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The Physics of Photosynthesis

The Physics of Photosynthesis

The Nature of Light

Light behaves both as a wave phenomenon and as discrete particles of energy called photons. If we look at light as a wave phenomenon, we can assign it a wavelength (the distance from one peak of the wave to the next) and an amplitude (the distance the wave oscillates from its centerline). Different wavelengths of light have different characteristic energies and properties. Light can also travel at various speeds in different media, producing a frequency at which the wave travels. The energy contained in a wave of light is related to its frequency.

Where E is energy, h is Planck's constant Energy = (6.626196 * 10^-34 Joule-seconds), and c is the speed of light. Short wavelengths have high energies and long wavelengths have lower energies.


How is light captured by living things? Molecules, when struck by a wave or photon of light, reflect some of its energy back out, or it can absorb the energy, and thus enter into a higher energy or excited state. Each molecule absorbs or reflects its own characteristic wavelengths of light. Molecules that have evolved to absorb wavelengths in the visible region of the spectrum very well are called pigments.

Absorption and Action Spectra

An absorption spectrum for a particular pigment describes the wavelengths at which it can absorb light and enter into an excited state. The following diagram represents the absorption spectrum of pure chlorophylls in solution:

An action spectrum, on the other hand, describes the efficiency of a particular molecule at acheiving its purpose in absorbing light; this measurement shows what wavelengths of light the molecule can trap to conduct photosynthesis. And action spectrum closely follows an absorption spectrum for a particular pigment because the molecule has to be able to absorb light to enter into its excited state and pass the energy along.

Chlorophylls and the Accessory Pigments

The two primary pigments involved in photosynthesis are chlorophyll a and chlorophyll b. These two molecules efficiently absorb light at the red and blue ends of the spectrum when purified and in solution, and not very efficiently in between (though this may not completely accurately reflect the situation in living cells). Photosynthesis has optimized its light-absorbing capabilities by making a series of pigments, covering more of the visible spectrum. These accessory pigments act as antennae to channel the energy they absorb into the reaction center. A molecule of chlorophyll at the reaction center can then transfer its excited state into biosynthetically useful energy.

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