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Lecture 2

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Western University
Biology 1002B
Denis Maxwell

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  Inversely proportional  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 processes  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 as fluorescence 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 absorbed.
  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
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