10.3 Calvin Benson Cycle

The calvin Benson cycle like the citric acid cycle

regenerates its starting material after molecules enter and leave the cycle

the calvin benson cycle builds

sugar from smaller molecules by using ATP as an energy source and consumes NADPH as a reducing power for adding high energy electrons to make the sugar

Carbon enters the Calvin-Benson Cycle

as CO₂

Carbon leaves the Calvin-Benson Cycle as

a three carbon sugar called glyceraldehyde 3-phospate

(G3P)

for the net synthesis of 1 G3P

the cycle must take place three times, fixing three molecules of CO₂

the Calvin-Benson Cycle has three phases

• carbon fixation

• reduction

• regeneration of the CO₂ acceptor (RuBP)

Carbon Fixation

CO₂ is incorporated into organic matter one at a time by attaching them to ribulose bisphosphate (RuBP), this is catalyzed by Rubisco enzyme

The product of carbon fixing is a

six carbon that is so unstable it breaks into two 3-phosphoglycerate

Reduction

ATP and NADPH are used to reduce 3-phosphoglycerate to glyceraldehyde 3-phosphate (G3P),

for every six G3P molecules

one G3P is used to build sugars

Regeneration of the CO₂ acceptor RuBP

ATP and 5 G3P molecules regenerate 3 RuBP molecules

For the net synthesis of one G3P molecules the Calvin-Benson Cycle consumes

9ATP and 6 NADPH

light reactions regenerate

the ATP and the NADPH

The G3 spun off from the Calvin-Benson Cycle becomes the starting

material for the metabolic pathways that synthesize other organic compounds like glucose and other carbohydrates

therefore neither

light reactions nor Calvin-Benson Cycle can make sugar from CO₂ alone

only about

50 percent of the sugar made during photosynthesis is consumed as fuel for cellular respiration in the plants mitochondria

Plants store excess sugar as starch in structures such

as roots, tubers, seeds, and fruits