Lecture 2 – Light
Thursday January 10, 2013
From shortest to longest wavelength:
Gamma Rays X-rays Ultraviolet Radiation Near-Infrared Radiation Infrared Radiation Microwaves Radio Waves
400 450 500 550 600 650 700
Wavelength of Visible Light (nm)
Gamma rays have more energy and shorter wavelength
Quanta= Photon, discrete or quantifiable amount of energy. Energy in quanta/photon of blue light is
more than quanta/photon of red light.
Pigments Absorb Light
- Have to absorb light to use it
- Pigment are the molecules that absorb light
- Conjugated system: alternation between double bond and single bond. Indicates a specific kind of
electron configuration. Many non-bonding electrons (pi orbital). These non-bonding electrons
interact with light.
- Couple exceptions: retinale, does involve bonding electrons in interaction of light
- Pigments aren’t free in cells, they are bound specifically to proteins.
- When isolate proteins you can keep pigment attached, giving the protein a color. Pigment protein
- Pigment bound non covalently to protein
- Mitochondrial proteins have no color. Stain the gel to view proteins. For gel electrophoresis
- Pigments proteins complexes can be seen without adding color. In gel electrophoresis.
Light Absorption and Emission
- Chlorophyll has two excited states; higher excited state and lower excited state.
- Some have more than that
- Shine light on chlorophyll, depending on the color and wavelength, can excite it.
- Lose energy as heat when excited. Higher excitation state decays to lower excited state.
- Arrows: Blue light and red light. X-axis = chlorophyll. - Doesn’t matter whether you absorb blue or red photon, except for the fact that energy released is
- 4 ways to get rid of excited state: Can lose it as heat, lose a little energy as heat and then lose
remaining as fluorescence (photon of light leaves, wavelength of light is different, different color,
lower energy, longer wavelength), can do work and work is photochemistry (use light to change
structure of a pigment), can transfer the energy of this excited state pigment to a neighbor
- Why is chlorophyll green? There is no green excited state, otherwise it would be able to absorb the
green light. Chlorophyll can’t absorb green light so it is reflected or transmit it through that
- One photon can excite one electron. One to one equivalency
- For energy to absorb, to trap the photon, the energy in that photon must match that amount of
energy required to get from ground state to excited state.
- Green photons don’t get absorbed because don’t match in en