Which of the following sequences correctly represents the flow of electrons during photosynthesis? a. NADPH→O2→CO2 b. H2O→NADPH→Calvin cycle c. H2O→photosystem I→photosystem II d. NADPH→electron transport chain→O2

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Identify the source of electrons in photosynthesis: In photosynthesis, water (H2O) is the initial electron donor which gets split into oxygen, protons, and electrons.

Understand the role of photosystems: Electrons from water first go to photosystem II (not I), where they are energized by sunlight.

Trace the path of electrons: After photosystem II, the energized electrons travel through an electron transport chain to photosystem I, where they are re-energized by sunlight.

Recognize the final electron acceptor: The energized electrons from photosystem I are then used to reduce NADP+ to NADPH.

Match the correct sequence: The sequence that correctly represents this flow is 'H2O→photosystem II→photosystem I→NADPH', which is closest to option b. H2O→NADPH→Calvin cycle, as it includes the correct initial and final electron states and their use in the Calvin cycle.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Photosynthesis Overview

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, primarily in the form of glucose. It occurs in two main stages: the light-dependent reactions, which capture energy from sunlight, and the light-independent reactions (Calvin cycle), which use that energy to synthesize glucose from carbon dioxide and water.

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes located in the thylakoid membrane of chloroplasts that facilitate the transfer of electrons derived from water. As electrons move through the chain, they release energy used to pump protons into the thylakoid lumen, creating a proton gradient that drives ATP synthesis and ultimately leads to the production of NADPH.

Role of NADPH and Oxygen

NADPH is a crucial electron carrier produced during the light-dependent reactions of photosynthesis, serving as a reducing agent in the Calvin cycle to help convert carbon dioxide into glucose. Oxygen is released as a byproduct of water splitting during these reactions, highlighting the importance of water as an electron source in the overall process of photosynthesis.