The C4 mechanism can pump up the concentration of CO2
around RuBisCO more than tenfold, making it highly unlikely
that RuBisCO will bind O2 rather than CO2.
3 The process is
often compared to a turbo-charger, which increases an engine's
power by pumping air into the combustion chamber. C4 also
has other advantages. The boost to photosynthesis means C4
plants can use less RuBisCO than C3 plants, saving on the
nitrogen needed to build the protein. In addition, C4 plants can
survive on a lower overall CO2 concentration in the leaf,
allowing them to keep stomata partially closed and so save
water. Researchers estimate that in semi-arid, warm regions, C4
allows plants to increase net photosynthesis by around 50%.1
Interestingly, a mechanism called CAM (Crassulacean Acid
Metabolism) that is similar to C4 has evolved in desert plants.
CAM plants open their stomata at night and fix CO2 into 4-
carbon molecules. During the day, they close their stomata to
save water, and release CO2 from the 4-carbon molecules to
allow photosynthesis.
The C4 Rice Project
by Stephen Day
CO2
CO2
RuBisCO
Mesophyll cell Bundle sheat