Aerobic respiration

A closer look at the reactions following glycolysis, should oxygen be available

Aerobic respiration uses oxygen to break down glucose, amino acids and fatty acids and is the main way the body generates adenosine triphosphate (ATP), which supplies energy to the muscles. After glycolysis (the anaerobic breakdown of glucose into pyruvate – see our separate article for more detail), pyruvate is converted to acetyl CoA in the matrix of the energy-transferring mitochondria, via the link reaction. Next is the Krebs cycle, which occurs twice per glucose molecule, producing – among other chemicals that feed into the aerobic part of the process – more ATP.

A diagram of the steps of aerobic respiration

A diagram showing the stages of the link reaction and Krebs cycle, taken from our cellular respiration poster.

Credit:

‘Big Picture: Exercise, Energy and Movement’ (2012)

The aerobic part of the process depends on a series of protein complexes that are organised along the folds of the inner membrane (cristae) of the mitochondria. These are arranged so that electrons pass from one reacting molecule to the next, in a series of steps known as the electron transport chain. This process ends with ATP synthase, an enzyme that produces ATP from adenosine diphosphate (ADP) and inorganic phosphate (up to around 30 molecules of ATP per molecule of glucose, according to current thinking), capturing the final portion of the energy released by the whole process in a form that the rest of the cell can use.

Below, we look a little closer at the specific reactions that take place during the steps of aerobic respiration.

Link reaction

The link reaction occurs in the mitochondrial matrix, and converts pyruvate into the two-carbon molecule acetyl CoA by removing carbon dioxide and hydrogen, through the process of decarboxylation

  1. Carbon dioxide and hydrogen are removed from two pyruvate molecules, producing two acetyl groups. The hydrogen removed is transferred to NAD, reducing it.
  2. Coenzyme A (CoA) combines with the acetyl group to form acetyl CoA.
Into the link reaction Out of the link reaction Net products

2 x pyruvate

2 x CoA

2 x NAD

2 x acetyl CoA

2 x CO2

2 x reduced NAD (NADH + H+)

2 x acetyl CoA

2 x CO2

2 x reduced NAD (NADH + H+)

Acetyl CoA is then able to enter the Krebs cycle. 

Krebs cycle

The Krebs cycle also occurs in the mitochondrial matrix, and works to completely oxidise acetyl CoA. As one molecule of glucose produces two pyruvate molecules, and therefore two acetyl CoA molecules, the cycle occurs twice per glucose.

  1. Acetyl CoA (a two-carbon molecule) combines with oxaloacetate (a four-carbon molecule) to produce citrate (a six-carbon molecule), releasing coenzyme A.
  2. Carbon dioxide is removed from citrate and NAD is reduced, which forms a five-carbon molecule.
  3. Carbon dioxide is removed from the five-carbon molecule and NAD is reduced. GDP is converted to GTP, which stimulates the conversion of ADP to ATP. A four-carbon molecule remains.
  4. FAD is then reduced to produce another four-carbon molecule.
  5. Finally, NAD is reduced to produce oxaloacetate from this four-carbon molecule, and the cycle starts again. 
Into the Krebs cycle Out of the Krebs cycle Net products

2 x acetyl CoA

6 x NAD

2 x FAD

2 x ADP (through GDP → GTP)

2 x oxaloacetate

4 x CO2

6 x reduced NAD (NADH + H+)

2 x reduced FAD (FADH2)

2 x ATP (through GDP → GTP)

4 x CO2

6 x reduced NAD (NADH + H+)

2 x reduced FAD (FADH2)

2 x ATP (through GDP → GTP)

Electron transport chain/oxidative phosphorylation

The electron transport chain in cellular respiration

A diagram showing the stages of the electron transport chain in cellular respiration, taken from our cellular respiration poster.

Credit:

‘Big Picture: Exercise, Energy and Movement’ (2012)

In the final stage of aerobic respiration, electron transport is used to power the transport of protons (H+), leading to the production of ATP. This process is known as oxidative phosphorylation.

As you may have noticed, the Krebs cycle does not produce much ATP. Instead, it produces a lot of high-energy electrons carried by reduced FAD and reduced NAD. The electron transport chain converts this energy into ATP through the actions and interactions of enzyme and protein complexes in the inner membrane.

  1. The protein complexes at the start of the chain accept electrons from reduced NAD and reduced FAD.  
  2. These electrons are then passed along the chain, from complex to complex. As they move along the chain, the electrons attract protons, drawing them out of the mitochondrial matrix, through the inner membrane, and into the intermembrane space.
  3. The final protein complex, cytochrome oxidase, passes the electrons to oxygen, forming water (together with protons present in the mitochondrial matrix).
  4. As the protons drawn into the intermembrane space are unable to penetrate the membrane and return to the matrix, the concentration gradient across the membrane increases dramatically. This gradient powers ATP synthase. The protons rapidly return to the matrix through ATP synthase, rotating a pair of motors that produce ATP from ADP and inorganic phosphate (Pi). (You can read more about how ATP synthase works in our ‘Focus protein: ATP synthase’ article.)

Net ATP yield

The amount of ATP made per molecule of glucose varies according to conditions. In theory, each glucose molecule can yield a maximum of 38 ATP according to current thinking, but around 30 is more likely.

Process ATP yield
Glycolysis and Krebs cycle 4
Reduced NAD in the electron transport chain 30
Reduced FAD in the electron transport chain 4
Total 38

 

Lead image:

A colour-enhanced scanning electron micrograph of three mitochondria (blue) fractured to reveal their internal structure. The internal membranes (cristae) are visible and are covered with enzyme complexes involved in the conversion of metabolites into energy. The surrounding cytoplasm is shown in gold.

Dr David Furness/Wellcome Images CC BY NC ND

References

Further reading

About this resource

This resource was first published in ‘Exercise, Energy and Movement’ in August 2016.

Topic:
Cell biology
Issue:
Exercise, Energy and Movement
Education levels:
16–19, Continuing professional development