Transcript Photosynthesis

Two Stages of Photosynthesis
Light Reaction
Calvin Cycle
aka light
dependent
rxn
aka light
independent
reaction
Transforming light
photons into ATP
and the electron
carrier NADPH
Use ATP/NADPH
to make glucose
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The Calvin Cycle : C3 carbon fixation
In stroma of chloroplast
Inputs
CO2
from
air
ATP
from
light
rxn
NADPH
e
Output
G3P: a 3C organic molecule used
to make glucose,
fructose, starch, cellulose…
2
The Calvin Cycle : C3 carbon fixation
3 turns
of the
Rubisco enzyme
RuBP
cycle
ATP
ATP
Most G3P
used to
regenerate
RuBP
Energy
input
e-
G3P
one
‘extra’
G3P
Energy stored
Calvin Cycle Accounting
Per turn
CO2
1
Consume ATP
3
Consume NADPH 2
Produces G3P
Produces Glucose
Consume
3 turn
3
9
6
1
6 turns
6
18
12
2
1
http://www.science.smith.edu/departments/Biology/Bio231/calv
in.html
http://faculty.nl.edu/jste/calvin_cycle.htm
http://highered.mcgrawhill.com/sites/0070960526/student_view0/chapter5/animation_
quiz_1.html
http://www.blinkx.com/watch-video/photosynthesis-bioflixanimation/zWVQ8TsGQEG7FGsd2MWPTw
http://www.wiley.com/college/boyer/0470003790/animations/p
hotosynthesis/photosynthesis.htm
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
1
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
What’s a plant to do?
CO2 uptake in mesophyll cells
Light reaction occurs
O2 produced stays here
4C molecule – PEP - made
C4 Plants
separate Cfixation and the
Calvin cycle into
different cells
PEP shuttles CO2 to
bundle-sheath cells
4C
3C + CO2 used in Calvin cycle
Therefore can keep stomata closed
2
C4 Plants
In mesophyll:
H20 input
light rxn
O2 , PEP fixes C
PEP yields CO2
In bundle-sheath
cells:
Calvin cycle
G3P sugar
corn, sugarcane
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
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