Citric Acid Cycle | HHMI BioInteractive Video

Summary

The citric acid cycle, also known as the Krebs cycle, is a critical biochemical pathway in cellular metabolism. It occurs in the mitochondria and involves a series of chemical reactions that break down carbon molecules from glucose, producing energy-rich molecules like NADH and GTP, while generating carbon dioxide as a byproduct. The cycle starts with the combination of acetyl-CoA and oxaloacetate to form citric acid, which undergoes a series of transformations to regenerate oxaloacetate. This process not only fuels energy production but also provides essential building blocks for biosynthesis, making it vital for cell function and survival.

Takeaways

• 😀 Mitochondria are the powerhouse of the cell, where energy production and biosynthesis occur.
• 😀 The citric acid cycle, also known as the Krebs cycle, involves a series of chemical reactions that break down carbon atoms and generate energy.
• 😀 The cycle begins with oxaloacetate, a four-carbon molecule, which combines with acetyl-CoA (from glycolysis) to form six-carbon citric acid.
• 😀 Citric acid plays a key role in biosynthesis, helping to create fatty acids, lipids, and cholesterol.
• 😀 Step 2 of the cycle involves converting citric acid into isocitrate by repositioning an oxygen atom.
• 😀 In Step 3, one carbon atom is removed from isocitrate, releasing carbon dioxide and generating NADH for the electron transport chain.
• 😀 Step 4 involves a large multi-enzyme complex that further breaks down ketoglutarate, generating another carbon dioxide and transferring electrons to NADH.
• 😀 Step 5 produces GTP (a molecule involved in RNA building and protein synthesis) by breaking a bond in succinyl-CoA.
• 😀 Step 6 is where succinate is oxidized to fumarate, producing electrons that are transferred to coenzyme Q and passed along the electron transport chain.
• 😀 Step 7 converts fumarate into malate through the addition of water.
• 😀 The final step of the cycle regenerates oxaloacetate from malate and produces more NADH for the electron transport chain, completing the loop.
• 😀 The citric acid cycle not only produces energy (ATP) but also provides essential building blocks for amino acids, DNA, and RNA synthesis.

Q & A

What is the role of mitochondria in energy production?

Mitochondria are responsible for energy production in cells. They contain metabolic enzymes that facilitate the citric acid cycle, which breaks down glucose to generate ATP, the cell's primary energy source.

What is the citric acid cycle, and why is it important?

The citric acid cycle is a series of eight chemical reactions that break down two carbon atoms from glucose, producing energy in the form of ATP, as well as carbon dioxide as a waste product. It is crucial for cellular metabolism and biosynthesis.

How does glycolysis relate to the citric acid cycle?

Glycolysis breaks down glucose into pyruvate, which is then converted into acetyl CoA. This acetyl CoA enters the citric acid cycle, providing the carbon atoms that are further processed to generate energy.

What happens during step 1 of the citric acid cycle?

In step 1, a two-carbon acetyl group from acetyl CoA combines with a four-carbon molecule, oxaloacetate, forming six-carbon citric acid, which initiates the cycle.

What is the role of NADH in the citric acid cycle?

NADH is produced during various steps of the citric acid cycle when electrons are transferred to NAD+, a coenzyme. NADH then carries these high-energy electrons to the electron transport chain, where they are used to generate ATP.

How is carbon dioxide produced in the citric acid cycle?

Carbon dioxide is produced in steps 3 and 4 of the citric acid cycle. During these steps, carbon atoms are removed from molecules, resulting in the release of carbon dioxide as a waste product.

What is the function of GTP in the citric acid cycle?

GTP is produced in step 5 of the cycle when succinyl-CoA is converted. GTP serves as an energy source, similar to ATP, and can also be used to generate ATP directly in the mitochondrial matrix.

How does the enzyme complex in step 4 facilitate the citric acid cycle?

The enzyme complex in step 4, which consists of multiple core enzymes, efficiently catalyzes the removal of a carbon atom from ketoglutarate, generating carbon dioxide and succinyl-CoA. This complex also transfers electrons to NADH.

What is the significance of oxaloacetate in the citric acid cycle?

Oxaloacetate is essential for starting the citric acid cycle by reacting with acetyl-CoA to form citric acid. It is regenerated at the end of the cycle, allowing the process to continue and also plays a key role in the synthesis of amino acids and nucleic acids.

How does the citric acid cycle contribute to the biosynthesis of fatty acids and cholesterol?

The citric acid cycle provides citric acid, which is a precursor for the biosynthesis of fatty acids, lipids, and cholesterol, key components of cell membranes and signaling molecules.