lecture 29 - Transduction of Light Energy in Chloroplasts

Question 1

What is the equation for photosynthesis?

Answer 1

CO2 + 2H2A light (CH2O) + H2O + 2A

“A” can be S, but is usually O

Question 2

What is a photon?

Answer 2

a particle carrying a quantum of light

Question 3

What is the primary photosynthetic

pigment in higher plants?

Answer 3

chlorophyll a and b
e.g. chl a:
A very hydrophobic molecule
localized exclusively
within the membrane.....
and flavonoids and carotenoids.

Question 4

Explain absorption by chlorophyll

Answer 4

Chlorophyll absorbs blue and red light
Absorption of “blue” photon pushes e- to 2nd excited state
Only sufficient energy in “red” photon to get to 1st state
[e- concerned is delocalised over tetrapyrrole ring by alternating single and double bands.]
Transitions 2nd to 1st excited state:
energy lost as heat
1st ex state to ground state: energy can be lost through emission of photon of longer wavelength = fluorescence.

Question 5

How is light harvesting accomplished?

Answer 5

through a 3rd form of Energy Loss:
Resonance Energy Transfer
•Neighbouring chl responds to electric field of excited
chl (*chl): e- of *chl excites a lower energy e- in
neighbouring chl, thereby losing its own energy.

Resonance Energy Transfer
•Time: 10-12 s (1 ps).
•There is a SMALL energy loss in this process (i.e. not quite 100% efficient): Thus, in chl aggregates, excitation tends to pass from species absorbing at shorter wavelengths to those absorbing at longer wavelengths of light.

Question 6

What is The Biological Importance of

Resonance Energy Transfer?

Answer 6

-proteins broaden absorption spectrum.
-other pigments absorb in other parts of spectrum are present
-energy is “funnelled” to chls absorbing lower energy photons
This process is known as light-harvesting

A broad spectrum of wavelengths of visible light
is capable of funnelling energy to chl absorbing
at long wavelengths

light harvesting complexes form an antenna consisting of light harvesting proteins. the chlorophylls within psii set of complexes absorb light with a wavelength >680 nm

Question 7

What are Antenna and Reaction Centre Chlorophylls?

Answer 7

-The bulk of chlorophyll is in the light-harvesting
complexes e.g. 1 protein (LHC II) typically binds:
8 molecules chlorophyll a
7 molecules chlorophyll b
5-7 molecules carotenoid

-Each reaction centre is associated with ~300 antenna
chlorophyll molecules (~20 LHCs)
-Each reaction centre has its own special chlorophyll molecules for participation in redox reactions

Question 8

Describe the properties of the photosystems

Answer 8

Properties:
1. Absorption properties of the 2 photosystems are
different
PS I: absorbs at 700 nm: Pigment is P700
PSII: absorbs at 680 nm: Pigment is P680
2. The 2 reaction centres are spatially-separated:
PS I: Stromal lamellae
PS II: Granal lamellae (stacks)

Question 9

What is The 4th way in which Energy Stored in
Excited Chlorophyll (*Chl) can be lost
(but only in a reaction centre)?

Answer 9

A redox reaction: the excited e- is lost from
the *chl to some other molecule
The redox reaction is facilitated by a change in E’o of
chl in the excited state
The Initial Redox Reactions in PS II and PS I
are Transmembrane Events

Question 10

calculate speed of light

Answer 10

frequency x wavelength

c = f x lamda

Question 11

calculate photon energy

Answer 11

6.63 x 10 to the -34 x frequency