25. Upon absorption of a photon, a chlorophyll molecule get converted to its excited state when the energy of photon is
(1) more than that of ground state of pigment molecule
(2) equal to that of pigment molecule’s excited state
(3) more than that of ground state but lesser then excited state of the pigment molecule
(4) equal to energy gap between ground state and excited state energy

The correct answer is (4): Upon absorption of a photon, a chlorophyll molecule gets excited when the energy of the photon is equal to the energy gap between the ground state and the excited state energy of the pigment molecule. The excitation only takes place when the photon energy precisely matches this energy gap, which is called resonance absorption.

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Chlorophyll Excitation: Photon Energy Requirements

Introduction

Key phrase: chlorophyll molecule excitation photon energy ground state excited state

In photosynthesis, the absorption of light by chlorophyll triggers electron excitation, which is the foundation of energy conversion. Only photons with energy exactly matching the energy gap can push an electron from the ground to the excited state, resulting in photochemical events.

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Explanation of Each Option

• (1) More than that of ground state of pigment molecule

  • Incorrect. Photons with energy greater than the ground state but not equaling the energy gap either do not excite the pigment or result in excess energy being lost as heat rather than causing excitation.

• (2) Equal to that of pigment molecule’s excited state

  • Incorrect wording. The “excited state” is a fixed property, but it is the energy difference between ground and excited states that matters. The photon must match the energy gap, not the excited state’s absolute energy.

• (3) More than that of ground state but lesser than excited state of the pigment molecule

  • Incorrect. This describes an energy in between the ground and excited state, which cannot cause the necessary excitation. Only photons at the right energy gap will suffice.

• (4) Equal to energy gap between ground state and excited state energy

  • Correct. Photons must have energy equal to the energy difference (gap) between the pigment’s ground and excited state. This ensures efficient absorption and excitation in photosynthesis.

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Key Facts: Chlorophyll and Photon Energy

• Resonant absorption means photon energy must precisely match the energy gap between ground and excited states.

• Excess photon energy is dissipated as heat or lost via other processes.

• The principle applies to all photo-activated molecules, not just chlorophyll.