Cellular respiration enables living organisms to turn food into energy. It revolves around the Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle.
The series of chemical reactions drives cellular metabolism, allowing bodies to extract energy from carbohydrates, fats, and proteins. This cycle illustrates how energy production supports life at a cellular level.
Cellular respiration is the process whereby cells convert nutrients into usable energy in the form of ATP (adenosine triphosphate). The enzymatic reactions of the Krebs cycle happen in the mitochondria of eukaryotic cells.
It's discovered in the 1930s by Hans Krebs.
The cycle starts with acetyl-CoA, a molecule derived from carbohydrates, fats, and proteins. This combines with oxaloacetate to form citrate, or citric acid.
Through a series of reactions, acetyl-CoA is completely oxidized, releasing carbon dioxide (CO2) as a byproduct. Enzymes catalyze the conversion of one molecule into another to keep the cycle continuous.
The cycle regenerates oxaloacetate, a four-carbon crystalline organic acid. It takes products of earlier metabolic stages, like glycolysis, and further reduces them to release high-energy electrons.
The electrons are captured by carrier molecules NADH and FADH2. These molecules proceed to the electron transport chain, the final stage of cellular respiration, where most ATP is generated.
The eight distinct steps of the Krebs cycle are driven by specific enzymes.
Citrate Formation: Acetyl-CoA merges with oxaloacetate to create citrate.
Citrate to Isocitrate: Citrate is rearranged into isocitrate by the enzyme aconitase.
Isocitrate to α-Ketoglutarate: Isocitrate undergoes oxidation and decarboxylation, releasing a carbon as CO2 and forming α-ketoglutarate. This step produces one molecule of NADH.
α-Ketoglutarate to Succinyl-CoA: Another carbon is released, another NADH is created as α-ketoglutarate transforms into succinyl-CoA.
Succinyl-CoA to Succinate: This step generates ATP (or GTP) as succinyl-CoA converts to succinate.
Succinate to Fumarate: Succinate is oxidized to fumarate, resulting in the production of FADH2.
Fumarate to Malate: Fumarate is converted to malate through hydration.
Malate to Oxaloacetate: Finally, malate is oxidized back to oxaloacetate, producing a third molecule of NADH and closing the cycle.
Importance of the Krebs Cycle
ATP Production: While the cycle itself only produces a small amount of ATP directly, it provides the NADH and FADH2 needed to fuel the electron transport chain and generate the large amounts of ATP cells need.
Provision of Precursors: The cycle provides important precursor molecules used in synthesis of compounds like amino acids, fatty acids and heme, integral to building and maintaining cell structures.
Waste Removal: The cycle helps remove waste products by oxidizing acetyl-CoA and releasing carbon dioxide.
Regulation: The Krebs cycle is tightly regulated by various factors, including availability of ATP and levels of key reactants and products. This makes sure energy production is matched to the cell's needs.
Electron Transport Chain
The energy carriers NADH and FADH2 help the electron transport chain (ETC), where oxidative phosphorylation occurs. These reduced coenzymes donate electrons to the ETC.
This begins the series of reactions in creation of ATP. Cells can generate about 34 ATP molecules from one glucose molecule through these combined processes.
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