TLDR;
This video explains the citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid cycle, which is a crucial part of aerobic respiration. It details how pyruvate molecules from glycolysis enter the mitochondrial matrix and are converted into acetyl CoA, which then enters the cycle. The cycle involves eight steps, each catalyzed by a specific enzyme, producing key molecules like NADH, FADH2, and ATP. These products then move on to oxidative phosphorylation to generate the majority of ATP in aerobic respiration.
- Glycolysis produces pyruvate molecules.
- Pyruvate converts to Acetyl CoA to enter the citric acid cycle.
- The citric acid cycle produces NADH, FADH2, and ATP.
- Products of the cycle move to oxidative phosphorylation for ATP production.
Introduction to the Citric Acid Cycle [0:00]
The lecture introduces the citric acid cycle as an essential aerobic process that generates significantly more energy than glycolysis. Glycolysis, an anaerobic process, yields only two ATPs per glucose molecule, which was sufficient for early organisms. However, the evolution of complex organisms required more efficient energy production through aerobic respiration, which became possible with the increase of oxygen in the atmosphere due to photosynthesis. This process occurs in the mitochondria, organelles within eukaryotic cells believed to have originated as separate organisms incorporated for their respiratory capabilities.
From Glycolysis to Acetyl CoA [1:20]
The process begins with pyruvate molecules, produced during glycolysis in the cytoplasm, entering the mitochondrial matrix. Here, pyruvate interacts with Coenzyme A in the presence of NAD+. Pyruvate undergoes decarboxylation, oxidation by NAD+, and attachment to Coenzyme A, resulting in the formation of acetyl CoA. This acetyl CoA then enters the citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid cycle.
Steps of the Citric Acid Cycle [1:50]
The citric acid cycle is an eight-step pathway, each step requiring a specific enzyme. First, citrate synthase removes the acetyl group from acetyl CoA and attaches it to oxaloacetate, forming citrate. Next, aconitase helps remove and add a water molecule to create isocitrate, a structural isomer of citrate. Isocitrate dehydrogenase then catalyzes the oxidation of isocitrate by NAD+ and decarboxylates it to form alpha-ketoglutarate. Subsequently, another CO2 molecule is lost, and further oxidation by NAD+ occurs with the help of ketoglutarate dehydrogenase, leading to the molecule joining with Coenzyme A to form succinyl-CoA.
Production of Key Molecules [2:44]
CoA is then displaced by a phosphate group to form succinate, catalyzed by succinyl-CoA synthetase, producing one molecule of guanosine triphosphate (GTP), which can be used to make one ATP. Succinate dehydrogenase then oxidizes succinate using FAD, resulting in fumarate and FADH2. Fumarase catalyzes hydration, producing malate. Lastly, malate dehydrogenase facilitates one more oxidation by NAD+ to regenerate oxaloacetate, which restarts the cycle by reacting with a new acetyl CoA.
Overall Yield and Next Steps [3:33]
Overall, each acetyl CoA that enters the citric acid cycle produces three NADHs, one FADH2, and one ATP. Since one glucose molecule yields two pyruvates during glycolysis, and thus two acetyl CoAs, these numbers are doubled per glucose molecule. While the energy payoff from the citric acid cycle itself isn't huge, the products (NADH and FADH2) proceed to oxidative phosphorylation, where the majority of ATP in aerobic respiration is generated.