Photosynthesis. Carbon reactions

RICARDO A. WOLOSIUK AND BOB B. BUCHANAN

Plant Physiology, 5th. ed

Sinauer Associates, Inc.

Lugar: Sunderland, Massachusetts; Año: 2009; p. 199 - 242

Because the amount of matter in our planet remains constant, energy must be continually supplied to maintain the circulation of nutrients through the biosphere. Without energy input, entropy would increase and the flow of matter would ultimately stop. Solar radiant energy that strikes the Earth’s surface is the ultimate source of energy for sustaining life in the biosphere. The capture of sunlight energy for transformation into various forms of chemical energy is one of the oldest biochemical reactions on Earth. One billion years ago, heterotrophic cells acquired the ability to convert sunlight into chemical energy through primary endosymbiosis with a cyanobacterium. In this chapter we will first examine the cyclic reactions that accomplish the incorporation of atmospheric CO2 into organic compounds appropriate for life: the Calvin–Benson cycle. Subsequently we will consider how the unavoidable phenomenon of photorespiration releases part of the assimilated CO2. Because photorespiration decreases the efficiency of photosynthetic CO2 assimilation, we will also describe biochemical mechanisms for mitigating the loss of CO2: CO2 pumps, C4 metabolism, and crassulacean acid metabolism (CAM). We will close the chapter with a consideration of the two major products of the photosynthetic fixation of CO2: starch, the reserve polysaccharide that accumulates transiently in chloroplasts; and sucrose, the disaccharide that is exported from leaves to developing and storage organs of the plant.