Krebs Cycle

Definition

The Krebs Cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA. This cycle plays a crucial role in cellular respiration by producing electron carriers that are essential for ATP synthesis and linking various metabolic pathways together.

5 Must Know Facts For Your Next Test

1. The Krebs Cycle takes place in the mitochondrial matrix and consists of eight main steps, starting with the condensation of acetyl-CoA with oxaloacetate to form citrate.
2. For each turn of the cycle, three molecules of NADH, one molecule of FADH2, and one molecule of GTP (or ATP) are produced, which are essential for energy transfer within the cell.
3. The cycle is regulated by key enzymes such as citrate synthase and isocitrate dehydrogenase, which respond to the energy needs of the cell by modulating their activity based on the concentrations of substrates and products.
4. Intermediates of the Krebs Cycle can be siphoned off for other metabolic pathways, including amino acid synthesis and gluconeogenesis, demonstrating its role in connecting various biochemical processes.
5. The Krebs Cycle is closely linked to the electron transport chain, where the NADH and FADH2 generated during the cycle donate electrons to produce a substantial amount of ATP through oxidative phosphorylation.

Review Questions

• Explain how the Krebs Cycle contributes to cellular respiration and energy production.
  • The Krebs Cycle is integral to cellular respiration as it processes acetyl-CoA to generate electron carriers like NADH and FADH2. These carriers transport high-energy electrons to the electron transport chain, where their energy is used to produce ATP through oxidative phosphorylation. This cycle not only provides energy but also serves as a hub for integrating various metabolic pathways by supplying intermediates for biosynthesis.
• Discuss the regulation of the Krebs Cycle and how it responds to cellular energy demands.
  • The Krebs Cycle is regulated by several key enzymes that respond to changes in cellular energy status. For example, high levels of NADH or ATP can inhibit enzymes such as isocitrate dehydrogenase, slowing down the cycle when energy is abundant. Conversely, an increase in ADP or calcium ions can activate these enzymes to enhance the cycle's activity during periods of high energy demand. This regulatory mechanism ensures that the Krebs Cycle aligns with cellular needs.
• Evaluate the role of anaplerotic reactions in maintaining proper function of the Krebs Cycle and how they relate to overall metabolism.
  • Anaplerotic reactions are vital for replenishing intermediates that may be depleted during various metabolic processes connected to the Krebs Cycle. For instance, when amino acids are converted into energy or used for protein synthesis, key intermediates like oxaloacetate can be diminished. Anaplerotic pathways, such as those involving pyruvate carboxylase, help maintain a steady supply of these intermediates. This interplay ensures that the Krebs Cycle can operate efficiently, supporting both energy production and biosynthetic needs within the cell.

Related terms

Acetyl-CoA: A central molecule in metabolism that enters the Krebs Cycle, derived from carbohydrates, fats, and proteins, and acts as a key substrate for energy production.

NADH:

An electron carrier produced during the Krebs Cycle that transfers electrons to the electron transport chain, leading to ATP production.

Substrate-level phosphorylation: A process occurring in the Krebs Cycle that generates ATP directly by transferring a phosphate group from a high-energy substrate to ADP.