Stage 3: Krebs Cycle (also called the Citric Acid Cycle)

🔹 Overview

Once glucose has been broken down during glycolysis (Stage 1), and pyruvic acid has been converted into a molecule called acetyl-CoA (Stage 2), we reach the Krebs cycle, which is the third stage of cellular respiration.

🧠 Why is it called the “Krebs Cycle”?
It was discovered by a British biochemist named Sir Hans Krebs, so it is named after him.

🍊 Why is it also called the “Citric Acid Cycle”?
Because the first major molecule formed in this cycle is citric acid (a 6-carbon molecule, which is also found in citrus fruits like oranges and lemons).

🏭 Where does this cycle happen in the cell?
All the steps of the Krebs cycle take place in a special part of the cell called the mitochondria. This is often called the “powerhouse” of the cell because it produces energy.

🔄 Purpose of the Krebs Cycle

The main purpose of the Krebs cycle is to:

• Complete the breakdown of glucose that started in glycolysis.
• Release energy in the form of molecules like NADH, FADH₂, and ATP, which will be used in the next stage to make even more energy.
• Produce carbon dioxide (CO₂) as a waste product.

Now, let’s break down each step of the cycle in a way that’s easy to follow.

🔹 Steps of the Krebs Cycle (explained simply)

Step 1: Acetyl-CoA joins the cycle

• The molecule acetyl-CoA is like a delivery truck—it delivers a 2-carbon group called the acetyl group into the cycle.
• It hands this acetyl group to a 4-carbon molecule called oxaloacetic acid.
• Together, they form a new 6-carbon molecule called citric acid.
• The CoA (coenzyme A) is released and goes back to pick up another acetyl group.

🧪 Imagine citric acid like a circle made of 6 carbon atoms—this starts the cycle.

Step 2: Citric acid loses a carbon (CO₂) and forms NADH

• Citric acid now goes through a process called oxidative decarboxylation:
  • Decarboxylation means it loses one carbon atom in the form of carbon dioxide (CO₂).
  • Oxidation means it loses electrons and hydrogen atoms.
  • These electrons are picked up by NAD⁺, which becomes NADH—a molecule that stores energy.

🔄 Now, the 6-carbon citric acid becomes a 5-carbon molecule called alpha-ketoglutaric acid.

Step 3: Another CO₂ is removed and another NADH is formed

• The alpha-ketoglutaric acid (5-carbon) also goes through oxidation and decarboxylation:
  • It loses another carbon atom (as CO₂).
  • It also produces another NADH molecule.
• What’s left is a 4-carbon molecule called succinic acid, which temporarily binds with CoA to become succinyl-CoA.