Cellular respiration converts nutrients into usable energy. The Kreb’s Cycle, also known as the Citric Acid Cycle or Tricarboxylic Acid (TCA) cycle, is a central metabolic pathway within this process. It takes place within the mitochondrial matrix of eukaryotic cells, playing a significant role in harnessing energy from carbohydrates, fats, and proteins.
Key Outputs of the Kreb’s Cycle
Each turn of the Kreb’s Cycle produces several important molecules. For every molecule of acetyl-CoA, three molecules of Nicotinamide Adenine Dinucleotide (NADH) and one molecule of Flavin Adenine Dinucleotide (FADH2) are generated. One molecule of adenosine triphosphate (ATP) or guanosine triphosphate (GTP) is also directly synthesized. Two molecules of carbon dioxide (CO2) are released as metabolic byproducts.
The Role of Electron Carriers
NADH and FADH2 are coenzymes that function as electron carriers, transporting high-energy electrons. They do not directly provide energy but act as intermediaries, carrying electrons and protons from glucose breakdown to the electron transport chain (ETC).
The ETC, located in the inner mitochondrial membrane, uses the energy from these electrons to pump protons, creating an electrochemical gradient. The flow of these protons back into the mitochondrial matrix through ATP synthase drives the synthesis of ATP, a process called oxidative phosphorylation. Each NADH molecule contributes to about 2.5 ATP molecules, while each FADH2 molecule yields about 1.5 ATP molecules.
Direct Energy Production
The Kreb’s Cycle also directly produces a small amount of usable energy as ATP or GTP through substrate-level phosphorylation. In this step, a phosphate group is directly transferred from an intermediate molecule to ADP or GDP to form ATP or GTP.
While only one ATP or GTP molecule is generated per cycle turn, this provides immediate energy for cellular functions. GTP is readily converted to ATP, contributing to the cell’s main energy currency.
Carbon Dioxide as a Byproduct
Carbon dioxide (CO2) is generated during the Kreb’s Cycle as a waste product of cellular respiration. This gas represents fully oxidized carbon atoms from the original glucose molecule.
Two CO2 molecules are released per cycle turn as carbon atoms are removed from intermediates. This metabolic byproduct diffuses out of cells into the bloodstream, then transports to the lungs for exhalation.
Overall Contribution to Cellular Energy
The products of the Kreb’s Cycle contribute significantly to the cell’s overall energy supply. Electron carriers, NADH and FADH2, are the primary energy-yielding outputs, fueling the majority of ATP production through oxidative phosphorylation.
Directly produced ATP or GTP provides immediate energy for various cellular processes. The cycle also serves as a metabolic hub, providing precursors for synthesizing other molecules like amino acids and fats, demonstrating its broader role beyond energy generation.