Summary
Highlights
Cellular respiration's primary goal is to create ATP (adenosine triphosphate), the cell's energy currency. ATP has three phosphate groups, and energy is stored in the bond between the second and third phosphates. The hydrolysis of ATP, where water breaks off a phosphate group, releases energy to power cellular processes.
ATP is generated through cellular respiration, primarily in the mitochondria. This process breaks down glucose from food to produce a large amount of ATP. The three main stages of cellular respiration are glycolysis, the Krebs cycle, and the electron transport chain.
Glycolysis occurs in the cytoplasm. Glucose is broken down into two molecules of pyruvate. This process requires two ATP molecules to start but generates four ATP molecules, resulting in a net gain of two ATP. During glycolysis, NAD+ is converted to NADH, which carries hydrogen to the mitochondria. The pyruvates can then enter either the aerobic pathway (Krebs cycle) or the anaerobic pathway (fermentation).
The Krebs cycle takes place in the mitochondrial matrix. Pyruvate is first converted into acetic acid, creating NADH and carbon dioxide. Acetic acid then combines with coenzyme A to form acetyl-CoA. Acetyl-CoA combines with a four-carbon molecule from the previous cycle to form citric acid (a six-carbon molecule). Through several steps, citric acid is broken down, producing more NADH, FADH2, and one ATP molecule, along with carbon dioxide waste. The four-carbon molecule is regenerated to continue the cycle. The main output of the Krebs cycle is not ATP but the electron carriers NADH and FADH2, which are crucial for the next stage.
The electron transport chain occurs in the inner mitochondrial membrane, involving protein complexes embedded within it. NADH and FADH2 deliver electrons and hydrogen ions. As electrons move through the protein complexes, they provide energy to pump hydrogen ions out of the mitochondrial matrix, creating a high concentration gradient. These hydrogen ions then flow back into the matrix through ATP synthase, which harnesses this movement to combine ADP and phosphate, producing a large amount of ATP (up to 34 molecules). Finally, oxygen acts as the final electron acceptor, combining with hydrogen ions and electrons to form water, which is a byproduct of cellular respiration.