Cellular respiration is a fundamental biological process that converts nutrients, primarily glucose, into adenosine triphosphate (ATP). ATP serves as the main energy currency of the cell, powering various cellular activities like muscle contraction, nerve impulse propagation, and the synthesis of essential molecules. This series of biochemical reactions releases stored chemical energy in a controlled manner, making it available for cellular functions.
Glycolysis
Glycolysis is the initial stage of cellular respiration, occurring in the cytoplasm. This process is anaerobic, operating without oxygen. A single six-carbon glucose molecule undergoes a series of ten enzymatic transformations.
These reactions convert glucose into two molecules of a three-carbon compound called pyruvate. This conversion yields a net gain of two ATP molecules and produces two molecules of NADH. NADH serves as an electron carrier, holding energy for later stages of cellular respiration.
Pyruvate Oxidation
Following glycolysis, the two pyruvate molecules undergo pyruvate oxidation, also called the “link reaction.” In eukaryotic cells, this process takes place within the mitochondrial matrix. Pyruvate must first be transported from the cytoplasm into this mitochondrial space.
During pyruvate oxidation, each three-carbon pyruvate molecule is converted into a two-carbon molecule called acetyl-CoA. This conversion releases one carbon dioxide molecule and generates one NADH molecule per pyruvate. For each original glucose molecule, two acetyl-CoA, two carbon dioxide, and two NADH molecules are produced, preparing the carbon atoms for entry into the next major cycle.
The Krebs Cycle
The Krebs cycle, also known as the Citric Acid Cycle or TCA cycle, is the third major stage of cellular respiration and occurs within the mitochondrial matrix. This cycle begins when the two-carbon acetyl-CoA molecule combines with a four-carbon molecule called oxaloacetate to form a six-carbon compound, citrate. Coenzyme A is then released to be reused.
Through a series of eight enzyme-catalyzed reactions, the citrate molecule is progressively oxidized. During each turn of the cycle, two carbon atoms are released as carbon dioxide. The cycle generates energy-carrying molecules: three NADH, one FADH2, and one ATP (or GTP, which is readily converted to ATP) per acetyl-CoA molecule. Since two acetyl-CoA molecules enter the cycle per glucose molecule, the Krebs cycle yields a total of six NADH, two FADH2, and two ATP molecules, while regenerating oxaloacetate to continue the cycle.
Oxidative Phosphorylation
Oxidative phosphorylation is the final and most substantial ATP-generating stage of cellular respiration, occurring on the inner mitochondrial membrane. This complex process consists of two interconnected components: the electron transport chain (ETC) and chemiosmosis. NADH and FADH2 molecules, carrying high-energy electrons from previous stages, deliver these electrons to the ETC.
As electrons move through a series of protein complexes in the inner mitochondrial membrane, energy is released incrementally. This energy is utilized to pump protons (hydrogen ions) from the mitochondrial matrix into the intermembrane space, creating a high concentration gradient of protons. This proton gradient stores potential energy, similar to a battery. Oxygen serves as the terminal electron acceptor at the end of the ETC, combining with electrons and protons to form water.
The stored energy in the proton gradient is harnessed by an enzyme called ATP synthase. Protons flow back into the mitochondrial matrix through ATP synthase, causing the enzyme to rotate and catalyze the synthesis of a large quantity of ATP from ADP and inorganic phosphate. This process, known as chemiosmosis, produces the majority of ATP during cellular respiration.