Introduction Flashcards
What are the two main types of photosynthesis?
Rhodopsin homologs and Reaction centre based photosynthesis
Where is most rhodopsin based photosynthesis done?
In archaea
What colour is the rhodopsin pigment in archaea?
Rhodopsin give the membrane a purple colour.
How is ATP generated following rhodopsin based photosynthesis?
Protons pumped across the membrane during photosynthesis flow back through ATP synthase to generate ATP.
Describe the structure of rhodopsin.
GPCR fold - 7 TM helices.
Describe the retinal cofactor of bacteriorhodopsin in the inactive state.
Trans retinal
What happens to retinal following light activation of bacteriorhodopsin?
Photoisomerisation - to 13-cis retinal.
Describe the key chemiosmosis experiment performed using bacteriorhodopsin.
Put bacteriorhodopsin and mitochondrial ATPase in a vesicle - spaced at a distance from one another. When light was shone on the membrane, ATP was produced.
Describe features of photosynthetic reaction centres.
Involve 5 TM domain proteins, often involve pseudodimers, derived from a common ancestor. Usually have an antenna region fused to the RC protein or located close by.
What are the reaction centre types? And what are they named after?
Type 1 (named after PSI) and Type 2 (named after PSII)
What are the terminal electron acceptors for the two types of reaction centre?
Type 1 - FeS cluster
Type 2 - quinone
Why does oxygenic photosynthesis require two enzymes, one of each reaction centre types?
Oxidising water requires a lot of energy - both reaction centres are needed to provide enough energy to split water and still have some left for carbon fixation.
What is the general equation for photosynthesis?
CO2 + 2H2A -> [CH2O] + H2O + 2A (where A is oxygen or sulfur)
Where does the free oxygen come from in the general equation for photosynthesis?
From the substrate water
Where did oxygenic photosynthesis evolve from?
Cyanobacteria
How can eukaryotes become photosynthetic?
By endosymbiosis with cyanobacteria.
Is the photosynthesis reaction endogonic or exergonic?
Endogonic - requires light energy.
What change in atomosphere was the carboniferous era responsible for?
More oxygen and high levels of coal formation.
Why do carbon dioxide levels fluctuate throughout the year?
When carbon dioxide reduces, it is thought to be due to the photosynthesis of plants in the northern hemisphere in the summer.
Describe the membranes in cyanobacteria.
Plasma membrane has normal respiratory chain and cyanobacteria also have a complex internal membrane system (like thylakoids).
What is special about thermophilic bacteria?
Their proteins are very stable and tend to crystallise easily.
What is the pH on either side of the thylakoid membrane in cyanobacteria?
pH 8 - outside of the thylakoid
pH 5 - inside the thylakoid membrane
What are chloroplasts derived from?
Endosymbiosis of cyanobacterium - this was monophyletic, meaning it only happened once.
Describe the structure of the chloroplast.
5μm ellipsoid structure. Outer membrane and inner membrane;
- continuous thylakoid membranes form stacks called grana
- grana linked by stromal lamellae
- stroma is the equivalent of cyanobacterial cytoplasm
Where are PSI and ATP synthase located within the thylakoid membranes?
In the stromal lamellae because they project from the membranes more - cannot fit within grana.
Where are PSII and Cytochrome b6f found within the thylakoid membranes?
PSII tends to be in the granal regions. Cytochrome b6f is distributed evenly throughout the membrane.
What did EM tomography tell us about the structure of the thylakoid membrane?
Shows spiral staircase arrangement of continuous thylakoid membrane.
Name some photosynthetic eukaryotes.
Higher plants, green algae, red algae and brown algae.
What gave rise to the different types of algae?
Secondary and tertiary endosymbiosis events.
Where are chloroplasts found and why?
Just underneath the mesophyll cell layer so that they can capture light easily.
What is the minimum energy requirement per photon in photosynthesis?
Red photon - 680 nm.
What happens if a blue photon is absorbed during photosynthesis?
The equivalent energy to a red photon is used and the extra energy provided by the blue photon is lost as heat.
How do antennae regions increase the efficiency of light harvesting?
Couple thousands of pigments to the reaction centre so that the reaction centre can be excited many times per second.
What happens to an antenna pigment when absorbing a photon?
This exciton makes downward energy jumps around the antenna region until it reaches the reaction centre where it can perform the chemistry of the reaction.
What other pathways can excited state electrons undergo (if not absorbed by an antenna pigment)?
Photochemical reactions, red fluorescence and resonance transfer to an acceptor molecule.
In decreasing energy levels, name the different coloured absorbing pigments.
highest energy is blue absorbing, then orange and red.
What are the major light harvesting pigments?
Chlorophylls a and b
Describe the structure of chlorophylls a and b.
Porin ring with a long hydrophobic phytol side chain. Lots of conjugated double bonds which absorb in the visible region of light.
Why is it difficult to get chlorophyll in solution? How is this overcome?
It has a long phytol side chain. Can get into solution if within a membrane or bound to another protein.
What are the accessory pigments?
Carotenoids and bilins
Where are bilins found and what are they similar to?
Found in cyanobacteria. They are essentially an open chlorophyll molecule as they are linear tetrapyrroles.
What is the main light harvesting protein in plants?
LHC2 - associated with PSII
What are the major light harvesting proteins of cyanobacteria?
Phycobiliproteins which contain bound bilin pigments.
Describe the phycobilisome.
Contains thousands of different coloured pigments that act as a funnel to the reaction centre. Fill in all gaps in the absorption spectrum - this makes a dense culture of cyanobacteria appear black.
How are proteins in the thylakoid membranes connected?
By mobile electron carriers that can diffuse through the whole membrane system.
