Citric acid cycle

Definition

The citric acid cycle, also known as the Krebs cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. This cycle plays a critical role in cellular respiration and is essential for producing ATP, the energy currency of cells, while also generating electron carriers that drive oxidative phosphorylation.

5 Must Know Facts For Your Next Test

1. The citric acid cycle occurs in the mitochondria of eukaryotic cells and is one of the key stages in aerobic respiration.
2. Each turn of the citric acid cycle produces three NADH, one FADH2, and one ATP (or GTP), which are vital for cellular energy production.
3. The cycle begins with the condensation of acetyl-CoA with oxaloacetate to form citric acid (citrate), which undergoes a series of transformations.
4. Intermediate compounds formed during the citric acid cycle can be used for biosynthesis of amino acids and other biomolecules.
5. Regulation of the citric acid cycle is crucial for metabolic homeostasis; key enzymes are activated or inhibited based on the energy needs of the cell.

Review Questions

• How does the citric acid cycle connect to other metabolic pathways within the cell?
  • The citric acid cycle is not just a standalone process; it serves as a central hub connecting various metabolic pathways. It takes in acetyl-CoA from carbohydrate, fat, and protein metabolism and produces key energy carriers like NADH and FADH2. These carriers feed into oxidative phosphorylation, linking it directly to ATP production. Additionally, intermediates from the cycle can be siphoned off to contribute to amino acid synthesis and other metabolic functions, illustrating its interconnected role in cellular metabolism.
• What are the main regulatory mechanisms that control the activity of enzymes in the citric acid cycle?
  • Enzymes in the citric acid cycle are regulated primarily through feedback inhibition and allosteric regulation. Key enzymes like citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase are sensitive to the levels of ATP, ADP, NADH, and NAD+. High levels of NADH indicate sufficient energy supply and inhibit these enzymes, while increased ADP or NAD+ signals a need for more ATP and activates them. This regulatory mechanism ensures that the cycle operates efficiently according to cellular energy demands.
• Evaluate the significance of the citric acid cycle in understanding early life forms and their metabolic pathways.
  • The citric acid cycle holds significant evolutionary importance as it suggests a common metabolic pathway among diverse life forms. Its presence in many organisms implies that this cycle may have emerged early in evolutionary history as an efficient means to extract energy from organic compounds. Studying this cycle can provide insights into how early life adapted to different environments and utilized available resources. The core reactions of the citric acid cycle reveal a complex interplay between biochemistry and environmental conditions that likely influenced life's evolution on Earth.

Related terms

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

Oxidative Phosphorylation: The process in which ATP is produced using energy derived from the electron transport chain and chemiosmosis, utilizing the electron carriers generated from the citric acid cycle.

Aerobic Respiration: A metabolic process that requires oxygen to produce energy by breaking down glucose and other substrates through glycolysis, the citric acid cycle, and oxidative phosphorylation.