Question

What do the electrons added to \(\mathrm{NAD}^{+}\) do? a. They become part of a fermentation pathway. b. They go to another pathway for ATP production. c. They energize the entry of the acetyl group into the citric acid cycle. d. They are converted into NADP.

Short answer

The correct answer is B: The electrons added to NAD+ go to another pathway for ATP production.

Step-by-step solution

Understand the Role of NAD+

NAD+ is a coenzyme present in the cells that accepts electrons during glycolysis and the Krebs cycle to form NADH. This reaction is reduction.

Recognize the Use of NADH in Cells

The NADH molecule, which now has additional electrons, moves to the Electron Transport Chain, a next stage in the cellular respiration. Here, these electrons contribute to the generation of a gradient for ATP synthesis, which is the energy currency of the cell.

Choose the Correct Answer

The proper function of the electrons added to NAD+ is most accurately represented by option B: 'They go to another pathway for ATP production.' because NADH (with its additional electrons) donates those electrons in the Electron Transport Chain, which contributes to ATP production.

Key concepts

Electron Transport Chain

Understanding the Electron Transport Chain (ETC) is crucial in grasping cellular respiration. The ETC is the final and most complex series of reactions in the process of cellular respiration, and it takes place on the inner mitochondrial membrane. The primary role of the ETC is to facilitate the transfer of electrons from electron donors to electron acceptors via redox (reduction-oxidation) reactions.

NADH plays a pivotal role in this process by serving as an essential electron donor. When NADH donates its electrons to the ETC, it does so through a series of protein complexes and small organic molecules embedded in the mitochondrial membrane. The transfer of electrons through these proteins leads to the pumping of protons (H+) across the membrane, creating an electrochemical gradient known as the proton motive force.

Role of Proton Motive Force

The proton motive force is critical for ATP synthesis. As protons flow back into the mitochondrial matrix through the enzyme ATP synthase, their movement drives the enzymatic action needed to synthesize ATP from ADP and inorganic phosphate (Pi). This process is known as oxidative phosphorylation.

By accepting and donating electrons, NADH functions like a rechargeable battery, playing a vital part in the cell's energy conversion mechanisms.

ATP Production

Adenosine Triphosphate (ATP) is often referred to as the energy currency of the cell, and its production is a key goal of cellular respiration. The process of generating ATP from substrates like glucose involves three main stages: glycolysis, the Krebs cycle (or citric acid cycle), and oxidative phosphorylation.

Glycolysis and the Krebs cycle generate a small amount of ATP directly, but the major ATP production occurs during oxidative phosphorylation, facilitated by the ETC. NADH and FADH2, produced in earlier stages, donate their electrons to the ETC, which leads to the generation of the proton gradient that powers ATP synthase.

Quantity of ATP Generated

Each molecule of NADH has the potential to yield approximately three molecules of ATP, while a molecule of FADH2 yields about two ATP molecules. This difference is due to the position at which each electron carrier enters the ETC. NADH contributes at an earlier complex, leading to more proton pumping and hence more ATP production.

The efficiency of this process is a testament to the intricate design of cellular mechanisms for energy production, optimized over millions of years of evolution.

Reduction of NAD+

The reduction of NAD+ to NADH is a fundamental step in both glycolysis and the Krebs cycle. Reduction, in chemical terms, refers to the gain of electrons by a molecule. NAD+ gains electrons (and a proton) to become NADH, indicating that it has been reduced.

This transformation is not only important for carrying high-energy electrons to the ETC but also for the continuation of glycolysis and the Krebs cycle. Without NAD+, these metabolic pathways would stall, as there would be no molecule capable of accepting the electrons that are removed from glucose or other organic molecules.

Enzymatic Catalysis of Reduction

A variety of dehydrogenase enzymes facilitate the reduction of NAD+. These enzymes catalyze the removal of hydrogen molecules from substrates, transferring the electrons to NAD+, thus turning it into the electron-carrying and energy-rich NADH. The continual regeneration of NAD+ from NADH during the ETC is essential for maintaining the cyclical nature of these metabolic reactions, allowing cells to efficiently produce the energy required for their survival.