The light reactions and the Calvin cycle cooperate in converting light energy to the chemical energy of food: an overview

The equation for photosynthesis is a deceptively simple summary of a very complex process. Actually, photosynthesis is not a single process, but two, each with multiple steps. These two stages of photosynthesis are known as the light reactions (the photo part of photosynthesis) and the Calvin cycle (the synthesis part) (FIGURE 10-4).

The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Light absorbed by chlorophyll drives a transfer of electrons and hydrogen from water to an acceptor called NADP+ (nicotin-amide adenine dinucleotide phosphate), which temporarily stores the energized electrons. Water is split in the process, and thus it is the light reactions of photosynthesis that give off 0, as a by-product. The electron acceptor of the light reactions, NADP+, is first cousin to NAD', which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an extra phosphate group in the NADP+ molecule.The light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons along with a hydrogen nucleus, or H The light reactions also generate ATP by powering the addition of a phosphate group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of two compounds: NADPH, a source of energized electrons ( "reducing power"); and ATP, the versatile energy currency o that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin cycle.

The Calvin cycle is named for Melvin Calvin, who began to elucidate its steps along with his colleagues in the late 1940s. The cycle begins by incorporating C02 from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH, which acquired energized electrons in the light reactions. To convert C02 to carbohydrate, the Calvin cycle also requires chemical engery in the form of ATP, which is also generated by the light reactions. Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the NADPH and ATP produced by the fight reactions. The metabolic steps of the Calvin cycle are sometimes referred to as the dark reactions, or fight-independent reactions, because none of the steps requires light directly. Nevertheless, the Calvin cycle in most plants occurs during daylight, for only then can the fight reactions regenerate the NADPH and ATP spent in the reduction Of C02 to sugar. In essence, the chloroplast uses light energy to make sugar by coordinating the two stages of photosynthesis. AS FIGURE 10.4 shows, the thylakoids of the chloroplast are the sites of the light reactions, while the Calvin cycle occurs in the stroma. As molecules of NADP' and ADP bump into the thylakoid membrane, they pick up electrons and phosphate, respectively, and then transfer their high-energy cargo to the Calvin cycle. The two stages of photosynthesis are treated in this figure as metabolic modules that take in ingredients and crank out products. Our next step toward understanding photosynthesis is to look more closely at how the two stages work, beginning with the light reactions.

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Figure 10.4

An Overview of photosythesis: cooperation of the light reactions and the Calvin cycle.

The light reactions use solar energy to make ATP and NADPH, which funcrtion as chemical energy and reducing power respectively, in the Calvin cycle.(notice that in contrast to ATP generated by cellular respiration, ATP produced in the light reactions of photosynthesis is usually dedicated to a single kind of cellular work, driving the Calvin cycle.) The Calvin cycle incorperates CO2 into organic molecules. Thylakoid membranes, especially those of the grana, are the sites of the light reactions, whereas the Calvin cycle occurs in the stoma.