Oxidative Phosphorylation

ETC, ATP synthase, complexes...

What is the main purpose of cellular respiration?

To break down glucose and produce ATP.

Where does glucose come from?

Food, mainly carbohydrates.

Where does oxygen come from?

The air we breathe.

Where does the CO₂ come from?

Pyruvate oxidation and the Krebs cycle.

Where is water produced in the ETC?

Complex IV, when oxygen accepts electrons and combines with H⁺.

What is the final electron acceptor in the ETC?

Oxygen (O₂).

What happens when oxygen is unavailable?

The ETC backs up, electron flow stops, and oxidative ATP production decreases.

What is chemiosmosis?

H⁺ diffuses down its gradient through ATP synthase to help produce ATP.

How does ATP synthase make ATP?

H⁺ flow causes rotation and shape changes, allowing ADP and Pi to join into ATP.

What is oxidative phosphorylation?

The ETC and chemiosmosis working together to produce ATP.

What is the electron transport chain (ETC)?

A series of protein complexes that transfer electrons and use their energy to pump H⁺.

What does it mean that electrons flow downhill?

Electrons move toward carriers with greater electronegativity, releasing energy in steps.

What is the energy from moving electrons used for?

Pumping H⁺ from the mitochondrial matrix into the intermembrane space.

What does it mean that the inner mitochondrial membrane is impermeable to H⁺?

H⁺ cannot freely cross it and needs ATP synthase to return.

Which ETC complexes pump H⁺?

Complexes I, III, and IV.

Which ETC complex does NOT pump H⁺?

Complex II.

What happens at Complex I (NADH dehydrogenase)?

NADH donates electrons; their energy helps pump H⁺ into the intermembrane space.

What happens at Complex II (succinate dehydrogenase)?

FADH₂ donates electrons; they enter the ETC, but Complex II does not pump H⁺.

What is cytochrome Q (ubiquinone)?

A mobile electron carrier that transfers electrons to Complex III.

What happens at Complex III (cytochrome bc1)?

Electrons pass through and energy is used to pump H⁺ into the intermembrane space.

What is cytochrome c?

A mobile electron carrier that transfers electrons from Complex III to Complex IV.

What happens at Complex IV (cytochrome c oxidase)?

Electrons are transferred to oxygen, and H⁺ is pumped across the membrane.

What happens to oxygen at Complex IV?

Oxygen accepts electrons and combines with H⁺ to form water.

Why doesn't FADH₂ produce as much ATP as NADH?

FADH₂ enters at Complex II, bypassing Complex I, so fewer H⁺ are pumped.

What is the proton motive force?

Stored energy created by the H⁺ electrochemical gradient across the inner membrane.

How do H⁺ ions return to the mitochondrial matrix?

They flow down their gradient through ATP synthase.

What happens when H⁺ flows through ATP synthase?

It drives rotation and shape changes in the enzyme.

How does ATP synthase use rotational energy?

Shape changes allow ADP and Pi to join and form ATP.

Where does the phosphate (Pi) used to make ATP come from?

The cell's available inorganic phosphate pool.

What is the difference between the ETC and oxidative phosphorylation?

The ETC transfers electrons and builds the H⁺ gradient; oxidative phosphorylation includes the ETC and chemiosmosis.

What are the main steps of oxidative phosphorylation?

1. Electrons enter the ETC. 2. H⁺ is pumped. 3. Oxygen forms water. 4. H⁺ returns through ATP synthase. 5. ATP forms.

Why can't glycolysis-produced NADH enter the mitochondrial matrix?

The inner mitochondrial membrane is not permeable to NADH.

What do shuttle systems do?

Transfer electrons from cytoplasmic NADH into the mitochondrion.

What are the two NADH shuttle systems?

Glycerol phosphate shuttle and malate-aspartate shuttle.

How does the glycerol phosphate shuttle affect ATP yield?

Electrons enter through FAD and bypass Complex I, producing less ATP.

How does the malate-aspartate shuttle affect ATP yield?

It regenerates NADH in the matrix, allowing electrons to enter at Complex I.

Which shuttle produces more ATP and why?

Malate-aspartate; electrons enter at Complex I, allowing more H⁺ to be pumped.

How much ATP does each shuttle produce per glucose in the older accounting used in class?

Malate-aspartate: about 38 ATP; glycerol phosphate: about 36 ATP.

Why is there a 2 ATP difference between the shuttles?

Glycerol phosphate bypasses Complex I, resulting in fewer H⁺ pumped and less ATP.

How much ATP comes from glycolysis?

A net gain of 2 ATP per glucose.

How much ATP comes directly from the Krebs cycle?

2 ATP per glucose.

How much ATP is attributed to oxidative phosphorylation in the older accounting?

Approximately 32–34 ATP, depending on the shuttle and accounting method.

Why do some sources give different total ATP yields?

Different shuttle systems and ATP accounting conventions produce different estimates.

Why is oxygen essential for the ETC?

It is the final electron acceptor, allowing electron flow to continue.

What happens if oxygen is unavailable?

Electron flow stops, NADH and FADH₂ cannot unload electrons, and oxidative ATP production decreases.

What is cyanide poisoning's effect on cellular respiration?

Cyanide inhibits Complex IV, blocking electron transfer to oxygen and stopping the ETC.

What happens to ATP synthase when the ETC is blocked?

The H⁺ gradient gradually dissipates, reducing the energy available to ATP synthase.

Where does the CO₂ released during cellular respiration come from?

Pyruvate oxidation and the Krebs cycle.

Where does the ATP made during cellular respiration come from?

Glycolysis, the Krebs cycle, and ATP synthase during oxidative phosphorylation.

What is the main purpose of cellular respiration?

To extract energy from glucose and make ATP.