Topic 3-L5 - Chemolithotrophs and phototrophs Flashcards
Lithotroph” =
rock eater - get their energy from oxidizing inorganic molecules (minerals, in many cases)
Chemolithotrophs can be either
aerobic or anaerobic – some can use O2 as an external electron acceptor for respiration.
Chemolithotrophs are mostly
Autotrophs, produce ATP, Need
a great deal of reducing power (NADH) for biosynthetic reactions
common electron donors for Chemolithotrophs (energy sources) include:
H2S, H2, Fe 2+, NH4+
An example of chemolithophic microbe –
Ralstoniaeutropha– H2 electron donor
Ralstoniaeutropha– H2 electron donor
- gram -, founder in soil and freshwater
- Can grow as a chemolithoautotroph on H2, CO2, and O2 – aerobic conditions
Ralstoniaeutropha– H 2electron donor
Produces
two hydrogenase enzymes that split H2 to H+ (oxidize H2) and donate electrons to produce ATP/NADH:
Ralstoniaeutropha contains two types of enzymes
- Membrane bound enzyme donates electrons to (reduces) quinones in ETC – generates proton motive force, ATP
- Soluble (cytoplasmic) enzyme reduces NAD+ to NADH – generates reducing power for biosynthetic reactions
Oxidation of sulfur compounds
Common electron donors include hydrogen sulfide (H2S), elemental sulfur (S0), thiosulfate (S2O32-) and sulfite (SO32-) – final oxidation product typically sulfate (SO42-)
- High energy electrons funneled into ETC , generates PMF, ATP
For oxidation of sulfur compounds, elemental sulfur can be stored
in the cell (sulfur storage granules) as an energy/electron reserve
“….almost any combination of electron donor and electron acceptor can be used to sustain life if these reactions are coupled to an electron transport chain used in oxidative phosphorylation and if
the ΔEo’ of the redox reaction releases sufficient free energy to form ATP”
Phototrophs Use
light energy (from sun) used instead of chemical reaction to drive electron flow – generate a proton motive force, produce ATP
Phototrophs
- ATP generated by photophosphorylation – in many ways similar to oxidative phosphorylation
- Some phototrophs are oxygenic - generate O2 as a biproduct of photosynthesis.
- Other phototrophs are anoxygenic – do not generate O2. Evolved first. (E.g. green sulfur bacteria, phototrophic purple bacteria)
photoheterotrophs
(rare) phototrophs that get carbon from organic molecules
purple bacteria – anoxygenic phototroph
- Photosynthetic reaction center contains a bacteriochlorophyll (P870) –absorbs light energy – goes from weak electron donor P870 (Eo’ +0.5) to very
strong electron donor P870* (Eo’ -1.0) - P870* donates electrons to a quinone, enters an electron transport chain (ETC) - generates PMF - ATP synthase makes ATP
- Electrons cycle back to P870 to return it to its original state – cyclic photophosphorylation
Photosynthetic reaction center contains a
bacteriochlorophyll (P870) –absorbs light energy
Photosynthetic reaction centers:
- Where electrons are excited and transferred to the ETC
- Contain light-sensitive pigments that absorb light & transfer energy to ETC
Light sensitive pigments are different in
- chlorophylls for oxygenic phototrophs
- bacteriochlorophylls for anoxygenic phototrophs
Antenna Pigments –
“light-harvesting complexes” of (bacterio)chlorophylls that capture light energy and transfer to reaction center
Different pigments with different absorption ranges allow different phototrophs to
coexist in the same habitat – make use of light others can’t use
Purple bacteria an example of a
“Q-type” reaction center – electrons transferred to a quinone
Unlike purple bacteria, other bacteria use
“FeS type” – electrons transferred to an Fe/S cluster carrier – lower Eo’, stronger electron donor
Not all anoxygenic bacteria have
cyclic electron flow – some transfer electrons to an external electron acceptor
Reducing power
- Electrons for ultimately come from
an external electron donor like H2S-enter quinone pool - Q-type Eo’ not low enough to reduce NAD+ – use “reverse electron flow”
Generating reducing power - NAD(P)H in autotrophs
Reverse electron transport – use proton motive force (costs a lot of energy) to drive electrons in opposite direction in electron transport chain – reduce
NAD(P)+ to NAD(P)H
In Addition to ATP, all organisms need
NAD(P)H – __________– for
biosynthetic reactions.
reducing power
Oxygenic phototrophs contain Two distinct photocenters
- photosystem I (PSI or P700 - FeS-type)
- photosystem II (PSII or P680, Q-type)
Rxn centres are found in
membranes (cyanobacteria – cytoplasmic membrane, eukaryotes like algae – chloroplast thylakoid membranes)
Chloroplasts contain stacks of thylakoid membranes which contain the
photosynthetic reaction centers
Oxygen is phototrophs steps
1). PSII (P680) excited by light transfers electrons to ETC – in doing so
becomes highly electropositive – can accept electrons from H2O to generate H+ and O 2
(no easy task, H2O = veryweak electron donor)
2) . P680 now back to original state, can be excited again
3) . Electrons from PSII passed to quinones, down ETC generating PMF. Ultimately low energy electrons transferred to PSI
4) . PSI (P700) excited by light transfers electrons, ultimately to reduce NADP+ to NADPH
5) . NADPH subsequently used as electron source for biosynthetic reactions – CO 2 fixation (see Calvin cycle on coming slides)
6) . CO2 therefore the ultimate electron acceptor (electrons come from water, pass through PSI/PSII, to NADPH, then to CO2)
prokaryotic phototrophs are either
Anoxygenic or oxygenic
Most chemolithotrophs and phototrophs are autotrophs (use CO2as carbon source)
????
YES
The Calvin cycle is used in
phototrophic bacteria, most chemolithotrophic bacteria, algae, some archaea – not the only way to fix CO2
In the Calvin cycle,
- CO2 converted to organic molecules
- Costs ATP and NAD(P)H
- For every 36C that go in – 6C get drawn off for biosynthesis
- RuBisCO enzyme does key carboxylation step
