Lecture 12: Generation and Use of Reducing Power (including photosynthesis) Flashcards
Explain how an electron transport chain is used to release redox energy.
- Electron transport is a series of redox reactions that resemble a relay race. Electrons are passed rapidly from one component to the next to the endpoint of the chain, where the electrons reduce molecular oxygen, producing water. This requirement for oxygen in the final stages of the chain can be seen in the overall equation for cellular respiration, which requires both glucose and oxygen.
- Electron transport chain converts reducing power into a pmf (membrane potential and proton gradient).
- Electron carrier: Accepts electrons and then ultimately donates them (both and electron acceptor and an electron donor).
Differentiate between electron carriers that carry electrons only and those that carry both H+ and electrons. Name a few carriers in each class.
- Ones that carry both electrons and protons
- –NAD+ and NADH (reduced)
- –FAD and FADH2 (reduced)
- –Quinone (oxidized) and Quinol (reduced)
- Ones that carry electrons
- –Hemes
- –Iron
- –Sulfur
- –FeS protein
Sketch a mitochondrial electron transport chain in a membrane, showing how the PMF is generated and how NADH is used as an electron donor.
- Never varies, always the same.
- Contains 4 multiple protein complexes but will only use 3 at a time.
- Complex I electron donor: NADH (which becomes NAD+)
- Ubiquinone (Q) carries electrons from carrier to complex III
- Cytochrome C carries electrons from complex III to complex IV
- O2 is reduced to H2O in complex IV
- Complexes I, III, and IV are complex pumps
Explain how alternating between electron carrier types can be used to pump H+ across the membrane against a gradient.
- Ones that carry hydrogens and ones that don’t
- Complex III Quinone loop: When Quinone gets reduced, it takes electrons from heme, 2 protons come from the cytoplasm and it becomes Quinol. Quinol donates electrons to FeS and heme and the protons are pumped out to periplasmic side of membrane.
- Protons are attracted to water parts*
- Complex I: Protons bind one side of rocker-switch, conformation switches, and protons are released on the other side. Energy comes from high energy electrons (reducing power) from NADH. Electron charge repulsion pushes down the loop which holds a finger that moves all of the shafts to open all of that channels.
Explain how the PMF is used to generate ATP vis chemiosmosis and the ATP synthase.
- ATP Synthase: Rotor turns when protons come in with their concentration gradient through the ATP synthase. Repulsion of positive charge on stator turns the rotor.
- Chemical energy in gradient is transformed to mechanical turning of the rotor and ADP and Phosphate are pushed near each other and ATP is produced
- If ATP concentration is high and proton concentration is low, the ATP synthase will reverse in order to hydrolyze ATP pump protons back against their concentration gradients.
- IMPORTANT*
- Always 10 protons for every rotation to make 3 ATP in mitochondria
List 4 differences between bacterial and mitochondrial electron transport chains.
- Bacterial electron transport is much more diverse than mitochondrial. Can use different transport complexes, can use different terminal electron acceptors, can pump variable numbers of H+(7-4 protons to make an ATP), can use more than one transport chain at a time (chain can be branched), may or may not have cytochrome c oxidase (complex IV)
- Bacteria can do aerobic respiration, fermentation, and anaerobic respiration (uses something other than O2 as a terminal electron acceptor).*
- Want to aerobic respiration if you can but if you can’t you get more energy from anaerobic respiration (via pmf) than fermentation.*
List 3 alternate terminal electron acceptors (other than O2) that can be used by bacteria.
1-Nitrate (NO3-)
2-Iron (Fe3+)
3-Sulfate (SO2-)
Explain the basis for the oxidase test in diagnostic microbiology.
The oxidase test identifies organisms that produce the enzyme cytochrome oxidase. Cytochrome oxidase participates in the electron transport chain by transferring electrons from a donor molecule to oxygen.
Given a table such as that in slide 13 of lecture 12, be able to predict whether a particular electron donor and acceptor pair could be used by bacteria to provide energy.
-Electron donor must have a higher energy than the electron acceptor (negative free energy change) and there must be some type of enzyme or enzyme series that transfers the electrons from the donor to the acceptor
Sketch the “Z-scheme” of oxygenic photosynthesis. Explain where the ATP and NADPH are formed, and show how water serves as the electron donor.
- Photosystem II => excited chlorophyll => proton pump => Photosystem I (evolved first)=> excited chlorophyll => NAD reduced to NADH
- Photosystems get hit by wavelength of light to excite chlorophyll.
- Generates O2 by taking e- from H20.
- Low energy electrons get excited with light. Chlorophyll or some similar molecule is required for this to happen.*
- Chlorophyll floresces red, if you excite them they will emit light and we catch the electrons before it decays.
- Antennas can accept light of a lot of different wavelengths and then excites the electrons.
- Requires pmf, reducing power, and ATP*
- Has similiar electron transport chain that we talked about in mitochondria. PS I is the electron acceptor for the first electron transport chain.
- Fd is reduced and at the end NADP is reduced to NADPH (reducing power)(final electron acceptor for entire scheme)
- ATP comes from pmf and ATP synthase.
- Reduces H2O to O2 for photosystem II (NOT THE ELECTRON ACCEPTOR)
Compare and contrast the photosynthetic and respiratory electron transport chains.
pretty much the same?
Discuss the difference between photosynthesis in purple bacteria and in cyanbacteria or plants.
Purple bacteria: Non-oxygenic photosynthesis. Uses only one chlorophyll. Pmf used to move electrons uphill because bacteriochlorophyll does not absorb a high enough energy of light to excite the electrons enough to make reducing power. Can use low light sources (purple light) such as deep down in the ocean. Does reverse electron transport.
Green Sulfur Bacteria: Only use one photosystem. Can accept higher energy light. Electrons get powered high enough to reduce Fd. Do not produce oxygen. Essentially Photosystem I used by itself. Electron originally comes from electron donor (sulfur=H2S; non sulfur=some other donor).
Know what we mean by “reverse” electron transport.
Similiar to complex I in the respiratory chain. Pmf builds up so much, that the “complex I” will run backward to remake NADH. Helps build up reducing power. Occurs in light.
Know why purple sulfur bacteria need to use reverse electron transport, but green sulfer bacteria do not.
Gren sulfur bacteria do not have to reduce NADH. It directly reduces Fd.
Explain the difference between cyclic and non-cyclic photosynthesis.
Non oxygenic vs oxygenic photosynthesis
