Week 2

Why do electrons flow from NADH to O₂?

Electrons flow from NADH to O₂ because NADH/NAD⁺ has a relatively negative reduction potential while O₂/H₂O has a very positive reduction potential. Electrons spontaneously flow from lower E to higher E.

Note that the Ecell’ = +1.14V for NADH to O2. Because Ecell is positive, delta G’ is negative, so electron transfer is thermodynamically ____. Cells do not release all this energy at once; the ETC captures it ___ by using electron transfer to pump protons and create a _______ that can drive ATP synthesis.

favorable/spontaneous

stepwise

proton gradient

Who wants electrons more, the redox couple with the more positive or negative reduction potential?

more positive reduction potential

• More positive E’→ stronger electron ______

• More negative E’→ stronger electron ______

• More positive E’→ stronger electron acceptor

• More negative E’→ stronger electron donor

How does biology capture redox energy?

Biology captures redox energy stepwise through the electron transport chain rather than allowing electrons to move directly from NADH to O₂ in one large energy-releasing step.

Electron flow through the ETC is coupled to H⁺ pumping across the _____. This creates a proton gradient. H⁺ then flows back through ATP synthase, and that energy is used to make ATP.

inner mitochondrial membrane

what is the relationship between Ecell and dG for knowing if an elecron transfer is thermodynamically favorable?

• Ecell > 0 → dG < 0 → spontaneous/favorable

• Ecell < 0 → dG > 0→ nonspontaneous/unfavorable

• 
  

Why do we use E’ in biology? (note: theres supposed to be a o as a superscript)

represents the biological standard reduction potential, which is adjusted to approximately pH 7

What does regular Eo indicate for chemical conditions

the reduction potential under chemical standard conditions.

• solutes at 1M

• gases at 1 bar

• for reactions involving protons, pH is 0

What changes the actual redox potential?

depends on the concentrations of the oxidized and reduced forms and, for proton-coupled reactions, pH.

What is a standard reduction potential?

measures how strongly a redox couple tends to accept electrons.

What does a more positive reduction potential mean? (in terms of the oxidized or reduced form)

The oxidized form is a stronger electron acceptor and has a greater tendency to be reduced.

What does a more negative reduction potential mean?

The reduced form is a stronger electron donor and has a greater tendency to give up electrons.

Why must reduction potentials be measured relative to something like SHE?

Reduction potentials are measured relative to a reference electrode rather than as absolute values.

Which reduced species is the best electron donor?

NADH, because the NAD⁺/NADH couple has the most negative E’ of the species shown.

Which species is the best electron acceptor?

O₂, because O₂/H₂O has the most positive E’.

In what direction will spontaneous electron flow occur? (in terms of negative and positive energy)

more negative E to more positive E

ETC carriers generally ____ in reduction potential as electrons move through the chain.

increase

What does Complex I do?

• accepts electrons from NADH

• transfers them into the ETC

• pumps H⁺

What does Complex II do?

• accepts electrons associated with FADH₂

• transfers them into the ETC

• does not pump H⁺

What does CoQ (ubiquinone) do?

CoQ, or ubiquinone, is a mobile electron carrier that transfers electrons from Complex I and/or Complex II toward Complex III. 2 electrons at a time

What does Complex III do?

Complex III transfers electrons from CoQ to cytochrome c and pumps H⁺.

What does cytochrome c do?

Cytochrome c is a mobile electron carrier that carries electrons from Complex III to Complex IV. one electron at a time

What does Complex IV do?

Complex IV transfers electrons to O₂, reducing O₂ to H₂O, and it also pumps H⁺.

What is the final electron acceptor in the ETC?

Oxygen is reduced to water.

What is the purpose of pumping H⁺ across the membrane?

H⁺ pumping creates a proton concentration/electrochemical gradient across the membrane. There are more protons on one side and fewer on the other, producing stored potential energy.

What does ATP synthase do?

ATP synthase allows H⁺ to flow back down its gradient and uses the released energy to produce ATP.

How do you calculate cell potential?

E cell = E cathode - E anode

What happens at the anode?

Oxidation occurs at anode

What happens at the cathode?

Reduction occurs at the cathode

What does a positive cell potential mean?

+E means -dG which means spontaneous

What does a negative cell potential mean?

-E means +dG which means nonspontaneous

if the Ecell’ = +1.14V, the electron transfer is favorable/not favorable

favorable

Why does pH matter for redox reactions?

Many biological redox reactions involve H⁺. Changing pH changes the concentration/activity of H⁺ and therefore changes the measured reduction potential

What is the purpose of the Nernst equation?

adjusts the reference reduction potential to account for actual, nonstandard concentrations, such as a modified concentration and pH

What is the relationship with increasing [Red] wrt [Ox], Q, and E?

↑[Red] = ↓[Ox] = ↑Q = ↓E

Is a redox couple always equally good at accepting or donating electrons?

No. Its actual reduction potential depends on its environment, especially the ratio of reduced to oxidized species and, when H⁺ participates, the pH.

When does pH directly affect a redox potential?

pH affects the potential when H⁺ appears in the redox half-reaction.

Decreasing/increasing pH generally makes the potential more negative

increasing

proton gradient

A difference in H⁺ concentration/electrochemical potential across a membrane that stores energy.

Electron Transport Chain (ETC)

A series of electron carriers that transfers electrons stepwise and uses the released energy to establish a proton gradient to allow for ATP synthesis