Oxidative phosphorylation

What is oxidative phosphorylation?

Oxidative phosphorylation is the process in cellular respiration where electrons from NADH and FADH2 are transferred through the electron transport chain, generating a proton gradient that drives ATP synthesis via chemiosmosis.

What are the two main components of oxidative phosphorylation?

The two main components are the electron transport chain and chemiosmosis.

Why is oxygen essential in oxidative phosphorylation?

Oxygen acts as the final electron acceptor at the end of the electron transport chain, combining with electrons and protons to form water; without oxygen, the chain stops, ATP production ceases, and cells cannot function.

Where do NADH and FADH2 deposit their electrons in the electron transport chain?

NADH deposits electrons at Complex I, turning into NAD+, while FADH2 deposits electrons at Complex II, turning into FAD.

Why does FADH2 contribute less to the proton gradient than NADH?

FADH2 enters at Complex II, which does not pump protons across the membrane, so fewer protons are pumped compared to NADH entering at Complex I.

What is the role of ubiquinone (Q) in the electron transport chain?

Ubiquinone (Q) is a small, mobile electron carrier that receives electrons from Complex I and II, gets reduced to QH2, and delivers electrons to Complex III.

What is the role of cytochrome C (cyt C) in the electron transport chain?

Cytochrome C is a mobile carrier that transfers electrons from Complex III to Complex IV.

How is water formed in oxidative phosphorylation?

At Complex IV, electrons are transferred to molecular oxygen, which splits and combines with protons (H+) to form water; 4 electrons, 1/2 O2, and 2 H+ produce one water molecule.

Which complexes in the electron transport chain pump protons?

Complexes I, III, and IV pump protons from the mitochondrial matrix to the intermembrane space, generating a proton gradient.

What is the proton gradient also called, and why is it important?

The proton gradient is called the proton-motive force; it stores energy that is later used to drive ATP synthesis through ATP synthase.

How does ATP synthase produce ATP?

Protons flow down their electrochemical gradient through ATP synthase, causing it to spin like a turbine; this mechanical motion catalyzes the addition of phosphate to ADP to form ATP.

Why can’t protons move directly through the phospholipid bilayer?

The membrane's hydrophobic core prevents ions like H+ from passing, so they require membrane proteins like ATP synthase to move down their gradient.

How is chemiosmosis defined?

Chemiosmosis is the process where energy stored in a proton gradient is used to do work, such as synthesizing ATP.

What percentage of cellular ATP is made via chemiosmosis during glucose breakdown?

Chemiosmosis accounts for over 80% of ATP made during glucose breakdown.

How can the proton gradient be used other than for ATP synthesis?

Some cells use the proton gradient to produce heat; uncoupling proteins allow protons to bypass ATP synthase, releasing energy as heat, important in brown fat cells of hibernating mammals.

What is the estimated ATP yield from one molecule of glucose?

The maximum ATP yield is about 30–32 ATP per glucose molecule.

How is ATP yield calculated from NADH and FADH2?

Each NADH moving through the chain pumps ~10 H+, producing ~2.5 ATP; each FADH2 pumps ~6 H+, producing ~1.5 ATP.

Why does NADH from glycolysis yield variable ATP amounts?

Glycolysis occurs in the cytosol, so NADH electrons must be shuttled into mitochondria; depending on the shuttle, 2 NADH can produce 3–5 ATP.

What are the direct ATP products of glycolysis and the citric acid cycle?

Glycolysis produces 2 ATP, pyruvate oxidation produces no direct ATP but 2 NADH, and the citric acid cycle produces 2 ATP/GTP, 6 NADH, and 2 FADH2.

What are the two important functions of the electron transport chain?

1) Regenerates NAD+ and FAD for glycolysis and the citric acid cycle. 2) Builds a proton gradient across the inner mitochondrial membrane, storing energy for ATP synthesis.

How does cyanide affect oxidative phosphorylation?

Cyanide inhibits Complex IV, stopping electron transport, collapsing the proton gradient, and halting ATP production.

How does DNP (dinitrophenol) affect ATP production?

DNP makes the inner mitochondrial membrane leaky to protons, decreasing ATP production while generating heat, which can dangerously raise body temperature.

Why is water only a product in oxidative phosphorylation?

Water forms when electrons reduce oxygen at Complex IV; it is not used further in the electron transport chain.

What is electron stability in the context of water formation?

Electron stability refers to electrons reaching a lower energy state in water molecules, forming stable covalent bonds with oxygen and hydrogen.

How many water molecules are formed per glucose molecule?

6 water molecules are formed net per glucose; although 12 are initially made in ETC, 6 are used earlier in cellular respiration.

What is the purpose of regenerating NAD+ and FAD in oxidative phosphorylation?

Regeneration ensures these electron carriers are available for glycolysis and the citric acid cycle, allowing continuous cellular respiration.

Where are the electron transport chain components located in eukaryotes and prokaryotes?

In eukaryotes, they are embedded in the inner mitochondrial membrane; in prokaryotes, they are in the plasma membrane.

What is the role of energy released during “downhill” electron transfers?

The energy is captured to pump protons across the membrane, forming an electrochemical gradient used for ATP synthesis.

Why do electrons move from higher to lower energy levels in the electron transport chain?

Electrons move from less electron-hungry to more electron-hungry molecules, releasing energy that is used to pump protons and form the gradient.

How do uncoupling proteins in brown fat cells function?

They provide an alternate path for protons to return to the matrix without ATP synthase, releasing energy as heat instead of making ATP.

Why is ATP considered the “biological currency” of energy?

ATP stores and transfers energy in cells, allowing them to perform necessary reactions and maintain life processes.

What happens if the proton gradient energy is not captured by ATP synthase or other cellular work?

The energy is released as heat; in some specialized cells, this heat is used for thermoregulation rather than ATP production.

How many H+ ions are required to synthesize one ATP molecule?

Four H+ ions must flow through ATP synthase to produce one ATP molecule.

How many H+ ions are pumped per NADH and FADH2?

About 10 H+ ions per NADH and 6 H+ ions per FADH2 are pumped across the membrane.

Summarize the ATP yield from one glucose molecule.

Glycolysis: 2 ATP, 2 NADH → 3–5 ATP; Pyruvate oxidation: 2 NADH → 5 ATP; Citric acid cycle: 2 ATP/GTP + 6 NADH + 2 FADH2 → 2 + 15 + 3 = total 30–32 ATP.