Question 1

Jan Baptista van Helmont’s willow tree experiment showed that a tree gained significant mass over several years, while the soil lost very little mass. Based on modern understanding and the results of heavy-oxygen experiments, what is the primary source of the mass gained by the tree?

Accepted Answer

Carbon dioxide from the air

Question 2

In Ruben’s heavy-water experiment, algae were given $$\mathrm{H_2$^{18}O}$$ and ordinary $$\mathrm{CO_2$}. $$After photosynthesis, the released $$\mathrm{O_2$}$$ was found to be heavy. What does this result demonstrate?

Accepted Answer

The oxygen released by plants comes from water

Question 3

Which of the following best describes the role of light in the light reactions of photosynthesis?

Accepted Answer

Light provides the energy to lift electrons from water to a higher energy state

Question 4

What is the correct sequence of electron flow during the light reactions of photosynthesis?

Accepted Answer

Water → Photosystem II → Electron Transport Chain → Photosystem I → NADPH

Question 5

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

Accepted Answer

It fixes carbon dioxide into organic molecules using ATP and NADPH

Question 6

A plant is placed in complete darkness but has access to water and carbon dioxide. What will happen to its metabolic processes within a few minutes?

Accepted Answer

The light reactions will stop, the Calvin cycle will soon stall, but respiration will continue

Question 7

Which enzyme is responsible for fixing carbon dioxide in the Calvin cycle, and what is a notable property of this enzyme?

Accepted Answer

Rubisco; it is the most abundant protein on Earth but also slow and sometimes binds O₂

Question 8

Why are leaves green, according to the absorption spectrum of chlorophyll?

Accepted Answer

Chlorophyll reflects green light and absorbs blue and red light

Question 9

Which of the following equations correctly represents the overall process of photosynthesis?

Accepted Answer

$$6\ \mathrm{CO_2$} + 6\ \mathrm{H_2O} \rightarrow \mathrm{C_6H_{12}O_6$} + 6\ \mathrm{O_2$}$$

Question 10

What is the main function of NADPH in photosynthesis?

Accepted Answer

To carry high-energy electrons from the light reactions to the Calvin cycle

Question 11

If a herbicide blocks photosystem II, what is the immediate effect on the plant’s metabolism?

Accepted Answer

O₂ release stops, and ATP and NADPH production cease

Question 12

Which of the following best explains why plants must also respire, even though they perform photosynthesis?

Accepted Answer

Respiration provides ATP for cellular processes when photosynthesis is not occurring

Question 13

During the light reactions, what is the direct source of electrons that are ultimately transferred to NADP⁺ to form NADPH?

Accepted Answer

Water

Question 14

Which of the following is a correct description of photophosphorylation?

Accepted Answer

The production of ATP using a proton gradient generated by the light reactions in chloroplasts

Question 15

A potato stored in a dark cupboard loses weight over time. Which process is primarily responsible for this loss of mass?

Accepted Answer

Respiration, converting stored starch to CO₂ and H₂O

Question 16

Which of the following best describes the relationship between the thylakoid membrane and ATP synthesis in the chloroplast?

Accepted Answer

A proton gradient across the thylakoid membrane drives ATP synthesis in the stroma

Question 17

Which of the following is NOT a product of the light reactions of photosynthesis?

Accepted Answer

Glucose

Question 18

What is the main reason two photosystems (II and I) are required in the light reactions of photosynthesis?

Accepted Answer

One photon is not enough energy to lift electrons from water to NADPH; two lifts are needed

Question 19

Which of the following best explains why the Calvin cycle cannot continue indefinitely in the dark?

Accepted Answer

It depends on ATP and NADPH produced by the light reactions, which stop in the dark

Question 20

Which of the following statements about the flow of matter and energy in photosynthesis and respiration is correct?

Accepted Answer

Matter cycles, but energy flows through and is eventually lost as heat

Photosynthesis: Lifting Electrons and the Calvin Cycle

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