Concept 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce to sugar

Question 1

citric acid cycle is catabolic, oxidizing acetyl CoA and using the energy to synthesize ATP, while the Calvin cycle is anabolic, building carbohydrates from

Answer 1

smaller molecules and consuming energy.

Question 2

Carbon enters the Calvin cycle in the form of and leaves in the form of

Answer 2

sugar.

Question 3

The cycle spends ATP as an energy source and consumes NADPH as reducing power for adding

Answer 3

high-energy electrons to make the sugar.

Question 4

As we mentioned in Concept 10.1, the carbohydrate produced directly from the Calvin cycle is not

Answer 4

glucose.

Question 5

It is actually a three-carbon sugar called

Answer 5

glyceraldehyde 3-phosphate (G3P)

Question 6

For the net synthesis of one molecule of G3P, the cycle must take place

Answer 6

three times, fixing three molecules of —one per turn of the cycle. (Recall that the term carbon fixation refers to the initial incorporation of into organic material.)

Question 7

For the net synthesis of one molecule of G3P, the cycle must take place

Answer 7

three times, fixing three molecules of —one per turn of the cycle. (Recall that the term carbon fixation refers to the initial incorporation of into organic material.)

Question 8

The Calvin cycle incorporates each molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate (abbreviated RuBP).

Answer 8

Phase 1: Carbon fixation.

Question 9

The enzyme that catalyzes this first step is RuBP carboxylase-oxygenase, or

Answer 9

rubisco. (This is the most abundant protein in chloroplasts and is also thought to be the most abundant protein on Earth.)

Question 10

The product of the reaction is a six-carbon intermediate that is short-lived because it is so energetically unstable that it immediately

Answer 10

splits in half, forming two molecules of 3-phosphoglycerate (for each fixed).

Question 11

Each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP, becoming 1,3-bisphosphoglycerate.

Answer 11

Phase 2: Reduction.

Question 12

Next, a pair of electrons donated from NADPH reduces 1,3-bisphosphoglycerate, which also loses a phosphate group in the process, becoming

Answer 12

glyceraldehyde 3-phosphate (G3P).

Question 13

Specifically, the electrons from NADPH reduce a carboyxl group on 1,3-bisphosphoglycerate to the aldehyde group of G3P, which stores more

Answer 13

potential energy.

Question 14

G3P is a sugar—the same three-carbon sugar formed in glycolysis by the splitting of

Answer 14

glucose

Question 15

In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP.

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Phase 3: Regeneration of the acceptor (RuBP).

Question 16

To accomplish this, the cycle spends three more molecules of ATP. The RuBP is now prepared to

Answer 16

receive again, and the cycle continues.

Question 17

For the net synthesis of one G3P molecule, the Calvin cycle consumes a total of

Answer 17

nine molecules of ATP and six molecules of NADPH.

Question 18

The light reactions regenerate the

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ATP and NADPH

Question 19

G3P spun off from the Calvin cycle becomes the starting material for metabolic pathways that synthesize other organic compounds, including

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glucose (from two molecules of G3P), the disaccharide sucrose, and other carbohydrates.

Question 20

Neither the light reactions nor the Calvin cycle alone can make sugar from

Answer 20

CO2.