Explain the phenomenon of photorespiration, including why it is thought to occur (evolutionarily speaking), and why plants have evolved to minimize its occurrence.

Photorespiration limits casualty products of light reactions that build up in the absence of the Calvin cycle. In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle.  The closing of stomata reduces access to CO2 and causes O2 to build up. These conditions favor a seemingly not useful process called photorespiration.  In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound.  In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle. Photorespiration eats up O2 and organic fuel and releases CO2 without producing ATP or sugar. Photorespiration can evolve relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2.