|FIGURE 3.9 Schematic drawing of the three phases of CBB cycle.
Carboxylation involves the addition of one molecule of CO2
to a 5-carbon “acceptor” molecule,
ribulose biphosphate (RuBP). This reaction is catalyzed by the enzyme RuBisCO. Plants invest
a huge amount of their available nitrogen into making this one protein. As a result, RuBisCO is
the most abundant protein in the biosphere. The resulting 6-carbon product splits into two
identical 3-carbon products. These products are 3-phosphoglycerate, or simply 3-PG. At this
point in the cycle, CO2
has been “fixed” into an organic product but no energy has been added
to the molecule.
The second step in the Calvin cycle is the reduction of 3-PG to the level of carbohydrate. This reaction
occurs in two steps: (1) phosphorylation of 3-PG by ATP to form a 1,3-biphosphoglycerate
(1,3-BPG) and (2) reduction of 1,3-BPG by NADPH to form glyceraldehyde-3-phosphate (G3P),
a simple 3-carbon carbohydrate, and its isomers collectively called triose phosphates. This reaction
requires both ATP and NADPH, the high energy chemical intermediates formed in the light
and ADP formed in this process return to the thylakoids to regenerate NADPH and
ATP in the light reactions.
The final stage in the Calvin cycle is the regeneration of the CO2
acceptor RuBP. This involves a
series of reactions that convert triose phosphate first to the 5-carbon intermediate Ru5P (ribulose 5-
phosphate), then phosphorylate Ru5P to regenerate RuBP (ribulose-bisphosphate). This final step
requires ATP formed in the light reactions.
Overall, for every three turns of the cycle one molecule of product (triose phosphate) is formed
:1G3P). The remaining 15 carbon atoms (5 G3P) re-enter the cycle to produce three molecules
The triose phosphate formed in the Calvin cycle can remain in the chloroplast where it is converted
to starch. This is why chloroplasts form starch grains. Alternatively, triose phosphate can be
exported from the chloroplast where it is converted to carbohydrates in the cytoplasm. Both reactions
involve the release of phosphate. In the case of carbohydrates, the phosphate must be returned
to the chloroplast to support continued photophosphorylation (ATP formation).
The net energy balance of six rounds of the Calvin cycle to produce one mole of hexose is thus:
+ 18ATP + 12NADPH + 12H2
O → C6
+ 18Pi + 12NADP+