Dark reaction of Photosynthesis | Calvin Cycle | Biology lecture

The Calvin cycle is
anabolic, building carbohydrates from smaller molecules and consuming energy. Carbon enters the Calvin cycle in
the form of CO2 and leaves in the form of sugar. The cycle
spends ATP as an energy source and consumes NADPH as
reducing power for adding high-energy electrons to make
the sugar.
Calvin cycle performed in three Phases,
Phase 1: Carbon fixation. The Calvin cycle incorporates
each CO2 molecule, one at a time, by attaching
it to a five-carbon sugar named ribulose bisphosphate
(abbreviated RuBP). The enzyme that catalyzes this first
step is RuBP carboxylase-oxygenase, or rubisco. (This
is the most abundant protein in chloroplasts and is also
thought to be the most abundant protein on Earth.) The
product of the reaction is a six-carbon intermediate that
is short-lived because it is so energetically unstable that
it immediately splits in half, forming two molecules of
3-phosphoglycerate (for each CO2 fixed).
Phase 2: Reduction. Each molecule of 3-phosphoglycerate
receives an additional phosphate group from ATP, becoming
1,3-bisphosphoglycerate. Next, a pair of electrons
donated from NADPH reduces 1,3-bisphosphoglycerate,
which also loses a phosphate group in the process, becoming
glyceraldehyde 3-phosphate (G3P). Specifically,
the electrons from NADPH reduce a carboyxl group on
1,3-bisphosphoglycerate to the aldehyde group of G3P,
which stores more potential energy. G3P is a sugar—the
same three-carbon sugar formed in glycolysis by the splitting
of glucose.
Phase 3: Regeneration of the CO2 acceptor (RuBP). In
a complex series of reactions, the carbon skeletons of five
molecules of G3P are rearranged by the last steps of the
Calvin cycle into three molecules of RuBP. To accomplish
this, the cycle spends three more molecules of ATP. The
RuBP is now prepared to receive CO2 again, and the cycle
continues.
For the net synthesis of one G3P molecule, the Calvin
cycle consumes a total of nine molecules of ATP and six
molecules of NADPH. The light reactions regenerate the
ATP and NADPH.

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