Lecture 2: Light
1. Relationship between excited states of a pigment and its absorption, fluorescence emission spectra.
Two major excited states for chlorophyll
White light is a mixture of wavelengths
Chlorophyll can absorb blue light to higher excited state
Heat loss 10^-12 s
o From higher excited state to the lower excited state
o If absorb red light = Low excited state
o Decay is so fast that the energy is always at lower
excited state regardless if absorb blue or red
2. Region of the electromagnetic spectrum known as “visible light”.
400 nm to 700 nm
3. Relationship between wavelength and energy content of a photon.
The shorter the wavelength the more energy it has
Photons/Quanta: discrete package of light with defined energy content
4. Molecular characteristic of visible pigments that make them able to absorb light.
Pigments absorb photons of light
o Conjugated system – double bond, single bond, double bond, single bond, etc.
o Abundance of electrons
o Pi orbital electrons that don’t bond
Readily available electrons to be excited to trap light
o Retinal is an exception but electrons that absorb light don’t involve in bonding
5. Relationship between pigments and associated protein.
Protein has colour because it is bound nonconvalently to pigments
o Pigment protein complexes e.g. photosystem 1 protein complex
Gel electrophoresis to isolate proteins
o Pigment protein complexes still attach so don’t need to use a stain
6. Four “fates” of the excited state of chlorophyll resulting from absorption of photons.
Four fates to lose the excited state
o Go back to ground state – lost as heat
o Lose some as fluorescence (deep red – longer wavelength, less energy since some lost as heat)
o Photochemistry – energy used to do work, change the molecule or structure
o Transfer that energy to another neighbouring pigment
7. Reason(s) why relative fluorescence is different in isolated chlorophyll vs. intact cells when exposed to light.
Cell requires a lot of energy to run cell processes to function properly
Energy produced by the excitation is used for essential molecules and cell organelles to perform those
Energy can also be transferred to the reaction center to drive photochemistry as part of photosynthesis
The cell may have developed a mechanism that helps utilize light energy by minimizing energy loss and
the amount of fluorescence given off
In isolated chlorophyll, there is no pathway for the energy produced to be used so most of it is released
8. Quantitative relationship between photons and excited electrons.
One photon excites one electron 9. Relationship between energy of photon and energy required to excite electrons in order for photons to be
Energy of photon must match with the electron’s excited state perfectly
There is no green excited state
It cannot absorb green so it reflects green (what colour we see)
Energy doesn’t match
Green photons have more energy than lower and less than higher
10. General structure of photosystem.
Photosystem is the unit of photochemistry (diagram on right)
o Purple is the antenna (pigment proteins)
Interaction between pigment and light
o Light purple is the reaction centre
Binds chlorophyll and other pigments
11. Draw the ground state and lowest excited state of four pigment molecules A,B,C,D that allows for stepwise
energy transfer from A to D.
each time energy is transferred to another