6.6

The energy in ATP and NADPH is used in the carbon-fixation reactions to ‘fix’ CO2 into a reduced form and convert it into carbohydrates

Most CO2 fixation occurs in the light, when ATP and NADPH are being generated.

This metabolic pathway occurs in the stroma, or central region, of the chloroplast

Calvin cycle: the energy in ATP and NADPH is used to “fix” CO2 in reduced form in carbohydrates

Each reaction is catalyzed by a specific enzyme.

The cycle is composed of three distinct processes.

1. Fixation of CO2

2. Reduction of 3PG to form glyceraldehyde 3-phosphate

3. Regeneration of the CO2

acceptor, RuBP

1. CO2 combines

with its acceptor,

RuBP, forming

3PG

2. 3PG is reduced

to G3P in a two-

step reaction

requiring ATP and

NADPH

3. About one-sixth of the

G3P molecules are used to

make sugars – the output

of the cycle

4. The remaining five-

sixths of the G3P

molecules are

processed in a series

of reactions that

reduce RuMP

5. RuMP is converted

to RuBP in a reaction

requiring ATP. RuBP

is ready to accept

another CO2

TEDEd – Natures Smallest Factory

Khan Academy – Calvin Cycle

The initial reaction of the Calvin cyleadds the one-carbon CO2 to an acceptor molecule, the five-carbon ribulose 1,5-biphosphate (RuBP)

This 6-carbon molecule quickly splits into two 3-carbon molecules: 3-phosphoglycerate (3PG)

Ribulose bisphosphate carboxylase/oxygenase (rubisco)catalyzes the reaction.

Rubisco is a slug, relatively speaking when talking about enzymes, therefore plants require a lot of it, which actually constituents almost half of all the protein in a leaf

The fate of the

carbon atom in CO2

is followed in red

The enzyme

rubisco catalyzes

the reaction of CO2

with RuBPThe reaction intermediate

splits into two molecules of

3-phosphoglycerate (3PG)

This series of reactions involves phosphorylation using the high energy P from ATP made in the light reaction and reduction from the NADPH also from the light reaction

The product is 3PG is reduced to form glyceraldehyde 3-phosphate (G3P).

This is a 3-carbon sugar phosphate, also known as a triose phosphate

Most of the G3P ends up as ribulose monophosphate (RuMP), and ATP is used to covert this compound into RuBP.

So with every turn of the Calvin cycle, one CO2 is fixed and the CO2 acceptor is regenerated.

The CO2 acceptor RuBP is regenerated from G3P.

What happens to extra G3P made by the Calvin cycle?

It has two fates: depending on the time of day and the needs of different parts of the plant

Some of the extra G3P is exported to the cytosol and is converted to hexoses (glucose and fructose).

These molecules can then be used as part of cellular respiration; used as carbon skeletons for synthesis of amino acids and other molecules; or converted to sucrose, which can be transported out of the leaf to another part of the plant

When glucose accumulates, it is linked to form starch, a storage carbohydrate.

This storage carbohydrate can be drawn upon at night or stored in the roots for growth and other cellular activites

The products are crucial to the biosphere of the Earth

The C—H bonds generated by the Calvin cycle provide almost all the energy for life on Earth.

Photosynthetic organisms (autotrophs) use most of this energy to support their own growth and reproduction.

Heterotrophs cannot photosynthesize and depend on autotrophs for chemical energy.