6 6 6 6 6 6 × 3-phosphoglycerate 6 × 1,3-bisphosphoglycerate 6 × glyceraldehyde 3-phosphate 1 MOLECULE OF GLYCERALDEHYDE 3-PHOSPHATE LEAVES THE CYCLE 5 × glyceraldehyde 3-phosphate 3 ×ribulose 1,5-bisphosphate 3C 5C 3C 3C3C Rubisco ATPADPATPADPNADPHNADP+Pi Pi CARBON-FIXATION (CALVIN) CYCLE NET RESULT: For every 3 molecules of CO2 that enter the cycle, 1 molecule of glyceraldehyde 3-phosphate is produced and 9 molecules of ATP + 6 molecules of NADPH are consumed REGENERATIONCARBOXYLATIONREDUCTION sugars, fats, amino acids glycolytic pathway (see Figure 2–46), where it is converted to pyruvate. Both that pyruvate and the fatty acids can enter the plant cell mitochondria and be fed into the citric acid cycle, ultimately leading to the production of large amounts of ATP by oxidative phosphorylation (Figure 14–42). Plants use this ATP in the same way that animal cells and other nonphotosynthetic organisms do to power a variety of metabolic reactions.
The glyceraldehyde 3-phosphate exported from chloroplasts into the cytosol can also be converted into many other metabolites, including the disaccharide sucrose. Sucrose is the major form in which sugar is transported between the cells of a plant: just as glucose is transported in the blood of animals, so sucrose is exported from the leaves to provide carbohydrate to the rest of the plant.
the thylakoid Membranes of Chloroplasts Contain the protein Complexes required for photosynthesis and atp Generation
We next need to explain how the large amounts of ATP and NADPH required for carbon fixation are generated in the chloroplast. Chloroplasts are much larger and less dynamic than mitochondria, but they make use of chemiosmotic energy conversion in much the same way. As we saw in Figure 14–38, chloroplasts and mitochondria are organized on the same principles, although the chloroplast contains a separate thylakoid membrane system in which its chemiosmotic mechanisms occur. The thylakoid membranes contain two large membrane protein complexes, called photosystems, which endow plants and other photosynthetic organisms with the ability to capture and convert solar energy for their own use. Two other protein complexes in the thylakoid membrane that work together with the photosystems in photophosphorylation—the generation of ATP with sunlight— have mitochondrial equivalents. These are the heme-containing cytochrome
Figure 14–41 The carbon-fixation cycle. this central metabolic pathway allows organic molecules to be produced from CO2 and h2O. In the first stage of the cycle (carboxylation), CO2 is added to ribulose 1,5-bisphosphate, as shown in Figure 14–40. In the second stage (reduction), atp and NaDph are consumed to produce glyceraldehyde 3-phosphate molecules. In the final stage (regeneration), some of the glyceraldehyde 3-phosphate produced is used to regenerate ribulose 1,5-bisphosphate. Other glyceraldehyde 3-phosphate molecules are either converted to starch and fat in the chloroplast stroma, or transported out of the chloroplast into the cytosol. the number of carbon atoms in each type of molecule is indicated in yellow. there are many intermediates between glyceraldehyde 3-phosphate and ribulose 5-phosphate, but they have been omitted here for clarity. the entry of water into the cycle is also not shown (but see Figure 14–40).