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Glucose + O2 → CO2 + H2O + ATP

Energy

  • Energy is primarily in C-H bonds (C-O too)
  • Chemical energy drives metabolism
    • Autotrophs: harvest energy through photosynthesis or related process (plants, algae, some bacteria)
    • Heterotrophs: live on energy produced by autotrophs (most bacteria and protists, fungi, animals)
  • Digestion: enzymatic breakdown of polymers into monomers
  • Catabolism: enzymatic harvesting of energy
  • Respiration: harvesting of high energy electrons from glucose

Respiration

  • Transfer of energy from high energy electrons of glucose to ATP
  • Energy depleted electron (with associated H+) is donated to acceptor molecule
    • Aerobic respiration: oxygen accepts electrons, forms water
    • Anaerobic respiration: inorganic molecule accepts hydrogen/electron
    • Fermentation: organic molecule accepts hydrogen/electron

Respiration of glucose

  • C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy
  • ΔG = -720 kcal/mole under cellular conditions
  • Largely from the 6 C-H bonds
  • Same energy whether burned or catabolized
  • In cells, some energy produces heat, most is transferred to ATP

Alternative anaerobic respiration

  • Methanogens (Archaebacteria).
    • CO2 is electron acceptor, forming CH4
  • Sulfur bacteria
    • SO4 reduced to H2S
    • Formation of H2S set stage for evolution of photosynthesis (H2S as electron donor before H2O)
    • About 2.7 by, based on ratio of 32S/34S, where only biological processes produce 32S enrichment

Glycolysis overview

Glycolysis accounting

  • Oxidation
    • Two electrons (one proton) are transferred from each G3P to NAD+ forming NADH

2NADH

  • Substrate level phosphorylation
    • G3P to pyruvate forms 2 ATP molecules
4 ATP (from 2 G3P)

–2 ATP (priming) 

 2 ATP (net gain)

Summary: The net input of glycolysis is 2 ATP molecules which are used to split one glucose molecule. The net yield of this step is 2 ATP and 2 pyruvate.

Regeneration of NAD+

  • Reduction of NAD+ to NADH can deplete NAD+ supply; it must be regenerated
  • Two pathways, coupled to fate of pyruvate
    • With oxygen: enter electron transport chain, forming water (and ATP)
    • Without oxygen: fermentation
  • lactate
  • ethanol

Alcohol fermentation

Lactate formation

Either lactic acid or alcohol can be formed as a result of anaerobic respiration in cells.

Krebs cycle: overview

  • Matrix of mitochondrion
  • Priming steps
    • Joining of acetyl-CoA to oxaloacetate
    • Isomerization reactions
  • Energy extraction steps in Krebs cycle
    • Per glucose
  • 6 NADH
  • 2 FADH2
  • 2 ATP (from GTP)
  • 4 CO2

ATP production

  • Chemiosmosis (Mitchell)
  • H+ (from NADH and FADH2) is pumped against a gradient into the intermembranal space of the mitochondrion (creates voltage potential)
  • Diffusion back into matrix through ATP synthase channels drives synthesis of ATP (ADP + Pi → ATP)
  • ATP exits mitochondrion by facilitated transport

Evolution of aerobic respiration

  • Preceded by evolution of photosynthesis (O2 needed; also, prior evolution of electron transport and chemiosmosis)
  • High efficiency of ATP production compared to glycolysis
    • Fostered evolution of heterotrophs
    • Fostered evolution of mitochondria by endosymbiosis in eukaryotes
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