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Cellular respiration is the process by which cells extract energy stored in food and transfer that energy to molecules of ATP. The process is highly exergonic (releases a lot of energy) and occurs in the mitochondria of cells. 

There are two types of cellular respiration: 

  • ❌ Anaerobic (when oxygen is NOT present) 
  • ✅ Aerobic (when oxygen is present) 

In this article, we will only discuss aerobic respiration. In aerobic respiration there are four steps:

  • Glycolysis
  • The Citric Acid Cycle (also known as the Krebs Cycle) 🔄
  • The Electron Transport Chain
  • Oxidative phosphorylation


Glycolysis is a ten-step process that breaks down 1 molecule of glucose (a six-carbon molecule) into 2 three-carbon molecules called pyruvates and releases 4 molecules of ATP. For the AP Biology exam, you do not need to memorize the steps of glycolysis. Here are some important things to remember for the process of glycolysis:

  • The simplified equation is: 2 ATP + 1 Glucose ➡ 2 Pyruvate + 4 ATP
  • Occurs in the cytoplasm
  • Releases ATP without using oxygen
  • The reaction is critical because the end product (pyruvate) is the raw material for the Citric Acid Cycle 
  • ATP is produced by substrate level phosphorylation
  • The enzyme that catalyzes the third step is an allosteric enzyme. This means that glycolysis can be stopped if the cell doesn't need any more ATP. 

The Citric Acid Cycle🔄

This cycle, also known as the Krebs cycle, takes place in the mitochondria and is important to producing lots of energy for the cell. Here are some important things to remember for the citric acid cycle: 

  • Takes place in the matrix of the mitochondria
  • Requires pyruvate (the product of glycolysis)
  • Generates 1 ATP per cycle by substrate-level phosphorylation
  • NAD+ and FAD and reduced to NADH and FADH2. This is important because these coenzymes are used as electron carriers in the electron transport chain. 

While you don't need to memorize all the steps to the cycle, here is a graphic to visualize what is going on: 

The Election Transport Chain (ETC)

The electron transport chain is essential to cellular respiration because this is the final step before we get all the ATP. We've been through all the steps and we're almost there! The ETC has many important aspects: 

  • The ETC is a collections of molecules embedded in the cristae membrane of the mitochondrion
  • The ETC carries electrons delivered by NAD and FA from glycolysis and the Krebs cycle to oxygen (the final electron acceptor)
  • The highly electronegative oxygen acts to pull electrons through the electron transport chain.

Oxidative Phosphorylation (The Final Step! 🎉🎉)

We made it! The last step of aerobic respiration is oxidative phosphorylation. Here are the important things to know about this step: 

  • Most energy of aerobic respiration is produced in this step
  • It occurs in the mitochondria
  • Phosphorylation of ADP to ATP by the carrier molecules (NADH and FADH2)
  • Powered by the redox reactions of the ETC
  • Protons are pumps from the matrix to the outer compartment by the ETC
  • A proton gradient is created between the outer compartment and the inner matrix
  • Protons cannot diffuse through the cristae membrane; they cna flow only down the gradient through the ATP synthase channels. This is called chemiosmosis. 
  • IMPORTANT: Chemiosmosis is the key to ATP production. As the protons flow through the ATP synthase channels, they generate energy to phosphorylate ADP to ATP. You can think of this process like how hydroelectric plants convert potential energy from water flowing through a dam into useable energy. 🌏 

Overall, it is important to understand that all of these processes function towards one goal: to produce ATP. By understanding the process of aerobic respiration, you'll be able to learn processes like anaerobic respiration and photosynthesis with ease. If you're ever confused, come chat with us at Fiveable. Lastly, here is a little graphic and some resources to help you remember what goes on during aerobic respiration: 

Here are some other resources to check out: 

Good Luck! 😊

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