The Citric Acid Cycle: An Overview -- Movie Narrative

Aerobic respiration is the most important process used to produce energy for a cell. It involves several pathways including: glycolysis, pyruvate oxidation, the citric acid cycle, and electron transport. Various sugars, fats, and proteins enter these pathways, and they can all be broken down to produce ATP energy for a cell. In this animation, we will focus on the citric acid cycle--also called the tricarboxylic acid (TCA) cycle or Krebs cycle.

The citric acid cycle takes place in the matrix, or fluid, of the mitochondrion. This is where mitochondrial DNA is found and where fatty acid breakdown takes place. The citric acid cycle involves eight chemical reactions that use acetyl CoA and oxaloacetate to produce carbon dioxide, NADH, ATP, and FADH2. The NADH and FADH2 are electron carriers that can be used in the electron transport chain to make more ATP for a cell.

Let’s take a look at the major steps of the citric acid cycle. In the first step of the cycle, a 2-carbon molecule and a 4-carbon molecule are combined to form a 6-carbon molecule. That 6-carbon molecule undergoes multiple biochemical changes during the cycle, and at the end the original 4-carbon molecule is produced. Each time a carbon molecule loses one carbon, a carbon dioxide is released. So, two carbon dioxides are formed during the conversion of the 6-carbon molecule back to the 4-carbon molecule.

So, where does the 2-carbon molecule come from to begin the citric acid cycle? This molecule, acetyl CoA, is made from pyruvate. Pyruvate is a product of glycolysis, the other major ATP producing cycle in the cell. Pyruvate is transported into the mitochondrial matrix where it is oxidized to acetyl CoA by pyruvate dehydrogenase. This step produces one NADH and one carbon dioxide for each of the two pyruvate molecules made from glucose. The acetyl CoAs are the starting point for the citric acid cycle.

In the citric acid cycle, the acetyl group from acetyl CoA is transferred to oxaloacetate to form citrate. Four different enzymatic reactions then lead to the formation of succinate. These steps produce two carbon dioxides, two NADHs, and one ATP. Succinate is then recycled back to oxaloacetate through three more reactions. These steps produce FADH2 and one more NADH. All of the energy molecules made in the citric acid cycle, along with those produced by glycolysis, are essential for fully functioning cells, as errors in these pathways can lead to life-threatening diseases.