3. Light-Independent Reactions
Learning Objectives for this Section
Photoautotrophs use sunlight as a source of energy and through the process of photosynthesis, reduce carbon dioxide to form carbohydrates such as glucose. The radient energy is converted to the chemical bond energy within glucose and other organic molecules.
The overall reaction for photosynthesis is as follows:
C6H12O6 + 6 O2 + 6 H2O
Note that carbon dioxide (CO2) is reduced to produce glucose (C6H12O6 ) while water (H2O) is oxidized to produce oxygen (O2).
Photosynthesis is composed of two stages: the light-dependent reactions and the light independent reactions. We will now look at the light-independent reactions.
The endergonic (def) light-independent reactions of photosynthesis use the ATP and NADPH synthesized during the exergonic (def) light-dependent reactions to provide the energy for the synthesis of glucose and other organic molecules (def) 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:
C6H12O6 (glucose) + 12 NADP+ + 18 ADP + 18 Pi + 6 H2O
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.
1. The Calvin (C3) Cycle
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.
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) (see Fig. 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 or 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 (see Fig. 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 (see Fig. 1). Some of the molecules of G3P, however, are used to synthesize glucose and other organic molecules. As can be seen in Fig. 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 (see Fig. 2).
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 (see Fig. 1). Ten molecules of the three-carbon compound G3P eventually form six molecules of the four-carbon compound ribulose phosphate (RP) (see Fig. 1). Each molecule of RP then becomes phosphorylated by the hydrolysis of ATP to produce ribulose bisphosphate (RuBP), the starting compound for the Calvin cycle (see Fig. 1).
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