shorter wavelength -> higher Energy
light is wave-particle duality
Photon: packages of light(quanta) defines E-content
i. Example: quanta/photon of red light. Blue has higher energy
2. Pigment absorbs light
Chlorophyll and Indigo (blue) are the examples. What they are in common is that
they are conjugated system means that they have double single double single
i. It has a lot of electrons.
ii. They are available to get excited by light
iii. Conjugated system results in many Non-bonding Pi orbital electrons. These electrons that will interact with the
iv. Retinal does the bonding.
3. Pigment-protein complex
Pigment binds on the protein non-covalently–result in-> Pigment protein complex. Pigment are not free, Pigment
are attached on the protein, it has colour because of the protein.
4. Light absorption and Emission
Chlorophyll has two excited states.
Blue and red have different energy contents.
Blue photon (has enough energy to bring the electron to the higher excite state), Thus, when blue light strikes an
electron, it goes to the higher excited state
1 photon excites 1 electron (1:1)
The electron is not stable then it goes to lower excited state through heat. Because the decay goes too fast and
electron eventually goes back down to lower excited state, thus, electron mostly is in lower excited state.
Competing Processes/ Four “fates” of the excited state of chlorophyll resulting from absorption of photons.
a) Heat loss
Electron that is from higher excited state, goes down to lower excited state and release heat , then eventually
goes to ground state and release the remaining heat when it is back to the ground state. There can’t be a lot of heat loss especially in chalmy, however, it is because the cell will heat up and die and the
goal to trap and use the Energy because heat is energy.
b) Heat and Fluorescence
Electron that is from the higher excited state goes down to the lower excited state and releases heat. Then, the
remaining emits fluorescence (deep red) when it is back to the ground state. It slightly has longer wavelength to
show that it loses some heat (energy) at the first process.
c) Photochemistry = to do WORK.
Is the work to change the structure of the pigment by using the light.
d) Energy transfer
transfer energy to the neighbour.
5. Why relative fluorescence is different in isolated chlorophyll vs. intact cells when exposed to light.
chlorophyll vs. intact cell.
Extracted more fluorescence – more red
Intact is less fluorescence - green
It is because if the chlorophyll is intact with the cells, the energy that is absorbed from exciting the electrons is used
to power photosynthesis, therefore causing no fluorescence to occur because the energy is already used up. When
the chlorophyll is isolated, the excited electrons have to fall back to their ground state which explains why
fluorescence is much higher.
Are you saying that in the isolated system 100% of the energy is going to fluorescence?
The reason we see fluorescence as a different color is because of the longer wavelength. The reason we have a
slightly longer wavelength is because there is a small amount of heat being lost as heat energy. Therefore, in the
isolated chlorophyll, the energy is lost through the re-emission of light as fluorescence and as heat energy
6. Chlorophyll is green because
There is no green excited state.
Green colour gets reflected instead of getting absorbed. Chlorophyll cannot absorb it therefore it only gets
Photon energy doesn’t equal the energy difference between the electron’s ground state and an excited state (the
wavelengths don’t match) therefore, they can’t absorb the photon and only can reflect. Therefore, energy in the
photon must match the amount of the energy required to get from the excited and ground state to absorb the
photon. Green photon doesn’t get absorb because the energy doesn’t match.
One photon can only excite 1 electron (1:1)
7. High absorption in blue and not so much in red.
8. Phototransduction vs. Photosynthesis
Photochemistry (2 options: trap the energy or not at all)
i. On Rod and Cone, the photochemistry takes place in the photoreceptor itself. This photochemical event is called
the isomerization of retinal.
Photosystem: light capture.
i. Two parts:
a. antenna – has a lot of pigment protein. Chlorophyll individually bounds on the protein
there is no photochemistry. Only energy transfer. Electrons get excited. These interactions happen here. Energy transfer within the neighbouring pigment and that pigment gets excited where the previous pigment decay. YOU
ARE NOT MOVING THE ELECTRON just THE EXCITED STATE. Photochemistry doesn’t occur here.
Chl*+ chl -> chl + chl+.
b. Middle is the reaction centre
Photochemistry occurs when that photon (E) reaches the reaction centre this is called oxidation of
Chl* -> chl+ +e-.
The electron is used for Electron Transport Chain.
9. HMWRK: 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!
10. Rhodopsin=Retinal + Opsin
a. Opsin binds retinal.
b. Recall: for many pigments they interact with non bonding electron, but not in retinal
c. trans vs cis.
Different in geometric structure. To rotate the double required a lot of energy because The double bond prevent
11 cis -> all trans retinal. The bond breaks.
The photon is captured, excited one of those pi bond. There is enough energy to break that bond. Then the pi
bond is reformed in trans configuration. This is photochemical event in eye. It is called PHOTOISOMERIZATION
d. you change the geometric structure in the eye.
e. Can the process of photo isomerization occur without light?
I need light to take the energy to break the C-C bond and reform.
But it can happen spontaneously every 1:10 thousands yrs without light but It takes longer time
11. Transductin interacts with rhodopsin.
-for this transduction pathway to become active, transductin needs to interact with opsin protein.
- in dark, it cannot get in the 11 cis retinal, it cannot bind upon photon absoption. - 11 cis retinal has pocket so it allows cis -> trans. The cis that has been converted into trans does not have any
pocket anymore. Thus, cannot bind. The opsin cannot absorb phton thus, it gets recycled.