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18.7C: Oxygenic Photosynthesis: Light-Independent Reactions

Skills to Develop

  1. Briefly describe the overall function of the light-independent reactions in photosynthesis and state where in the chloroplast they occur.
  2. State how the light-dependent and light-independent reactions are linked during photosynthesis.
  3. State the reactants and the products for the light-independent reactions.
  4. Briefly describe the following stages of the Calvin cycle:
    1. CO2 fixation
    2. production of G3P
    3. regeneration of RuBP
  5. State the significance of glyceraldehyde-3-phosphate (G3P) in the Calvin cycle.

The endergonic light-independent reactions of photosynthesis use the ATP and NADPH synthesized during the exergonic light-dependent reactions to provide the energy for the synthesis of glucose and other organic molecules from inorganic carbon dioxide and water. This is done by "fixing" carbon atoms from CO2 to the carbon skeletons of existing organic molecules. These reactions occur in the stroma of the chloroplasts.

The light-independent reactions can be summarized as follows:

\[12 NADPH + 18 ATP + 6 CO_2 \rightarrow \underbrace{C_6H_{12}O_6}_{glucose} + 12 NADP^+ + 18 ADP + 18 P_i + 6 H_2O\]

Most plants use the Calvin (C3) cycle to fix carbon dioxide. C3 refers to the importance of 3-carbon molecules in the cycle. Some plants, known as C4 plants and CAM plants, differ in their initial carbon fixation step. There are three stages to the Calvin cycle: 1) CO2 fixation; 2) production of G3P; and 3) regeneration of RuBP. We will now look at each stage.

Figure 18.7C.1: The Calvin Cycle

Stage 1: CO2 Fixation

To begin the Calvin cycle, a molecule of CO2 reacts with a five-carbon compound called ribulose bisphosphate (RuBP) producing an unstable six-carbon intermediate which immediately breaks down into two molecules of the three-carbon compound phosphoglycerate (PGA) (Figure 18.7C.1). The carbon that was a part of inorganic CO2 is now part of the carbon skeleton of an organic molecule. The enzyme for this reaction is ribulose bisphosphate carboxylase (Rubisco). A total of six molecules of CO2 must be fixed this way in order to produce one molecule of the six-carbon sugar glucose.

Stage 2: Production of G3P from PGA

The energy from ATP and the reducing power of NADPH (both produced during the light-dependent reactions) is now used to convert the molecules of PGA to glyceraldehyde-3-phosphate (G3P), another three-carbon compound (Figure 18.7C.1). For every six molecules of CO2 that enter the Calvin cycle, two molecules of G3P are produced. Most of the G3P produced during the Calvin cycle - 10 of every 12 G3P produced - are used to regenerate the RuBP in order for the cycle to continue. Some of the molecules of G3P, however, are used to synthesize glucose and other organic molecules. As can be seen in Figure 18.7C.1, two molecules of the three-carbon G3P can be used to synthesize one molecule of the six-carbon sugar glucose. The G3P is also used to synthesize the other organic molecules required by photoautotrophs (Figure 18.7C.2).

Figure 18.7C.2: Products Synthesized from Glyceraldehyde-3-Phosohate. Glyceraldehyde-3-phosphate (G3P), the end product of the Calvin Cycle, can be converted to many different organic molecules required by photoautotrophs.

Stage 3: Regeneration of RuBP from G3P

As mentioned in the previous step, most of the G3P produced during the Calvin cycle - 10 of every 12 G3P produced - are used to regenerate the RuBP so that the cycle may continue (Figure 18.7C.1). Ten molecules of the three-carbon compound G3P eventually form six molecules of the four-carbon compound ribulose phosphate (RP). Each molecule of RP then becomes phosphorylated by the hydrolysis of ATP to produce ribulose bisphosphate (RuBP), the starting compound for the Calvin cycle.

Summary

  1. Photoautotrophs absorb and convert light energy into the stored energy of chemical bonds in organic molecules through a process called photosynthesis.
  2. Plants, algae, and cyanobacteria are known as oxygenic photoautotrophs because they synthesize organic molecules from inorganic materials, convert light energy into chemical energy, use water as an electron source, and generate oxygen as an end product of photosynthesis.
  3. Oxygenic photosynthesis is composed of two stages: the light-dependent reactions and the light-independent reactions.
  4. The light-independent reactions use the ATP and NADPH from the light-dependent reactions to reduce carbon dioxide and convert the energy to the chemical bond energy in carbohydrates such as glucose.
  5. The light-independent reactions can be summarized as follows: 12 NADPH + 18 ATP + 6 CO2 yields C6H12O6 (glucose) + 12 NADP+ + 18 ADP + 18 Pi + 6 H2O.
  6. Most plants use the Calvin cycle to fix CO2. To begin the Calvin cycle, a molecule of CO2 reacts with a five-carbon compound called ribulose bisphosphate (RuBP) producing an unstable six-carbon intermediate which immediately breaks down into two molecules of the three-carbon compound phosphoglycerate (PGA).
  7. The energy from ATP and the reducing power of NADPH (both produced during the light-dependent reactions) is now used to convert the molecules of PGA to glyceraldehyde-3-phosphate (G3P), another three-carbon compound.
  8. Most of the G3P produced during the Calvin cycle are used to regenerate the RuBP so that the cycle may continue, however, some of the molecules of G3P, however, are used to synthesize glucose and other organic molecules.

Contributors

  • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)