electron transport systems and the proton motive force

in respiration, electrons move from

reduced food molecules into energy carriers then into membrane protein carriers and final acceptors

membrane-embedded protein carriers in respiration form

the electron transport system

three major classes of prokaryotic energy-aquiring processes using an ETC

1. organotrophy/chemoorganotrophy - organic electron donor

2. lithotrophy/chemolithotrophy - inorganic electron donor

3. phototrophy - light capture usually to split H2S, H2O, organic molecules

a positive E (redox potential) means

the gain of electrons yields energy; negative delta G; increasing in electron affinity

a negative E (redox potential) means

loss of electrons yields energy; positive delta G; decreasing in electron affinity

redox couple

oxidized and reduced states of a compound; a complete redox reaction combines two redox couples where one accepts e and becomes reduced while the other donates e and becomes oxidized

oxidoreductases

• electron transport proteins

• form complexes that form an electron transport system

• contains cytochromes, non-cytochrome proteins

cytochrome

colored proteins in the electron transport system; absorbance spectrum shifts when there is a change in redox state

chemiosmotic theory

process where proton pumps in the ETC generate a proton-motive force that drives ATP synthesis

proton potential

combination of electrical potential and pH difference that drives gradients; may drive many cell processes like rotation of flagella, uptake of nutrients, efflux of toxic drugs

summary of the electron transport chain

dehydrogenase receives electrons from a substrate like NADH → electrons move to a mobile electron carrier like quinone → reduction of quinone pumps H across the membrane → terminal oxidase (cytochrome) receives electrons → electrons transferred to terminal electron acceptor

obligate aerobes are organisms that grow only using

O2 as a terminal electron acceptor

anaerobic respiration is used in environments where

oxygen is scarce; typically only unique to prokaryotes

dissimilatory denitrification

reduction of oxidized states of nitrogen (anaerobic respiration)

NO3 → NO2 → NO → 1/2N2O → 1/2N2

sulfur reduction in anaerobic bacteria

SO4 → SO3 → S2O3 → S → H2S

dissimilatory metal reduction

reduction of metal cations, commonly iron (Fe3 → Fe2) and manganese (Mn4 → Mn2)

nanowires

• pili that relay electric current across a biofilm

• reaches insoluble or distant electron acceptors

• used by metal-reducing bacteria

obligate lithotrophs

oxidize only inorganic molecules

nitrogen oxidation

• type of lithotrophy

• NH4 → NH2OH → HNO2 → HNO3

microbial sulfur oxidation is dangerous because it can cause

severe environmental acidification

hydrogenotrophy

use of molecular hydrogen H2 as an electron donor; has sufficient reducing potential to donate e to nearly all biological electron acceptors (can be coupled with many unfavorable forms to make them favorable)

methanogenesis

reduction of CO2 and other 1C compounds to methane; done by archaea