Citric acid cycle

Definition

The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is a crucial metabolic pathway that takes place in the mitochondria, where acetyl-CoA is oxidized to produce energy in the form of ATP and electron carriers. This cycle is not only essential for energy production but also serves as a hub for various metabolic pathways, connecting carbohydrate, fat, and protein metabolism while generating important biochemical intermediates.

5 Must Know Facts For Your Next Test

1. The citric acid cycle consists of eight enzymatic reactions that begin with the condensation of acetyl-CoA and oxaloacetate to form citrate.
2. Each turn of the citric acid cycle produces three NADH molecules, one FADH2 molecule, and one ATP (or GTP) molecule, which are essential for cellular energy.
3. The cycle also releases two carbon dioxide molecules as waste products for each acetyl-CoA that enters, which are expelled during respiration.
4. Regulation of the citric acid cycle is crucial and occurs through various mechanisms including substrate availability and feedback inhibition by its end products.
5. The intermediates produced in the citric acid cycle can be utilized in various biosynthetic pathways, making it a central metabolic hub for cellular metabolism.

Review Questions

• How does the citric acid cycle integrate different metabolic pathways and why is this integration important?
  • The citric acid cycle serves as a key intersection for carbohydrate, fat, and protein metabolism by utilizing acetyl-CoA derived from different sources. This integration allows cells to efficiently convert various macromolecules into usable energy while maintaining metabolic flexibility. By providing intermediates for biosynthetic processes, the citric acid cycle supports not only energy production but also the synthesis of essential compounds required for cell growth and function.
• Discuss how the production of NADH and FADH2 during the citric acid cycle contributes to ATP synthesis.
  • NADH and FADH2 generated during the citric acid cycle are critical for ATP synthesis in oxidative phosphorylation. These electron carriers transport high-energy electrons to the electron transport chain, where their energy is used to pump protons across the mitochondrial membrane. This creates a proton gradient that drives ATP synthesis as protons flow back through ATP synthase. Essentially, NADH and FADH2 act as energy-rich molecules that facilitate efficient ATP production following the energy-releasing reactions of the citric acid cycle.
• Evaluate how changes in environmental conditions such as oxygen availability might affect the efficiency of the citric acid cycle.
  • Oxygen availability significantly impacts the efficiency of the citric acid cycle because it relies on aerobic respiration processes. Under conditions of low oxygen (hypoxia), cells may switch to anaerobic metabolism, limiting their ability to fully oxidize substrates in the cycle. This shift can lead to decreased production of NADH and FADH2, ultimately reducing ATP yield from oxidative phosphorylation. Therefore, understanding these environmental influences is essential for comprehending how cells adapt their metabolic strategies to ensure survival under varying conditions.

Related terms

Acetyl-CoA: A central metabolite that enters the citric acid cycle and is derived from carbohydrates, fats, and proteins, serving as the starting substrate for energy production.

NADH:

A key electron carrier generated during the citric acid cycle, which plays an important role in transporting electrons to the electron transport chain for ATP synthesis.

Oxidative Phosphorylation:

The process that occurs after the citric acid cycle, where the energy stored in NADH and FADH2 is used to produce ATP through the electron transport chain.