tailieunhanh - Plant physiology - 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 the amount 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. | 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 the amount 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 CO2 to 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 saw how 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 stroma reactions 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 that accomplish fixation and reduction