Energy Generation in Mitochondria and Chloroplasts Flashcards
(22 cards)
What are the ě stages of oxidative phosphorylation?
- Stage = electron-transport chain
- high-energy electrons derived from sunlight, glucose or other sources -> transferred through multitude of electron carriers embedded in the inner mitochondrial membrane -> releases energy used to pump H+ outside the cell -> generating electrochemical proton gradient
- Stage = ATP generation
- protons flow back down their electrochemical gradient through ATP synthase -> catalyzes ADP+Pi
=> together called Chemiosmotic coupling = combination of ATP synthesis (chem) with transport of protons across membrane (osmotic)
How come both eukaryotic cells and bacteria house this ATP producing mechanism?
- The mechanism was so successful that it was decided to be kept in both
- Specifically in eukaryotic cells - mitochondria and chloroplasts
- derived from bacteria engulfed by ancestral cell -> still can reproduce just like bacteria, has the same RNA machinery (independent of the cell), its own genes (although some of the genes for indepepndent functioning were moved to the nucleus)
Why are mitochondria so important?
Without them organisms would need to rely on just glycolysis to get ATP - extra inefficient
- glycolysis produces only 2 ATPs while with mitochondria we can get 30 ATPs
- there are cells that would be extra sensitive to it crucial for normal functioning i.e. muscle cells, neurons
- NOTE: people with myoclonic epilepsy and ragged red fiber disease (MERRF) = disruption in proteins responsible for electron transport -> epilepsy, dementia, muscle weakness
True/False: Mitochondria are always constant/stable organelles of the cell.
No. They are very adaptable -> can change its shape, number and even location depending on the cell’s requirements
- E.g. heart muscle cells have their mitochondria grouped at the site close to the contractile apparatus X in other cells they can form fused tubular networks all across the cytosol
- E.g. muscle cells can immensely increase the number of mitochondria
What is the basic structure of mitochondria i.e. membranes, compartments -> their content?
- bound by 2 membranes (creates 2 compartments - intermembrane space and matrix):
- The outer:
- houses large number of transport proteins = Porins -> highly permeable to small molecules => the intermembrane approxiamates contents of cytosol
- The inner:
- doesn’t let just any small molecule pass through (only those that fit the transport proteins e.g. fatty acids, pyruvates) => matrix is highly specialized
- proteins for electron transfer, proton pumps, ATP synthase
- additionally the inner membrane is convoluted forming Cristae -> increases the surface area
How is glycolysis and citric acid cycle connected to mitochondria?
Products of the glycolysis (pyruvates and fatty acids) eneter the outer mitochondrial membrane via porins -> contrinue into the matrix through specialized transport proteins within the inner membrane -> there, converted into acetyl CoA -> via cutric acid cycle converted into CO2 and activated carriers NADH and FADH2
-> these will be further supplying the electron transport chain
Yet again recall what is oxidative phosphorylation - and why is it called that way?
NADH and FADH2 donate its electrons to the electron-transport chain (and become NAD+ and FAD again)
-> electrons bind to O2 to form H2O
-> releases energy used to pump protons out -> establishing a membrane potential -> proton gradient drives ATP synthase
Name - consumption of O2 (oxidative) and addition of phosphate group (phosphorylation)
See the whole picture:
What could be the sources of high-energy electrons?
- In mitochondria - sugars, fats
- In chloroplasts - chlorophyll which captures the energy from the sunlight
- In single-celled organisms like archae - inorganic substances e.g. iron, sulfur, hydrogen
What actually makes up the electron-transport chain (what molecules)?
Inner membrane contains multiple copies of this chain all over -> about 40 proteins (enzymatic and transmembrane anchoring) are grouped into 3 respiratory enzyme complexes
- each is accompanied by the pumping of H+ out of the cell
1. NADH dehydrogenase complex
- extracts electrons from NADH in the form of hydride ion (H-) -> catalyzes converting it into H+ and 2 high-energy electrons i.e. H- => H+ + 2e-
-> passed along by electron carriers going from e. carriers with weaker electron affinity to those with stronger affinity
2. Cytochrome c reductase complex
3. Cytochrome c oxidase complex
- combining the electrons with O2 to form H2O
= energetically favourable (high-energy electrons going into low-energy)
What are the consequences of protons running out?
- pH gradient
-> ph in the matrix becomes 0.7 units higher than it is in the intermembrane space - Voltage gradient
-> the matrix side of the membrane becomes more negative while the intermembrane space more positive
=> Creating strong electrochemical gradient (also called “proton-motive force”)
-> H+ would like to cross to more negative environment
-> H+ would like to add to lower concentration of itself
What is the ATP synthase? Does it always produce ATP?
= a large, multisubunit protein embedded in inner mitochondria that is capapble of converting ADP into ATP
- the transmembrane domain = H+ carrier -> it keeps on accepting H+ and transferring them inside the matrix -> this drives a rotation “motor” (F0 rotor)
- the motor is connected to a central stalk which rotates with the motor -> bumps into ->
- the proteins of F1 ATPase head that keep on changing conformation
-> those enable ADP+Pi and release of ATP
No. It can also function the other way around and use up ATP in order to pump out H+
=> It all depends on the electrochemical gradient
What other processes take advantage of the proton influx?
- Anti-port of ATP out and ADP in enabled via the membrane potential
- Co-transporters that bring pyruvate and Pi in along the proton gradient
=> So proton gradient drives both formation of ATP and transport of essential metabolites
Elaborate on this: “Cell maintains high ATP/ADP ratio”
- the statement refers to the fact that cellular environmetn needs to keep ATP and ADP in balance
- there has to be a rapid influx of ADP into the mitochondria as well as rapid eflux of ATP out
- since ATP is used for all kinds of biosynthetic unfavourable reactions all over the cell -> there has to be markably high concentration of ATP in the cytosol (10x higher than ADP)
- If mitochondria stopped functioning -> dramatic fall of ATP levels -> cells cannot conduct chemical reactions and die
How does cyanid work?
Cyanid has high affinity for iron in the heme group and copper
-> Cytochrome c oxidase uses heme domain with copper to bind O2
-> if cyanid binds to it, O2 cannot be used
-> blocking electron-transport chain
-> H+ gradient cannot be established
-> ATP synthase cannot produce ATP
-> cell will rapidly use all of its ATP
=> and die
- especially prominent in cells that conduct energetically difficult actions e.g. neurons, heart mucle cells -> leading to immediate symptoms like loss of consciousness, seizures (Na+/K+ pump failure, accumulation of Ca+ -> overexcitation), heart failure
What is the energetical difference between NADH and FADH2?
- NADH enter the electron-transport early on by being used by the very first complex in the chain (i.e. NADH dehydrogenase) -> ends up producing more ATP
- FADH2 enters only after, it binds to the membrane embedded mobile carrier ubiquinone -> produce less ATP
= depending on when is each carrier produced and used do we get number of ATP
How are H+ protons different from K+ or Na+?
Although both in this case move across the membrane, H+ are still special
- abundant everywhere, especially in water
- mobile in H2O meaning they can keep on dissociating and bonding
- they are often involved in oxidation X reduction
- reduction = molecule gains e -> to balance it out -> takes up H+ from H2O (if electron carriers gift electrons to protein it can take up H+)
- oxidation = molecule looses e -> proton is passed onto water (if electron carrier takes the e away -> H+ would be relased)
What is the basis of electron-transport chain?
Redox reactions - i.e. reduction and oxydation
- electrons will pass spontaneously from low affinity (i.e. giving out readily) to high affinity molecules
- e.g. NADH has low electron affinity -> readily passed on to NADH dehydrogynase complex
NOTE:
- affinity can be measured by determining the redox potential = tendency of a redox pair to donate or accept electrons
- e will spontaneously move from a pair (e.g.NADH/NAD+) with low redox potential to molecule with high redox potential (O2/H2O)
-> NADH = a great starting molecule
-> O2 = great ending
Read a paragraph on “Electron transfers release large amounts of energy”
How exactly are electrons transfered along the elctron-transport chain?
Within complex
Electron gets donated to the respiratory complex -> binds to metal ion imbedded in it -> moves from one metal ion to the next towards the ones with stronger affinity (e.g. NADH dehydroxygenase starts with iron-sulfur centers -> the redox potentials of the next ones increase)
Between complex
Electrons are ferried by electron carriers diffusing freely within the lipid bilayer
- E.g. Ubiquinone picks up electrons from NADH dehtdroxygenase complex and delivers them to cytochrome c reductase complex (also serves as first step for FADH2)
- NOTE: can accept 1-2 electrons and can pick up H+ from the H2O corresponding to this number
Why is it called cytochrome c oxidase? What does it do?
= final electron carrier complex with the highest redox potential
- removes 4 electrones from the cytochrome c (thereby oxidizing it) -> handed towards O2 which produces H2O
- in addition to 4 electrones gifted to O2, 4 protons are pumped across the membrane during this cytochrome to O2 transfer
- the transfer causes allosteric changes in conformation of the protein that move protons out
How can this cytochrome-O2 electron transfer be dangerous?
O2 binds to a specific oxygen-binding site that contains heme group and copper atom
- once it receives 1 electrone it becomes superoxide radical O2- = dangerously reactive (could attack macromolecules throughout the cell)
-> that’s why it is held there in the binding site until 3 other e bind to it
-> with 4 e it can form 2 molecules of water and gets released
