Carbon Dioxide Assimilation Occurs in Three Stages

The first stage in the assimilation of CO2 into biomole-cules (Fig. 20-4) is the carbon-fixation reaction:

condensation of CO2 with a five-carbon acceptor, ribu-lose 1,5-bisphosphate, to form two molecules of 3-phosphoglycerate. In the second stage, the 3-phos-phoglycerate is reduced to triose phosphates. Overall, three molecules of CO2 are fixed to three molecules of ribulose 1,5-bisphosphate to form six molecules of glyc-eraldehyde 3-phosphate (18 carbons) in equilibrium with dihydroxyacetone phosphate. In the third stage, five of the six molecules of triose phosphate (15 carbons) are used to regenerate three molecules of ribu lose 1,5-bisphosphate (15 carbons), the starting material. The sixth molecule of triose phosphate, the net product of photosynthesis, can be used to make hex-oses for fuel and building materials, sucrose for transport to nonphotosynthetic tissues, or starch for storage. Thus the overall process is cyclical, with the continuous conversion of CO2 to triose and hexose phosphates. Fructose 6-phosphate is a key intermediate in stage 3 of CO2 assimilation; it stands at a branch point, leading either to regeneration of ribulose 1,5-bisphosphate or to synthesis of starch. The pathway from hexose phosphate to pentose bisphosphate involves many of the same reactions used in animal cells for the conversion of pentose phosphates to hexose phosphates during the nonoxidative phase of the pentose phosphate pathway (see Fig. 14-22). In the photosynthetic assimilation of CO2, essentially the same set of reactions operates in the other direction, converting hexose phosphates to pentose phosphates. This reductive pentose phosphate cycle uses the same enzymes as the oxidative pathway, and several more enzymes that make the reductive cycle irreversible. All 13 enzymes of the pathway are in the chloroplast stroma.

Types Plastids

Amyloplast

FIGURE 20-3 Plastids: their origins and interconversions. All types of plastids are bounded by a double membrane, and some (notably the mature chloroplast) have extensive internal membranes. The internal membranes can be lost (when a mature chloroplast becomes a proplastid) and resynthesized (as a proplastid gives rise to a pregranal plastid and then a mature chloroplast). Proplastids in nonphotosyn-thetic tissues (such as root) give rise to amyloplasts, which contain large quantities of starch. All plant cells have plastids, and these organelles are the site of other important processes, including the synthesis of essential amino acids, thiamine, pyridoxal phosphate, flavins, and vitamins A, C, E, and K.

Amyloplast

FIGURE 20-3 Plastids: their origins and interconversions. All types of plastids are bounded by a double membrane, and some (notably the mature chloroplast) have extensive internal membranes. The internal membranes can be lost (when a mature chloroplast becomes a proplastid) and resynthesized (as a proplastid gives rise to a pregranal plastid and then a mature chloroplast). Proplastids in nonphotosyn-thetic tissues (such as root) give rise to amyloplasts, which contain large quantities of starch. All plant cells have plastids, and these organelles are the site of other important processes, including the synthesis of essential amino acids, thiamine, pyridoxal phosphate, flavins, and vitamins A, C, E, and K.

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