# General Biology/Cells/Respiration

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Glucose + O$_2$ → CO$_2$ + H$_2$O + 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

* C$_6$H$_{12}$O$_6$ + 6 O$_2$ → 6 CO$_2$ + 6 H$_2$O + 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).
** CO$_2$ is electron acceptor, forming CH$_4$
* Sulfur bacteria
** SO$_4$ reduced to H$_2$S
** Formation of H$_2$S set stage for evolution of photosynthesis (H$_2$S as electron donor before H$_2$O)
** 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
–2 ATP (priming)
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 FADH$_2$
* 2 ATP (from GTP)
* 4 CO$_2$

## ATP production

* Chemiosmosis (Mitchell)
* H$^+$ (from NADH and FADH$_2$) 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