Question

Why does FADH \(_{2}\) yield two ATPs, using the electron transport system, but NADH yields three ATPs?

Short answer

NADH yields more ATPs than FADH₂ because it causes more protons to be pumped across the membrane, leading to higher ATP generation.

Step-by-step solution

- Understand the Role of NADH and FADH₂

NADH and FADH₂ are both electron carriers used in the electron transport chain (ETC) to produce ATP during cellular respiration. They donate electrons to the ETC, initiating a series of redox reactions that ultimately create a proton gradient used to generate ATP.

- Location in the Electron Transport Chain

NADH donates electrons to Complex I of the ETC, while FADH₂ donates electrons to Complex II. This difference in the entry points of the ETC influences the ATP yield.

- Proton Pumping Differences

When NADH donates electrons at Complex I, it causes proton pumping at Complexes I, III, and IV. In contrast, FADH₂ donations at Complex II result in proton pumping only at Complexes III and IV. Fewer protons are pumped by FADH₂ compared to NADH.

- ATP Yield Correlation

Each proton entering through ATP synthase generates ATP. Because NADH results in more protons being pumped across the membrane, it generates more ATP. Specifically, NADH results in the formation of approximately 3 ATPs, while FADH₂ results in about 2 ATPs.

Key concepts

NADH

NADH stands for Nicotinamide Adenine Dinucleotide (NAD) with one Hydrogen atom attached. It functions primarily as an electron carrier in the electron transport chain (ETC). NADH is formed during earlier stages of cellular respiration, such as glycolysis and the citric acid cycle. When NADH donates its electrons to Complex I of the ETC, it initiates a series of reactions that pump protons across the inner mitochondrial membrane. This proton pumping creates a gradient used to produce ATP. Because NADH pumps protons at Complexes I, III, and IV, it is highly efficient, yielding around 3 ATP molecules for each NADH molecule.

FADH₂

FADH₂ stands for Flavin Adenine Dinucleotide with two Hydrogen atoms attached. Similar to NADH, FADH₂ acts as an electron carrier but is less efficient. FADH₂ is produced primarily during the citric acid cycle. When FADH₂ donates its electrons to Complex II of the electron transport chain, fewer protons are pumped across the membrane. This results in a smaller proton gradient. Because FADH₂ pumps protons only at Complexes III and IV, it yields about 2 ATP molecules per FADH₂ molecule. That's the main reason why FADH₂ generates less ATP compared to NADH.

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane. It is the final stage of cellular respiration, where most ATP is produced. Electrons from NADH and FADH₂ are transferred through complexes I-IV in the chain. The movement of electrons through these complexes enables the pumping of protons from the mitochondrial matrix to the intermembrane space. This creates an electrochemical gradient (proton gradient) that drives ATP synthesis when protons flow back into the matrix through ATP synthase. The efficiency of the ETC is heavily dependent on the entry points of the electrons, which is why NADH and FADH₂ generate different amounts of ATP.

Cellular Respiration

Cellular respiration is a multi-step process cells use to convert the energy stored in glucose into ATP, which is used for various cellular activities. This process consists of three main stages: glycolysis, the citric acid cycle (also known as the Krebs cycle), and the electron transport chain (ETC). Glycolysis occurs in the cytoplasm and breaks down glucose into pyruvate, producing a small amount of ATP and NADH. The citric acid cycle occurs in the mitochondrial matrix, generating more NADH and FADH₂. Finally, the ETC uses these electron carriers to produce a significant amount of ATP. Hence, cellular respiration is crucial for energy production.

Proton Pumping

Proton pumping is a critical step in the electron transport chain, where the energy from electron transfers is used to transport protons (H⁺ ions) across the inner mitochondrial membrane. Each time an NADH or FADH₂ molecule donates electrons, energy is released, enabling the movement of protons to the intermembrane space. This creates a high concentration of protons outside the inner membrane, forming a proton gradient. The stored energy in this gradient is then harnessed to produce ATP when protons flow back into the mitochondrial matrix through ATP synthase. Proton pumping at multiple complexes (Complexes I, III, and IV for NADH) explains the higher ATP yield from NADH compared to FADH₂.