General Biology/Cells/Respiration

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 2NADH 4 ATP (from 2 G3P) –2 ATP (priming) 2 ATP (net gain)
 * Oxidation
 * Two electrons (one proton) are transferred from each G3P to NAD+ forming NADH
 * Substrate level phosphorylation
 * G3P to pyruvate forms 2 ATP molecules

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