Two Stages of Photosynthesis. Light Reaction aka light dependent rxn. Calvin Cycle aka light independent reaction. Transforming light photons into ATP and the electron carrier NADPH. Use ATP/NADPH to make glucose. The Calvin Cycle : C 3 carbon fixation . In stroma of chloroplast.
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aka light dependent
Calvin Cycle aka light independent reaction
Transforming light photons into ATP and the electron carrier NADPH
Use ATP/NADPH to make glucose
In stroma of chloroplast
CO2 ATP NADPH
from light rxn
G3P: a 3C organic molecule used to make glucose,
fructose, starch, cellulose…
of the cycle
Most G3P used to regenerate
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
This version of photosynthesis is used by 85% of plants - the C3 plantsC3 plants include grains, soy, legumes, cotton, most trees, and more
Selection produces functional adaptations, but not necessarily at optimal efficiency
What about the other 15%?
i.e. the C4 and CAM plants
• to conserve water, leaf stomata close, preventing CO2 intake and O2 release.
• as CO2 concentration in the chloroplasts drops,
Rubisco fixes oxygeninstead 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
PEP shuttles CO2 to
4C 3C + CO2 used in Calvin cycle
Therefore can keep stomata closed
O2 , PEP fixes C
PEP yields CO2
In bundle-sheath cells:
Avoiding evaporative loss
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