Topic 3-L5 - Chemolithotrophs and phototrophs

Question 1

Lithotroph” =

Answer 1

rock eater - get their energy from oxidizing inorganic molecules (minerals, in many cases)

Question 2

Chemolithotrophs can be either

Answer 2

aerobic or anaerobic – some can use O2 as an external electron acceptor for respiration.

Question 3

Chemolithotrophs are mostly

Answer 3

Autotrophs, produce ATP, Need

a great deal of reducing power (NADH) for biosynthetic reactions

Question 4

common electron donors for Chemolithotrophs (energy sources) include:

Answer 4

H2S, H2, Fe 2+, NH4+

Question 5

An example of chemolithophic microbe –

Answer 5

Ralstoniaeutropha– H2 electron donor

Question 6

Ralstoniaeutropha– H2 electron donor

Answer 6

• gram -, founder in soil and freshwater

- Can grow as a chemolithoautotroph on H2, CO2, and O2 – aerobic conditions

Question 7

Ralstoniaeutropha– H 2electron donor

Produces

Answer 7

two hydrogenase enzymes that split H2 to H+ (oxidize H2) and donate electrons to produce ATP/NADH:

Question 8

Ralstoniaeutropha contains two types of enzymes

Answer 8

• 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

Question 9

Oxidation of sulfur compounds

Answer 9

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

Question 10

For oxidation of sulfur compounds, elemental sulfur can be stored

Answer 10

in the cell (sulfur storage granules) as an energy/electron reserve

Question 11

“….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

Answer 11

the ΔEo’ of the redox reaction releases sufficient free energy to form ATP”

Question 12

Phototrophs Use

Answer 12

light energy (from sun) used instead of chemical reaction to drive electron flow – generate a proton motive force, produce ATP

Question 13

Phototrophs

Answer 13

• 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)

Question 14

photoheterotrophs

Answer 14

(rare) phototrophs that get carbon from organic molecules

Question 15

purple bacteria – anoxygenic phototroph

Answer 15

• 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

Question 16

Photosynthetic reaction center contains a

Answer 16

bacteriochlorophyll (P870) –absorbs light energy

Question 17

Photosynthetic reaction centers:

Answer 17

• Where electrons are excited and transferred to the ETC

- Contain light-sensitive pigments that absorb light & transfer energy to ETC

Question 18

Light sensitive pigments are different in

Answer 18

• chlorophylls for oxygenic phototrophs

- bacteriochlorophylls for anoxygenic phototrophs

Question 19

Antenna Pigments –

Answer 19

“light-harvesting complexes” of (bacterio)chlorophylls that capture light energy and transfer to reaction center

Question 20

Different pigments with different absorption ranges allow different phototrophs to

Answer 20

coexist in the same habitat – make use of light others can’t use

Question 21

Purple bacteria an example of a

Answer 21

“Q-type” reaction center – electrons transferred to a quinone

Question 22

Unlike purple bacteria, other bacteria use

Answer 22

“FeS type” – electrons transferred to an Fe/S cluster carrier – lower Eo’, stronger electron donor

Question 23

Not all anoxygenic bacteria have

Answer 23

cyclic electron flow – some transfer electrons to an external electron acceptor

Question 24

Reducing power

Answer 24

• 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”

Question 25

Generating reducing power - NAD(P)H in autotrophs

Answer 25

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

Question 26

In Addition to ATP, all organisms need
NAD(P)H – __________– for
biosynthetic reactions.

Answer 26

reducing power

Question 27

Oxygenic phototrophs contain Two distinct photocenters

Answer 27

• photosystem I (PSI or P700 - FeS-type)

- photosystem II (PSII or P680, Q-type)

Question 28

Rxn centres are found in

Answer 28

membranes (cyanobacteria – cytoplasmic membrane, eukaryotes like algae – chloroplast thylakoid membranes)

Question 29

Chloroplasts contain stacks of thylakoid membranes which contain the

Answer 29

photosynthetic reaction centers

Question 30

Oxygen is phototrophs steps

Answer 30

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)

Question 31

prokaryotic phototrophs are either

Answer 31

Anoxygenic or oxygenic

Question 32

Most chemolithotrophs and phototrophs are autotrophs (use CO2as carbon source)
????

Answer 32

YES

Question 33

The Calvin cycle is used in

Answer 33

phototrophic bacteria, most chemolithotrophic bacteria, algae, some archaea – not the only way to fix CO2

Question 34

In the Calvin cycle,

Answer 34

• 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