Turbo-charged photosynthesis
C4 photosynthesis evolved as a patch to fix a fundamental flaw
at the heart of normal photosynthesis.3 During photosynthesis,
plants capture carbon dioxide from the air and use it to build
sugars. Unfortunately, the enzyme that first binds CO2 - called
RuBisCO - sometimes binds oxygen by mistake. When this
happens, it triggers a series of reactions called
photorespiration that costs the plant both energy and carbon
(released as CO2), effectively undoing the work of
photosynthesis.
Given equal amounts of CO2 and O2 this would not be much of a
problem. RuBisCO is much more likely to bind CO2. But RuBisCO
isn't given an even choice-there's far more O2 than CO2 in the
atmosphere. Things get worse as the temperature increases.
When it's hot, RuBisCO is more likely to mistake O2 for CO2, and
the solubilities of the two gases shift, increasing the ratio of
dissolved O2 to dissolved CO2. At temperatures above 30oC, this
can mean photorespiration reduces the net rate of
photosynthesis by over 30%.4