2.1 Essential ideas

2.1.8 Cell respiration

  • Cell respiration is the release of energy from organic compounds (i.e. glucose) to produce energy, in the form of ATP (adenosine triphosphate) for use in cell processes.
  • Cell respiration involves many metabolic pathways, and the release of energy is controlled at each step by specific enzymes.
  • In eukaryotes, cell respiration takes place in the cytoplasm and mitochondria.

Figure 2.18a – Overview of cell respiration in eukaryotesFigure 2.18a – Overview of cell respiration in eukaryotes

  • The first step of cell respiration is glycolysis, which takes place in the cytoplasm. During glycolysis, one glucose molecule is split into two molecules of pyruvate.
Glycolysis Glucose 2 Pyruvate + 2 ATP
  • After glycolysis, pyruvate may stay in the cytoplasm to undergo anaerobic respiration or enter the mitochondria for aerobic respiration. 

Anaerobic respiration occurs in the absence of oxygen

  • Anaerobic respiration, or fermentation, takes place in the cytoplasm. It produces no further ATP: 

Alcohol fermentation

Pyruvate Ethanol + Carbon dioxide

Lactic acid fermentation
(animals, some bacteria)

Pyruvate Lactic acid

Aerobic respiration involves many chemical reactions

  • Aerobic cell respiration is more efficient at releasing energy from glucose than anaerobic respiration.
  • Once pyruvate enters the mitochondria, it undergoes the link reaction:

Link reaction

Pyruvate Acetyl CoA
+ Carbon dioxide
  • Acetyl CoA enters the Kreb’s cycle. During the Kreb’s cycle, ATP is produced and many electron carrier molecules are reduced.
  • The electron carrier molecules interact directly with oxygen in the electron transport chain (ETC). The result is a large yield of ATP.
  •  There are many reactions involved in aerobic respiration, but the entire process can be summarised as follows:
Glucose + Oxygen Carbon dioxide + Water + Energy
C6H12O6 + 602 6CO2  + 6H20 + 38ATP

ATP is the energy currency of the cell

Figure 2.1.8b – ATP is formed during cell respiration and hydrolyzed to provide energy for cell processesFigure 2.1.8b – ATP is formed during cell respiration and hydrolyzed to provide energy for cell processes

  • ATP, or adenosine triphosphate, is made of one adenosine nucleoside bonded to three inorganic phosphates.
  • When chemical bonds are broken during cell respiration, the energy released is used to bond adenosine diphosphate (ADP) with inorganic phosphate (Pi), to form ATP (shown on the left side of Figure 2.1.8b).
  • ATP hydrolysis (shown on the right side of Figure 2.1.8b) releases energy for use in cellular processes.
  • ATP is not used to store energy – it provides an immediate energy source for living things.

atpFigure 2.1.8c – ATP
Energy from ATP is immediately available to a cell


  • Cell respiration is the controlled release of energy from organic compounds to produce ATP (adenosine triphosphate).
  • Anaerobic cell respiration gives a small yield of ATP from glucose.
  • Aerobic respiration requires oxygen and gives a large yield of ATP from glucose.
  • ATP from cell respiration is immediately available as a source of energy in the cell.

Concept help

  • Aerobic respiration in bacteria takes place in the cytoplasm.
  • The Kreb’s cycle is also known as the citric acid cycle.

sauerkrautFigure 2.1.8d – Sauerkraut

Food for thought

  • Lactic acid fermentation by bacteria causes cabbage to ‘sour’ into sauerkraut and turns milk into yogurt. There is evidence that eating fermented foods can help balance the gut ecosystem.

biofuelsFigure 2.1.8e – Biofuels

Science and social responsibility (Aim 8)

  • Yeasts are used with additional enzymes to ferment corn and sugar cane to produce biofuels. What are the ethical implications of using food crops in the large-scale production of biofuels?

Think about it

  • How many carbons atoms are in one molecule of glucose? Pyruvate? Lactic acid? Ethanol? Acetyl CoA?
  • Is glycolysis an aerobic or anaerobic process?

In the lab

Modify an experiment to test your own hypothesis about factors affecting respiration rates in germinating seeds. See 2.2.8 and 9.2.3.

Course link

  • In 8.1.2, HL students can learn more about:
    • NAD+ and FAD, the electron carrier molecules
    • how control of metabolism is achieved by end product inhibition.