In green plants, Photosystem I has a reaction center chlorophyll with a maximum absorption at approximately which wavelength?

Introduction / Context:
Each photosystem is named after the approximate wavelength (in nanometers) at which its special pair of chlorophyll molecules absorbs maximally. Knowing these “P numbers” helps you map energy flow through PSI and PSII.

Given Data / Assumptions:

• Photosystem I reaction center is termed P700.
• Photosystem II reaction center is termed P680.
• Absorption maxima are approximate and context-dependent but widely taught values.

Concept / Approach:
By definition, P700 indicates a chlorophyll special pair that absorbs best near 700 nm in the far-red region. Thus, PSI’s reaction center peaks around 700 nm, enabling it to accept electrons from plastocyanin and deliver high-energy electrons to ferredoxin.

Step-by-Step Solution:

Verification / Alternative check:
Spectral measurements of isolated PSI complexes show strong far-red absorption near 700 nm, consistent across plant species with minor shifts due to environment/protein context.

Why Other Options Are Wrong:

• 550, 600, 650, 750 nm: do not match the well-known P700 designation for PSI in plants.

Common Pitfalls:
Swapping P680 and P700 between PSII and PSI; remember the order in the Z-scheme: PSII (P680) → PSI (P700).

Final Answer:
700 nm.