During photosynthesis, green plants convert solar energy, water (H_2O), and carbon dioxide… — Armed Services Vocational Aptitude Battery (ASVAB)

During photosynthesis, green plants convert solar energy, water ( $H_2O$ ), and carbon dioxide ( $CO_2$ ) into chemical energy and gas byproducts. If a plant is provided with water labeled with a heavy oxygen isotope ( $^{18}O$ ), in which photosynthetic product will this labeled oxygen primarily be detected?

A
Glucose ( $C_6H_{12}O_6$ ) molecules synthesized during the Calvin cycle
Molecular oxygen gas ( $O_2$ ) released during the light-dependent reactionsAnswer
C
Carbon dioxide ( $CO_2$ ) gas released directly by chloroplasts
D
Cellulose fibers incorporated into the animal-like cell membrane

Answer

Molecular oxygen gas (

O_2

) released during the light-dependent reactions

During the light-dependent reactions of photosynthesis within the thylakoid membranes of chloroplasts, photon energy drives photolysis, splitting

H_2O

into hydrogen ions, electrons, and molecular oxygen. Consequently, all oxygen atoms released as atmospheric

O_2

originate strictly from water. Therefore, labeling water with

^{18}O

results directly in labeled

O_2

gas.

Step-by-Step Solution

Identify the stage of photosynthesis where water (

H_2O

) is consumed.

Water is utilized during the light-dependent reactions occurring on the thylakoid membranes of chloroplasts.

Light energy is absorbed by chlorophyll to power the photolysis (splitting) of water molecules.

Determine the biochemical fate of the atoms derived from water splitting.

Water is split into protons (

H^+

), electrons (

e^-

), and diatomic oxygen gas (

O_2

Electrons replace those excited in photosystem II, while the oxygen atoms combine and are liberated into the atmosphere as

O_2

Trace the isotope

^{18}O

from labeled water.

The

^{18}O

label appears directly in the released

O_2

gas rather than organic sugars.

The oxygen atoms in carbohydrates like glucose (

C_6H_{12}O_6

) are derived from the carbon dioxide (

CO_2

) fixed during the Calvin cycle.

Key Concept

Photolysis of water in the light-dependent reactions of photosynthesis as the source of atmospheric oxygen

Estimated Time:1m 30s

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