L4 - Origins of Photosynthesis

Question 1

Sketch the Z scheme and outline the general workings of the two photosystems in 8 steps.

Answer 1

(See Z scheme in pg 1 L4)
1) e- transferred from LHC to heterodimer P680 in PSII.
2) P680 oxidised to P680+ .
3) OEC splits H20 to O2, H+ ions and e- which reduce P680+ to P680.
4) e- moves through e- transport chain via Cyt b and Cyt f, moving H+ into thylakoid.
5) LHCs in PSI transfer e- to P700.
6) Excited P700 donates e- to electron chain, along with e- from PSII.
7) e- chain finally produces NADPH, used to convert CO2 to CH2O in Calvin Cycle.
8) ATP also produced as H+ ions return to lumen.

Question 2

Describe in which domains of life photosynthesis is found. List the groups of photosynthetic organisms within each domain.

Answer 2

• Photosynthetic organisms in all 3 domains - Eukaryotes, Eubacteria and Archaebacteria.
• Archaebacteria (1): Halobacteria (primitive photosynthesis)
• Eubacteria (5): Green non-sulphur, green sulphur, heliobacteria, cyanobacteria, purple bacteria.

Question 3

Give an example of a halobacteria and its environment.

Outline photosynthesis in the halobacteria in 3 steps (sketch diagram).

Give 3 key differences between this and complex photosynthesis .

Answer 3

• Halobacterium halobium, high salt environments (very halophilic).
  1) Light captured by purple pigment bacteriorhodopsin in outer membrane.
  2) Conformational change induced, driving H+ out of cell from cytosol to periplasm.
  3) ATP formed as H+ diffuse back into cell.
• Water not split, no LHC and no RCs

Question 4

Where are purple bacteria found?

Outline photosynthesis in purple bacteria in 5 steps. Draw a sketch (see annotated in pg 3 L4)

Answer 4

• Anoxic water in sediment with light present.

1) Bacteriochlorophylls (BChls) found in LHs, absorb light at longer wavelength than Chl.
2) 8 LHII rings channel excitation to central LHI ring and then to RC.
3) RC made of two core proteins - M and L - and 14 cofactors. SIMILAR TO PSII
4) Electrons move through RC causing protons to move from cytoplasm to periplasm.
5) Proton gradient drives ATPase, forming ATP. NADPH also occasionally produced.

Question 5

Give differences between the two types of Green Bacteria

What are the 2 key points about the photosystem seen in Green Sulphur Bacteria?

Answer 5

• Green non-sulphur: Filamentous and facultatively aerobic.
• Green sulphur: Exclusively ANAEROBIC environments, obligately phototrophic.

1) Use BChl c, d, e which form large LHCs called chlorosomes.
2) RCs contain dimer core, RESEMBLING PSI

Question 6

Give an overview of cyanobacteria, when evidence for them exists and their range of habitats.

Outline their important photosynthetic properties in 3 points.

Answer 6

• Evidence of cyanobacteria with oxygenic photosynthesis 2.5bya. Range of habitats: oceanic, freshwater, terrestrial. Often in extreme environments: UV, dry, high temp., low light etc…

1) Some use bilins as LH pigment, forming mobile antenna systems called phycobilisomes.
- Mobility seen via florescence microscopy.
2) Some use chlorophylls e.g Prochlorococcus.
3) All possess two photosystems
4) All use oxygenic photosynthesis
5) All use PSII and PSI acting together to split water and reduce NADPH (As seen in algae and plants!)

Question 7

What are the two hypotheses that explain the different RCs seen in oxygenic and anoxygenic photosynthetic organisms?

Answer 7

1) Two photosystems developed early in a single organism and various anoxygenic forms arose by loss of one or other of the photosystems.

2) Two classes of photosystem developed independently (but ultimately from a common ancestor) and became linked via genetic fusion or lateral transfer.

Question 8

How can different types of prokaryotes be put into two classes depending on their RC structure?

Give these classes and explain what the similarities are that link them.

Answer 8

• The RCs of each prokaryote resembles either PSI or PSII of higher plants.
• Looks like PSII derived from RCs of purple bacteria
• Both have Pheophytin-quinone RCs (similar e- donors and acceptors).
• Looks like PSI derived from RCs of green sulphur or heliobacteria.
• All have Fe-S RCs (similar e- donors and acceptors).

Question 9

Sketch redox potential reaction diagrams for each group, detailing the e- donors and acceptors.

Sketch a phylogenetic tree diagram of each group as well as their RC type.

Answer 9

See sketches on L4 pg 5.