C4 Pathway

The C4 pathway is identical to the C3 pathway except for the

way in which the pathway begins. Instead of carbon dioxide

being directly fixated to RuBP as it is in C3, it first joins a

molecule of pyruvate that has been converted into

phosphoenolpyruvate. This takes place in a mesophyll cell

that sits right next to a bundle sheath cell. The resultant

molecule is a four-carbon molecule that then travels to the

bundle sheath cell and goes through the Calvin Cycle. The

four-carbon molecule splits into a three carbon and a one

carbon molecule. The three-carbon molecule travels back

to the mesophyll cell while the one carbon is fixated by

RuBisCO to RuBP. There is a much higher concentration of

carbon dioxide present in the bundle sheath cell in C4 than

in C3 because of the transprotation from the mesophyll cell,

which decreases oxygenation.

Because the carbon dioxide is fixated before it arrives at the

beginning of the Calvin Cycle, it bypasses the stage at which

oxygen can be accidently fixated in its place, which increases

that plant's efficiency. However, it takes more energy to

transport the molecule from the mesophyll cell to the bundle

sheath cell, so it does come with a cost.

Continuing with the example of an assembly line worker,

the C4 pathway is like a very careful employee. This worker

never makes mistakes because of how careful he/she is,

but he/she is not able to work as quickly as the careless

worker and requires more energy to make sure no errors

are made.