8.6 Photosynthesis  (Page 2/9)

Light-independent reactions

After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules (having lifespans of millionths of a second), photoautotrophs have the fuel needed to build multicarbon carbohydrate molecules, which can survive for hundreds of millions of years, for long-term energy storage. The carbon comes from CO ₂ , the gas that is a waste product of cellular respiration.

The Calvin-Benson cycle (named for Melvin Calvin [1911–1997] and Andrew Benson [1917–2015]), the biochemical pathway used for fixation of CO ₂ , is located within the cytoplasm of photosynthetic bacteria and in the stroma of eukaryotic chloroplasts. The light-independent reaction s of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration (see Appendix C for a detailed illustration of the Calvin cycle).

• Fixation : The enzyme ribulose bisphosphate carboxylase (RuBisCO) catalyzes the addition of a CO ₂ to ribulose bisphosphate (RuBP) . This results in the production of 3-phosphoglycerate ( 3-PGA ).
• Reduction : Six molecules of both ATP and NADPH (from the light-dependent reactions) are used to convert 3-PGA into glyceraldehyde 3-phosphate (G3P). Some G3P is then used to build glucose.
• Regeneration : The remaining G3P not used to synthesize glucose is used to regenerate RuBP, enabling the system to continue CO ₂ fixation. Three more molecules of ATP are used in these regeneration reactions.

The Calvin cycle is used extensively by plants and photoautotrophic bacteria, and the enzyme RuBisCO is said to be the most plentiful enzyme on earth, composing 30%–50% of the total soluble protein in plant chloroplasts. A. Dhingra et al. “Enhanced Translation of a Chloroplast-Expressed Rbc S Gene Restores Small Subunit Levels and Photosynthesis in Nuclear Rbc S Antisense Plants.” Proceedings of the National Academy of Sciences of the United States of America 101 no. 16 (2004):6315–6320. However, besides its prevalent use in photoautotrophs, the Calvin cycle is also used by many nonphotosynthetic chemoautotrophs to fix CO ₂ . Additionally, other bacteria and archaea use alternative systems for CO ₂ fixation. Although most bacteria using Calvin cycle alternatives are chemoautotrophic, certain green sulfur photoautotrophic bacteria have been also shown to use an alternative CO ₂ fixation pathway.

Key concepts and summary

• Heterotrophs depend on the carbohydrates produced by autotrophs, many of which are photosynthetic, converting solar energy into chemical energy.
• Different photosynthetic organisms use different mixtures of photosynthetic pigments , which increase the range of the wavelengths of light an organism can absorb.
• Photosystems (PSI and PSII) each contain a light-harvesting complex , composed of multiple proteins and associated pigments that absorb light energy. The light-dependent reactions of photosynthesis convert solar energy into chemical energy, producing ATP and NADPH or NADH to temporarily store this energy.
• In oxygenic photosynthesis , H ₂ O serves as the electron donor to replace the reaction center electron, and oxygen is formed as a byproduct. In anoxygenic photosynthesis , other reduced molecules like H ₂ S or thiosulfate may be used as the electron donor; as such, oxygen is not formed as a byproduct.
• Noncyclic photophosphorylation is used in oxygenic photosynthesis when there is a need for both ATP and NADPH production. If a cell’s needs for ATP outweigh its needs for NADPH, then it may carry out cyclic photophosphorylation instead, producing only ATP.
• The light-independent reactions of photosynthesis use the ATP and NADPH from the light-dependent reactions to fix CO ₂ into organic sugar molecules.

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