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

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