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Lecture 2

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Biology 1001A
Tom Haffie

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-protein complex. 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? Chlorophyll 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 excited state. 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 photochemistry. 4) You can transfer the energy of this excited state pigment to a neighbouring pigment. Energy transfer. 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. Absorption’s factors 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 Photochemistry There must be a photochemical event. The photochemical event take place here: in the photoreceptor molecule itself withi
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