Carbon enters the cycle in the form of CO 2 and leaves in the form of sugar (C 6 H 12 O 6 ) ATP and NADPH are consumed

the carbohydrate produced is actually a 3-C sugar: G3P  same sugar from Cell. Resp. to make 1 molecule of G3P, the cycle occurs 3X (consumes 3CO 2 )

The Calvin Cycle can be divided into 3 phases 1.Carbon Fixation 2.Reduction 3.Regeneration of CO 2 acceptor (RuBP)

Phase 1: Carbon Fixation each CO 2 is attached to a 5-C sugar, RuBP, by the enzyme RUBISCO This forms an unstable 6-C sugar which immediately splits into two 3- C molecules of 3-phosphoglycerate (GP or PGA) 3-C (GP) + 3-C (GP)

Phase 2: Reduction each GP receives an additional P i from ATP, forming 1,3-bisphosphoglycerate Next, a pair of electrons from NADPH reduces the 1,3-bisphosphoglycerate to G3P

3CO G3P 1 G3P (a sugar 3 5-C RuBP to be incorporated into glucose, etc.) 5 G3P (returned to cycle) P P P P P P

unstable intermediate NADPH NADP+ CO 2 RuBP G3P -1 to glucose -5 recycled GP

Phase 3: Regeneration of RuBP the 5 molecules of G3P are rearranged through a series of reactions to regenerate RuBP (this uses an additional 3 ATP) –This is the reason for Cyclic electron flow!! Calvin cycle consumes: –9 ATP –6 NADPH Leads to negative feedback!

more ATP than NADPH is used per molecule of CO 2

unstable intermediate NADPH NADP+ 3 CO 2 RuBP G3P -1 to glucose -5 recycled more ATP than NADPH is used per molecule of CO 2

Net Equation for Photosynthesis : 6CO 2 + 6H 2 O + light C 6 H 12 O 6 + 6O 2 energy chloroplast

Mechanisms of C-Fixation C 3 Plants : plants which combine CO 2 to RuBP, so that the first organic product of the cycle is the 3-C molecule 3-phosphoglycerate –examples: rice, wheat, soybeans –produce less food when their stomata close on hot, dry days; this deprives the Calvin cycle of CO 2 and photosynthesis slows down

PHOTORESPIRATION: consumes O 2, releases CO 2 into peroxisomes, generates no ATP, decreases photosynthetic output O 2 replaces CO 2 in a non-productive, wasteful reaction; Oxygen OUTCOMPETES CO 2 for the enzyme’s active site! Product leaves chloroplast

CO 2 not able to be used in Calvin Cycle

1.C 4 Plants: use an alternate mode of C- fixation which forms a 4-C compound as its first product *Calvin Cycle still occurs! This 1 st product is an intermediate step prior to Calvin Cycle.

–examples: sugarcane, corn, grasses –the 4-C product is produced in mesophyll cells –It is then exported to bundle sheath cells (where the Calvin cycle occurs); Has a higher affinity for CO 2 than RuBP does

–once here, the 4-C product releases CO 2 which then enters the Calvin cycle & combines with RuBP (w/ the help of Rubisco) –this process keeps the levels of CO 2 sufficiently high inside the leaf cells, even on hot, dry days when the stomata close

2.CAM Plants: stomata are open at night, closed during day (this helps prevent water loss in hot, dry climates) –at night, plants take up CO 2 and incorporate it into organic acids which are stored in the vacuoles until day (intermediate step, prior to Calvin Cycle); –then CO 2 is released once the light reactions have begun again –examples: cactus plants, pineapples

C 4 and CAM Pathways are: Similar: in that CO 2 is first incorporated into organic intermediates before Calvin cycle Different: -in C 4 plants the first and second steps are separated spatially (different locations) -in CAM plants the first and second steps are separated temporally (occur at different times)

What happens to the products of photosynthesis? O 2 : consumed during cellular respiration 50% of sugar made by plant is consumed by plant in respiration some are incorporated into polysaccharides: -cellulose (cell walls) -starch (storage; in fruits, roots, tubers, seeds)

What “powers” the Calvin Cycle (light independent reactions)? A.Wavelengths of light B.ATP & NADH C.ATP & NADPH D.ATP

Where in the chloroplast does the Calvin Cycle take place? A.Stroma B.Thylakoid membranes C.Granum D.Cristae

Why is Phase 2 of the Light Independ. Reactions titled “reduction?” A.Because an inorganic compound becomes organic. B.Because RuBP gets reduced by G3P. C.Because energy is stored in the bonds of a 3-C sugar. D.Because the ending 6 molecules of G3P are reduced to 5 when 1 is removed from the cycle to become part of a glucose

Why do C-4 plants have to use 2 cells for Photosynthesis? A.Because the stomata close during the heat of the day B.Because rubisco has an almost equal affinity for oxygen & carbon dioxide C.Because RuBP will only bind to carbon dioxide in a particular cell D.Because Photosystem II & I are located in different cells