Lecture 2 Notes: Light YOU NEED TO REWATCH ARCHIVE
Gamma, X-ray, UV, Visible (indigo, blue, green, yellow, orange, red), near-infrared, infrared, microwaves, radio
**Energy of light is inversely related to its wavelength.
(shortest wavelength=highest energy of light) ➔ gamma rays
blue light energy > red light energy
Quantum of light⇒ using particles to describe light
particles⇒ photons = discrete packages of light
Particle-wave duality⇒ light behaves as both waves and particle (photons)
Light must be absorbed to be used.
➔ trap energy of photon in molecule (typically pigment)
Photosynthesis and eye use visible light (very narrow part of spectrum)
Why? What’s so special about visible light?
1. Visible Light is the most dominant form of light on the earth’s surface
➔ By the time light hits the earth’s surface after going through atmosphere, very
little types of light other than visible light are still present.
➔ Ozone absorbs a lot of UV radiation
➔ evolution occurs to use most abundant molecule (not just w/ light!)
2. Energy from visible light is perfect energy for photochemistry
➔ just the right amount of light needed to excite pigment and/or change isomer of molecule
ie. expose clamy to X-rays & pigments are destroyed
expose clamy to microwave and they won’t do anything (maybe just vibrate a little)
⇒e- won’t be excitable without sufficient energy Pigments absorb light
Pigments contain a conjugated system
=>(double bond-single bond repeated for carbons (covalent))
⇒ non-bonding e- left. molecule has pi bonds ∴ lots of available e- to trap light
⬇ Indigo-used to make blue jeans
Retinal is the
Pigments aren’t free moving. Pigments are bound to protein in protein-pigment complex
Even after spun in a centrofuge, the pigments are still attatched to the protein (as photo in slide
Pigment can be seen during protein gel electrophoresis ➔ but careful not to detach pigment from
Light absorbtion and Emission (& the 4 Phates of Light for Cell)
Photochemical equilavence- ONE photon can excite ONE e- (that’s the rule!)
When chlorophyll molecule absorbs 1 photon of light there are 2 possible excited states it can
reach. (maybe ≠reach either if not enough NRG)
⇒ higher excited state or lower excited state
BUT When photon (absorbing blue light) excites to the higher energy state, there is instant
energy decay from heat loss ➔ falls ball to lower excited state
Whether blue or red light is absorbed, the e- will reach same excited state because decay is too
fast to account. (ie. decay takes 10E-12 sec)
➔ red was lower energy so no decay (heat loss occurs quickly)
4 Fates of Light: Competing Processes 1) Complete Energy Loss as HEAT
-decays right back down to ground state
2) Lose just a little energy as heat & lose rest as FLUORESCENCE
3) PHOTOCHEMISTRY- use light to do work (ie. photosynthesis)
4) ENERGY TRANSFER
REVIEW BEFORE EXAM!
➔ ➔ ➔
In the demo where Maxwell shines light on tubes of chlorophyll
(either still intact in cell or extracted on its own)
does the extracted shine bright red (flourescence)
and the intact shines bright green?? Chlorophyl on its own will not cause
photosynthesis- it has all this excess energy with no pathway, energy ≠utilized... that will now
be released through flourescence.
Intact cell will utilize energy for organelle functions and to drive photosynthesis
(still wont use hreen energy though➔ it is reflected back out of cell)
Chlorophyll has excited state for red and blue lights (absorbs them)
Chlorophyll appears green b/c there is no excited state for green!
Green energy levels don’t match the needed energy for e- to get excited to higher state.
➔ Green photon wave passes through & now chor