Page 124 - Demo 1

THE CALVIN CYCLE

The Calvin cycle consists of the following events that occur in the
stroma:
• CO2 is taken up by one of the molecules in the cycle

• ATP and NADPH from the light reacons reduce CO2 to a
carbohydrate (G3P)

In plants, carbon dioxide enters the interior of a leaf via pores called stomata
and diffuses into the stroma of the chloroplast — the site of the Calvin Cycle
reacons, where sugar is synthesized. The events of the Calvin Cycle do not
require an input of solar energy; thus, they are oen referred to as light-
independent reacons. These set of reacons is also known as the dark reacons,
C3 cycle, Calvin-Benson-Bassham (CBB) cycle, or reducve pentose phosphate
cycle. The cycle was discovered in 1950 by Melvin Calvin, James Bassham, and
Andrew Benson at the University of California, Berkeley. They used radioacve
carbon-14 to trace the path of carbon atoms in carbon fixaon.

The Calvin cycle reacons can be divided into three main stages: (1) carbon
dioxide fixaon, (2) carbon dioxide reducon, and (3) regeneraon of the first
substrate, RuBP (ribulose-1, 5-biphosphate). These reacons produce
molecules of G3P (glyceraldehyde 3-phosphate), which are used by plants to
produce glucose and other organic molecules. The ATP and NADPH generated
by the light reacons power the Calvin cycle reacons.

1. Fixation of carbon dioxide -- A CO2 molecule combines with a
five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP).
The enzyme for this reacon is called RuBP carboxylase (rubisco). This
step makes a six-carbon compound that splits into two molecules of
a three-carbon compound, 3-phosphoglyceric acid (3-PGA).

2. Reduction of carbon dioxide -- In the second stage, ATP and NADPH
are used to supply the needed electrons and energy for CO2
reducon, respecvely. The 3-PGA molecules produced from carbon
dioxide fixaon are converted into molecules of a three-carbon sugar,
glyceraldehyde-3-phosphate (G3P). This stage gets its name
because NADPH donates electrons to, or reduces, a three-carbon
intermediate to make G3P.

3. Regeneration of RuBP -- Some G3P molecules go to make glucose,
while others must be recycled to regenerate the RuBP acceptor.

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