General Biology/Cells/Respiration

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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

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