The Calvin-Benson cycle “fixes” CO2. That is, it takes chemically unreactive, inorganic CO2
and incorporates it into an organic molecule that can be used in biological systems. The biosynthetic
pathway involves over a dozen products. The function of the pathway is to produce a single
molecule of glucose (C6H12O6). In order to accomplish this, the Calvin-Benson cycle must
repeat six times, and use 6 CO2 molecules. Thus, in Figure 3-1 and the discussion that follows,
all the molecules involved have been multiplied by 6. Only the most important molecules are
discussed.
1. Carboxylation: 6 CO2 combine with 6 RuBP to produce 12 PGA. The enzyme RuBP
carboxylase, or rubisco, catalyzes the merging of CO2 and RuBP (ribulose bisphosphate).
The Calvin-Benson cycle is referred to as C3 photosynthesis because the first product
formed, PGA (phosphoglycerate), contains three carbon atoms. Other names are the
Calvin cycle and the carbon reduction cycle.
2. Reduction: 12 ATP and 12 NADPH are used to convert 12 PGA to 12 PGAL. The
energy in the ATP and NADPH molecules is incorporated into PGAL (glyceride
3-phosphate), thus making PGAL a very energy-rich molecule. ADP, Pi, and NADP+ are
released and then re-energized in noncyclic photophosphorylation.
3. Regeneration: 6 ATP are used to convert 10 PGAL to 6 RuBP. Regenerating the 6
RuBP originally used to combine with 6 CO2 allows the cycle to repeat.
4. Carbohydrate synthesis. Note that 12 PGAL were created in step 2, but only 10 were
used in step 3. What happened to the remaining two? These two remaining PGAL are
used to build glucose, a common energy-storing molecule. Other monosaccharides like
fructose and maltose can also be formed. In addition, glucose molecules can be combined
to form disaccharides like sucrose and polysaccharides like starch and cellulose.
You should recognize that no light is directly used in the Calvin-Benson cycle. Thus, these reactions
are often called the light-independent reactions or even the dark reactions. But be
careful—the process cannot occur in the dark. This is because it is dependent upon the energy
from ATP and NADPH, and these two energy-rich molecules can be created only during photophosphorylation,
which can occur only in light.
In summary, the Calvin-Benson cycle takes CO2 from the atmosphere and the energy in ATP
and NADPH to create a glucose molecule. Of course, the energy in ATP and NADPH represents
energy from the sun captured during photophosphorylation.
Keep in mind that the reactions that occur during photosynthesis (and in any biosynthetic pathway)
do not occur spontaneously. Every product formed in every reaction is catalyzed by an
enzyme. In some reactions, coenzymes or metal-ion cofactors may also be involved.
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