Lecture 3C

Question 1

What is the terminal electron acceptor in aerobic respiration?

Answer 1

Oxygen

Question 2

What is chemolithotrophy?

Answer 2

A process where inorganic compounds (like H₂, reduced sulfur, or ammonia) serve as electron donors.

Question 3

What is carbon dioxide fixation?

Answer 3

The process of incorporating CO₂ into organic molecules.

Question 4

What is the reductive acetyl-CoA pathway?

Answer 4

An alternative carbon fixation pathway used by some anaerobes.

Question 5

Why are strict anaerobes unable to survive in oxygen?

Answer 5

They lack enzymes to detoxify toxic aerobic metabolism products.

Question 6

Give an example of anaerobic respiration.

Answer 6

Nitrate reduction or denitrification.

Question 7

What is denitrification?

Answer 7

The conversion of nitrate (NO₃⁻) to nitrogen gas (N₂), often through intermediates like NO and N₂O.

Question 8

Why is denitrification ecologically significant?

Answer 8

It removes fixed nitrogen from ecosystems, affecting agriculture and contributing to greenhouse gas emissions.

Question 9

What are common electron acceptors in anaerobic respiration?

Answer 9

Nitrate, sulfate, and ferric iron.

Question 10

What is a true denitrifier?

Answer 10

A microorganism that can convert NO₃⁻ fully to N₂, like Pseudomonas stutzeri.

Question 11

Why is NO₂⁻ (nitrite) important in nitrate reduction?

Answer 11

It is an intermediate in the reduction of NO₃⁻ to N₂.

Question 12

What type of bacteria typically perform denitrification?

Answer 12

Facultative anaerobes, such as Pseudomonas species.

Question 13

What gases are released during denitrification?

Answer 13

NO, N₂O, and N₂.

Question 14

Why is N₂O a concern for the environment?

Answer 14

It is a strong greenhouse gas.

Question 15

What is the role of nitrate reductase in nitrate reduction?

Answer 15

It catalyzes the first step of NO₃⁻ reduction.

Question 16

What represses nitrate reductase activity?

Answer 16

Oxygen (O₂).

Question 17

What happens when NO₂⁻ is further reduced?

Answer 17

It can form NO, N₂O, or N₂.

Question 18

Why is denitrification undesirable in agriculture?

Answer 18

It removes nitrate fertilizer from the soil.

Question 19

How does denitrification impact water bodies?

Answer 19

It can prevent algal blooms by removing excess nitrogen.

Question 20

Which microorganism is capable of nitrate-based anaerobic respiration?

Answer 20

E. coli.

Question 21

What is the electron donor in nitrate reduction?

Answer 21

NADH.

Question 22

What is the final electron acceptor in denitrification?

Answer 22

Nitrogen gas (N₂).

Question 23

Where does nitrate reduction occur in bacteria?

Answer 23

In the cytoplasmic membrane.

Question 24

What is ubiquinol’s role in nitrate reduction?

Answer 24

It helps transfer electrons from NADH to the electron transport chain.

Question 25

What happens in the periplasmic space during denitrification?

Answer 25

Electron ions accumulate, creating a proton motive force.

Question 26

What enzyme reduces NO₂⁻ to NO?

Answer 26

Nitrite reductase.

Question 27

What enzyme reduces NO to N₂O?

Answer 27

Nitric oxide reductase.

Question 28

What enzyme reduces N₂O to N₂?

Answer 28

Nitrous oxide reductase.

Question 29

What is the key difference between nitrate reduction and denitrification?

Answer 29

Nitrate reduction only produces NO₂⁻, while denitrification converts NO₃⁻ to N₂.

Question 30

What enzyme catalyzes sulfate reduction?

Answer 30

Sulfate reductase.

Question 31

What is the end product of sulfate reduction?

Answer 31

Hydrogen sulfide (H₂S).

Question 32

What is an example of a sulfate-reducing bacterium?

Answer 32

*Desulfovibrio desulfuricans*.

Question 33

What is the role of hydrogen in sulfate reduction?

Answer 33

It serves as an electron donor.

Question 34

What are the two activated sulfate intermediates in sulfate reduction?

Answer 34

APS (adenosine 5'-phosphosulfate) and PAPS (phosphoadenosine 5'-phosphosulfate).

Question 35

Why is APS important in sulfate reduction?

Answer 35

It activates sulfate to make reduction more thermodynamically favorable.

Question 36

What enzyme reduces APS to SO₃²⁻?

Answer 36

APS reductase.

Question 37

What enzyme reduces SO₃²⁻ to H₂S?

Answer 37

Sulfite reductase.

Question 38

What is the role of cytochromes in sulfate reduction?

Answer 38

They help transfer electrons through the electron transport chain.

Question 39

What is hydrogenase's function in sulfate reduction?

Answer 39

It oxidizes H₂ to release electrons.

Question 40

How does sulfate reduction contribute to ATP production?

Answer 40

It generates a proton motive force for ATP synthesis.

Question 41

What is the difference between assimilative and dissimilative sulfate metabolism?

Answer 41

Assimilative is for biosynthesis, while dissimilative is for energy conservation.

Question 42

What is the major environmental impact of sulfate reduction?

Answer 42

Production of H₂S, which can contribute to acid rain and corrosion.

Question 43

What is anammox?

Answer 43

Anaerobic ammonia oxidation, an alternative nitrogen removal process.

Question 44

Why are denitrifiers considered metabolically diverse?

Answer 44

They can use multiple electron acceptors and even ferment.

Question 45

What is the role of Fe/S complexes in anaerobic respiration?

Answer 45

They transfer electrons from NADH to the electron transport chain.

Question 46

What happens to nitrate in the presence of oxygen?

Answer 46

Nitrate reductase activity is repressed.

Question 47

What alternative electron acceptor can denitrifying bacteria use besides nitrate?

Answer 47

Ferric iron (Fe³⁺).

Question 48

What is the key function of cytochrome b in denitrification?

Answer 48

It helps transfer electrons to the final electron acceptor.

Question 49

What is the consequence of excess nitrogen in water systems?

Answer 49

Algal blooms and eutrophication.

Question 50

What is the overall importance of anaerobic respiration in ecosystems?

Answer 50

It recycles nutrients, removes excess nitrogen, and contributes to greenhouse gas emissions.