Microbial Metabolism Questions and Answers

Answer: A

Correct answer: regenerate NAD+ so that glycolysis can continue

Glycolysis needs a supply of oxidised NAD+, and without a terminal electron acceptor the cell cannot regenerate it through the electron transport chain. Fermentation solves this by reducing pyruvate or a derivative, so the ATP yield stays at the 2 from glycolysis.

Answer: C

Correct answer: Nitrogenase

Nitrogenase reduces atmospheric N2 to ammonia, an energy-expensive reaction requiring around 16 ATP per molecule fixed. The enzyme is irreversibly inactivated by oxygen, which is why nitrogen-fixing cells protect it inside heterocysts or root nodules.

Answer: A

Correct answer: removal of electrons and hydrogens

In biological systems oxidation almost always means the removal of hydrogen atoms - each an electron with its proton - transferred to a carrier such as NAD⁺ or FAD. That is why the enzymes are called dehydrogenases. The older definition, addition of oxygen, describes only a minority of biological oxidations.

Answer: D

Correct answer: both a and b

Glycolysis yields a net 2 ATP, and also 2 molecules of water, released by enolase when 2-phosphoglycerate is dehydrated to phosphoenolpyruvate. Its other products are 2 NADH and 2 pyruvate. No CO₂ is released - that comes later, at pyruvate dehydrogenase and in the TCA cycle.

Answer: B

Correct answer: 2

Fermentation yields a net 2 ATP per glucose: glycolysis makes 4 by substrate-level phosphorylation and spends 2 in the preparatory phase. Nothing further is gained, because the fermentation steps that follow exist only to reoxidise NADH so that glycolysis can continue.

Answer: C

Correct answer: 38

Complete aerobic oxidation yields about 38 ATP in the classical accounting: 4 from substrate-level phosphorylation, and the rest from reoxidising 10 NADH and 2 FADH₂ through the electron transport chain. Modern measurements put the real figure nearer 30, because the proton gradient also pays for transport across the membrane.

Answer: C

Correct answer: acetyl coenzyme A

Glycolysis ends at pyruvate, which is not itself an intermediate of the cycle. The link reaction - pyruvate dehydrogenase - removes a carbon as CO₂, reduces NAD⁺ and attaches the remaining two-carbon acetyl group to coenzyme A. It is that acetyl-CoA which condenses with oxaloacetate to form citrate and so enters the cycle.

Answer: C

Correct answer: coenzyme Q

FADH₂ is bound to succinate dehydrogenase, which sits in the membrane as Complex II and passes its electrons straight to coenzyme Q - entering the chain below NADH dehydrogenase. It therefore bypasses one proton-pumping site, which is why FADH₂ yields about 2 ATP against NADH’s 3.

Answer: C

Correct answer: 3

Each NADH delivers its electrons at the top of the chain, and their passage down it pumps enough protons to drive roughly 3 ATP through ATP synthase. FADH₂, entering lower down at coenzyme Q, yields about 2. (Measured values are closer to 2.5 and 1.5, since the gradient also pays for transport across the membrane.)

Answer: B

Correct answer: gene

A gene is the smallest unit of heredity - the stretch of DNA that specifies one product and is inherited as a unit. A chromosome carries many genes; a codon is three bases within one; a nucleotide is a single chemical building block and carries no heritable information alone.