Principles of terrestrial ecosystem ecology.pdf

100 5. Carbon Input to Terrestrial Ecosystems
Light-harvesting
reactions
chl
Thylakoid
Thylakoid
H 2 O
H + + O 2
Thylakoid
e
NADP
- transportchain NADPH
ATP
ADP
Stroma
Figure 5.3. A chloroplast showing the location of the
major photosynthetic reactions. The light-harvesting
reactions occur in the thylakoid membranes; chlorophyll
(chl) absorbs visible light and funnels it to reaction
centers (Photosystems I and II). In Photosystem
II, water is split to H + and O2, and the resulting electrons
are then passed down an electron-transport
chain inside the thylakoid, ultimately to NADP, producing
NADPH. During this process, protons move
across the thylakoid membrane to the stroma,and the
proton gradient drives the synthesis of ATP.ATP and
NADPH provide the energy to regenerate ribulosebisphosphate
(RuBP) within the carbon fixation
to 40% of the carbon fixed by C 3 photosynthesis
and regenerates ADP and NADP in the
process. Why do C3 plants have such an inefficient
system of carbon acquisition, by which
they immediately lose a third of the carbon that
they acquire from photosynthesis? Although
we have no definite answer to this question, the
most likely explanation is that photorespiration
acts as a safety valve. It provides a supply of
reactants (ADP and NADP) to the light reaction
under circumstances in which an inadequate
supply of CO2 limits the rate at which
these reactants can be regenerated by carbonfixation
reactions. In the absence of photores-
O 2
Light
H +
Carbon-fixation
reactions
RuBP
(C 5 sugar)
Photorespiration
2 C 3 sugars
Starch
2 C 2 compounds
Sugar
export
CO 2
O 2
C 2
export
reactions. RuBP reacts either with CO 2 to produce
sugars and starch (carbon-fixation reactions of
photosynthesis) or with O 2 to produce two-carbon
intermediates (photorespiration). These two-carbon
intermediates are exported from the chloroplast to
mitochondria or peroxisomes, where they are again
converted to sugars, with loss of CO 2 and ATP.
Through either carbon fixation or photorespiration,
ADP and NADP again become reactants available to
produce additional ATP and NADPH.The net effect
of photosynthesis is to convert light energy into chemical
energy (sugars and starches) that is available to
support plant growth and maintenance.
piration, continued light harvesting produces
oxygen radicals that destroy photosynthetic
pigments.
Plants have additional lines of defense
against excessive energy capture, which are
at least as important as photorespiration. One
such photoprotection mechanism involves pigments
that change from one form to another in
the xanthophyll cycle. When excess excitation
energy is present and cannot be processed to
generate ATP and NADPH, xanthophyll
pigment is converted to a form that can receive
excess absorbed energy from the excited
chlorophyll (Demming-Adams and Adams