The discussion of photosynthesis to this point has concentrated more on the light reactions that occur in photosynthesis. The organic components in bio­mass are formed during the dark reactions. Some discussion of the biochemical pathways and organic intermediates involved in the reduction of C02 to sugars is beneficial because they play a significant role in our understanding of the molecular events of biomass growth, and in differentiating between the various kinds of biomass.

Before discussion of these pathways, it is important to note that the photo­synthetic pathways also involve several dark reactions that occur in the glycoly­sis of glucose. The metabolic pathways provide energy for cellular maintenance and growth, form 3-phosphoglyceric acid and 3-phosphoglyceraldehyde, from which almost all other cellular organic components are synthesized, and are virtually identical in a large variety of living organisms, for example, in corn, wheat, oats, legumes, algae, many bacteria, the muscle, brain, liver, and other organs of humans and animals, and many birds, insects, and reptiles. Several reactions in the dark reactions on C02 uptake are the same as those that occur in the metabolism of foodstuffs by these organisms. Seven reactions of photosynthesis are common to the Embden-Meyerhof metabolic pathway, and three reactions are common to the pentose phosphate metabolic pathway. Each pathway converts glucose to pyruvic acid.

In photosynthesis, C02 generally enters the leaves or stems of biomass through the stoma, the small intercellular openings in the epidermis. These openings provide the main route for both photosynthetic gas exchange and for water vapor loss in transpiration. At least three different biochemical path­ways can occur during C02 reduction to sugars (Rabinovitch, 1956; Loomis et ah, 1971; Osmond, 1978).

One pathway is called the Calvin or Calvin-Benson cycle and involves the three-carbon intermediate 3-phosphoglyceric acid. This cycle, which is sometimes referred to as the reductive pentose phosphate cycle, is used by autotrophic photochemolithotrophic bacteria, algae, and green plants. As shown in Fig. 3.2, ribulose-1,5-diphosphate (1) and C02 react to form 3- phosphoglyceric acid (11), which in turn is converted via 1,3-diphosphoglyceric acid (111) and 3-phosphoglyceraldehyde (IV) to glucose (V) and ribulose-5- phosphate (VI), from which I is regenerated. For every 6 molecules of C02 converted to 1 molecule of glucose in a dark reaction, 18 ATP, 12 NADPH, and 24 Fd molecules are required. Twelve molecules of II are formed in the chloroplasts from 6 molecules each of C02 and I. After these carboxylation reactions, a reductive phase occurs in which 12 molecules of II are successively transformed into 12 molecules of III and 12 molecules of IV, a triose phosphate. Ten molecules of the triose phosphate are then used to regenerate 6 molecules of I, which initiates the cycle again. The other 2 triose phosphate molecules are used to generate glucose.

Plant biomass species that use the Calvin-Benson cycle are called C3 plants. The cycle is common in many fruits, legumes, grains, and vegetables. C3 plants usually exhibit low rates of photosynthesis at light saturation, low light

CHO

6 co2

FIGURE 3.2 Biochemical pathway from carbon dioxide to glucose for Cj biomass. (Net process: 3 ATP, 2 NADPH2, 4 Fd+2/C02 assimilated.) saturation points, sensitivity to oxygen concentration, rapid photorespiration, and high C02 compensation points (about 50 ppm). The C02 compensation point is the C02 concentration in the surrounding environment below which more C02 is respired by the plant than is photosynthetically fixed. Typical C3

biomass species are alfalfa, barley, Chlorella, cotton, Eucalyptus, Euphorbia lathyris, oats, peas, potato, rice, soybean, spinach, sugar beet, sunflower, tall fescue, tobacco, and wheat.

The second pathway is called the C4 cycle because C02 is initially converted to the four-carbon dicarboxylic acids, malic or aspartic acids (Fig. 3.3). Phos — phoenolpyruvic acid (I) reacts with one molecule of C02 to form oxaloacetic acid (II) in the mesophyll of the biomass, and then malic or aspartic acid (III) is formed. The C4 acid is transported to the bundle sheath cells, where decarboxylation occurs to regenerate pyruvic acid (IV), which is returned to

C02H

C=0

I

CH3 IV. ,

NADP

co2

FIGURE 3.3 Biochemical pathway from carbon dioxide to glucose for C4 biomass. (Net process: 5 ATP, 2 NADPH2, 4 Fd+2/C02 assimilated.)

the mesophyll cells to initiate another cycle. The C02 liberated in the bundle sheath cells enters the C3 cycle in the usual manner. Thus, no net C02 is fixed in the portion of the C4 cycle shown in Fig. 3.3, and it is the combination with the C3 cycle which ultimately results in C02 fixation.

The subtle differences between the C4 and C3 cycles are believed responsible for the wide variations in biomass properties. In contrast to C3 biomass, C4 biomass is usually produced at higher yields and has higher rates of photosyn­thesis, high light saturation points, insensitivity to atmospheric oxygen concen­trations below 21 mol %, low levels of respiration, low C02 compensation points, and greater efficiency of water usage. C4 biomass often occurs in areas of high insolation, hot daytime temperatures, and seasonal dry periods. Typical C4 biomass includes important crops such as com, sugarcane, and sorghum, and forage species and tropical grasses such as Bermuda grass. Even crabgrass is a C4 biomass. At least 100 genera in 10 plant families are known to exhibit the C4 cycle.

The third pathway is called crassulacean acid metabolism, or CAM. CAM refers to the capacity of chloroplast-containing biomass tissues to fix C02 via phosphoenolpyruvate carboxylase in dark reactions leading to the synthesis of free malic acid. The mechanism involves the /З-carboxylation of phosphoe — nolpyruvic acid by this enzyme and the subsequent reduction of oxaloacetic acid by malate dehydrogenase. CAM has been documented in at least 18 families, including the family Crassulaceae, and at least 109 genera of the Angeospermae. Biomass species in the CAM category are typically adapted to arid environments, have low photosynthesis rates, and have high water usage efficiencies. Examples are cactus plants and the succulents, such as pineapple. The information developed to date on CAM biomass indicates that CAM has evolved so that the initial C02 fixation can take place in the dark with much less water loss than the C3 and C4 pathways. CAM biomass also conserves carbon by recycling endogenously formed C02. Several CAM species show temperature optima in the range 12 to 17°C for C02 fixation in the dark. The stomates in CAM plants open at night to allow entry of C02 and then close by day to minimize water loss. The carboxylic acids formed in the dark are converted to sugars when the radiant energy is available during the day. Relatively few CAM plants have been exploited commercially.