Cellular respiration is the foundational process by which living cells extract usable energy from nutrient molecules like glucose. This process breaks down larger molecules, releasing stored chemical energy. The immediate, universal energy currency produced is adenosine triphosphate (ATP), a high-energy molecule that fuels nearly all cellular activities. Without the continuous generation of ATP, a cell cannot maintain its structure or perform the tasks necessary for life.
The Role of Oxygen and Cellular Location
The defining difference between the two types of cellular respiration lies in the requirement for a final electron acceptor. Aerobic respiration is strictly dependent on the presence of molecular oxygen (\(\text{O}_2\)), which serves this role at the end of the energy-generating pathway. Conversely, anaerobic respiration proceeds entirely without oxygen, utilizing a different terminal electron acceptor or ending the process with an organic molecule.
Aerobic respiration, the mechanism used by most complex organisms, begins in the cytoplasm but primarily shifts to the mitochondria. The later, higher-yielding stages of this process are localized to the mitochondrial matrix and inner membrane. In contrast, anaerobic respiration is confined entirely to the cytoplasm.
Contrasting Metabolic Pathways
Both aerobic and anaerobic respiration begin with a shared sequence of reactions known as glycolysis, which occurs in the cytoplasm. Glycolysis breaks a six-carbon glucose molecule into two molecules of three-carbon pyruvate, generating a small net gain of two ATP molecules. This initial stage does not require oxygen, but the subsequent fate of the pyruvate molecule separates the two pathways.
In the presence of oxygen, pyruvate is transported into the mitochondrial matrix to begin the aerobic sequence, which includes the Krebs cycle (or Citric Acid Cycle). Before entering the cycle, pyruvate is converted into Acetyl-CoA, releasing carbon dioxide (\(\text{CO}_2\)). The Krebs cycle then fully oxidizes the Acetyl-CoA, producing additional ATP, \(\text{CO}_2\), and high-energy electron carriers, specifically NADH and \(\text{FADH}_2\).
The final and most productive stage of aerobic respiration is oxidative phosphorylation, which includes the electron transport chain (ETC) on the inner mitochondrial membrane. The electron carriers from the Krebs cycle deliver their high-energy electrons to the ETC, a series of protein complexes. As electrons move down the chain, their energy is used to pump hydrogen ions, creating a concentration gradient that drives the synthesis of ATP. Oxygen acts as the final acceptor in this chain, combining with electrons and hydrogen ions to form water (\(\text{H}_2\text{O}\)).
When oxygen is absent, the pyruvate from glycolysis cannot enter the mitochondria or the Krebs cycle, and the ETC cannot function. Instead, the cell initiates fermentation to regenerate the \(\text{NAD}^+\) electron carrier, which is necessary to keep glycolysis running. In human muscle cells during intense exercise, pyruvate is converted to lactic acid (lactate) via lactic acid fermentation. Other organisms, such as yeast, perform alcoholic fermentation, converting pyruvate into ethanol and \(\text{CO}_2\).
Energy Output and Final Products
The starkest quantifiable difference between the two processes is the total energy yield, measured in the number of ATP molecules produced per glucose molecule. Aerobic respiration is vastly more efficient, generating a total yield of 30 to 36 ATP molecules per molecule of glucose. This high output is a direct result of the complete oxidation of glucose through the Krebs cycle and the high ATP production via the electron transport chain.
Anaerobic respiration, relying only on glycolysis and fermentation, produces a meager net yield of only two ATP molecules per glucose molecule. This low yield occurs because the glucose molecule is only partially broken down, and the energy held in the pyruvate and the electron carriers is not fully utilized. The final products released by the two types of respiration also differ significantly, reflecting the extent of fuel breakdown.
Aerobic respiration results in the production of simple, easily excretable inorganic molecules: carbon dioxide (\(\text{CO}_2\)) and water (\(\text{H}_2\text{O}\)). The carbon atoms from the original glucose are fully oxidized and released as \(\text{CO}_2\). Anaerobic respiration, by contrast, produces organic molecules that still contain chemical energy, such as lactic acid in mammals or ethanol and \(\text{CO}_2\) from yeast fermentation.