Cellular respiration is a multi-step process that occurs in three main stages: glycolysis, the citric acid cycle (also known as the Krebs cycle), and oxidative phosphorylation (which includes the electron transport chain and chemiosmosis). Here’s a brief overview of each step:
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Glycolysis:
- Location: Cytoplasm
- Process: Glucose (a six-carbon sugar) is broken down into two molecules of pyruvate (a three-carbon compound). This process generates a small amount of ATP and NADH.
- Products: 2 ATP (net gain), 2 NADH, and 2 pyruvate.
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Citric Acid Cycle (Krebs Cycle):
- Location: Mitochondrial matrix
- Process: Each pyruvate is converted into Acetyl-CoA, which then enters the Krebs cycle. Through a series of enzymatic reactions, Acetyl-CoA is further oxidized, releasing carbon dioxide as a byproduct. The cycle generates high-energy electron carriers (NADH and FADH2) and a small amount of ATP.
- Products: Per Acetyl-CoA, the cycle produces 3 NADH, 1 FADH2, and 1 ATP, as well as 2 CO2 as byproducts (since two Acetyl-CoA molecules are produced from one glucose molecule, this cycle runs twice per glucose).
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Oxidative Phosphorylation:
- Location: Inner mitochondrial membrane
- Process: NADH and FADH2 donate electrons to the electron transport chain, where electrons are passed through a series of proteins and eventually used to reduce oxygen to form water. The energy from these electron transfers is used to pump protons (H+) into the intermembrane space, creating a proton gradient. ATP synthase uses this gradient to produce ATP from ADP and inorganic phosphate.
- Products: Approximately 26-28 ATP, and water.
Carbon Dioxide Release: Carbon dioxide is released during the citric acid cycle. More specifically, it occurs during the decarboxylation reactions as Acetyl-CoA is processed through the cycle, and it is produced as a waste product when carbon atoms are removed from the organic substrates being metabolized.