BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 19. PHOTOSYNTHESIS

Summary

The first step in Photosynthesis is the absorption of light by chlorophyll molecules organized into photosynthetic units within the thylakoid membranes of METABOLISM/14.html">Chloroplasts. The excitation Energy is transferred from one chlorophyll molecule to another until it is trapped by a reaction center. The critical event occurring in the reaction center is the light-driven transfer of a single electron to an acceptor against a chemical potential gradient. Photosynthesis in green plants requires the cooperation of two light reactions. Photosystem I generates a strong reductant (bound ferredoxin) that leads to The formation of NADPH. Photosystem II generates a strong oxidant that produces O₂ from H₂O.

The flow of electrons through the electron-transport chain from photosystem II to photosystem I establishes a proton gradient, which in turn drives the synthesis of ATP. Thus, light powers the movement of electrons from H₂O to NADPH with the concomitant generation of ATP. In addition, the proton gradient and consequently ATP can be generated without the formation of NADPH via a process known as cyclic Photophosphorylation, following Light absorption by photosystem I. Photophosphorylation, much like Oxidative Phosphorylation, is driven by a proton-motive force and therefore requires a closed compartment.

ATP and NADPH produced in the light reactions of photosynthesis are utilized to convert CO₂ into hexoses and other Organic compounds. The Dark Phase of photosynthesis, known as The Calvin Cycle, begins with the reaction between CO₂ and ribulose 1,5-bisphosphate, yielding two molecules of 3-phosphoglycerate. The steps converting 3-phosphoglycerate into fructose 6-phosphate and glucose 6-phosphate are analogous to those of Gluconeogenesis, with the sole exception that chloroplast glyceraldehyde-3-phosphate dehydrogenase is specific for NADPH rather than NADH. Through a complex sequence of reactions, ribulose bisphosphate is regenerated from fructose 6-phosphate, glyceraldehyde 3-phosphate, and dihydroxyacetone phosphate. Several steps in the generation of ribulose 1,5-bisphosphate are similar to those of the Pentose Phosphate Pathway. The conversion of CO₂ into hexose consumes three ATP and two NADPH molecules. To generate two NADPH, four protons are absorbed by photosystem I and another four by photosystem II.

Tropical plants possess an auxiliary pathway for concentrating CO₂ at the site of the Calvin cycle. This C₄ pathway provides tropical plants with an advantage under high light intensity and minimizes the competing reaction, the oxygenation of ribulose 1,5-bisphosphate.