Photosynthetic organisms must cope with a competing reaction that inhibits photosynthesis known
as photorespiration. Unlike photosynthesis, this process involves the uptake of oxygen and the
release of carbon dioxide.
Recall that mitochondrial respiration involves the uptake of O2 and the evolution of CO2 and is
associated with the burning of cellular fuel to obtain energy in the form of ATP. In contrast, photorespiration
starts in the chloroplast and wastes energy.
Photorespiration can be defined as the light-dependent uptake of O2 in the chloroplast. It is
caused by a fundamental “inefficiency” of RuBisCO.
During photosynthesis RuBisCO catalyzes the carboxylation of RuBP to give two molecules of
PGA. However, it can also catalyze the oxygenation of RuBP to give one molecule of PGA and one
molecule of a 2-carbon compound called phosphoglycolate. This reaction occurs because O2 can
compete with CO2 at the active site of RuBisCO. As oxygenation of RuBP competes with carboxylation,
it lowers the efficiency of photosynthesis. A significant portion (25%) of the carbon in
phosphoglycolate is lost as CO2. Algae must use energy to “recover” the remaining 75% of this
carbon, which further limits the efficiency of photosynthesis.
If photorespiration lowers the yield of photosynthesis, why has such a process been maintained
throughout the course of evolution? The answer to this intriguing question has to do with the origin
of RuBisCO and the CBB cycle. RuBisCO is an ancient enzyme, having evolved over 2.5 billion
years ago in cyanobacteria. During this period in Earth’s history, the atmosphere contained high
levels of CO2 and very little oxygen.
Thus, photorespiration did not present a problem for early
photosynthetic organisms. By the time oxygen levels accumulated to significant levels in the atmosphere
(ironically, by the process of photosynthesis!), the catalytic mechanism of RuBisCO was
apparently “fixed.” In other words, because both O2
compete for the same active site of
the enzyme, algae could not decrease the efficiency of oxygenation without also decreasing the
efficiency of carboxylation. To compensate, algae evolved an elaborate pathway, known as the
photorespiratory pathway, to recover at least some of the carbon that would otherwise be lost.
This pathway involved biochemical reactions in the chloroplast, mitochondria, and peroxisome.
The importance of photorespiration is easily demonstrated by the fact that nearly all plants grow
better under high CO2
versus low CO2
. Conditions that favor carboxylation (photosynthesis)
over oxygenation (photorespiration) include high CO2
, moderate light intensities, and moderate
temperatures. Conditions that favor oxygenation over carboxylation include low CO2
temperatures, and high light intensities.