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

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

References

1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.