Photosynthesis(Dark Rxns)

Review: the light reactions
• ____ energy is converted to ____ energy
– Light… chemical
• What are the products?
– NADPH, ATP (both used in the Calvin cycle) & oxygen
• Why not stop after light reactions?
– NADPH & ATP aren’t stable; energy can not be stored or
transported in these forms
– carbon compounds are needed for organisms to grow
Calvin Cycle (dark reactions)
General equation for photosynthesis:
6CO2 + 6H2O + Sunlight  C6H12O6 + 6O2
Note: This is for the light reactions and the
Calvin cycle combined
The mission:
Store chemical energy in the form of 3-carbon
sugars made from CO2
Calvin Cycle, a.ka. “dark reactions”
• “light independent reactions”
• The Calvin cycle does not operate only at
night, it doesn’t require darkness 
• requires ATP and NADPH, which are produced
in the light reactions
• several of the Calvin cycle enzymes are activated
by light
• little CO2 is available when the stomates are
closed at night.
The Players
 RuBP (Ribulose bisphosphate) - a 5-carbon sugar that binds to the
CO2 entering the Calvin Cylce
 CO2 - Carbon Dioxide (comes from the atmosphere, produced
during cellular respiration)
 Rubisco (Ribulose bisphosphate carboxylase-oxygenase) – a BIG
name for a little enzyme that allows CO2 to attach to RuBP- this is
called “carbon fixation”
 PGA- (3-Phosphoglyceric acid)- 3-carbon carboxylic acid
 ATP – (Adenosine triphosphate) - the main energy source
 DPG (1,3-Disphosphoglycerate)- 3-carbon molecule that has a
phosphate group on both ends
 NADPH- (Nicotinamide adenine dinucleotide phosphate) product
of light reaction
 PGAL OR G3P- (Glyceraldehyde-3-phosphate)- the simplest
sugar known, food for plants
The Scene
These reactions occur in the stroma of chloroplasts
Scene 1: Carbon Fixation
• CO2 comes into the stroma of the chloroplast via the
stomata of the leaves.
• Rubisco catalyzes the bonding of CO2 to RuBP to create
an unstable 6-carbon molecule that instantly splits into
two 3-carbon molecules of PGA
• CO2 + RuBP -------> unstable molecule-------> 2 PGA
Scene 2: Reduction
• ATP phosphorylates (adds a phosphate group to) each
PGA molecule and creates DPG.
– This in turn results in the loss of the terminal
phosphate group from ATP thus making ADP
• NADPH reduces DPG which causes the phosphate
group to break off once again. The molecule then
picks up a proton (H+) from the medium to become
• NADPH is oxidized (loses an electron) by this process
and becomes NADP+.
Scene 3: Regeneration
• For every six molecules of PGAL created, five
molecules continue on to phase 3 which leave one
to be used to make glucose.
• ATP is once again needed. However, this time it
phosphorylates PGAL to regenerate RuBP after
some rearrangement.
• For every 6 CO2s IN, there is 1 molecule of
glucose OUT.
– 2 PGAL (3C) molecules combine and leave
the Calvin Cycle, where they are linked to
form glucose;
– 10 PGAL (3C) molecules are rearranged to
form RuBP
• For every glucose made, 18 ATP and 12
NADPH are used.
Why bother with all this work??!
• Photosynthesis is very "costly" to the cell, requiring a lot
of energy from the sun as well as a cast of molecules that
make the needed energy and rearrange the chemical
bonds needed to make sugar.
• The payoff - when sugar is broken down, it yields even
more chemical energy needed to do all other cellular
reactions in cells - growth, reproduction, metabolism....
This is the subject of the next chapter, Cellular
What affects
photosynthesis rates?
• Light intensity
• Temperature
• CO2 concentration
How does light intensity
affect the rate of photosynthesis?
• The rate of photosynthesis
levels off. The light reactions
are saturated.
• Photoinhibition can occur
when plants are exposed to full
light where some of the extra
energy gets passed to oxygen
molecules and hydrogen
peroxide forms which damages
How does temperature
affect the rate of photosynthesis?
• The rate of the
photosynthesis reaction
speeds up as higher
temperatures provide
more energy, however
if the temperature gets
too high, the proteins
become denatured.
How does carbon dioxide concentration
affect the rate of photosynthesis?
• An increase in CO2
increases the rate of
photosynthesis to a
maximum point, after
which the rate levels
• What effect will
climate change
What’s photorespiration?
• O2 can have an inhibitory
effect upon photosynthesis
• In the presence of elevated
O2 levels, photosynthesis
rates are lower due to
competition between O2
and CO2 on the Rubisco
Photorespiration cont.
• Recall, the "normal" reaction to start the Calvin cycle has CO2 joined with
RUBP to form 2 molecules of 3PGA.
• In the process called photorespiration, O2 replaces CO2 in a non-productive,
wasteful reaction
• It is believed that photorespiration in plants has increased over geologic time
and is the result of increasing levels of O2 in the atmosphere--the byproduct
of photosynthetic organisms themselves.
• The appearance of C4-type plants appears to be an evolutionary mechanism
by which photorespiration is suppressed.
C3vs. C4 vs. CAM
C3 Photosynthesis : C3 plants
•Called C3 because the CO2 is first incorporated into
a 3-carbon compound.
•Stomata are open during the day.
•Uses RUBISCO to fix CO2.
Photosynthesis takes place throughout the interior
of the leaf.
Adaptive Value: more efficient than C4 and CAM plants
under cool and moist conditions and under normal light
because it requires less machinery (fewer enzymes
and no specialized anatomy).
95% of all plants are C3.
C4 Photosynthesis : C4 plants
Called C4 because the CO2 is first incorporated into a 4-carbon compound.
• Stomata open & close during the day.
• Uses PEP Carboxylase in the uptake of CO2, then it "delivers" the CO2
directly to RUBISCO
• The Calvin Cycle takes place in inner cells of leaf (called the bundle sheath)
• Adaptive Value:
– Photosynthesizes faster than C3 plants under high light intensity and high
temperatures because the CO2 is delivered directly to RUBISCO, not
allowing it to grab oxygen and undergo photorespiration.
– Is more efficient because PEP Carboxylase brings in CO2 faster and so
does not need to keep stomata open as much for the same amount of CO2
gain for photosynthesis.
• Examples: sugarcane, corn, and many of our summer annual plants.
CAM Photosynthesis : CAM Plants
(Crassulacean Acid Metabolism)
 The CO2 is stored in the form of an acid before use in the Calvin Cycle.
 Stomata open only at night (when evaporation rates are usually lower).
 CO2 is converted to an acid and stored during the night. During the day, the
acid is broken down and the CO2 is released to RUBISCO for photosynthesis
 Adaptive Value:
• More efficient with water than C3 plants due to opening stomata at
night (no sunlight, lower temperatures, lower wind speeds, etc.).
• When conditions are extremely arid, CAM plants can just leave their
stomata closed night and day.
• Examples: succulents such as cacti and agaves and also some orchids and
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