Citric acid cycle

Definition

The citric acid cycle, also known as the Krebs cycle or TCA cycle, is a fundamental metabolic pathway that takes place in the mitochondria of cells. It is responsible for the oxidation of acetyl-CoA to produce energy in the form of ATP, as well as key intermediates used in various biosynthetic pathways. This cycle plays a crucial role in cellular respiration, linking carbohydrate, fat, and protein metabolism to energy production and maintaining metabolic homeostasis.

5 Must Know Facts For Your Next Test

1. The citric acid cycle involves a series of eight enzymatic reactions that convert acetyl-CoA into carbon dioxide while capturing high-energy electrons.
2. For each turn of the citric acid cycle, three NADH and one FADH2 are produced, which are critical for the electron transport chain.
3. The cycle is regulated by key enzymes, including citrate synthase and isocitrate dehydrogenase, which respond to the availability of substrates and energy needs of the cell.
4. The citric acid cycle not only generates energy but also provides precursors for biosynthetic pathways that synthesize amino acids and nucleotides.
5. Disruptions in the citric acid cycle can lead to metabolic disorders, highlighting its importance in maintaining cellular function and overall homeostasis.

Review Questions

• How does the citric acid cycle integrate various macronutrients into energy production?
  • The citric acid cycle serves as a central hub in metabolism by allowing carbohydrates, fats, and proteins to be converted into acetyl-CoA, which enters the cycle. Carbohydrates are broken down into glucose and then converted to pyruvate before forming acetyl-CoA. Similarly, fatty acids undergo beta-oxidation to produce acetyl-CoA. Proteins are degraded into amino acids that can also enter the cycle after being converted into intermediates. This integration ensures efficient energy production from different sources.
• What roles do NADH and FADH2 play in cellular respiration following the citric acid cycle?
  • NADH and FADH2 produced during the citric acid cycle are essential for oxidative phosphorylation. They act as electron carriers that transfer electrons to the electron transport chain located in the inner mitochondrial membrane. The electrons flow through this chain, driving protons across the membrane and creating a proton gradient. This gradient is then utilized by ATP synthase to produce ATP from ADP and inorganic phosphate. Therefore, NADH and FADH2 are crucial for maximizing ATP yield from glucose metabolism.
• Evaluate the significance of regulatory enzymes in maintaining metabolic homeostasis during the citric acid cycle.
  • Regulatory enzymes such as citrate synthase and isocitrate dehydrogenase play a vital role in controlling the rate of the citric acid cycle based on cellular energy demands and substrate availability. For instance, when ATP levels are high or NADH accumulates, these enzymes are inhibited to slow down the cycle. Conversely, when energy levels drop, these enzymes are activated to enhance metabolite flow through the cycle. This regulation ensures that energy production aligns with cellular needs, thus maintaining metabolic homeostasis and preventing excess substrate accumulation.

Related terms

Acetyl-CoA: A central metabolite that enters the citric acid cycle, formed from carbohydrates, fats, and proteins after their breakdown.

ATP (Adenosine Triphosphate): The primary energy carrier in cells, generated during the citric acid cycle and used to power various cellular processes.

Oxidative Phosphorylation:

The final stage of cellular respiration that follows the citric acid cycle, where most ATP is produced through the electron transport chain.