Aerobic Respiration

Aerobic Respiration Definition

Aerobic respiration is the process by which oxygen-breathing creatures turn fuel, such as fats and sugars, into energy.

Respiration is a process used by all cells to turn fuel, which contains stored energy, into a usable form. The product of respiration is a molecule called ATP, which can easily use the energy stored in its phosphate bonds to power chemical reactions the cell needs to survive.

Aerobic respiration is respiration that uses oxygen as a reactant. Aerobic respiration is much more efficient, and produces ATP much more quickly, than anaerobic respiration (respiration without oxygen). This is because oxygen is an excellent electron acceptor for the chemical reaction.

The complex process of aerobic respiration is illustrated in this graphic. You may wish to reference this image as you study the different parts of the process of cellular respiration.

Here, we will break down the process into simpler steps to illustrate how cellular respiration turns energy from glucose into a form that the cell can use to power its life functions.

Common Steps Between Aerobic Respiration and Anaerobic Respiration

Both aerobic respiration and anaerobic respiration use an electron transport chain to move energy from its long-term storage in sugars to a more usable form.

In respiration, the energy from sugar is moved into ATP, which can be used to power many chemical reactions necessary to a cell’s survival.

Both aerobic and anaerobic respiration start with the process of glycolysis. “Glycolysis,” which literally means “sugar splitting,” breaks a sugar molecule down into two smaller molecules.

In the process of glycolysis, two ATP molecules are consumed and four are produced. This results in a net gain of two ATP molecules produced for every sugar molecule broken down through glycolysis.

In cells that use oxygen, a sugar molecule is broken down into two molecules of pyruvate. In cells that do not have oxygen, the sugar molecule is broken down into lactate.

Although our cells normally use oxygen for respiration, which is much more efficient than anaerobic respiration, when we use ATP faster than we are getting oxygen molecules to our cells, our cells can perform anaerobic respiration to supply their needs for a few minutes.

Fun fact: Buildup of lactate from anaerobic respiration is one reason why muscles can feel sore after intense exercise!

Differences Between Aerobic Respiration and Anaerobic Respiration

After glycolysis, different respiration chemistries take a few different paths:

• Cells that are deprived of oxygen but are not made for anaerobic respiration, like our own muscle cells, may leave the end products of glycolysis sitting around, obtaining only two ATP per sugar molecule they split.


• Cells that are made for anaerobic respiration may continue the electron transfer chain to extract more energy from the end products of glycolysis.


• Cells using aerobic respiration continue their electron transfer chain in a highly efficient process that ends up yielding 38 molecules of ATP from every sugar molecule!

After glycolysis, cells that do not use oxygen may use a different electron acceptor, such as sulfate or nitrate, to drive their reaction forward.

These processes are called “fermentation.” Some types of fermentation reactions actually have alcohol as their end product. So now you know where alcoholic drinks come from: the respiration processes of yeasts splitting sugars to produce energy!

Aerobic respiration, on the other hand, sends the pyruvate left over from glycolysis down a very different chemical path.

Aerobic Respiration and Weight Loss

Aerobic respiration is the process by which many cells, including our own, produce energy using food and oxygen. It also gives rise to carbon dioxide, which our bodies must then get rid of.

This equation explains why we need both food and oxygen, as both are reacted together to produce the ATP that allows our cells to function.

This equation also explains why we breathe out carbon dioxide – and how we lose weight!

Hint: We breathe in O₂ and we breathe out the same number of molecules of CO₂. Where did the carbon atom come from? It comes from the food, such as sugar and fat, that you’ve eaten!

This is also why you breathe harder and faster while performing calorie-burning activities: your body is using both oxygen and food at a faster-than-normal rate, and is producing more ATP to power your cells, along with more CO₂ waste product, as a result.

Unfortunately, simply breathing faster doesn’t mean you’ll unload more carbon: to lose carbon faster, your cells need to be consuming energy at a faster-than-normal rate. So get out those running shoes!

Aerobic Respiration Equation

The equation for aerobic respiration describes the reactants and products of all of its steps, including glycolysis. That equation is:

1 glucose + 6O₂ → 6CO₂+ 6 H₂O + 38 ATP

The reactions of aerobic respiration can be broken down into four stages, described below:

Steps of Aerobic Respiration

1. Glycolysis. In aerobic cells, the equation for glycolysis is:

Glucose + 2 HPO₄^2- + 2 ADP^3- + 2 NAD⁺ → 2 Pyruvate^– + 2 ATP^4- + 2 NADH + 2 H⁺ + 2 H₂O

As discussed above, glycolysis in aerobic respiration refers to the splitting of a sugar molecule into two pyruvate molecules. This process creates two ATP molecules.

You will notice that this process also creates NADH from NAD⁺. This is important because later in the process of cellular respiration, NADH will power the formation of much more ATP through the mitochondria’s electron transport chain.

Pyruvate is then processed to turn it into fuel for the citric acid cycle, using the process of oxidative decarboxylation.

2. Oxidative decarboxylation of pyruvate

2 (Pyruvate^– + Coenzyme A + NAD⁺ → Acetyl CoA + CO₂ + NADH)

In this process, pyruvate is combined with coenzyme A to produce acetyl-CoA.

You will note that more NADH is created in this step. This means more fuel to create more ATP later in the process of cellular respiration!

This is important because acetyl-CoA is an ideal fuel for the citric acid cycle, which can in turn power the process of oxidative phosphorylation in the mitochondria, which produces huge amounts of ATP!


3. Citric acid cycle

2 (Acetyl CoA + 3 NAD⁺ + FAD + GDP^3- + HPO₄^2- + 2H₂O → 2 CO₂ + 3 NADH + FADH₂ + GTP^4- + 2H⁺ + Coenzyme A)

In the citric acid cycle, both NADH and FADH₂ – another carrier of electrons for the electron transport chain – are created. All the NADH and FADH₂ created in the preceding steps now come into play in the process of oxidative phosphorylation.

4. Oxidative phosphorylation

34 (ADP^3- + HPO₄^2- + NADH + 1/2 O₂ + 2H⁺ → ATP^4- + NAD⁺ + 2 H₂O)

Oxidative phosphorylation uses the folded membranes within the cell’s mitochondria to produce huge amounts of ATP.

In this process, NADH and FADH₂ donate the electrons they obtained from glucose during the previous steps of cellular respiration to the electron transport chain in the mitochondria’s membrane.

The electron transport chain consists of a number of complexes in the mitochondrial membrane, including complex I, Q, complex III, cytochrome C, and complex IV.

All of these ultimately serve to pass electrons from higher to lower energy levels, harvesting bits of their energy in the process. This energy is used to power proton pumps, which in turn power ATP formation.

Just like the sodium-potassium pump of the cell membrane, the proton pumps of the mitochondrial membrane are used to create a concentration gradient which can be used to power other processes.

In the case of the mitochondria’s proton gradient, the protons that are transported across the membrane using the energy harvested from NADH and FADH₂ “want” to pass through channel proteins from their area of high concentration to their area of low concentration.

These channel proteins are actually ATP synthase – the enzyme that makes ATP. When protons pass through ATP synthase, they drive the formation of ATP.

This process is why mitochondria are referred to as “the powerhouses of the cell.” The mitochondria’s electron transport chain makes nearly 90% of all the ATP produced by the cell from breaking down food.

This is also the process that requires oxygen. Without oxygen molecules to accept the depleted electrons at the end of the electron transport chain, the electrons would back up and the process of ATP creation would not be able to continue.

No wonder we need oxygen to live!

Function of Aerobic Respiration

Aerobic respiration produces ATP, which is then used to power other life-sustaining functions, such as the action of the sodium-potassium pump, which allows us to move, think, and perceive the world around us; the actions of many enzymes; and the actions of countless other proteins that sustain life!

Related Terms

• ATP – The cellular “fuel” which can be used to power countless cellular actions and reactions.


• Mitochondria – An important organelle in animal cells which efficiently extracts energy from sugars.


• Sodium-potassium pump – An important transport protein which uses about 20-25% of all ATP in the human body. It illustrates the importance of ATP because of what happens if this pump runs out of fuel.

Test Your Knowledge

1. All cells perform glycolysis.
A. True
B. False

Answer to Question #1

True! All cells split sugars to release some of the chemical energy stored in the sugar molecules.

Some cells stop there, while others go on to use processes of fermentation or aerobic respiration to obtain much more energy from the sugar fragments left over after glycolysis.

2. The process of aerobic respiration explains how we lose weight when we diet and exercise.
A. True
B. False

Answer to Question #2

True. The equation for aerobic respiration shows how we break down sugars and consume oxygen in order to produce energy. We then breathe out the carbon from the sugars we have broken down in the form of CO₂.

When the cell needs more energy, such as when we exercise, it turns oxygen into CO₂ faster in order to generate more ATP.

Likewise, if we take in less sugar than we need to meet our energy needs, our body breaks down fat molecules – ultimately into carbon, which we breathe out.

3. The only important product of the citric acid cycle is ATP.
A. True
B. False

Answer to Question #3

False! NADH and FADH₂ are also very important products of the citric acid cycle, because they are used to make much more ATP in the mitochondria later, during oxidative dephosphorylation.

Aerobic Respiration