Chapters 12 and 13 have described how plants
use light energy to assimilate carbon into sugars
and starch and how these molecules are subsequently
broken down and converted into organic acids and
other compounds. In this chapter we examine aerobic
respiration— the further oxidation of these compounds
to CO2 and H2O in the mitochondrion—and
review the mechanisms by which the energy released
during respiration is conserved as ATP, a process
called oxidative phosphorylation. We examine how
plants can minimize ATP production while maintaining
respiration rates, a mitochondrial attribute that
may affect plant responses to environmental stress.
We also describe how mitochondria and other cellular
compartments interact by means of substrate shuttles
across the inner mitochondrial membrane.
In addition to
aerobic respiration, another respiratory process
takes place in the leaves of many plants. This
novel process, photorespiration, is the light-dependent
release of CO2 and uptake of O2.
The O2 uptake occurs in the chloroplast
as a result of the oxygenase reaction of Rubisco,
which leads to production of phosphoglycolate.
Metabolism of this compound involves a complex
interaction among chloroplasts, peroxisomes, and
mitochondria and leads to release of CO2.
Although photorespiration in effect drains carbon
from plants and adversely affects growth, the
process seems to be an unavoidable side reaction
of CO2 fixation in most plants. Some
plants, however, have evolved special anatomical
and biochemical features that minimize the oxygenase
reaction. These plants, known as C4
plants and Crassulacean acid metabolism (CAM)
plants (see Chapter 12), demonstrate very low
rates of photorespiration.