomata but is released in bundle-sheath cells by decarboxylation of a four-carbon acid (C4 acid). This C4 acid is supplied by the mesophyll cells that surround the bundle sheath cells. The C4 pathway for the transport of CO2 starts in a mesophyll cell with the condensation of CO2 and phosphoenolpyruvate to form oxaloacetate, in a reaction catalyzed by phosphoenolpyruvate carboxylase (PEPCase), and the reduction of oxaloacetate to malate [5]. Figure 2.4 shows the C4 cycle of CO2 fixation in photosynthesis.

Due to the elimination of the photorespiration process, C4 plants are proposed to be ideal for increased biomass production especially in mar­ginal conditions. Grasses are suitable for this purpose as they can be

grown on a repetitive cropping mode for continuous and maximum production of biomass. Grasses such as Bermuda grass, Sudan grass, sugarcane, and sorghum are good candidates for energy generation from biomass. A comparison of the characteristics of C3 and C4 plants, in terms of leaf anatomy, is shown in Table 2.1.