The Dark Reactions (Carbon Fixation)
A) Carbon fixation
1) Light Independent Reactions = Carbon fixation
Chemical energy from light reactions used to fix carbon dioxide to produce sugars.
2) Occurs in stroma of chloroplasts
3) Calvin cycle. Like Krebs cycle in that starting compound
(ribulose 1,5 biphosphate - RuBP) is regenerated at the end.
a) C3 (typical) pathway: CO2 fixed to RuBP, which then splits to form 2 3-phosphoglycerate
(PGA) molecules. ATP and NADPH used to generate glyceraldehyde 3-phosphate (GP:
for glucose synthesis) and to regenerate RuBP.
b) Done by the enzyme RuBP carboxylase/oxygenase (Rubisco): catalyzes
carboxylation of RuBP. Rubisco is the most abundant plant protein - up to 15%
of leaf protein.
c) Stomata open in day to allow CO2 uptake, closed at night to conserve water.
d) GP transported to cytoplasm where it is used to make glucose, sucrose, or
starch.
B) C4 or Hatch-Slack pathway:
1) First product of carbon fixation is not the 3-C PGA, but rather the 4-C
oxaloacetate. Phosphoenolpyruvate (PEP) + CO2 = oxaloacetate. Occurs in
mesophyll cells.
2) Oxaloacetate is either reduced to malate or converted to aspartate.
3) Malate or aspartate moves to bundle sheath cells, where it is decarboxylated
to regenerate CO2 + pyruvate. This CO2 enters the Calvin cycle in the bundle
sheath cells.
4) Pyruvate returns to mesophyll cells where it is phosphorylated to regenerate
PEP.
5) More costly than C3 pathway. 5 ATP vs. 3 ATP per glucose, but avoids
photorespiration. Photorespiration:
O2 oxidizes RuBP to release CO2. Wasteful. Also done by Rubisco. At low
CO2/high O2 levels, high light, high temperatures, pathway turns away from
carbon fixation and toward photorespiration.
6) In C4 plants, Calvin cycle takes place in bundle sheath cells, away from excess oxygen (high CO2/O2 ratios are maintained there).
C4 plants can also recycle CO2 released during
photorespiration.
D) CAM plants (Crassulacean Acid Metabolism)
1) Mainly in succulent desert plants. Need to maintain low transpiration rates
in the day (minimize water loss during the day). This limits gas exchange
during the day. So, these plants open stomata only at night.
2) At night, with stomata open, PEP + CO2 --> oxaloacetate --> malate.
Malate stored in large central vacuole.
3) In day, with stomata closed, malate decarboxylated to release CO2. This CO2
enters the Calvin cycle where it is fixed to form sugars.
4) Main advantage is greatly increased water use efficiency (water used per
glucose formed). CAM plants are well adapted to high light, high temperatures,
high O2, low CO2, low water levels (they also avoid photorespiration). BUT,
these plants have a low net photosynthetic rate when in the CAM mode. They grow
much more slowly than either C3 or C4 plants. Present in 23 families, mostly
dicots. During periods when water balance is not a problem, these plants may
switch back to C3 (typical) metabolism, which allows higher growth rates.
E) Light Saturation Curves: show maximum rate of photosynthesis as a
function of varying light levels. Plot irradiance (uE/m2/hr) vs photosynthesis
(usually CO2 uptake/some unit of leaf area/hr).
1) Species vary in light saturation curves. C3 plants typically have lower
levels of light saturation than do C4 and CAM plants.
2) "Sun" leaves have higher levels of light saturation than do
"shade" leaves.
F) Photosynthetic Response to Temperature:
1) Plants often have temperature optima for photosynthesis that are very close
to the ambient temperatures that they typically experience during the growing
season. Desert plants have higher temperature optima than do tundra plants, for
instance.
2) Plants that typically experience large daily or seasonal changes in
temperature (e.g., desert plants) are more likely to be plastic in their
photosynthetic response to temperature than plants in less variable
environments (e.g., tropical forest plants). Such plants can acclimate to
temperature changes.
3) CAM plants may switch to C3 metabolism with high nighttime temperatures. CAM
plants do not typically live in areas where nighttime temperatures are high --
unless water balance is not a problem. They are usually confined to areas with
high daytime temperatures, low water availability, and cool nighttime
temperatures.
G) IRGA - Infrared Gas Analyzer
1) Used to measure flux of CO2 to or from a plant part. Measures net
photosynthetic rate by subtracting CO2 released by respiration (dark conditions).
2) CO2 pumped to a chamber, where a leaf is held, then to an analyzer. Another
line bypasses the chamber and goes directly to the analyzer (reference line).
Difference between chamber and reference line is the net CO2 uptake.
3) Plot net CO2 uptake at different light levels. Extrapolate to the zero light
level (dark). CO2 flux in dark = respiration. Difference between respiration
and CO2 flux in the light is the net photosynthetic rate.
H) 14C technique:
1) Expose plant part to 14C for a short period of time under a particular set
of environmental conditions.
2) Sample plant part and measure radioactivity (liquid scintillation counter).
3) Less commonly used because of danger of using 14C.
1) The entire base to the “economy” (food web) of forests and other plant communities starts with photosynthesis.
2) Variation among species in photosynthetic pathways and responses to environmental conditions (light, O2, CO2, water, temperature) help determine their niches.
3) Variation in niches helps explain both the structure and dynamics of communities. For example, plants with lower light saturation curves dominate the understory of climax forests.