Photosynthesis: the Calvin Cycle

Photosynthesis: the Calvin Cycle
• Light Independent Reactions
– Can occur in the absence of light
– Energy that was stored in ATP and NADPH during the
light reactions is used to produce organic compounds
in the form of sugars
• Carbon fixation
– Calvin cycle- series of enzyme- assisted chemical
reactions that produces a three-carbon sugar
– Carbon fixation- incorporation of CO2 into organic
• Carbon fixation
– Steps of the calvin cycle
1. each of the three CO2 molecules combines
with a molecule of ribulose biphosphate (RuBP).
Each resulting six-carbon molecule immediately
splits into two molecules of 3-phophoglycerate
2. ATP and NADPH are used to convert each
molecule of 3-PGA into glyceraldehyde 3phosphate (G3P)
3. One molecule of G3P is used to make organic
compounds like glucose
• 4 the rest of the G3P is converted back into
RuBP– C3 plants- species that fix carbon exclusively
through the Calvin cycle
• Alternative pathways
– C4 pathway
• Found in plants that grow in hot climates and
need to keep stomata closed to conserve
water; low CO2
• Enzyme used to fix CO2 into four-carbon
compounds, which are used for the Calvin cycle
when needed
• Such plants lose only about half as much water
as C3 plants when producing the same amount
of carbohydrates
• Alternative pathway
– CAM pathway
1. CAM = crassulacean acid metabolism
2. Plants open stomata at night to absorb CO2 and close
them during the day
3. CO2 converted into other molecules and stored until
4. Grow slowly, but they lose less water than either C3 or
C4 plants