Fase bioquimica fotosintesis

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Chapter

8

Photosynthesis: Carbon Reactions

IN CHAPTER 5 WE DISCUSSED plants’ requirements for mineral nutrients and light in order to grow and complete their life cycle. Because living organisms interact with one another and their environment, mineral nutrients cycle through the biosphere. These cycles involve complex interactions, and each cycle is critical in its own right. Because theamount of matter in the biosphere remains constant, energy must be supplied to keep the cycles operational. Otherwise increasing entropy dictates that the flow of matter would ultimately stop. Autotrophic organisms have the ability to convert physical and chemical sources of energy into carbohydrates in the absence of organic substrates. Most of the external energy is consumed in transforming CO2to a reduced state that is compatible with the needs of the cell (—CHOH—). Recent estimates indicate that about 200 billion tons of CO2 are converted to biomass each year. About 40% of this mass originates from the activities of marine phytoplankton. The bulk of the carbon is incorporated into organic compounds by the carbon reduction reactions associated with photosynthesis. In Chapter 7 we sawhow the photochemical oxidation of water to molecular oxygen is coupled to the generation of ATP and reduced pyridine nucleotide (NADPH) by reactions taking place in the chloroplast thylakoid membrane. The reactions catalyzing the reduction of CO2 to carbohydrate are coupled to the consumption of NADPH and ATP by enzymes found in the stroma, the soluble phase of chloroplasts. These stromareactions were long thought to be independent of light and, as a consequence, were referred to as the dark reactions. However, because these stroma-localized reactions depend on the products of the photochemical processes, and are also directly regulated by light, they are more properly referred to as the carbon reactions of photosynthesis. In this chapter we will examine the cyclic reactions thataccomplish fixation and reduction of CO2, then consider how the phenomenon of photorespiration catalyzed by the carboxylating enzyme alters the effi-

146

Chapter 8
The Calvin cycle proceeds in three stages (Figure 8.2): 1. Carboxylation of the CO2 acceptor ribulose-1,5-bisphosphate, forming two molecules of 3-phosphoglycerate, the first stable intermediate of the Calvin cycle 2. Reduction of3-phosphoglycerate, forming gyceraldehyde-3-phosphate, a carbohydrate 3. Regeneration of the CO2 acceptor ribulose-1,5-bisphosphate from glyceraldehyde-3-phosphate The carbon in CO2 is the most oxidized form found in nature (+4). The carbon of the first stable intermediate, 3phosphoglycerate, is more reduced (+3), and it is further reduced in the glyceraldehyde-3-phosphate product (+1). Overall, theearly reactions of the Calvin cycle complete the reduction of atmospheric carbon and, in so doing, facilitate its incorporation into organic compounds.

H2O Light

ADP + Pi

NADP+

(CH2O)n

Chlorophyll

Triose phosphates

O2

ATP

+ NADPH

CO2 + H2O Carbon reactions

Light reactions

FIGURE 8.1 The light and carbon reactions of photosynthe-

sis. Light is required for thegeneration of ATP and NADPH. The ATP and NADPH are consumed by the carbon reactions, which reduce CO2 to carbohydrate (triose phosphates).

ciency of photosynthesis. This chapter will also describe biochemical mechanisms for concentrating carbon dioxide that allow plants to mitigate the impact of photorespiration: CO2 pumps, C4 metabolism, and crassulacean acid metabolism (CAM). We will closethe chapter with a consideration of the synthesis of sucrose and starch.

The Carboxylation of Ribulose Bisphosphate Is Catalyzed by the Enzyme Rubisco
CO2 enters the Calvin cycle by reacting with ribulose-1,5bisphosphate to yield two molecules of 3-phosphoglycerate (Figure 8.3 and Table 8.1), a reaction catalyzed by the chloroplast enzyme ribulose bisphosphate carboxylase/oxygenase, referred...
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