Aerobic cellular respiration (ACR) is the process by which cells convert chemical energy stored in nutrients into adenosine triphosphate (ATP), the primary energy currency of life. This metabolic pathway requires oxygen to efficiently break down fuel molecules, typically glucose, to provide sustained energy for cellular activities. The process transforms glucose and oxygen into carbon dioxide and water, capturing energy as ATP.
The First Stage Glycolysis
Glycolysis is the first stage, consisting of ten reactions that occur within the cytoplasm. A single molecule of glucose, a six-carbon sugar, is rearranged and split into two three-carbon molecules called pyruvate.
The process requires an initial investment of two ATP molecules but generates four ATP, resulting in a net gain of two ATP. Simultaneously, two molecules of NAD+ are reduced to NADH. This phase does not require oxygen, making it a universal metabolic pathway found in almost all organisms.
The Transition Phase
The transition phase prepares the two pyruvate molecules to enter the next stage. In eukaryotic cells, pyruvate moves from the cytoplasm into the mitochondrial matrix. An enzyme complex catalyzes the removal of one carbon atom from each pyruvate molecule.
This carbon atom is released as carbon dioxide (\(\text{CO}_2\)). The remaining two-carbon fragment attaches to a coenzyme, forming acetyl coenzyme A (acetyl-CoA). This reaction generates one NADH molecule for each pyruvate, resulting in two NADH molecules produced per glucose.
The Citric Acid Cycle
The citric acid cycle, also known as the Krebs cycle, is a closed loop of eight reactions that takes place within the mitochondrial matrix. Acetyl-CoA enters the cycle and joins a four-carbon acceptor molecule. The cycle’s primary purpose is the complete chemical oxidation of the original glucose carbon atoms.
As the acetyl group moves through the cycle, its carbon atoms are systematically removed and released as four molecules of \(\text{CO}_2\). This breakdown produces high-energy electron carriers. For every turn, three NADH, one FADH\(_2\) (flavin adenine dinucleotide), and one ATP are generated. Since the cycle turns twice per glucose molecule, the total yield is six NADH, two FADH\(_2\), and two ATP.
Generating the Bulk of Energy
The final and most productive stage of aerobic cellular respiration is oxidative phosphorylation, which generates the vast majority of the cell’s ATP. This process occurs across the inner mitochondrial membrane and involves two steps: the electron transport chain (ETC) and chemiosmosis. The high-energy electron carriers, NADH and FADH\(_2\), deliver their electrons to protein complexes embedded in this membrane.
As electrons are passed along the ETC, energy is released. This energy is harvested by the protein complexes to pump protons (\(\text{H}^+\)) from the mitochondrial matrix into the intermembrane space. The continuous pumping of these protons creates a high concentration gradient across the inner membrane.
This gradient is a form of stored potential energy. Protons seek to flow back into the matrix to equalize the gradient, but the membrane is impermeable to them except through a specific enzyme called ATP synthase.
As the protons flow back into the matrix through ATP synthase, the energy of their movement is harnessed to power the synthesis of ATP from adenosine diphosphate (ADP) and an inorganic phosphate group. This process of generating ATP by coupling the proton gradient to ATP synthase is called chemiosmosis. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water (\(\text{H}_2\text{O}\)).
The Final Energy Tally
Aerobic cellular respiration is an efficient process for energy generation. The entire process can be summarized by the overall chemical equation: \(\text{C}_6\text{H}_{12}\text{O}_6\) (glucose) + \(6\text{O}_2\) (oxygen) \(\rightarrow\) \(6\text{CO}_2\) (carbon dioxide) + \(6\text{H}_2\text{O}\) (water) + Energy (ATP).
The total yield of usable ATP from one glucose molecule generally falls within the range of 30 to 32 ATP molecules. This includes two ATP from glycolysis, two ATP from the citric acid cycle, and 26 to 28 ATP generated by oxidative phosphorylation. This lower, more realistic figure accounts for the energy costs associated with transporting molecules into the mitochondria.