Two stages of photosynthesis
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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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Two Stages of Photosynthesis

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Two Stages of Photosynthesis

Light Reaction

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

The Calvin Cycle : C3 carbon fixation

In stroma of chloroplast



from air

from light rxn



G3P: a 3C organic molecule used to make glucose,

fructose, starch, cellulose…

The Calvin Cycle : C3 carbon fixation

3 turns

of the cycle

Rubisco enzyme




Energy input

Most G3P used to regenerate




one ‘extra’


Energy stored

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

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 oxygeninstead of CO2.

• Photorespiration wastes radiant energy

no glucose is made, no ATP is made

What’s a plant to do?

C4 Plantsseparate 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

C4 Plantscorn, sugarcane

In mesophyll:

H20 input

light rxn

O2 , PEP fixes C

PEP yields CO2

In bundle-sheath cells:

Calvin cycle

G3P sugar

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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