Question

For each NADH produced from the citric acid cycle, about how many ATP are formed? a. 38 b. 36 c. 32 d. 2.5 e. 2.0

Short answer

Each NADH produced from the citric acid cycle forms about 2.5 ATP.

Step-by-step solution

Understand the process of ATP formation

The citric acid cycle, also known as the Krebs cycle, generates energy through the oxidation of acetate into carbon dioxide. The energy from this process is captured in the form of ATP (adenosine triphosphate), NADH (nicotinamide adenine dinucleotide), and FADH2 (flavin adenine dinucleotide). NADH generated in the citric acid cycle is used in the electron transport chain to produce ATP.

Know the yield of ATP from NADH

Each NADH molecule that enters the electron transport chain is used to generate ATP. Research has shown that the theoretical yield of ATP per NADH is about 3. However, due to the cost of transporting the NADH into the mitochondria and other factors within the cell, the actual yield is often considered to be closer to 2.5 ATP per NADH.

Select the correct answer

Based on the average actual yield of ATP from a single NADH molecule, we must find the closest answer that represents this yield. The options are given in integer numbers and in fractions. We look for the option that best represents the yield of about 2.5 ATP per NADH.

Key concepts

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle, is a cornerstone of cellular metabolism. It plays a critical role in converting the carbohydrates, fats, and proteins that we eat into energy that our cells can use. This cycle takes place in the mitochondria, often referred to as the powerhouse of the cell.

To understand how the cycle works, imagine it as a circular assembly line. Each step involves a different enzyme and leads to the transformation of molecules, eventually resulting in the production of two molecules of carbon dioxide, three molecules of NADH, one molecule of FADH2, and one molecule of ATP from each acetyl-CoA molecule that enters the cycle. These electron carriers, NADH and FADH2, are vital for the next stage of energy production—the electron transport chain.

NADH

NADH is like a currency of energy within the cell. It stands for nicotinamide adenine dinucleotide (NAD) plus hydrogen (H). This molecule plays a pivotal role in metabolism, acting as an electron carrier. It collects electrons during the citric acid cycle, and then, like a little energy-packed delivery truck, it transports these high-energy electrons to the electron transport chain.

There is a common misconception that NADH directly translates into ATP, but it is more accurate to say that NADH is used to generate ATP in the later stages of cellular respiration. Our bodies are efficient but not perfect machines. Typically, we estimate that for every NADH molecule, approximately 2.5 ATP are produced, though this number can vary slightly depending on the cell's condition and efficiency.

Electron Transport Chain

The electron transport chain (ETC) is where the real magic of ATP production happens. Located in the inner mitochondrial membrane, it is composed of a series of protein complexes that work together to transfer electrons from NADH and FADH2 to oxygen, the final electron acceptor. As these electrons move through the chain, they power the pumping of protons across the membrane, creating a gradient.

Then enters ATP synthase, a remarkable enzyme that uses this proton gradient to synthesize ATP from ADP and inorganic phosphate (Pi). Think of it as a hydroelectric dam, using the flow of protons to generate ATP—the energy currency of the cell. Given the complex nature of the electron transport chain and its reliance on a gradient to produce ATP, it's remarkable that each NADH can result in the production of around 2.5 ATP, illustrating the incredible intricacy and efficiency of cellular processes.