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Calvin Cycle (Carbon Fixation)

Carbon enters the cycle as CO2 and leaves as sugar (glyceraldehyde 3-

phosphate (G3P)). 3 molecules of CO2 1 molecule of G3P.

In the first step of the Calvin cycle, CO2 attaches to a 5 C sugar RibuloseBiphosphate (RuBP) using the enzyme Rubisco. (Most abundant protein inplants and on Earth)

The product of this is a 6C unstable intermediate that splits into 2 molecules

of 3-phosphoglycerate

Each 3-phosphoglycerate receives an additional Pi from the breakdown of

ATP. This forms the molecule 1, 3-biphosphoglycerate

A pair of electrons are donated from NADPH reducing 1, 3

biphosphoglycerate into G3P. NADPH gets oxidized into NADP+ and H+

An oxygen atom is released from 1, 3 BPG

The proton from the oxidation of NADPH, the oxygen from the reduction of 1,

3-BPG and an extra proton from the stroma combine to form one watermolecule

G3P is the same sugar compound that is found in glycolysis when glucose

splits into two molecules

2 molecules of G3P are fromed/CO2 used in the cylce. Therefore _____

molecules of G3P are fromed per turn of the Calvin cycle (3/CO2/turn)

the plant uses only one of these6 molecules of G3P. The other 5 molecules

are recycled back into the Calvin cycle.

The C atoms of the 5 remaining G3P molecules are rearranged into 3

molecules of RuBP (5C/molecule). This rearrangement requires the use of 3

molecules of ATP. RuBP is now ready to accept more CO2 molecules and the cycle continues.

The one G3P that was released from the cycle then combines with another

G3P molecule to make sugar (glucose and other carbohydrates)

The plant, for nutritional needs, can then use this glucose molecule

Note: 3 CO2 molecules go into the cycle at one timeThe cycle needs to occur twice in order to produce enough G3P to makeup one glucose molecule

Number Recap

For each G3P molecule released by Calvin Cycle: ATP are consumed

NADPH are consumed

For each glucose molecule made:

ATP are consumed

NADPH are consumed

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The equation of photosynthesis now looks like this:

The net equation for photosynthesis is: