Two Stages of Photosynthesis

1
Two Stages of Photosynthesis
Calvin Cycle aka light independent reaction
Light Reaction aka light dependent rxn

Transforming light photons into ATP and the
electron carrier NADPH
Use ATP/NADPH to make glucose
2
The Calvin Cycle C3 carbon fixation

• In stroma of chloroplast

Inputs
CO2 ATP
NADPH
from air
from light rxn
e
Output
G3P a 3C organic molecule used to make glucose,
fructose, starch, cellulose
3
The Calvin Cycle C3 carbon fixation
3 turns of the cycle
Rubisco enzyme
RuBP
ATP
ATP
Energy input
Most G3P used to regenerate RuBP
e-
G3P
one extra G3P
Energy stored
4
Calvin Cycle Accounting
Per turn 3 turn 6
turns Consume CO2 1 3 6 Consume
ATP 3 9 18 Consume NADPH 2 6
12 Produces G3P 1
2 Produces Glucose 1
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http//www.science.smith.edu/departments/Biology/B
io231/calvin.html http//faculty.nl.edu/jste/calv
in_cycle.htm http//highered.mcgraw-hill.com/site
s/0070960526/student_view0/chapter5/animation_quiz
_1.html http//www.blinkx.com/watch-video/photosy
nthesis-bioflix-animation/zWVQ8TsGQEG7FGsd2MWPTw
http//www.wiley.com/college/boyer/0470003790/anim
ations/photosynthesis/photosynthesis.htm
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This version of photosynthesis is used by 85 of
plants - the C3 plants C3
plants include grains, soy, legumes, cotton,
most trees, and more
What about the other 15? i.e. the C4 and CAM
plants

• Selection produces functional adaptations,
  but not necessarily at optimal efficiency

7
In hot, dry conditions, plant will undergo
Photorespiration

• to conserve water, leaf stomata close,
  preventing CO2 intake and O2 release.

as CO2 concentration in the chloroplasts drops,
Rubisco fixes oxygen instead of CO2.
Photorespiration wastes radiant energy no
glucose is made, no ATP is made
Whats a plant to do?
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C4 Plants separate C-fixation and the Calvin
cycle into different cells
CO2 uptake in mesophyll cells Light reaction
occurs O2 produced stays here 4C molecule PEP -
made

PEP shuttles CO2 to bundle-sheath cells 4C
3C CO2 used in Calvin cycle Therefore can
keep stomata closed
9
C4 Plants corn, sugarcane
In mesophyll
H20 input light rxn O2 , PEP fixes C PEP yields
CO2

In bundle-sheath cells
Calvin cycle G3P sugar
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CAM Plants C-fixation and Calvin at different
times of the day

• Avoiding evaporative loss
• Stomates open at night
• CO2 is converted to an
• acid and stored

hottest, driest conditions
During the day, the acid is broken down CO2
released to RUBISCO for photosynthesis
CAM plants can idle a while