2. The carbon-fixation reactions do not require sunlight. Here the ATP and NADPH generated by the light reactions serve as the source of energy and reducing power, respectively, to drive the conversion of CO2 to carbohydrate. These carbon-fixation reactions begin in the chloroplast stroma, where they generate the three-carbon sugar glyceraldehyde 3-phosphate. This simple sugar is exported to the cytosol, where it is used to produce sucrose and many other organic metabolites in the leaves of the plant. The sucrose is then exported to meet the metabolic needs of the nonphotosynthetic plant tissues, serving as a source of both carbon skeletons and energy for growth.
Thus, the formation of ATP, NADPH, and O2 (which requires light energy directly) and the conversion of CO2 to carbohydrate (which requires light energy only indirectly) are separate processes (Figure 14–39). However, they are linked by elaborate feedback mechanisms that allow a plant to manufacture sugars only when it is appropriate to do so. Several of the chloroplast enzymes required for carbon fixation, for example, are inactive in the dark and reactivated by light-stimulated electron-transport processes.
Carbon Fixation Uses atp and NaDph to Convert CO2 into Sugars
We have seen earlier in this chapter how animal cells produce ATP by using the large amount of free energy released when carbohydrates are oxidized to CO2 and H2O. The reverse reaction, in which plants make carbohydrate from CO2 and H2O, takes place in the chloroplast stroma. The large amounts of ATP and NADPH produced by the photosynthetic electron-transfer reactions are required to drive this energetically unfavorable reaction.
Figure 14–39 A summary of the energy-converting metabolism in chloroplasts.
Chloroplasts require only water and carbon dioxide as inputs for their light-driven photosynthesis reactions, and they produce the nutrients for most other organisms on the planet. Each oxidation of two water molecules by a photochemical reaction center in the thylakoid membrane produces one molecule of oxygen, which is released into the atmosphere. at the same time, protons are concentrated in the thylakoid space. these protons create a large electrochemical gradient across the thylakoid membrane, which is utilized by the chloroplast atp synthase to produce atp from aDp and phosphate. the electrons withdrawn from water are transferred to a second type of photochemical reaction center to produce NaDph from NaDp+. as indicated, the NaDph and atp are fed into the carbon-fixation cycle to reduce carbon dioxide, thereby producing the precursors for sugars, amino acids, and fatty acids. the CO2 that is taken up from the atmosphere here is the source of the carbon atoms for most organic molecules on Earth.
In a plant cell, a variety of metabolites produced in the chloroplast are exported to the cytoplasm for biosyntheses. Some of the sugar produced is stored in the form of starch granules in the chloroplast, but the rest is transported throughout the plant as sucrose or converted to starch in special storage tissues. these storage tissues serve as a major food source for animals.
carbon ribulose 1,5-bisphosphate intermediate 2 molecules of dioxide 3-phosphoglycerate