They are thus known as the "supporting cells" of the nervous system. The four main functions of glial cells are: to surround neurons and hold them in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to destroy and remove the carcasses of dead neurons (clean up).
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.
