生命科學(一) Ch10-4 Cellular Respiration and Fermentation

Summary

This lecture explains the final and most crucial step of aerobic respiration: oxidative phosphorylation. It highlights how electron carriers like NADH and FADH2, produced in earlier stages like glycolysis and the citric acid cycle, transfer electrons down the electron transport chain to ultimately produce ATP. The process involves creating a proton gradient across the mitochondrial membrane, which drives ATP synthesis via ATP synthase. The summary also explains why a glucose molecule yields approximately 30-32 ATP due to variations in NADH and FADH2 contributions and other cellular factors.

Takeaways

• 🔋 Oxidative Phosphorylation is the final and most important step in aerobic respiration, generating the majority of ATP.
• 🔄 NADH and FADH2, produced from glycolysis and the citric acid cycle, act as electron carriers in the electron transport chain (ETC).
• ⚙️ The ETC consists of several protein complexes (Complex I, II, III, IV) that facilitate the transfer of electrons in a controlled manner.
• 🌬 Oxygen, as a strong electron acceptor, plays a key role in driving the ETC, eventually forming water.
• 🔬 The energy released during electron transfer is used to pump protons (H+) across the mitochondrial inner membrane, creating a proton gradient.
• 🌊 The return of protons through ATP synthase, a molecular motor, enables the production of ATP via chemiosmosis.
• 💡 ATP production via oxidative phosphorylation is not substrate-level phosphorylation but rather a complex, indirect process involving electron transfer and proton gradients.
• 📉 The process releases free energy in small, manageable steps, making ATP synthesis efficient.
• 💪 NADH contributes more significantly to the proton gradient than FADH2, resulting in more ATP per NADH (2.5 ATP) than per FADH2 (1.5 ATP).
• ⚖️ Variability in ATP production (30-32 ATP per glucose) is due to differences in how NADH from glycolysis enters the mitochondria, the efficiency of coupling, and how proton gradients are used for other mitochondrial functions.

Q & A

What is the role of oxidative phosphorylation in cellular respiration?

Oxidative phosphorylation is the final and most important step in cellular respiration, as it generates a large amount of ATP by using energy from electrons carried by NADH and FADH2. These electrons pass through the electron transport chain, ultimately producing water and creating the energy needed to drive ATP synthesis.

How do NADH and FADH2 contribute to ATP production in oxidative phosphorylation?

NADH and FADH2 are electron carriers that donate electrons to the electron transport chain. NADH enters the chain at Complex I, while FADH2 enters later at Complex II. As the electrons move through the complexes, they help pump protons across the mitochondrial membrane, generating a proton gradient that powers ATP production.

What is the electron transport chain and how does it work?

The electron transport chain is a series of protein complexes (Complexes I-IV) embedded in the inner mitochondrial membrane. Electrons are transferred through these complexes, releasing energy used to pump protons from the mitochondrial matrix to the intermembrane space. This creates a proton gradient that drives ATP synthesis.

What is chemiosmosis, and how is it related to oxidative phosphorylation?

Chemiosmosis is the process by which the energy stored in a proton gradient is used to drive ATP synthesis. In oxidative phosphorylation, the proton gradient created by the electron transport chain powers ATP synthase, the enzyme responsible for converting ADP into ATP.

Why is oxygen important in oxidative phosphorylation?

Oxygen acts as the final electron acceptor in the electron transport chain. It combines with electrons and protons to form water, which is a crucial step for maintaining the flow of electrons and preventing the chain from stalling.

How does ATP synthase function as a molecular motor?

ATP synthase is a protein complex that functions like a molecular motor. It uses the energy from protons moving down their concentration gradient (from the intermembrane space to the matrix) to catalyze the conversion of ADP to ATP.

What is the proton gradient, and why is it crucial for ATP production?

The proton gradient, also known as the proton motive force, is the difference in proton concentration between the intermembrane space and the mitochondrial matrix. It stores potential energy that is used by ATP synthase to produce ATP as protons flow back into the matrix.

How many ATP molecules can be produced from one molecule of glucose during oxidative phosphorylation?

During oxidative phosphorylation, 26 to 28 ATP molecules can be produced from one molecule of glucose, depending on how efficiently the process occurs and whether NADH or FADH2 is used to drive proton pumping.

Why is the total number of ATP produced from one glucose molecule not a fixed number?

The total ATP yield can vary between 30 to 32 ATP per glucose molecule because different cells may use different electron carriers (NADH or FADH2), and some of the energy from the proton gradient may be used for other mitochondrial functions, not just ATP synthesis.

What is the overall efficiency of ATP production from glucose, and how does it compare to other energy conversion systems?

Approximately 34% of the energy from one glucose molecule is converted into ATP, with the rest lost as heat. This is more efficient than many mechanical systems, such as car engines, which typically operate at around 20% efficiency.