How to Teach the Kreb Cycle

The Krebs cycle, also known as the citric acid cycle, is a critical process of enzyme-driven chemical reactions for all cells that use oxygen for cellular respiration. Understanding the Krebs cycle is crucial for any student hoping to gain a full understanding of cell biology. The cycle itself is complex and must be taught in an effective and memorable manner to be fully understood. One effective method is to teach the steps in the cycle one by one, using a circular diagram. Write on a chalkboard, whiteboard or overhead projector to show your students the processes.

Instructions

• 1

  Describe the formation of acetyl CoA. Show students that pyruvic acid and CoA react across pyruvic acid dehydrogenase to form acetyl CoA and carbon dioxide, as well as releasing two hydrogen atoms to NAD to form NADH.

• 2

  Describe the formation of citric acid. Show students that acetyl CoA joins with oxaloacetic acid, using citric acid synthetase as a catalyst, to form citric acid with six carbon atoms.

• 3

  Describe how dehydration, or losing a water molecule, affects the citric acid. Show students that the enzyme aconitase causes citric acid to forfeit a water molecule, converting the citric acid to aconitic acid.

• 4

  Describe how rehydration, or regaining a molecule of water, affects the aconitic acid. Show students that enzyme aconitase causes the aconitic acid to regain a molecule of water, coverting the aconitic acid to isocitric acid.

• 5

  Describe the dehydrogenization of the isocitric acid and its effects. Show students that isocitric acid dehydrogenase dehydrogenates the isocitric acid, converting it to oxalo succinic acid and releasing two hydrogen atoms in the process, which are accepted by NAD to form NADH.

• 6

  Describe how decarboxylase causes the decarboxylation of the oxalo succinic acid. Show students that decaboxylase eliminates one molecule of carbon dioxide, converting the oxalo succinic acid to a-ketoglutaric with only five remaining carbon atoms.

• 7

  Describe how a-ketoglutaric acid dehydrogenase causes oxidative decarboxylation of the a-ketoglutaric acid, converting it to succinyl CoA. Show students that a-ketoglutaric acid dehydrogenase causes the release of two hydrogen atoms, which are accepted by NAD, converting it to NADH.

• 8

  Describe how succinic acid thiokinase causes decarboxylation of the succinyl CoA, converting it to succinic acid. Show students that acid thiokinase causes the release of CoA.

• 9

  Describe how succinic acid dehydrogenase causes oxidization of the succinic acid, converting it to fumaric acid. Show students that succinic acid dehydrogenase causes the release of two hydrogen atoms, accepted by FAD to form FADH.

• 10

  Describe how fumarase causes hydration of the fumaric acid, converting it to malic acid. Show students that fumarase combines fumaric acid with a water molecule.

• 11

  Describe how malic acid dehydrogenase in the presence of NAD causes dehydrogenation of malic acid, converting it to oxalo acetic acid. Show students that malic acid dehydrogenase in the presence of NAD causes the release of two hydrogen atoms which are accepted by NAD, forming NADH. Show students further that the oxalo acetic acid condenses with acetyl CoA to produce citric acid, beginning the cycle anew.