Citric Acid Cycle

The Citric Acid Cycle is also called the "Tricarboxylic Acid Cycle (TCA) and the "Krebs Cycle." This cycle is a series of biochemical reactions that completes the catabolic pathway for the Glucose molecule that started glycolysis. Energy from the Citric Acid Cycle is captured by electron carriers (NAD and FAD). Also, ATP is generated at one of the steps in this cycle.

After the completion of this phase of metabolism, the following molecules and ATP are made as a byproduct:


Net Yield through Glycolysis







Below is a more detailed figure showing the citric acid cycle. Keep in mind that this is per glucose molecule. Two pyruvate are produced per glucose so the Krebs cycle will run twice per molecule of glucose.


Description automatically generated

Metabolism Summary Part 2: Citric Acid Cycle

Below is an image of the process of the Citric Acid (Krebs) Cycle magnified from the Metabolism Summary. A continuation of our summary on metabolism follows below for the Citric Acid Cycle up until we reach the Electron Transport Chain where we will return to get the detailed information on this process first before continuing with our summary on all of the processes of metabolism. The green numbers in the image correlate with each of the steps listed below:


Description automatically generated

Citric Acid (Krebs) Cycle, from the “Big Picture” of Metabolism: Glycolysis, Citric Acid (Krebs) Cycle, Electron Transport Chain, Beta Oxidation and Lipolysis. Image created at BYU-Idaho by JS 2010

8  Steps 8-12 complete the story of aerobic metabolism of glucose. After pyruvate is transported into the mitochondria, another dehydrogenase enzyme (actually a very large enzyme complex) will accomplish several things. It will remove 2 protons and 2 electrons from pyruvate. This creates NADH (actually 2 NADHs because there are 2 pyruvates).  Also, the reaction results in the loss of a carbon and two oxygen atoms (released as CO2) from pyruvate.  Finally, the remaining 2 carbon molecule is attached to Coenzyme A.  

Coenzyme A (often referred to as simply CoA) is derived from pantothenic acid (Vitamin B5). Acetyl CoA is the name used for the product of the reaction in step 8. The “Acetyl” prefix specifically refers to the 2-carbon group that is being transported by the CoA.  Black dots in the summary figure help us keep track of the carbons that originated from the glucose molecule way back at the beginning of glycolysis. Notice that ultimately all the black dots are released as CO2 so that the metabolism of glucose leaves us with no accumulation of carbons in the cell. Acetyl CoA will enter and participate in the reactions of the Citric Acid Cycle.  

9  Step 9 represents the activities of the Citric Acid (or Krebs) Cycle. The Citric Acid Cycle involves a lot of steps. 

10   The important things to remember about the Citric Acid Cycle are…

  1. A 4-carbon molecule called oxaloacetate combines with the acetyl (2 carbon) group of Acetyl CoA (which came from glucose or fatty acids or possibly even some of the amino acids). This will yield a 6-carbon molecule called citric acid. Citric acid will be changed and manipulated as this 6-carbon molecule ends up recycled back to oxaloacetate – thus the term “Citric Acid Cycle”. 
  2. During the reactions of the citric acid cycle, CO2 will be lost twice. This means that if you are counting, you will realize that every carbon of the original Glucose or Fat is ultimately lost as CO2. This is the reason complex organisms need to breathe continuously.  
  3. Hydrogens with electrons are transferred to NAD+. This creates 6 NADH molecules.  
  4. FAD is reduced to FADH2. This yields 2 FADH2 molecules.    
  5. Substrate-Level Phosphorylation will yield an ATP for each turn of the Citric Acid Cycle (or 2 total for each glucose).

This content is provided to you freely by BYU-I Books.

Access it online or download it at https://books.byui.edu/bio_180/83___citric_acid_cyc.