Cellular respiration is the process by which living cells extract chemical energy from nutrient molecules, primarily glucose, and convert it into a usable form known as adenosine triphosphate (ATP). This metabolic process powers nearly all life functions, including muscle contraction, nerve impulses, and the synthesis of complex biomolecules. It involves a sequence of interconnected chemical reactions that systematically break down energy sources. This controlled, step-by-step process allows the cell to capture energy efficiently.
Glycolysis
The process begins with glycolysis, a metabolic pathway occurring within the cell’s cytoplasm. This initial stage breaks down a single six-carbon glucose molecule into two three-carbon molecules of pyruvate. Glycolysis does not require oxygen, distinguishing it as an anaerobic process.
The pathway involves two phases: energy investment and energy payoff. The initial steps utilize two ATP molecules to prepare the glucose for breakdown. The later steps yield a total of four ATP molecules, resulting in a net gain of two ATP per glucose molecule.
Glycolysis also generates two molecules of the electron carrier NADH. These carriers temporarily hold high-energy electrons that will be utilized later in respiration. The resulting pyruvate molecules are then directed toward further oxidation if oxygen is present.
The Citric Acid Cycle
Following glycolysis, the two pyruvate molecules enter the mitochondria for further energy extraction. In the mitochondrial matrix, each pyruvate undergoes a transition step, converting it into Acetyl-CoA. This conversion releases a carbon atom as carbon dioxide and creates one NADH electron carrier.
The resulting Acetyl-CoA enters the Citric Acid Cycle (Krebs Cycle), a continuous sequence of eight reactions. The cycle’s primary function is to complete the breakdown of the glucose remnant by systematically harvesting the remaining high-energy electrons stored in Acetyl-CoA.
For every turn of the cycle, the two carbon atoms that entered as Acetyl-CoA are fully oxidized and released as two molecules of carbon dioxide. The cycle generates three molecules of NADH and one molecule of the secondary electron carrier, \(\text{FADH}_2\). Only one molecule of ATP is produced directly per turn. Since the initial glucose yielded two pyruvates, the cycle runs twice, doubling the total output of NADH and \(\text{FADH}_2\) for the final stage of respiration.
Oxidative Phosphorylation and ATP Synthesis
Oxidative phosphorylation is the final stage of aerobic respiration, producing the vast majority of the cell’s energy supply. This complex process occurs along the inner mitochondrial membrane, utilizing the \(\text{NADH}\) and \(\text{FADH}_2\) molecules generated previously. The process consists of two coupled parts: the Electron Transport Chain (ETC) and chemiosmosis.
The Electron Transport Chain (ETC)
The ETC is a series of protein complexes embedded within the inner mitochondrial membrane. The electron carriers, \(\text{NADH}\) and \(\text{FADH}_2\), drop off their high-energy electrons, becoming re-oxidized so they can return to earlier cycles. As electrons move down the chain, they release small amounts of energy.
This energy is harnessed by the protein complexes to pump protons (\(\text{H}^+\)) from the mitochondrial matrix into the intermembrane space. This continuous pumping creates a powerful electrochemical gradient across the membrane. Oxygen acts as the final electron acceptor at the end of the chain, combining with electrons and protons to form water as a byproduct.
Chemiosmosis and ATP Synthesis
The energy stored in this proton gradient is then utilized through chemiosmosis. Protons, driven by both the concentration and electrical gradient, attempt to flow back into the matrix. They can only pass through a specialized enzyme complex called ATP synthase, which spans the inner membrane.
The flow of protons through this enzyme causes a part of the complex to rotate, similar to a turbine. This mechanical rotation provides the energy needed to combine adenosine diphosphate (\(\text{ADP}\)) with an inorganic phosphate group (\(\text{P}_i\)) to synthesize ATP. This stage is highly efficient, yielding approximately 30 to 32 ATP molecules per glucose, accounting for about 90% of the total energy output.
Anaerobic Pathways
When oxygen is unavailable or insufficient, the cell relies on alternative metabolic routes known as anaerobic pathways or fermentation. Without oxygen to act as the final electron acceptor, the Electron Transport Chain halts. Consequently, the \(\text{NADH}\) and \(\text{FADH}_2\) carriers cannot be re-oxidized, causing the Citric Acid Cycle to stop functioning.
The primary purpose of fermentation is to regenerate the \(\text{NAD}^+\) molecules required to keep glycolysis running. Since glycolysis can proceed without oxygen, it becomes the cell’s sole source of energy, yielding two net ATP molecules. Without this regeneration mechanism, glycolysis would quickly cease.
There are two primary types of fermentation:
- Lactic acid fermentation occurs in certain bacteria and in human muscle cells during intense exercise. Pyruvate accepts electrons from \(\text{NADH}\), regenerating \(\text{NAD}^+\) and converting the pyruvate into lactate.
- Alcoholic fermentation is performed by organisms like yeast. Pyruvate is first converted into acetaldehyde and then into ethanol, which regenerates \(\text{NAD}^+\). This pathway releases carbon dioxide as a byproduct.
Both fermentation pathways provide a small, immediate supply of two ATP molecules from glycolysis, which is significantly less efficient than the 30 to 32 ATP molecules generated by the complete aerobic pathway.