Photosynthesis converts solar energy into chemical energy stored in carbohydrates through … — Armed Services Vocational Aptitude Battery (ASVAB)

Photosynthesis converts solar energy into chemical energy stored in carbohydrates through a series of coordinated biological stages. In what chronological sequence do these biochemical events occur, from the initial capture of sunlight to the synthesis of stable sugars?

1Chlorophyll pigments absorb photons in photosystem II, energizing electrons and photolyzing water ( $H_2O$ ) to yield oxygen gas ( $O_2$ ) and protons ( $H^+$ ).
2Excited electrons traverse the thylakoid electron transport chain, actively pumping hydrogen ions to establish an electrochemical proton gradient across the thylakoid membrane.
3Protons flow through ATP synthase to produce ATP, while terminal electron transfer reduces $NADP^+$ to NADPH in the stroma.
4The enzyme RuBisCO fixes atmospheric carbon dioxide ( $CO_2$ ) by combining it with ribulose bisphosphate (RuBP) in the stroma.
5Chemical energy from ATP and electrons from NADPH reduce 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P) for glucose assembly.

Answer

The proper chronological sequence begins with photon absorption and water splitting in photosystem II, proceeds through the electron transport chain and proton gradient formation, continues with ATP and NADPH synthesis via chemiosmosis and terminal reduction, moves to carbon fixation catalyzed by RuBisCO, and concludes with the reduction of 3-phosphoglycerate to glyceraldehyde-3-phosphate (G3P).

Photosynthesis functions strictly in two sequential phases: the light-dependent reactions occurring in the thylakoid membrane followed by the light-independent Calvin cycle occurring in the stroma. First, photosystem II absorbs light to excite electrons and split water. Second, the electron transport chain creates a trans-thylakoid proton gradient. Third, ATP synthase produces ATP and terminal electron transfer yields NADPH. Fourth, RuBisCO catalyzes carbon fixation in the stroma. Fifth, ATP and NADPH reduce the fixed intermediates to produce triose phosphate sugars (G3P).

Step-by-Step Solution

Identify the primary light absorption and electron excitation phase.

Light strikes chlorophyll in photosystem II, splitting

H_2O

and releasing

O_2

and

H^+

Photosynthesis requires an initial input of photon energy to extract electrons from water.

Trace the movement of high-energy electrons through membrane carriers.

Electrons pass down the electron transport chain, generating a high concentration of

H^+

in the thylakoid lumen.

Electron movement drives proton pumping to store potential energy across the membrane.

Determine the synthesis of mobile chemical energy carriers.

ATP synthase converts the proton gradient into ATP, and

NADP^+

reductase produces NADPH.

These products act as the indispensable energy currencies and reducing agents for dark reactions.

Locate the entry point of inorganic carbon into organic compounds.

RuBisCO fixes

CO_2

to RuBP in the stroma.

Carbon fixation must precede the reduction phase of the Calvin cycle.

Identify the reduction and sugar formation step.

ATP and NADPH reduce 3-phosphoglycerate to G3P molecules, which are used to build glucose.

This completes the energy transfer from short-lived carriers (ATP/NADPH) to long-term storage carbohydrates.

Key Concept

The two-stage photosynthetic mechanism coupling thylakoid light-dependent energy harvesting with stroma light-independent Calvin cycle sugar synthesis

Estimated Time:1m 30s

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