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.
Pigments
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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