The Biology Project: Biochemistry

Photosynthesis Problem Set 1

Problem 9 Tutorial: Fate of excited pigment molecules

The conversion of light energy to chemical energy during photosynthesis begins when an excited pigment molecule:

What occurs when a photon of light energy is absorbed?

The energy of the photon is absorbed, and the photon disappears. Photon absorption boosts an electron from ground state to a higher orbital, in an all-or-none fashion. An "excited state" pigment is produced, with a higher energy level than the ground state pigment, but this excited state is very short-lived, about a billionth of a second.

Conversion of light energy to chemical energy

Light energy is converted to chemical energy when a photochemically excited special chlorophyll molecule of the photosynthetic reaction center loses an electron, undergoing an oxidation reaction.

Primary Event in Photosynthesis

The conversion of light energy to chemical energy is summarized in the diagram to the right. Light energy from the absorption of a photon excites an electron of P680 (photosystem II) or P700 (photosystem I) to a higher energy level. The reducing power of the electron is increased approximately 1 volt. Within about a billionth of a second, the excited pigment molecule undergoes an oxidation reaction. There is a corresponding reduction reaction of an electron carrier in the thylakoid membrane. The oxidized pigment is subsequently reduced by accepting an electron from water (P680+ of photosystem II) or plastocyanin (P7600+ of photosystem I). A good analogy of the reaction centers is to a photovoltaic cell, that converts sunlight into an electrical current.

The Biology Project
University of Arizona
Thursday, October 3, 1996
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