Microbial Metabolism Questions and Answers

Answer: A

Microbial plastics are made from polyhydroxyalkanoates.

Polyhydroxyalkanoates (PHAs), such as poly-β-hydroxybutyrate, are polyesters that many bacteria accumulate as carbon and energy reserves when a nutrient other than carbon is limiting. They can be extracted and processed like conventional thermoplastics, and they are biodegradable.

Polystyrene, polyurethane and polyvinyl chloride are synthetic polymers made from petrochemical monomers by chemical polymerisation, not by microbes, and they resist biodegradation.

Answer: B

The answer is chemolithotrophs.

Chemolithotrophs obtain energy by oxidising inorganic compounds such as ammonia (Nitrosomonas), nitrite (Nitrobacter), hydrogen sulfide or sulfur (Thiobacillus), ferrous iron or hydrogen gas. The prefix litho, rock, refers to the inorganic energy source.

Heterotrophs and chemoorganotrophs take energy from organic compounds. Photoheterotrophs use light for energy and organic compounds as their carbon source.

Answer: B

The stringent response is associated with guanosine tetraphosphate (ppGpp).

When bacteria run short of amino acids, uncharged tRNAs enter the ribosome's A site and activate RelA, which makes (p)ppGpp, the 'alarmone'. ppGpp binds RNA polymerase and shuts down synthesis of rRNA and tRNA while switching on amino acid biosynthesis genes, so the cell saves resources until conditions improve.

The other nucleotides signal different things. Cyclic di-GMP controls the switch between motile and biofilm lifestyles; cAMP, with CRP, mediates catabolite repression; and cGMP is mainly a second messenger in eukaryotic cells. None of them is the mediator of the stringent response.

Answer: C

The bacterium is a chemoautotroph.

Nutritional groups are named from two choices: the energy source (light, photo-, or chemicals, chemo-) and the carbon source (CO₂, auto-, or organic compounds, hetero-). This organism gets energy by oxidising nitrite, a chemical (inorganic) source, and takes its carbon from CO₂, so it is a chemoautotroph, specifically a chemolithoautotroph. Nitrite-oxidising bacteria such as Nitrobacter live this way in the nitrogen cycle.

Photoautotrophs and photoheterotrophs use light, which this bacterium does not. A chemoheterotroph would draw its carbon from organic compounds rather than CO₂.

Answer: C

The bacterium is an obligate anaerobe.

Whenever a cell uses oxygen, toxic by-products form: superoxide radicals and hydrogen peroxide. Superoxide dismutase converts superoxide to hydrogen peroxide, and catalase and peroxidase destroy the peroxide. An organism lacking all three cannot detoxify these species, so it can survive only where oxygen is absent.

An obligate aerobe must have these enzymes, since it lives on oxygen. 'Does not produce superoxide' gets the cause backwards: superoxide forms on exposure to oxygen whatever the enzymes, and the lack of enzymes is what makes oxygen lethal. Nothing about these enzymes indicates virulence.

Answer: C

The phosphate comes from phosphoenol pyruvate (PEP).

In the bacterial phosphotransferase system (PTS), the phosphoryl group of PEP is passed along a relay of proteins: enzyme I, then HPr, then the sugar-specific enzyme II complex. Enzyme II phosphorylates glucose as it carries it across the membrane, so glucose enters the cell already as glucose 6-phosphate. This is group translocation, and PEP becomes pyruvate.

ATP is not the direct donor in the PTS, inorganic phosphate is not used, and creatine phosphate is an energy store of vertebrate muscle, not a bacterial metabolite.

Answer: A

The organism that gets energy only by fermentation is Lactobacillus bulgaricus.

Lactobacillus delbrueckii subsp. bulgaricus, the yoghurt bacterium, is an aerotolerant anaerobe. It lacks cytochromes and an electron transport chain, so it cannot respire even in air. It makes its ATP by substrate-level phosphorylation, fermenting lactose to lactic acid.

Bacillus subtilis is a respiring bacterium that can use oxygen and, under some conditions, nitrate. E. coli is a facultative anaerobe that respires when oxygen is present and ferments only without it. Pseudomonas putida is a strict aerobe that depends on respiration.

Answer: C

Correct answer: both (a) and (b)

The two enzymes unique to the glyoxylate cycle are isocitrate lyase, which splits isocitrate into succinate and glyoxylate, and malate synthase, which joins glyoxylate to a second acetyl-CoA. Between them they bypass the two decarboxylation steps of the TCA cycle, which is what conserves the carbon.

Answer: B

Correct answer: exoenzyme

Cellulose is a huge insoluble polymer that cannot cross the cell membrane, so the fungus secretes an exoenzyme to break it down outside the cell and then absorbs the sugars released. Endoenzymes work within the cell.

Answer: A

Correct answer: oxygen

In aerobic respiration the electron transport chain passes electrons down a series of carriers and finally to oxygen, which is reduced to water. Oxygen’s high reduction potential is what makes the drop in free energy so large, and so allows aerobic respiration to yield far more ATP than fermentation or most anaerobic respirations. Nitrate is the acceptor in anaerobic respiration by denitrifiers.