Question 1

Which enzyme is responsible for the initial fixation of CO2 in the Calvin cycle, and why is its activity considered inefficient?

Accepted Answer

Rubisco; because it can catalyze the addition of both CO2 and O2 to RuBP, leading to photorespiration

Question 2

What is the main ecological significance of the reaction catalyzed by Rubisco?

Accepted Answer

It fixes inorganic CO2 into organic molecules, making carbon available for life

Question 3

During the Calvin cycle, what is the product of the fixation phase when CO2 reacts with ribulose bisphosphate (RuBP)?

Accepted Answer

Two molecules of 3-phosphoglycerate (3PG)

Question 4

Which equation best represents the overall process of photosynthesis?

Accepted Answer

$$\mathrm{CO_2$ + H_2O + light\ energy \rightarrow C_6H_{12}O_6$ + O_2$ + H_2O}$$

Question 5

Why do C4 plants have an advantage over C3 plants in hot, dry environments?

Accepted Answer

C4 plants use PEP carboxylase to fix CO2, which reduces photorespiration by maintaining high CO2 concentrations near Rubisco

Question 6

In CAM plants, how is carbon fixation separated from the Calvin cycle?

Accepted Answer

By temporal separation: carbon fixation occurs at night, Calvin cycle occurs during the day

Question 7

What is photorespiration, and under what conditions is it favored?

Accepted Answer

Photorespiration is the process where Rubisco uses O2 instead of CO2, producing CO2 and consuming O2; it is favored when CO2 is low and O2 is high

Question 8

Which of the following best describes the role of stomata in photosynthesis?

Accepted Answer

Stomata allow CO2 to enter and O2 to exit the leaf, enabling gas exchange necessary for photosynthesis

Question 9

What happens to the G3P molecules produced by the Calvin cycle?

Accepted Answer

They are used to make glucose and fructose, which can be combined to form sucrose or polymerized to form starch

Question 10

Which regulatory mechanism ensures that photosynthesis proteins are produced only when needed?

Accepted Answer

Presence of light triggers production of photosynthesis proteins, while high sugar inhibits their production

Question 11

A plant is exposed to high temperatures and closes its stomata to prevent water loss. What is the most likely consequence for its photosynthetic efficiency?

Accepted Answer

Photosynthetic efficiency decreases due to increased photorespiration as CO2 levels drop and O2 levels rise inside the leaf

Question 12

Which pathway is used by C4 plants to initially fix CO2, and what is the enzyme involved?

Accepted Answer

C4 pathway; PEP carboxylase

Question 13

In the Calvin cycle, which molecule is regenerated to keep the cycle going?

Accepted Answer

Ribulose bisphosphate (RuBP)

Question 14

Which of the following statements about the cuticle of plant leaves is correct?

Accepted Answer

The cuticle prevents water evaporation and restricts gas transport across the plasma membrane

Question 15

Which of the following best explains why photorespiration is considered wasteful for plants?

Accepted Answer

It consumes O2 and releases CO2, effectively undoing the work of photosynthesis

Question 16

Which process occurs in the stroma of chloroplasts and involves the reduction of CO2 to sugar?

Accepted Answer

The Calvin cycle

Question 17

A scientist discovers a plant that fixes CO2 at night and performs the Calvin cycle during the day. Which type of photosynthetic pathway does this plant use?

Accepted Answer

CAM pathway

Question 18

Which molecule is produced during the reduction phase of the Calvin cycle after 3-phosphoglycerate is phosphorylated and reduced?

Accepted Answer

Glyceraldehyde 3-phosphate (G3P)

Question 19

Which of the following is a correct description of the spatial separation of carbon fixation in C4 plants?

Accepted Answer

PEP carboxylase fixes CO2 in mesophyll cells, and Rubisco operates in bundle sheath cells

Photosynthesis: Carbon Fixation and Sugar Production (Chapter 10, Part 2)

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