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Process of Cellular Respiration

Explains molecular processes cells use to make energy in the presence of oxygen.

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Process of Cellular Respiration

Why do you need to breathe?

Of course if you didn't breathe, you couldn't survive. Why do you need air to live? You need the gas oxygen to perform cellular respiration to get energy from your food.

The Process of Cellular Respiration

Cellular respiration is the process of extracting energy in the form of ATP from the glucose in the food you eat. How does cellular respiration happen inside of the cell? Cellular respiration is a three step process. Briefly:

  1. In stage one, glucose is broken down in the cytoplasm of the cell in a process called glycolysis.
  2. In stage two, the pyruvate molecules are transported into the mitochondria. The mitochondria are the organelles known as the energy "powerhouses" of the cells (Figure below). In the mitochondria, the pyruvate, which have been converted into a 2-carbon molecule, enter the Krebs cycle. Notice that mitochondria have an inner membrane with many folds, called cristae. These cristae greatly increase the membrane surface area where many of the cellular respiration reactions take place.
  3. In stage three, the energy in the energy carriers enters an electron transport chain. During this step, this energy is used to produce ATP.

Oxygen is needed to help the process of turning glucose into ATP. The initial step releases just two molecules of ATP for each glucose. The later steps release much more ATP.

Diagram of the mitochondria

Most of the reactions of cellular respiration are carried out in the mitochondria.

The Reactants

What goes into the cell? Oxygen and glucose are both reactants of cellular respiration. Oxygen enters the body when an organism breathes. Glucose enters the body when an organism eats.

The Products

What does the cell produce? The products of cellular respiration are carbon dioxide and water. Carbon dioxide is transported from your mitochondria out of your cell, to your red blood cells, and back to your lungs to be exhaled. ATP is generated in the process. When one molecule of glucose is broken down, it can be converted to a net total of 36 or 38 molecules of ATP. This only occurs in the presence of oxygen.

The Chemical Reaction

The overall chemical reaction for cellular respiration is one molecule of glucose (C6H12O6) and six molecules of oxygen (O2) yields six molecules of carbon dioxide (CO2) and six molecules of water (H2O). Using chemical symbols the equation is represented as follows:

C6H12O6 + 6O2 → 6CO2 + 6H2O

ATP is generated during the process. Though this equation may not seem that complicated, cellular respiration is a series of chemical reactions divided into three stages: glycolysis, the Krebs cycle, and the electron transport chain.


Stage one of cellular respiration is glycolysis. Glycolysis is the splitting, or lysis of glucose. Glycolysis converts the 6-carbon glucose into two 3-carbon pyruvate molecules. This process occurs in the cytoplasm of the cell, and it occurs in the presence or absence of oxygen. During glycolysis a small amount of NADH is made as are four ATP. Two ATP are used during this process, leaving a net gain of two ATP from glycolysis.  The NADH temporarily holds energy, which will be used in stage three.

The Krebs Cycle

In the presence of oxygen, under aerobic conditions, pyruvate enters the mitochondria to proceed into the Krebs cycle. The second stage of cellular respiration is the transfer of the energy in pyruvate, which is the energy initially in glucose, into two energy carriers, NADH and FADH2. A small amount of ATP is also made during this process. This process occurs in a continuous cycle, named after its discover, Hans Krebs. The Krebs cycle uses a 2-carbon molecule (acetyl-CoA) derived from pyruvate and produces carbon dioxide.

The Electron Transport Chain

Stage three of cellular respiration is the use of NADH and FADH2 to generate ATP. This occurs in two parts. First, the NADH and FADH2 enter an electron transport chain, where their energy is used to pump, by active transport, protons (H+) into the intermembrane space of mitochondria. This establishes a proton gradient across the inner membrane. These protons then flow down their concentration gradient, moving back into the matrix by facilitated diffusion. During this process, ATP is made by adding inorganic phosphate to ADP.  Most of the ATP produced during cellular respiration is made during this stage.

For each glucose that starts cellular respiration, in the presence of oxygen (aerobic conditions), 36-38 ATP are generated. Without oxygen, under anaerobic conditions, much less (only two!) ATP are produced.


  • Most of the steps of cellular respiration take place in the mitochondria.
  • Oxygen and glucose are both reactants in the process of cellular respiration.
  • The main product of cellular respiration is ATP; waste products include carbon dioxide and water.

Explore More

Use the resources below to answer the following questions

Explore More I

  1. Where does glycolysis occur?
  2. When glucose is broken down what is produced?
  3. Does glycolysis require oxygen?

Explore More II

  1. Which types of cells have mitochondria?
  2. What is the cristae? Where does it occur? Why is this structure important?
  3. What high energy electron carriers are produced by the Krebs cycle? Where do they carry their electrons?
  4. What is the role of acetyl-CoA? Where does it fit into the Krebs cycle?
  5. How much ATP is made by the Krebs cycle for every molecule of Pyruvate that enter the cycle?

Explore More III

  1. What is the name of the protein complex that makes ATP?
  2. What is the final electron acceptor at the end of the electron transport chain?
  3. What is a "mobile transfer molecule"? What is their function?
  4. How is the hydrogen ion gradient formed?
  5. What is the purpose of the proton (hydrogen ion) gradient?


  1. Where is glucose broken down to form ATP? What is this process called? Does this process need oxygen?
  2. Write the chemical reaction for the overall process of cellular respiration.
  3. What is necessary for the Krebs cycle to proceed?
  4. What happens during the Krebs cycle?
  5. What is pyruvate?
  6. What happens during the electron transport chain?
  7. How is ATP made during the third stage of cellular respiration?

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aerobic In the presence of oxygen.
anaerobic In the absence of oxygen.
ATP Adenosine triphosphate; usable form of energy inside the cell.
cellular respiration Process of breaking down glucose to obtain energy in the form of ATP.
cristae Inner membrane folds of the mitochondrion.
electron transport chain Series of electron-transport molecules that pass high-energy electrons from molecule to molecule and capture their energy.
glucose Simple sugar with the chemical formula C6H12O6; a product of photosynthesis.
glycolysis First stage of cellular respiration in which glucose is split to form two molecules of pyruvate.
Krebs cycle Second stage of cellular respiration in which two pyruvate molecules react to form NADH, and FADH2 and a small amount of ATP.
mitochondria Organelle of the cell in which energy is generated.
mitochondrion (plural mitochondria): Organelle of the cell in which energy is generated.
nucleus Cell structure that contains the genetic material, DNA.
pyruvate Three-carbon product of glycolysis.
reactants Raw ingredients (starting materials) in a chemical reaction.

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