Krebs cycle

Definition

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a crucial metabolic pathway that takes place in the mitochondria of cells. It plays a central role in cellular respiration, where it oxidizes acetyl-CoA derived from carbohydrates, fats, and proteins into carbon dioxide and high-energy electron carriers like NADH and FADH2. This cycle connects various metabolic processes, including glycolysis and oxidative phosphorylation, while also providing intermediates for biosynthesis.

5 Must Know Facts For Your Next Test

1. The Krebs cycle occurs in the mitochondrial matrix and consists of a series of eight enzymatic reactions that ultimately regenerate oxaloacetate.
2. During each turn of the Krebs cycle, one acetyl-CoA molecule is oxidized, resulting in the production of three NADH molecules, one FADH2 molecule, and one GTP or ATP molecule.
3. The carbon dioxide produced in the Krebs cycle is a waste product that is exhaled by organisms during respiration.
4. The intermediates generated in the Krebs cycle are not only used for energy production but also serve as building blocks for amino acids, nucleotides, and other essential biomolecules.
5. The regulation of the Krebs cycle is critical for maintaining cellular energy balance, influenced by factors such as substrate availability and energy needs of the cell.

Review Questions

• How does the Krebs cycle integrate with glycolysis and oxidative phosphorylation in cellular respiration?
  • The Krebs cycle serves as a bridge between glycolysis and oxidative phosphorylation. Glycolysis breaks down glucose into pyruvate, which is then converted into acetyl-CoA to enter the Krebs cycle. The cycle generates high-energy electron carriers NADH and FADH2, which are essential for oxidative phosphorylation. This process harnesses the energy from these carriers to produce ATP, illustrating how these three stages work together to efficiently convert glucose into usable energy.
• Discuss how the intermediates produced in the Krebs cycle can influence biosynthesis within cells.
  • The intermediates of the Krebs cycle play vital roles in biosynthesis by providing key building blocks for various biomolecules. For instance, citric acid can be converted into fatty acids, while alpha-ketoglutarate serves as a precursor for amino acids like glutamate. This dual function highlights how the Krebs cycle not only contributes to energy production but also supports the synthesis of essential compounds needed for growth and maintenance within cells.
• Evaluate the significance of regulation in the Krebs cycle and its impact on cellular metabolism under varying energy demands.
  • Regulation of the Krebs cycle is crucial for adapting to the changing energy needs of a cell. Key enzymes within the cycle are influenced by the availability of substrates and products, as well as cellular energy status indicated by levels of ATP or ADP. When energy demands increase, such as during intense exercise, elevated levels of ADP stimulate the cycle to produce more ATP. Conversely, when energy is abundant, feedback inhibition can slow down the cycle to prevent excess production. This dynamic regulation ensures metabolic flexibility and efficiency.

Related terms

Acetyl-CoA: A key molecule in metabolism that enters the Krebs cycle; it is derived from the breakdown of carbohydrates, fats, and proteins.

NADH:

An electron carrier that stores energy during the Krebs cycle and is utilized in oxidative phosphorylation to generate ATP.

Oxidative Phosphorylation: The final stage of cellular respiration, where ATP is produced using energy from the electrons carried by NADH and FADH2 generated during the Krebs cycle.