Our bodies constantly need energy for all life processes. This energy is primarily generated through cellular respiration, a complex series of chemical reactions. This process breaks down nutrients, particularly glucose, to produce adenosine triphosphate (ATP), the direct energy currency for cells.
From Glucose to Pyruvic Acid
Cellular respiration begins with a pathway called glycolysis, which occurs in the cytoplasm of a cell. In this initial stage, a single six-carbon glucose molecule is broken down into two three-carbon molecules of pyruvic acid, also known as pyruvate. This process happens without the need for oxygen, making it an anaerobic pathway. Glycolysis yields a small amount of ATP and also produces molecules of NADH, which are temporary energy carriers.
The Crucial Conversion
Pyruvic acid does not directly enter the Krebs cycle. Instead, it undergoes an intermediate step called pyruvate oxidation, sometimes referred to as the “link reaction.” This conversion takes place in the mitochondrial matrix in cells that use oxygen. During this process, each three-carbon pyruvic acid molecule transforms into a two-carbon molecule called acetyl-CoA.
This transformation involves three main events. First, a carboxyl group is removed from pyruvic acid and released as carbon dioxide. Second, the remaining two-carbon structure is oxidized, transferring electrons to NAD+ and forming NADH. Third, the oxidized two-carbon molecule, now an acetyl group, attaches to Coenzyme A, forming acetyl-CoA. This acetyl-CoA molecule then acts as fuel to enter the subsequent stage of cellular respiration.
Journey Through the Krebs Cycle
The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, takes place within the mitochondrial matrix. This cycle begins when the two-carbon acetyl-CoA molecule produced from pyruvic acid combines with a four-carbon molecule called oxaloacetate. This union forms a six-carbon compound known as citrate, which gives the cycle its alternative name. The cycle then proceeds through a series of reactions that regenerate the starting oxaloacetate, allowing the process to continue.
During each turn of the Krebs cycle, carbon atoms from acetyl-CoA are further broken down. Two molecules of carbon dioxide are released as waste products. The cycle generates high-energy electron carrier molecules: three molecules of NADH and one molecule of FADH2. A small amount of ATP is also produced directly within the cycle. Since two pyruvic acid molecules form from each glucose molecule, the Krebs cycle completes two turns for every original glucose molecule, doubling these outputs.
The Energy Harvest
The NADH and FADH2 molecules generated during glycolysis, pyruvate oxidation, and the Krebs cycle are important for the final stage of energy production. These molecules function as “electron carriers” because they hold high-energy electrons. They transport these electrons to the electron transport chain (ETC), located in the inner mitochondrial membrane.
Within the electron transport chain, the energy from these electrons drives the synthesis of a large quantity of ATP. This process, called oxidative phosphorylation, generates the majority of the cell’s energy currency. The transfer of electrons through the chain ultimately leads to the production of water and a significant energy yield, making the processing of pyruvic acid important for ATP production.