Light Harvesting Flashcards
When are photos created?
When radiation is emitted
What happens to energy when a photon is emitted?
It is lost
Why is UV light not used to power biology?
- photodamage of DNA has serious consequences
- UV photons have too much energy
- UV photons do not penetrate water well
- UV if screened out by the ozone layer
Why is IR light not used to power biology?
IR photons promote changes in the vibrational structure of electron states but don’t have enough energy to produce electronic transitions
What makes chlorophyll soluble inside a membrane?
The phytyl side chain
What is the structure of chlorophyll?
- Substituted tetrapyrole
- 4 pyrole N which coordinate an Mg2+
- conugated delocalised electron system
Give the structure of carotenoids
- Linear polyenes with 1 or 2 cyclic ends
In addition to their role as accessory pigments, what other role do carotenoids have in some organisms?
photoprotection, preventing the formation of singlet oxygen through non-photochemical quenching
What alters the colour of light absorbed by carotenoids?
the length of the conjugated system
How many conjugated double bonds does β-carotene have?
11
What pigments do red algae contain?
- phycoerrythrin
- phycocyanin
- allophycocyanin
What colour of light do bilin pigments absorb?
Yellow and green
What is the evolutionary advantage of bilin pigments?
They absorb the light between high and low chlorophyll pigments
What are the light harvesting structures in red algae and cyanobacteria called?
Phycobilisomes
Where are phycobilisomes found?
The outer face of thylakoid membranes
What are phycobilisomes?
Large assemblies of protein with covalently attached bilin pigments
What is the structure of phycobilisomes?
Water soluble proteins sit on top of the membrane surface, these are attached by cysteine side chains
What is the main difference between bacteriochlorophyll and chlorophyll?
Ring II is saturated making the system of conjugated double bonds even more asymmetric than chlorophyll
Why does bacteriochlorophyll have a different absorbance spectrum compared to chlorophyll?
Increased asymmetry in the conjugated double bonds
What accounts for the absorbance spectrum of chlorophyll?
The π to π* transitions between π HOMO and π* LUMO
What does HOMO stand for?
Highest occupied molecular orbital
What does LUMO stand for?
Lowest unoccupied molecular orbital
What occurs to electrons when a photon is absorbed?
An electron moves from HOMO to LUMO
What makes up a reaction centre?
Proteins, chlorophylls, quinones, and other redox centres involved in electron transfer
What makes up the antenna of a photosystem?
Proteins, antenna carotenoids and chlorophylls
What part of the photosystem makes light harvesting efficient?
The antenna
What is the primary role of reaction centres in photosystems?
Energy transduction
What is the primary role of antenna in photosystems?
Energy supply
How is excited state energy funnelled to reaction centres?
by FRET
What does FRET NOT involve?
Electron transfer, emission/absoption of photons
