Cellular respiration is the fundamental process by which living cells transform the chemical energy stored in nutrients into adenosine triphosphate (ATP), the primary energy currency of the cell. This process involves a series of interconnected chemical reactions, extracting energy from organic molecules through controlled oxidation.
Understanding Oxidation in Cellular Respiration
Oxidation, in biology, refers to the loss of electrons or hydrogen atoms from a molecule. Conversely, reduction describes the gain of electrons or hydrogen atoms. These two processes always occur together in redox reactions: if one molecule is oxidized (loses electrons), another molecule must be reduced (gains those electrons). Cellular respiration involves a continuous series of these redox reactions, systematically removing electrons from nutrient molecules.
When a molecule is oxidized during cellular respiration, it releases energy-rich electrons. These electrons are captured by specific electron carrier molecules, such as NAD+ and FAD. The energy from these captured electrons is then used to generate ATP, converting the chemical energy from the nutrient into a usable form for cellular activities.
The Primary Molecule Undergoing Oxidation
Glucose, a simple six-carbon sugar, is the most common fuel source oxidized during cellular respiration. The breakdown of glucose begins in the cytoplasm with glycolysis, which splits one glucose molecule into two molecules of pyruvate. During these initial steps, some ATP is directly produced, and electrons along with hydrogen atoms are removed from glucose, picked up by NAD+ to form NADH.
Following glycolysis, if oxygen is available, pyruvate moves into the mitochondria for further processing. Each pyruvate molecule converts into acetyl-CoA, and a carbon atom is released as carbon dioxide. Acetyl-CoA then enters the Krebs cycle, also known as the citric acid cycle, in the mitochondrial matrix. In this cycle, the remaining carbon atoms from the glucose molecule are completely oxidized and released as carbon dioxide. Throughout these stages, more electrons and hydrogen atoms are collected by NADH and FADH2, which are important for the next phase of energy production.
The Path of Electrons and Oxygen’s Role
The electrons and hydrogen atoms extracted from glucose, carried by NADH and FADH2, are delivered to the electron transport chain (ETC). This chain consists of protein complexes embedded within the inner mitochondrial membrane. As electrons move along this chain, they pass from one complex to the next in redox reactions, gradually releasing their energy.
This stepwise energy release pumps hydrogen ions across the inner mitochondrial membrane, creating an electrochemical gradient. Oxygen serves as the final electron acceptor at the end of the electron transport chain. Molecular oxygen accepts the “spent” electrons and combines with hydrogen ions to form water, a waste product. Without oxygen to accept these electrons, the electron transport chain would halt, and major ATP production would cease.
Oxidation of Other Fuel Sources
While glucose is the primary fuel, cells can also oxidize other macromolecules like fats (lipids) and proteins to generate ATP. These alternative fuel sources break down into smaller components that can enter the cellular respiration pathways at various points.
Fats break into glycerol and fatty acids. Glycerol converts into an intermediate of glycolysis to proceed through the pathway. Fatty acids undergo beta-oxidation, breaking down into two-carbon units of acetyl-CoA, which directly enters the Krebs cycle, similar to that from glucose. Proteins break into individual amino acids, with their amino groups removed; their remaining carbon skeletons can enter the cellular respiration pathway at different stages, including as intermediates in glycolysis or the Krebs cycle. Regardless of the initial fuel source, oxidation—the removal and transfer of electrons—remains central to energy extraction.