The process of aerobic respiration

Aerobic respiration is the process by which cells convert glucose and oxygen into ATP (energy), carbon dioxide, and water.

The overall reaction can be summarised as:

Glucose + Oxygen → Carbon Dioxide + Water + 30/32 ATP

alt-text: A graphic depicting glucose plus oxygen leads to carbon dioxide plus water plus energy. An image labeled glucose is at the far left-side of the graphic. To the right of this image is a plus symbol. To the right of the plus symbol is an image labeled oxygen. To the right of this image is an arrow pointing to the right. To the right of this arrow is an image labeled carbon dioxide. To the right of this image is a plus sign. To the right of the plus sign is an image labeled water. To the right of this image is a plus sign. To the right of this plus sign is a final image labeled energy.

It occurs in three stages:

  1. Glycolysis (in the cell’s cytoplasm), where glucose is broken down into pyruvate, NADH and 2 ATP;
  2. The Krebs cycle (in the mitochondria), where pyruvate is further processed to produce NADH and FADH2, 2 ATP; and
  3. The electron transport chain (in the inner mitochondrial membrane), where electrons are transferred to produce large amounts of ATP and water.

The breakdown of glucose is most efficient in the presence of oxygen (aerobic respiration) and produces the most ATP, which is essential for powering cellular functions.

The mitochondria

The mitochondria are the key organelle responsible for aerobic respiration and generates most of the cell's ATP.

The mitochondria are membrane-bound organelles found in eukaryotic cells.

They have a distinctive double membrane structure:

  • Outer membrane: Smooth and permeable to allow small molecules and ions to pass through.
  • Inner membrane: Folded into structures called cristae, which increase surface area for energy production.

Within the inner membrane is a fluid-like substance called the mitochondrial matrix, where the Krebs cycle takes place.

Click on the key parts of the mitochondria to find out more about its function.

Glycolysis - the first stage of aerobic respiration

Glycolysis is the first stage of aerobic cellular respiration and occurs in the cytoplasm of the cell.

This step does not require oxygen to occur.

It breaks down one molecule of glucose (a 6-carbon sugar) into two molecules of pyruvate (a 3-carbon compound), producing two molecules of ATP.

For every one glucose molecule split, glycolysis has a net yield of two ATP molecules produced, and two NADH molecules.

We specify net yield of two ATP, as the initial stages of glycolysis are endergonic and first require the consumption of 2 ATP molecules to begin to break down each glucose molecule. Overall, 4 ATP are gained by glycolysis, for a net gain of 2 ATP.

A graphic depicting the process of glycolysis. On the left-hand side of the graphic is an image depicting glucose. To the right of this image is an arrow that splits in 2 directions. To the right of this arrow a series of images follow along each line of the arrow indicating the production of 2 ATP molecules and 2 NADH molecules. At the end of this sequence of parallel images is the label 2 pyruvate.

Glycolysis inputsGlycolysis outputs

Glucose

ADP + Pi

NAD+

Pyruvate

2 ATP

NADH

The Krebs cycle  - the second stage of aerobic cellular respiration

The Krebs cycle is the second stage of aerobic respiration and takes place in the mitochondrial matrix .

It processes the products of glycolysis (i.e. pyruvate) and generates high-energy electron carriers (i.e. NADH and FADH₂) that are essential for ATP production.

A graphic depicting the Krebs cycle. On the left-hand side of the image is a 3 carbon graphic labeled 2 pyruvate. To the right of this graphic is an arrow pointing to the right that splits in two. The first branch of the arrow continues pointing to the right and below this arrow is an image depicting 2 NAD plus connected to 2 NADH via another arrow. To the right of the first arrow is an image depicting a 2 carbon connected to CoA with the label Acetyl CoA below. This image is connected by another arrow to an image labeled 2 Coenzyme A. The second branch of the initial arrow points up and above this arrow is another image. This image displays 2 Coenzyme A, then a 4 carbon symbol, and then the chemical symbol for carbon dioxide. On the right hand side of the graphic is a circular image labeled the Kreps cycle. Below the circle is an image depicting 2 ADP connected via an arrow to 2 ATP and then to the right of these two images is a plus sign and to the right of that is an image labeled 2 P. Above the circle is an image of 6 NAD plus connected via an arrow to 6 NADH. There is also an image of 2 FAD connected via an arrow to 2 FADH2. Branching off from the circle itself is another arrow pointing to an image of a 4 carbon and the chemical symbol for carbon dioxide.

The Krebs cycle is made up of many steps in a biochemical pathway. One of the intermediate steps is the conversion of pyruvate to acetyl CoA.

The final yield of ATP for this stage of aerobic respiration is 2 ATP molecules, however it is crucial for producing loaded electron carriers for ATP production in the next stage.  The Krebs cycle does not use oxygen.

The cycle releases CO2 as a waste product, which is exhaled by animals or used by plants in photosynthesis.

Krebs cycle inputsKrebs cycle outputs

Pyruvate

ADP + Pi

NAD+

FAD

Carbon Dioxide

2 ATP

NADH

FADH2

The electron transport chain - the third stage of aerobic respiration

The third stage of aerobic respiration is the electron transport chain (ETC).

The ETC uses a series of protein molecules embedded in the inner mitochondrial membrane, as depicted below.

A graphic depicting a cross-section of the inner mitochondrial membrane. The graphic is split into 5 horizontal sections. The top section is labeled cytoplasm. The next section below is labeled outer membrane. The next section below is labeled intermembrane space. The next section below is labeled inner membrane space. The final section below is labeled mitochondrial matrix. Along the inner member section are a series of images representing the electron transport chain. The farthest left image is a longitudinal oval shape and has the label 2e in the centre. Below this image is an image of NADH connected via an arrow to NAD+ with a carbon image below. There is also an arrow connecting the NADH connection to the 2e image. Above the 2e image is an arrow pointing into the intermembrane space with a carbon image labeled H+ at the top of the arrow. To the right of the 2e image is another image with the label 2e in the centre. This image is a horizontal oval shape. Above this shape is another smaller oval shape connected to the larger horizontal oval by an arrow. Below the larger horizontal oval labeled 2e is an image depicting FADH2 connected to FAD via an arrow. To the right of this oval shape is another longitudinal oval shaped image that is unlabeled. It has an arrow at the top pointing toward the intermembrane space. Below this image is an unlabeled carbon. To the right of this unlabeled oval image is an unlabeled circle shape. To the right of this shape is another image of an unlabeled shape. Below this unlabeled shape is an image depicting the equation 2e plus 2H+ plus 1/202 connection to the chemical symbol for water via an arrow. Another arrow flows from the very first image labeled 2e all through the intermediate images to point towards the water symbol. At the top of this unlabeled shape is another arrow pointing into the intermembrane space pointing to a carbon labeled H+. Below this entire series of images is a label marking them as the electron transport chain. To the right of this series of images is one final image of an unlabeled kidney shape. Below this final image is an arrow pointing down to a carbon. To the left of this arrow is an image labeled ADP connected via a plus sign to an image labeled P. To the right of the arrow is an image labeled ATP.

The electron transport chain’s primary role is to transfer electrons from NADH and FADH₂ to oxygen, forming water as a byproduct.

The hydrogen ions from NADH and FADH₂ move through the series of protein molecules embedded in the inner mitochondrial membrane to form a proton gradient across the inner mitochondrial membrane. ATP synthase uses the energy of this proton gradient to synthesise ATP from ADP + Pi.

The net ATP yield from the ETC is 26 or 28 ATP molecules.

Electron transport chain inputsElectron transport chain outputs

O2

ADP + Pi

NADH

FADH2

H2O

26/28 ATP

NAD+

FAD

Putting the three stages of aerobic respiration together

In summary, aerobic respiration is the process by which cells convert glucose and oxygen into usable energy in the form of ATP, along with carbon dioxide and water as byproducts.

This summary table and diagram overview the three key stages of aerobic respiration.

A graphic depicting the relationship and connections between the 3 stages of aerobic respiration. On the bottom left of the graphic is a cloud-shaped image labeled oxygen. An arrow comes out of the oxygen image pointing upwards. Above and slightly to the left of the oxygen image is an image of a carrot, broccoli, apple and loaf of bread all grouped together, labeled food. From the top of this image is another arrow pointing up. The arrows from the images of oxygen and food point towards a circular image labeled cell. This image has a variety of cellular elements depicted within it. One is labeled cytoplasm and another is labeled mitochondrion. From these two labels there are lines that link to an image depicting a close up of these elements. The cytoplasm surrounds the mitochondrion. Within the image of the enlarged mitochondrion there is a circular image labeled the Krebs cycle with an arrow pointing down from the circle to an image labeled ATP. Another arrow points from the right side of the Krebs cycle circle and is labeled NADH. This arrow points to another image labeled electron transport. Below the electron transport image is an arrow pointing down to an image labeled ATP. Above both the Krebs cycle and electron transport images is another image labeled glycolysis. At the top of the glycolysis image is a shape labeled glucose which is attached, via an arrow to another shape labeled pyruvate. To the left of the glucose image is an arrow pointing to a shape labeled ATP. From the pyruvate shape one arrow points back down to the Krebs cycle image and another arrow, labeled NADH, points toward the electron transport image. On the far right of the close-up mitochondrion image is an arrow pointing out of the mitochondrion labeled carbon dioxide and another arrow pointing out of the mitochondrion labeled water.

StageLocationInputOutputATP yield
GlycolysisCytoplasm

Glucose

NAD+

ADP + Pi

Pyruvate

NADH

ATP


2
Krebs cycleMitochondrial matrix

Pyruvate

NAD+

FAD

ADP + Pi

CO2

NADH

FADH2

ATP

2
Electron transport chainMitochondrial inner membrane

O2

NADH

FADH2

ADP + Pi

H2O

NAD+

FAD

ATP

26/28

The overall input and output equation of aerobic respiration is:

Glucose + Oxygen → Carbon Dioxide + Water + 30/32 ATP