Cellular respiration is the foundational process by which living organisms convert the biochemical energy stored in nutrients into adenosine triphosphate (ATP), the cell’s universal energy currency. This process takes in energy-rich molecules like glucose and extracts the energy required for every cellular function. The process is highly regulated and involves multiple stages, ensuring that the energy locked within food is efficiently harvested for use in activities ranging from muscle contraction to the synthesis of new proteins.
The Initial Energy Conversion
The first stage of energy extraction from glucose is a process called glycolysis, which occurs in the cytoplasm. This initial conversion is anaerobic, meaning it does not require the presence of oxygen to proceed. Glycolysis begins with a single six-carbon glucose molecule and ends with two three-carbon molecules of pyruvate. The process requires an investment of two ATP molecules but ultimately produces a net gain of two ATP. Beyond the pyruvate, glycolysis also generates two molecules of NADH. These electron carriers hold high-energy electrons that will be used in a later stage of respiration.
Energy Generation in the Mitochondria
Following glycolysis, the two pyruvate molecules move from the cytoplasm into the mitochondria. Inside the mitochondria, pyruvate is first converted into a compound called acetyl-CoA, releasing carbon dioxide and generating more NADH.
This acetyl-CoA then enters the Citric Acid Cycle, also known as the Krebs cycle, which takes place in the mitochondrial matrix. The cycle’s primary function is not to produce large amounts of ATP directly, but rather to complete the oxidation of the original glucose molecule. The cycle systematically strips electrons from the intermediate molecules, producing a significant quantity of the electron carriers NADH and \(\text{FADH}_{2}\). For each molecule of glucose, the Krebs cycle turns twice, yielding two ATP molecules, along with six NADH and two \(\text{FADH}_{2}\) molecules.
The final and most productive phase is the Electron Transport Chain (ETC) and Oxidative Phosphorylation, which occurs on the inner mitochondrial membrane. The NADH and \(\text{FADH}_{2}\) molecules donate their high-energy electrons to a chain of protein complexes. As electrons move down the chain, their energy is used to pump hydrogen ions (\(\text{H}^{+}\)) from the mitochondrial matrix into the intermembrane space, creating a steep concentration gradient. The \(\text{H}^{+}\) ions then flow back into the matrix through a specialized enzyme complex called ATP synthase. This flow of protons drives the rotation of the ATP synthase enzyme, which mechanically converts adenosine diphosphate (ADP) and a phosphate group into ATP.
Oxygen acts as the final electron acceptor at the end of the chain, combining with electrons and hydrogen ions to form water, which is why this entire process is dependent on oxygen. The ETC is responsible for producing the vast majority of the cell’s energy, generating 30 to 34 ATP molecules per glucose molecule.
When Oxygen is Scarce
When oxygen levels are insufficient, cells must rely on a different, temporary strategy called fermentation. Fermentation’s primary purpose is not to generate energy directly, but to regenerate the \(\text{NAD}^{+}\) molecules needed to keep glycolysis running, ensuring the limited production of two net ATP per glucose continues.
There are two main types of fermentation. Lactic acid fermentation occurs in human muscle cells during intense exercise. In this pathway, pyruvate is converted into lactate, which regenerates the \(\text{NAD}^{+}\) required for glycolysis to continue.
The second common type is alcoholic fermentation, used by organisms like yeast. In this process, pyruvate is converted into ethanol and carbon dioxide. Both forms of fermentation are significantly less efficient than aerobic respiration, yielding only two ATP molecules compared to the 30 or more ATP produced with oxygen.
Why This Process Matters
The significance of cellular respiration lies in adenosine triphosphate (ATP). ATP is accurately described as the cell’s energy currency because it captures the energy released from the breakdown of food molecules and shuttles it to where work needs to be done. The bonds between the three phosphate groups in ATP store a considerable amount of energy, which is released when the outermost phosphate is broken off, converting ATP into ADP (adenosine diphosphate).
This energy release powers biological functions necessary for life. For instance, ATP is required for muscle cells to contract, for nerve cells to transmit electrical impulses, and for the active transport of substances across cell membranes. The synthesis of complex macromolecules, such as DNA, RNA, and proteins, is dependent on the continuous supply of ATP generated by cellular respiration.