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
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 c 3 carbon fixation
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 c 3 carbon fixation1
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
In hot, dry conditions, plant will undergo the CPhotorespiration

• 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?

C 4 plants separate c fixation and the calvin cycle into different cells
C the 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

C 4 plants corn sugarcane
C the 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
CAM the CPlants 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