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Lecture 2

Biology 1002B Lecture Notes - Lecture 2: Chlorophyll, Nuclear Membrane, Photon


Department
Biology
Course Code
BIOL 1002B
Professor
Tom Haffie
Lecture
2

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Lecture 2
Ribosomes: where protein synthesis occurs on genes which are coded for by
the nucleus
oSome ribosomes are free in the cell and others are attached to the ER
Proteins synthesized in the ribosomes on the endoplasmic reticulum nd
themselves within the ER itself
Proteins synthesized on free ribosomes are found in the cytosol of the cell
ER is continuous with the nuclear envelope
oHard to distinguish between them  connected
LIGHT ABSOPRTION
Chlorophyll is very good at absorbing photons of light
Energy of a photon is transferred to an electron which goes to a higher
excited state
When single molecule of chlorophyll absorbs light:
o2 excited states in chlorophyll for which electrons can go
oIf blue photon of light is absorbed: electron goes to higher excited state
Energy of blue photon matches perfectly the energy gap
between the ground state and higher excited state
oHeat loss drops electron to the lower excited state in (10-12s)
oIf chlorophyll absorbed red light, in lower excited state
When chlorophyll absorbs photon of light, possible outcomes for excited
states:
oLose the energy as heat (excited state decays to ground state)
energy lost to environment as heat
oSpit the energy back out as a photon  chlorophyll can absorb it, and
the reverse can also occur  pigment spits it back out  3uorescence
Fluorescence is di5erent from the energy of the initial light that
was absorbed: it has a longer wavelength and less energy
because energy is lost as heat
oPhotochemistry: occurring in our eye, eyespot and chloroplast: using
energy for photochemistry  using light to drive a chemical change
oxidizing and changing the chemistry of the molecule using light
oEnergy transfer: occurs in a photosystem with chlorophylls (not in our
eye): chlorophylls can be so close together that when a photon is
absorbed and an electron goes to the excited state, the nearby
chlorophyll can raise up to excited face too and the initial electron
drops
Not transferring electron, but energy of the electron (not
oxidation reaction)
oCOMPETING PROCESSES
The environment changes the rate at which the 4 fates above occur: pigment
in solution, in water, etc… conditions changes how much it occurs

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oRate at which competing reactions occur is not xed  one reaction
could occur more often than the other in one environment but less in
another
WHY IS CHLOROYLL GREEN?
Has no excited state that matches energy of a green photon of light
No capacity to absorb green light (green photon passes through it or is
re3ected o5 of it)
oGives rise to the colour
Chlorophyll absorption spectrum:
oStrong absorption for blue  high excited state
oStrong absorption for red  low excited state
CHANNELRHODOPSIN = RETINAL + OPSIN
Pigments are never free in a cell, they are always attached to proteins
Channelrhodopsin has a pigment (retinal) and a protein (opsin)
Photoreceptor: need both the pigment and the protein for it to be functional
oRetinal bound to the opsin
Absorption spectrum: if isolated retinal from the opsin
Rhodopsin in our eye also has retinal  bound to opsin too (not identical
amino acid sequence)
oSimilar to channelrhodopsin
Opsin: group of proteins that make up the important parts of a photoreceptor
WHAT IS THE LIGHT ACTUALLY DOING TO CHANNELRHODOPSIN?
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