Cellular respiration is a fundamental biological process where living organisms convert chemical energy stored in nutrients, particularly carbohydrates, into adenosine triphosphate (ATP). This complex series of reactions allows cells to power various activities, from muscle contraction to the synthesis of new molecules. Carbohydrates serve as a primary fuel source, undergoing a step-by-step breakdown to release their stored chemical energy. The overall process transforms glucose and oxygen into carbon dioxide and water, capturing energy in ATP molecules for cellular use.
The First Split: Glycolysis
The initial stage of carbohydrate breakdown in cellular respiration is glycolysis, occurring in the cytosol. This stage begins with a single glucose molecule, a six-carbon sugar, which undergoes a series of chemical transformations. Through ten enzyme-catalyzed reactions, glucose splits into two molecules of a three-carbon compound called pyruvate.
Glycolysis directly produces a net gain of two ATP molecules per glucose. High-energy electrons are also captured by nicotinamide adenine dinucleotide (NAD+), forming two molecules of NADH. This phase does not require oxygen, making it an ancient metabolic pathway found in nearly all organisms.
Further Breakdown: Pyruvate and the Citric Acid Cycle
Following glycolysis, the two pyruvate molecules move from the cytosol into the mitochondria. Inside the mitochondrial matrix, pyruvate undergoes pyruvate oxidation. Each three-carbon pyruvate molecule transforms into a two-carbon acetyl-CoA molecule, releasing one carbon dioxide. During this conversion, NAD+ is reduced to NADH, capturing more high-energy electrons.
Acetyl-CoA then enters the citric acid cycle, also known as the Krebs cycle or TCA cycle. This cycle is a series of eight reactions where the two-carbon acetyl group from acetyl-CoA combines with a four-carbon molecule, regenerating the starting four-carbon molecule. For each acetyl-CoA molecule, the cycle produces two carbon dioxide molecules, completing the breakdown of original glucose carbons.
The cycle also generates high-energy electron carriers: three NADH and one FADH2, along with a small amount of ATP. These electron carriers carry most of the energy extracted from the carbohydrate, setting the stage for the final phase of ATP synthesis.
Energy Harvest: The Electron Transport Chain
The final substantial stage of energy production in cellular respiration is the electron transport chain (ETC), located on the inner mitochondrial membrane. This stage harnesses the energy stored in the NADH and FADH2 molecules generated in previous steps. These electron carriers deliver their high-energy electrons to a series of protein complexes embedded within the membrane.
As electrons pass through these protein complexes, they move from a higher to a lower energy level, releasing energy. This released energy is used to pump hydrogen ions (protons) from the mitochondrial matrix into the intermembrane space, creating a high concentration of protons in this narrow region. This difference in proton concentration across the inner membrane establishes an electrochemical gradient, often referred to as the proton motive force.
Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. Without oxygen, the electron transport chain cannot function, halting ATP synthesis.
The accumulated protons in the intermembrane space then flow back into the mitochondrial matrix through ATP synthase. This enzyme acts like a molecular turbine; proton flow drives its rotation, powering the synthesis of a large amount of ATP from adenosine diphosphate (ADP) and inorganic phosphate. This process, known as oxidative phosphorylation, generates the vast majority of the cell’s ATP, converting stored carbohydrate energy into usable cellular energy.