Cell Homeostasis

To describe the ~composition and control of intracellular fluid and the~ mechanisms by which cells maintain their homeostasis and integrity

Cellular respiration describes the production of ATP through a series of redox reactions. Oxygen is used as the oxidising agent, whilst the catabolic fuel may be glucose, fat, or protein.

Cellular respiration can be broken down into:

  • Glycolysis/Lipolysis/Proteolysis
  • Citric Acid Cycle
  • Electron Transport Chain


Glycolysis, or the Embden-Meyerhof pathway, describes the production of pyruvate from glucose. Glycolysis:

  • Occurs in the cytoplasm
  • Begins with the phosphorylation of glucose to glucose-6-phosphate
  • Produces:
    • 2 ATP
    • 2 Pyruvate
    • 2 NADH
  • Note that oxygen is not consumed and carbon dioxide is not produced
  • In aerobic conditions:
    NADH exchanges electrons across the mitochondrial wall, regenerating NAD+ and allowing glycolysis to continue
  • In anaerobic conditions:
    NAD+ is regenerated through the production of lactate
    • When aerobic conditions are restored, lactate can be oxidised back to pyruvate and enter the CAC
    • Transported to the liver and converted back to pyruvate (and enter the CAC), or produce glucose (Cori cycle)

Citric Acid Cycle/Kreb's Cycle

  • Takes place in the mitochondria
  • Complicated
  • Can take many various substrates:
    • Acetyl CoA
      Produced by β-oxidation of fatty acids and pyruvate.
    • Pyruvate
    • Ketoacids
  • Does not consume oxygen but also doesn't function under anaerobic conditions, due to its requirement on fresh NAD+ from the ETC
  • Produces:
    • NADH
    • FADH2
    • CO2

Electron Transport Chain

  • Final stage of carbohydrate, fat, and protein catabolism
  • ETC consists of five protein complexes
  • Electrons are passed along the chain and combine with oxygen, releasing energy which stimulates the movement of hydrogen ions
  • Each time a hydrogen ion crosses the mitochondrial matrix, an ATP is produced
    • This is called coupled phosphorylation
    • Uncoupled phosphorylation allows hydrogen ions to travel down their gradient without generating ATP, which produces excess heat instead
  • 36-38 ATP are produced by aerobic glycolysis
    Sources disagree on exactly how much ATP is produced.
    • 2 from the Embden-Meyerhof pathway
    • 34-36 from the CAC and ETC


  1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
Last updated 2019-07-18

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