Light has dual nature:
Light behaves like a wave.
Discrete particles of energy (photons)
Shorter the wavelength, stronger energy the wave has (inversely related)
The amount of energy in the blue photon is higher than the amount of energy in the red photon.
Before use light whether it is for information or energy, you have to absorb that light. Molecules
which absorb light are called pigments. Pigments absorb photons of lights.
Indigo: blue (color blue genes)
What pigments have in common: a conjugated (ring) system – the alternation of double bonds
and single bonds. This conjugated system represents or indicates the specific kind of electron
configuration, these are non-bonding electrons (Pi orbital electrons). Those electrons will interact
with the photons of light. They are not required for bonding.
Exception: retinal (involve bonding electrons)
Pigments are not free. They are bound very specifically to proteins. When you isolated protein carefully enough, you can keep the pigments attached:
Pigment is bound non-covalently to the protein.
FP: free pigment
PSⅠ: pigment-protein photosystem 1
PSⅡ: pigment-protein photosystem 2
Gel electrophoresis of proteins
Light absorption and emission
What actually happens when a pigment molecule absorb a photon of light?
This is one of those Pi order electrons (non-bonding). The electron can exist in the ground state
or one of the two excited states. (for chlorophyll, there are only two excited states. Other
molecules may have one, other pigments may have more than two…)
Let’s shine white light on this single molecule of chlorophyll.
If the electron absorbs the blue photon of light, it gets enough energy to get in to the higher
excited state. (blue photons have lots of energy, so that’s enough energy to get the electron from
the ground state all the way up to the higher excited states) Then the electron loses some energy as heat very quickly. The higher excitation state decays to
the lower excited states. And this is the energy state you will get if the chlorophyll absorbs the
red photon of lights.
So red photon doesn’t packed as much energy and only gets to the lower excited state. So it
doesn’t matter whether you absorb the blue photon or red photon, the energy content is
different, but very fast after absorption, you are really dealing with the energy being the lower
There are four ways to get rid of this lower excited state.
1) Lose heat (not a lot in chlamy in normal conditions)
2) Lose a little energy as heat, to the sub excited states, and then lose the energy as
fluorescence (emission of light). So there actually is a photon of light which would leave and
we call it as fluorescence. The wavelength of this red light is different than the initial one. The
fluorescence’s wavelength is a little longer. And the energy is slightly lower because some energy is lost as heat.
3) Do work. (vision and photosynthesis, trapping the energy to do work) Photochemistry. Use
the light to change the molecule, to change the structure of the pigment. That’s
4) You can transfer the energy of this excited state pigment to a neighbouring pigment. Energy
Why is chlorophyll green?
Because there is no green excited state. There is no excited state between the red photon
absorption and the blue one. So green photons are just lost. Chlorophyll cannot absorb them.
So the photon will either reflect it or transmit through that pigment.
One photon can excite only one electron. One to one
For the energy to absorb, to trap the photon, the energy that’s in the photon, whatever that
energy is, must match the amount of energy required to get from ground state to the excited
state. So to absorb red photon, the energy in that red photon has to match the energy
difference here between the ground state and the lower excited state. That’s required for
absorption to take place. That’s why the green photons don’t get absorbed. These energy
don’t match. There is no excited state for green photon.
So: one to one; the energy must match.
Light absorption spectra as a function of wavelength. This strong absorption band in the certain color represents the specific excited state.
The fluorescence emission shifted to the longer wavelength is decay from this excited state
here. So absorption characteristics really reflect the excited states of the pigments.
Phototransduction versus photosynthesis
There must be a photochemical event. The photochemical event take place here:
in the photoreceptor molecule itself withi