The Calvin Cycle

What is the Calvin cycle?

The Calvin cycle is the second stage of photosynthesis, also called the light-independent reactions, where carbon atoms from CO₂ are fixed into organic molecules to synthesize sugars, using ATP and NADPH produced in the light reactions.

Where does carbon enter the plant for the Calvin cycle?

Carbon enters the interior of a leaf via pores called stomata and diffuses into the stroma of the chloroplast, which is the site of the Calvin cycle reactions.

Why are Calvin cycle reactions called light-independent reactions?

They are called light-independent reactions because they are not directly driven by light, although they rely on ATP and NADPH produced by the light reactions.

What molecules fuel the Calvin cycle?

ATP and NADPH produced in the light reactions of photosynthesis fuel the Calvin cycle.

Where in the chloroplast does the Calvin cycle occur?

The Calvin cycle occurs in the stroma, the inner space of the chloroplast, unlike the light reactions which occur in the thylakoid membrane.

What are the three main stages of the Calvin cycle?

The three main stages are: 1) Carbon fixation, 2) Reduction, 3) Regeneration of the starting molecule (RuBP).

What happens during carbon fixation?

During carbon fixation, a CO₂ molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP), forming an unstable six-carbon compound that splits into two molecules of 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme rubisco (RuBP carboxylase/oxygenase).

What is the role of rubisco in the Calvin cycle?

Rubisco catalyzes the fixation of CO₂ to RuBP, producing a six-carbon compound that immediately splits into two molecules of 3-PGA.

What occurs during the reduction phase of the Calvin cycle?

In the reduction phase, six ATP and six NADPH are used to convert six 3-PGA molecules into six molecules of glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. NADPH donates electrons to reduce the intermediate to G3P.

Why is the reduction phase called “reduction”?

It is called reduction because NADPH donates electrons to a three-carbon intermediate, thereby reducing it to form G3P.

What occurs during the regeneration phase of the Calvin cycle?

During regeneration, five molecules of G3P are recycled through a complex series of reactions requiring ATP to regenerate three molecules of RuBP, the CO₂ acceptor, while one G3P exits the cycle to contribute to glucose synthesis.

How many CO₂ molecules are needed for one G3P molecule to exit the Calvin cycle?

Three CO₂ molecules must enter the cycle to provide three new atoms of fixed carbon, producing six G3P molecules, one of which exits the cycle.

How many turns of the Calvin cycle are required to produce one G3P molecule that can exit the cycle?

Three turns of the Calvin cycle are required to produce one G3P molecule that exits the cycle to contribute to glucose synthesis.

How many ATP molecules are used in producing one net G3P molecule?

Nine ATP molecules are used in total: six during the reduction phase and three during the regeneration phase.

How many NADPH molecules are used in producing one net G3P molecule?

Six NADPH molecules are used during the reduction phase.

How is glucose produced from the Calvin cycle?

Two molecules of G3P, each containing three carbon atoms, are combined to form one six-carbon glucose molecule. Since one G3P exits per three CO₂ molecules, six turns of the cycle are needed to produce one glucose molecule.

How many ATP and NADPH molecules are required to produce one molecule of glucose?

To produce one glucose molecule, six turns of the Calvin cycle require 18 ATP and 12 NADPH molecules.

What happens to the oxygen atom from CO₂ during the Calvin cycle?

The transcript indicates that the oxygen atom from CO₂ is retained in the G3P molecule and is not released as O₂; O₂ production occurs during the light reactions, not the Calvin cycle.

Why is the Calvin cycle considered a cycle?

It is considered a cycle because it regenerates its starting molecule, RuBP, allowing continuous fixation of CO₂.

Where exactly in the chloroplast is glucose synthesized from G3P?

Glucose is synthesized from G3P in the stroma of the chloroplast.

What nickname did a college professor give to the regeneration phase?

The regeneration phase was humorously called the “carbohydrate scramble” due to the complex series of reactions that recycle G3P to regenerate RuBP.

How many G3P molecules are produced when three CO₂ molecules enter the Calvin cycle?

Six G3P molecules are produced; one exits the cycle to form glucose and five are recycled to regenerate RuBP.