Lecture 14: Photosynthesis
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How much energy from sunlight hits a square meter of Earth’s surface per day?
~3.5-7.0 kWh/m^2
What are the three processes used by bacteria to generate energy via photosynthesis?
• Electron independent ATP generation a. produces ATP • Cyclic electron transport a. produces ATP • Linear electron transport a. produces ATP b. & NADPH
What are the three processes used by bacteria to generate energy via photosynthesis?
• Electron independent ATP generation a. produces ATP • Cyclic electron transport a. produces ATP • Linear electron transport a. produces ATP b. & NADPH
What are the 3 key compounds capable of absorbing light energy?
• Carotenoids • Chlorophylls • Bilins Contain a “chromophore” a. can absorb light E and transfer into electrons
How do rhodopsin proteins aid energy generation for bacteria and archaea?
• Transmembrane protein • Central carotenoid capable of absorbing light E (e.g., retinal) • Causes conformational change (e.g., goes from all-trans-retinal to 13-cis- retinal) • Causes release of proton (e.g., from Lys residue and then past to Asp to release inside) • ESSENTIALLY A SIMPLE FORM OF PROTON PUMP • Creates proton motive force
What is “photoheterotrophy”
Make energy through photosynthesis and respiration
What carotenoids do most photosynthetic organisms use? What’s the difference between them?
• Chlorophyll a a. absorbs light most at ~410 and 680nm • Chlorophyll b a. Abs. light most at ~460 and 630nm • THE ONLY DIFFERENCE a. Aldehyde group b. Has methyl group
What is purple non-sulfur bacteria?
• Utilize chlorophyll carotenoids • Belong to “Rhodospirillaceae group” (Alphaproteobacteria) • Include: a. Rhodobacter sphaeroides b. Rhodopseudomonas palutris • Can grow aerobically in dark by respiring on organic carbon sources
What are purple sulphur bacteria?
• Belong to groups: a. Chromatiaceae b. Ectothiorhodosiraceae • Can tolerate high sulphur environments • STRICTLY ANAEROBIC • Must fix CO2 to survive a. Utilise the “reverse tricarboxylic acid cycle (TCA)”
How can purple bacteria absorb infrared light?
Absorb INFRARED LIGHT via BACTERIOCHLOROPHYLLS 1. Light hits: "Light harvesting complex 2" (LH2) a. large complex containing bacteriochlorophylls (abs. infrared light) 2. Passes E onto LH1 a. circular array of proteins b. central "Reaction centre (RC)" 3. LH1 passes E onto RC 4. RC can use energy to: a. Excite an electron b. Pass electron onto ubiquinone (UQ) c. UQ passes electron onto cytochrome bc1 complex - pumps protons across membrane d. Electron passes onto cytochrome c to be passed back to the RC CYCLIC ELECTRON TRANSPORT
How do purple bacteria use their energy?
• Fix carbon dioxide into sugars a. Carboxysome inside the cell (high [CO2]) b. Fixation occurs using rubisco c. Can be done with nitrogen also
How does fixation of nitrogen and CO2 occur in purple bacteria?
1. Requires an electron donor (Hydrogen sulphide / H2S) a. restricts where they can survive 2. Electron arrival at RS 3. e- passed onto ferredoxin 4. e- can then be passed to either: a. Nitrogenase: N2 -> ammonia (8e-) b. Ferredoxin NAD(P)+ reductase: NAD(P)+ + H+ -> NAD(P)H (2e-)
How does fixation of nitrogen and CO2 occur in purple bacteria?
1. Requires an electron donor (Hydrogen sulphide / H2S) a. restricts where they can survive 2. Electron arrival at RS 3. e- passed onto ferredoxin 4. e- can then be passed to either: a. Nitrogenase: N2 -> ammonia (8e-) b. Ferredoxin NAD(P)+ reductase: NAD(P)+ + H+ -> NAD(P)H (2e-)
What are green sulphur bacteria?
• Only anaerobic • V. low light environments • E.g., Chlorobiaceae • Utilise “CHLOROSOMES” a. Stacks of bacteriochlorophyll c b. layer of bacteriochlorophyll a c. sit on a base plate connected to plasma membrane and reaction centre
What’s the problem with using reduced sulphur compounds for source of e-?
Limited to environment
What’s the problem with using reduced sulphur compounds for source of e-?
Limited to environment
How does oxygenic photosynthesis work?
Linear electron transport • Water as an electron donor for PS2 a. pair of chlorophylls get excited + release electron b. e- removed from H2O by 4Mn complex to replace lost e- c. O2 release + 4H+ made • e- passed to plastoquinone (PQ), reducing it to plastoquinol (PQH2) • Passed onto cytochrome b6f • Passed onto plastocyanin (PC) • Transferred to PS1 to replace a high energy electron removed from it a. High E electron goes to ferredoxin b. 2 electrons go to ferredoxin NADP reductase to form NADPH
How does cyclic electron transport work?
Same electron being used over and over again so does not require a source of electrons Similar process in algae (+ some plants) and cyanobacteria (+ some plants) a. cyanobacteria = NAD(P)H dehydrogenase b. Algae = PGR5 BOTH ONLY USE PS1
What are light harvesting complexes?
• Proteins encasing carotenoid “Bilins” • 2 types of bilin a. phycoeythrobilin b. phycocyanobilin • Attached to PS2 to energize electrons • Chlorophylls absorb light in red/blue spectra (little in green) vs bilins absorbing light inbetween • THEREFORE CYANOBACTERIA CAN ABSORB LIGHT IN A MUCH WIDER SPECTRUM • ALSO FOUND IN ALGAE
Light harvesting complexes in other plants and algae
1. Chlorophyll = same wavelength absorbance 2. found in membrane = taking up more space 3. increases surface area to increase chance of excitation