Lecture 2 Outcomes
Relationship between wavelength and energy content of a photon.
• Light behaves as discrete particles of energy: we call these particles photons or
quanta (packages of light); they have no mass, but they have energy
• Wavelength is inversely proportional to energy (ie the energy of a photon is
inversely related to its wavelength)
• Photon of blue light (short wavelength) has more energy than a photon of red light
• Proteins don’t absorb light
• Pigment absorbs light from the visible spectrum (not damaging)
Molecular characteristic of pigments that make them able to absorb light.
• Channelrhodopsin is a protein; it does NOT absorb light
• Thus channelrhodopsin must have a pigment associated with it that actually
absorbs the light
• It is important to note that pigments don’t just float freely; PIGMENTSARE
COVALENTLY BONDED TO PROTEINS
• Pigments absorb light because they have a conjugated ring system: alternating
bonding of double-single-double bonds
• CRS is a very planar shape that provides non-bonding electrons (delocalized
• These electrons are available to interact with photons of light; THEYARE NOT
INVOLVED IN BONDING
Relationship between pigments and associated protein.
• See above
• Proteins provide organization to pigments
• Most proteins (ie in mitochondria or cytosol) don’t have pigments associated with
• We usually stain proteins via blue dye
Four “fates” of the
excited state of
chlorophyll resulting from
• What happens when
absorbs a photon
of light? Energy excites an electron!
• Chlorophyll has 2 excited states • Excited electrons are used to do workmakeATP
• Environment of the pigment dictates which process is going to occur more of the
• 1) Heat Loss: losing all energy, doesn’t happen very often
• 2) Fluorescence: you lose a little of the excited state thus wavelength becomes
smaller =higher energy
• 3) Photochemistry: using excited state to get work done, such as break bonds;
happens more than anything else
• 4) Energy Transfer: if two pigments are close enough together, excited state can
fall to neighbouring pigment; electron doesn’t actually move, you’re simply
moving energy; this is NOT oxidation or reduction.
Relationship between energy of photon and electron excited states to explain
pigment colour and absorption spectrum.
• HOW does pigment interact with the photon?
• Step 1) Photon must be absorbed by the pigment
• Step 2) photon excites an electron in pigment from ground state to higher state
• 1 photon of light can excite only 1 electron; this is called Photochemical
• For a pigment to absorb a p