Lecture 3B

Question 1

What drives the electron transport chain (ETC) in respiration?

Answer 1

Redox reactions where electrons flow from low-potential donors to high-potential acceptors.

Question 2

What are the major components of the ETC?

Answer 2

Cytochromes, quinones, and iron-sulfur proteins

Question 3

What force drives ATP synthesis in respiration?

Answer 3

Proton motive force (PMF).

Question 4

What is the role of ATP synthase?

Answer 4

It uses the PMF to synthesize ATP from ADP and Pi.

Question 5

What distinguishes aerobic from anaerobic respiration?

Answer 5

Aerobic uses oxygen as the terminal electron acceptor; anaerobic uses alternatives like nitrate or sulfate.

Question 6

How is energy conserved in respiration vs fermentation?

Answer 6

Respiration uses ion motive force (oxidative phosphorylation); fermentation uses substrate-level phosphorylation.

Question 7

Which respiration type yields more energy?

Answer 7

Aerobic respiration, due to the high redox potential of the O2/H2O couple

Question 8

What is the redox span of aerobic bacterial ETCs?

Answer 8

More than 1V between NADH and O2.

Question 9

What is the redox potential of NADH/NAD+?

Answer 9

−320 mV.

Question 10

What is the redox potential of O2/H2O?

Answer 10

+815 mV.

Question 11

How many protons can be pumped per electron transferred from NADH to O2?

Answer 11

Up to five protons.

Question 12

What maintains the proton motive force (PMF) in bacterial respiration?

Answer 12

The cell membrane acts as an isolating layer, storing the pmf

Question 13

Which lipids are abundant in E. coli membranes?

Answer 13

Phosphatidylethanolamine (75%), phosphatidylglycerol (20%), cardiolipin (5%) and phospatidylinositol and phosphatidylinositol mannoside

Question 14

What are the two components of PMF?

Answer 14

Electrical gradient (ΔΨ) and pH gradient (ΔpH).

Question 15

How does bacterial ETC differ from mitochondrial ETC?

Answer 15

It is branched and modular, allowing flexibility based on environmental conditions

Question 16

What is the advantage of branched bacterial ETCs?

Answer 16

They allow bacteria to adapt their electron transfer routes to different conditions.

Question 17

Which ion gradients besides H+ may be used in bacteria?

Answer 17

Na+ gradients can also be used.

Question 18

What is the function of Complex I in the ETC?

Answer 18

It oxidizes NADH and transfers electrons to quinone.

Question 19

What is the function of Complex III in the ETC?

Answer 19

It oxidizes quinol and transfers electrons to cytochrome c.

Question 20

What is the function of Complex IV in the ETC?

Answer 20

It oxidizes cytochrome c and reduces O2 to water.

Question 21

What type of organism switches between aerobic and anaerobic respiration?

Answer 21

Facultative anaerobes.

Question 22

What is chemolithotrophy?

Answer 22

Energy conservation by oxidizing inorganic electron donors (e.g., H2, Fe2+, NH3)

Question 23

What is the main energy source for chemolithotrophs?

Answer 23

Inorganic compounds like hydrogen, sulfur, iron, or ammonia

Question 24

What is the ΔG°’ of H2 + ½O2 → H2O?

Answer 24

−237 kJ/mol.

Question 25

Which enzyme catalyzes hydrogen oxidation?

Answer 25

Hydrogenase.

Question 26

What electron carrier receives electrons from H2 in chemolithotrophs?

Answer 26

Quinone.

Question 27

Why are H2 bacteria considered facultative chemolithotrophs?

Answer 27

They can switch between H2 oxidation and organic metabolism.

Question 28

Where do H2 bacteria thrive best?

Answer 28

In microaerobic or oxic-anoxic interface environments.

Question 29

What is the SOX system?

Answer 29

A sulfur oxidation system involving proteins SoxXA, SoxYZ, SoxCD, and SoxB

Question 30

What does SoxCD do?

Answer 30

Removes 6 electrons from the sulfur compound bound to SoxYZ.

Question 31

What happens when SoxCD is absent?

Answer 31

Sulfur is stored in granules and later oxidized in the cytoplasm.

Question 32

Which enzyme oxidizes stored sulfur in the cytoplasm?

Answer 32

Reverse-acting DsrAB or adenosine phosphosulfate reductase.

Question 33

What is the energy yield of ferrous iron (Fe2+) oxidation?

Answer 33

Very low; bacteria must oxidize large amounts of Fe2+.

Question 34

What is the redox potential of Fe3+/Fe2+ at acidic pH?

Answer 34

+0.77 V.

Question 35

Which protein accepts electrons from Fe2+?

Answer 35

Rusticyanin.

Question 36

Why is iron oxidation thermodynamically inefficient?

Answer 36

Because of the small energy yield per Fe2+ oxidized.

Question 37

What is the ecological sign of iron-oxidizing bacteria?

Answer 37

Presence of ferric iron precipitates (Fe(OH)3).

Question 38

What is the electron acceptor in anaerobic iron oxidation?

Answer 38

Nitrate (NO3−), producing nitrite or nitrogen gas

Question 39

What do purple and green bacteria use as electron donors?

Answer 39

Fe2+ or FeS (iron sulfide).

Question 40

What is the key enzyme in ammonia oxidation?

Answer 40

Ammonia monooxygenase (AMO).

Question 41

What is the key enzyme in nitrite oxidation?

Answer 41

Nitrite oxidoreductase.

Question 42

What is the redox potential of the NO2−/NH3 couple?

Answer 42

+0.34 V.

Question 43

What is the redox potential of the NO3−/NO2− couple?

Answer 43

+0.43 V.

Question 44

What is the final electron acceptor in aerobic nitrification?

Answer 44

Oxygen (O2).

Question 45

What is the reaction of anammox bacteria?

Answer 45

NH4+ + NO2− → N2 + 2H2O (ΔG°’ = −357 kJ/mol).

Question 46

What group performs anammox?

Answer 46

Obligate anaerobic bacteria (e.g., Brocadia anammoxidans)

Question 47

What are ladderane lipids?

Answer 47

Specialized lipids in anammoxosomes with cyclobutene rings that form a dense membrane.

Question 48

Where does the anammox reaction occur?

Answer 48

In the anammoxosome, a membrane-enclosed compartment in Planctomycetes

Question 50

Why is the O2/H2O redox couple the most favorable for respiration?

Answer 50

Because it has the most electropositive redox potential, allowing maximum energy extraction

Question 51

How much free energy is released by a 1V redox span?

Answer 51

Approximately −96 kJ/mol (or 23 kcal/mol).

Question 52

What is the thermodynamic cost of pumping one proton across a membrane?

Answer 52

About 4.6 kcal/mol (~19.2 kJ/mol).

Question 53

Why do cell membranes play a key role in respiration?

Answer 53

They maintain the proton motive force by acting as an insulating barrier for charge separation.

Question 54

What is the role of phosphatidylethanolamine in E. coli membranes?

Answer 54

It is the most abundant membrane lipid and contributes to membrane structure and function.

Question 55

How do membrane lipids influence bacterial respiration?

Answer 55

They stabilize protein complexes and help guide protons along the membrane surface.

Question 56

How does Salmonella’s PMF differ from E. coli’s?

Answer 56

It maintains a higher pH gradient (ΔpH) with an alkaline cytoplasm and acidic exterior.

Question 57

What is the advantage of modular ETCs in bacteria?

Answer 57

They allow flexible energy generation routes depending on available substrates and environmental conditions.

Question 58

Which bacteria use nitrite or nitrate as terminal electron acceptors?

Answer 58

Facultative anaerobes like *E. coli* and *Salmonella*.

Question 59

Why are some anaerobes obligate?

Answer 59

Because their respiratory enzymes are oxygen-sensitive and only function in anoxic conditions.

Question 60

Which electron acceptors lock organisms into anaerobic lifestyles?

Answer 60

Highly electronegative acceptors like sulfate (SO4^2−), sulfur (S0), or carbon dioxide (CO2)

Question 61

What determines an organism's respiration flexibility?

Answer 61

The redox potential of its electron acceptors and enzyme sensitivity to oxygen.

Question 62

Why must iron oxidizers like *A. ferrooxidans* process large amounts of Fe2+?

Answer 62

Because energy yield per Fe2+ oxidized is very low, making cell growth inefficient

Question 63

What do large ferric precipitates indicate in the environment?

Answer 63

The activity of iron-oxidizing bacteria in acidic or microoxic habitats.

Question 64

What is the role of SoxYZ in the Sox system?

Answer 64

It is a carrier protein that binds and shuttles sulfur compounds throughout the oxidation pathway.

Question 65

What sulfur forms can SoxXA initially act on?

Answer 65

Hydrogen sulfide (HS−), elemental sulfur (S0), and thiosulfate (S2O3^2−)

Question 66

Why do some sulfur oxidizers lack SoxCD?

Answer 66

Because they store sulfur as granules for later oxidation via alternative cytoplasmic enzymes.

Question 67

What is the fate of sulfur granules in bacteria lacking SoxCD?

Answer 67

Sulfur is oxidized in the cytoplasm to sulfite (SO3^2−) then to sulfate (SO4^2−).

Question 68

Which bacteria commonly use sulfur for CO2 fixation?

Answer 68

Phototrophic sulfur bacteria (e.g., purple and green sulfur bacteria)

Question 69

What is the energy yield of nitrite oxidation?

Answer 69

Very small; nitrifiers must oxidize large quantities to maintain growth.

Question 70

What is the function of hydroxylamine oxidoreductase (HAO)?

Answer 70

Converts NH2OH (hydroxylamine) to NO2− in ammonia-oxidizing bacteria.

Question 71

What is the full reaction sequence of nitrification?

Answer 71

NH3 → NH2OH → NO2− → NO3−

Question 72

What is unique about anammoxosomes?

Answer 72

They are membrane-bound compartments where toxic intermediates are safely contained during anammox.

Question 73

What structural feature makes ladderane lipids special?

Answer 73

They contain multiple cyclobutane rings, making the membrane highly impermeable

Question 74

Why is the anammox reaction biologically significant?

Answer 74

It removes fixed nitrogen from ecosystems by converting ammonium and nitrite to nitrogen gas.