8.1 Essential ideas

8.1.2 Cell respiration

Chemical energy is converted to a usable form during cellular respiration. In 2.1.8, you learned that anaerobic respiration produces a small amount of energy in the form of ATP.  On this page, we will learn why aerobic respiration produces much more energy.

Recall the summary equation for aerobic cell respiration:


Figure 8.1.2a – Overview of processes and ATP production during aerobic cell respirationFigure 8.1.2a – Overview of processes and ATP production during aerobic cell respiration

Oxidation and reduction

  • Cellular respiration involves a series of oxidation-reduction (redox) reactions. During a redox reaction, electrons are transferred between two molecules so that one molecule loses electrons, and another gains electrons.
  • Electron carriers are molecules that link oxidation and reduction within cells. They can accept and give up electrons.
  • NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are two important electron carriers in cell respiration.

Electron carrier

Oxidised form

Electrons exchanged

Reduced form**











  • In glycolysis, one glucose molecule is converted to two molecules pyruvate. This occurs in the cytoplasm and oxygen is not required.
  • The first step in glycolysis is phosphorylation. The energy from two molecules of ATP is needed to add two high-energy phosphates to the glucose molecule. The result is a 6-carbon sugar that is phosphorylated at the terminal carbons.
  • Phosphorylation makes molecules less stable – the phosphorylated molecule reacts easily, and the rest of the glycolytic pathway occurs without the input of energy.

Figure 8.1.2b – Overview of glycolysisFigure 8.1.2b – Overview of glycolysis
Glycolysis gives a small net gain of ATP without the use of oxygen.

  • Two molecules of reduced NAD, and four molecules of ATP are other products of glycolysis.
  • ATP is formed by substrate-level phosphorylation during glycolysis.

The Link Reaction

  • If oxygen is present, pyruvate from glycolysis diffuses into the mitochondrial matrix, where the link reaction occurs.

Figure 8.1.2c – The Link reactionFigure 8.1.2c – The Link reaction

  • During the link reaction:
    • Pyruvate is decarboxylated (loses one molecule of carbon dioxide) and oxidised (it loses two electrons).
    • NAD+ is reduced to NADH + H+ (gains two electrons).
    • The resulting 2-carbon compound is added to a coenzyme complex.
  • The product of the link reaction is a two-carbon molecule called Acetyl CoA.
  • The link reaction does not produce ATP – after two pyruvate molecules go through the link reaction, two molecules of reduced NAD are produced.

The Krebs cycle

  • At the start of the Krebs cycle, the coenzyme from acetyl CoA is released, and the acetyl group is attached to a 4-carbon acid to form a 6-carbon acid.
  • During the Krebs cycle, which also takes place in the mitochondrial matrix, the acetyl group is oxidised and releases carbon dioxide, while electron carriers are reduced.

Figure 8.1.2d – The Krebs cycleFigure 8.1.2d – The Krebs cycle

  • Like glycolysis, during the Krebs cycle, ATP is formed through substrate-level phosphorylation.
  •  At the end of the Krebs cycle, the 4-carbon acid is regenerated. For every two turns of the cycle (two molecules of acetyl CoA), two molecules of ATP, six molecules of reduced NAD and two molecules of reduced FAD are produced.

The electron transport chain, oxidative phosphorylation and chemiosmosis

  • Reduced NAD (NADH +H+) and FAD (FADH2) carry energy in the form of bound electrons to the cristae of the mitochondria.

Figure 8.1.2e – The electron transport chain and ATP synthaseFigure 8.1.2e – The electron transport chain and ATP synthase

  • The electron transport chain (ETC) is a series of protein complexes that span the inner membrane of the mitochondria, and have different affinities for electrons.
  • The electron carriers that were reduced during glycolysis and the Krebs cycle, transfer their electrons to proteins in the ETC. The electrons from reduced NAD are slightly more energetic than those of reduced FAD.
  • Electrons are transferred between proteins in the ETC, at each stage releasing energy that is used to pump protons from the matrix into the intermembrane space.
  • The concentration of protons increases in the intermembrane space, creating an electrochemical gradient.
  • Oxygen must bind to free protons in the mitochondrial matrix in order to maintain the proton gradient.
  • When the electrons reach the end of the ETC, they are accepted by oxygen and joined with free protons to form water – the ATP formed in the electron transport chain is oxidative phosphorylation.
  • A large integral protein channel, called ATP synthase, allows the protons from the intermembrane space to diffuse back into the matrix.
  • ATP synthase is an enzyme that catalyses the formation of ATP from ADP. This process of proton diffusion, coupled with the generation of ATP via ATP synthase is called chemiosmosis.

Essential idea

Energy is converted to a usable form in cell respiration.

Figure 8.1.2f – Mitochondria 3DFigure 8.1.2f – Mitochondria 3D
How is the structure of mitochondria adapted to the functions they perform?

Figure 8.1.2g – RedoxFigure 8.1.2g – Redox

Concept help

  • Oxidation and reduction always occur together in redox reactions.
  • Electrons are negatively charged, so adding electrons reduces a molecule’s oxidation number. Here’s an easy way to remember this: 'LEO the lion says GER' (Lose Electrons =  Oxidised; Gain Electrons = Reduced)
  • **Some textbooks, as well as the IB biology guide, use the terms 'reduced NAD' and 'reduced FAD' to avoid the confusing use of charges and H+ ions on the electron carrier molecules.  Both terminologies are acceptable.
  • FADH2 and NADH + H+ lose electrons in the ETC. They are oxidized into FAD and NAD+ .

Figure 8.1.2h – Leo says gerFigure 8.1.2h – Leo says ger

Exam tip

You do not need to name the intermediates of the Kreb’s cycle, but you should be able to look at the pathway and deduce where carboxylation and oxidation occur.


Chemiosmosis theory faced years of opposition before finally being accepted. For what reasons does falsification not always result in an immediate acceptance of new theories or a paradigm shift?