Question 1

Which enzyme is responsible for catalyzing the fixation of CO$$_2 to $$ribulose-1,5-bisphosphate (RUBP) in the Calvin cycle?

Accepted Answer

Rubisco

Question 2

What is the primary role of ATP and NADPH in the Calvin cycle?

Accepted Answer

They provide energy and reducing power for the reduction of 3-phosphoglycerate to glyceraldehyde-3-phosphate.

Question 3

If a plant is exposed to light but deprived of CO$$_2$$, what would be the most immediate effect on the Calvin cycle?

Accepted Answer

ATP and NADPH would accumulate because the Calvin cycle cannot proceed without CO$$_2.$$

Question 4

Which of the following best describes the three main phases of the Calvin cycle?

Accepted Answer

Fixation, reduction, regeneration

Question 5

How does light indirectly regulate the activity of Rubisco in the Calvin cycle?

Accepted Answer

By increasing the pH of the stroma, which alters hydrogen bonds in Rubisco and increases its activity.

Question 6

What would happen to the Calvin cycle if the light reactions were inhibited?

Accepted Answer

The Calvin cycle would slow down or stop due to a lack of ATP and NADPH.

Question 7

Which of the following statements about the products of the Calvin cycle is correct?

Accepted Answer

The main product is a highly energized three-carbon sugar, glyceraldehyde-3-phosphate (G3P).

Question 8

Why is it inaccurate to refer to the Calvin cycle as the 'dark reactions'?

Accepted Answer

Because the Calvin cycle depends on ATP and NADPH produced by the light reactions, and is indirectly dependent on light.

Question 9

If three CO$$_2$$ molecules enter the Calvin cycle, how many molecules of 3-phosphoglycerate (3-PG) are produced?

Accepted Answer

$$6$$

Question 10

Which bonds within the Rubisco protein are most likely to change as the pH of the stroma increases during the light reactions?

Accepted Answer

Hydrogen bonds

Question 11

What is the immediate fate of the unstable 6-carbon intermediate formed when CO$$_2 is $$fixed to RUBP?

Accepted Answer

It splits into two molecules of 3-phosphoglycerate (3-PG).

Question 12

Which of the following best explains why RUBP must be regenerated in the Calvin cycle?

Accepted Answer

RUBP is required as a substrate for Rubisco to fix additional CO$$_2.$$

Question 13

A mutation causes a plant to produce a Rubisco enzyme that cannot change shape in response to pH. What is the most likely consequence?

Accepted Answer

Rubisco activity would not increase in the light, reducing the rate of carbon fixation.

Question 14

Which of the following is a direct product of the reduction phase of the Calvin cycle?

Accepted Answer

Glyceraldehyde-3-phosphate (G3P)

Question 15

If the pH of the stroma drops from 8 to 6, what is the most likely effect on Rubisco activity?

Accepted Answer

Rubisco activity would decrease because the enzyme is most active near pH 8.

Question 16

Which of the following best describes the connection between the light reactions and the Calvin cycle?

Accepted Answer

The light reactions produce ATP and NADPH, which are used in the Calvin cycle for carbon fixation and sugar production.

Question 17

How many molecules of RUBP are required to fix three molecules of CO$$_2 in $$the Calvin cycle?

Accepted Answer

$$3$$

Question 18

Which of the following would most likely occur if a plant was placed in complete darkness?

Accepted Answer

The Calvin cycle would slow down or stop due to a lack of ATP and NADPH.

Question 19

Which molecule is NOT a direct product or intermediate of the Calvin cycle?

Accepted Answer

Oxygen gas (O$$_2)$$

Question 20

A researcher adds a chemical that blocks ATP synthesis in a plant cell. What is the most immediate effect on the Calvin cycle?

Accepted Answer

The reduction and regeneration phases would halt due to lack of ATP.

Photosynthesis: The Calvin Cycle and Carbon Fixation

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