Cellular respiration is how cells convert the chemical energy stored in nutrient molecules, primarily glucose, into a usable form. This energy takes the form of adenosine triphosphate (ATP), which acts as the cell’s main energy currency. The overall goal is to fully break down the starting molecule through a series of controlled, multi-step chemical reactions. This complex metabolic pathway occurs in different locations within the cell.
Setting the Scene: The Products of Glycolysis
The energy extraction process begins with glycolysis, the first stage, which occurs in the cell’s cytosol. A single six-carbon glucose molecule is broken down into two three-carbon molecules of pyruvate. This initial breakdown yields a small net gain of two ATP and two molecules of the high-energy carrier NADH.
Pyruvate must be actively transported across the mitochondrial membranes to reach the inner compartment, the mitochondrial matrix. This transport sets the stage for the crucial transition that prepares the carbons for the main energy-releasing cycle.
Pyruvate Oxidation: The Transition Stage
Pyruvate Oxidation is the immediate second stage of cellular respiration, sometimes called the link reaction because it connects glycolysis to the next major cycle. This process occurs entirely within the mitochondrial matrix. Its function is to convert the three-carbon pyruvate molecule into a two-carbon compound that can enter the central energy-harvesting cycle.
The conversion begins when a carboxyl group is removed from pyruvate and released as carbon dioxide. The remaining two-carbon molecule is then oxidized, losing high-energy electrons captured by NAD+, reducing it to NADH. Finally, the two-carbon fragment, now called an acetyl group, is attached to Coenzyme A, forming Acetyl-CoA.
This transition step happens twice per glucose molecule. Therefore, Pyruvate Oxidation yields two molecules of Acetyl-CoA, two molecules of carbon dioxide, and two molecules of NADH per glucose.
The Citric Acid Cycle
The Citric Acid Cycle, also known as the Krebs Cycle or TCA Cycle, is the functional core of the middle phase of cellular respiration. This eight-step cycle takes place in the mitochondrial matrix and is responsible for the complete oxidation of the remaining carbon atoms. It is a closed loop, where the starting molecule is regenerated at the end of each turn.
The cycle begins when the two-carbon Acetyl-CoA combines with the four-carbon oxaloacetate to form citrate. Through a sequence of enzymatic reactions, the citrate molecule is systematically broken down and rearranged. The purpose of these steps is to strip away high-energy electrons and hydrogen ions from the carbon intermediates.
In one complete turn, two molecules of carbon dioxide are released, completing the breakdown of the entering Acetyl-CoA. The cycle generates three molecules of NADH and one molecule of FADH2, which are high-energy electron carriers. It also produces one molecule of ATP (or GTP) through substrate-level phosphorylation.
Since the cycle must turn twice per glucose, the total yield is six NADH, two FADH2, and two ATP (or GTP). At the conclusion of this cycle, all six carbon atoms from the original glucose have been released as carbon dioxide.
The Importance of Electron Carrier Molecules
The NADH and FADH2 molecules generated during Pyruvate Oxidation and the Citric Acid Cycle represent the majority of the energy harvested from glucose up to this point. These molecules are specialized coenzymes that function as mobile shuttles, temporarily holding onto high-energy electrons. NADH is produced in all three preceding stages, while FADH2 is produced only within the Citric Acid Cycle.
These carriers are necessary to fuel the final stage of cellular respiration. Their sole purpose is to deliver the captured high-energy electrons to the inner mitochondrial membrane, where the Electron Transport Chain resides. This delivery system ensures that the energy can be used efficiently to produce a large amount of ATP.