NADH, or nicotinamide adenine dinucleotide, functions as a vital coenzyme within living cells, playing a fundamental role in electron transfer. Cellular respiration is a complex process where cells convert nutrients, primarily glucose, into adenosine triphosphate (ATP), the primary energy currency of the cell. This article explains NADH’s function within this energy-generating process.
Understanding Cellular Respiration
Cellular respiration is a metabolic pathway that breaks down glucose and other organic molecules to generate ATP. It typically begins in the cytoplasm and concludes within the mitochondria.
The overall process involves four main stages: glycolysis, pyruvate oxidation, the Krebs cycle (also known as the citric acid cycle), and the electron transport chain combined with oxidative phosphorylation. Each stage contributes to the gradual extraction of energy from nutrient molecules, with NADH playing a role in transferring this energy.
NADH Production Pathways
NADH is generated at several points during cellular respiration, acting as an electron carrier. Glycolysis, the initial stage, occurs in the cytoplasm, breaking down a six-carbon glucose molecule into two three-carbon pyruvate molecules. This process produces two NADH molecules per glucose, along with two ATP.
Following glycolysis, two pyruvate molecules undergo pyruvate oxidation in the mitochondrial matrix. Each pyruvate converts to acetyl-CoA, releasing carbon dioxide and generating one NADH molecule. Thus, two NADH molecules are produced for each original glucose molecule.
Acetyl-CoA then enters the Krebs cycle within the mitochondrial matrix. This cyclical series of reactions further oxidizes carbon compounds, releasing more carbon dioxide. Each turn of the cycle generates three NADH molecules, one FADH2, and one ATP (or GTP). Since two acetyl-CoA molecules enter per glucose, six NADH molecules are produced during this stage.
NADH’s Critical Role in Energy Production
NADH’s primary function involves carrying high-energy electrons to the electron transport chain (ETC) on the inner mitochondrial membrane. Each NADH molecule delivers two electrons to the first protein complex within the ETC.
As electrons move through protein complexes in the inner mitochondrial membrane, they release energy. This energy pumps protons, or hydrogen ions (H+), from the mitochondrial matrix into the intermembrane space, creating a concentration gradient.
The proton gradient represents stored potential energy. Protons flow back into the mitochondrial matrix through ATP synthase, which drives the synthesis of ATP from ADP and inorganic phosphate. This process, oxidative phosphorylation, generates the majority of ATP during cellular respiration.
After donating electrons, NADH oxidizes back to NAD+. This regeneration is essential because NAD+ acts as a substrate for earlier stages like glycolysis and the Krebs cycle. Continuous recycling of NAD+ ensures ongoing glucose breakdown and ATP production.