Which one of the following statements about proteins in mammalian cells is correct?

The correct answer is A, Aspergillus niger, a fungus grown on a sugar medium to make most of the world's citric acid, which is used in soft drinks, food and medicines. Option B is wrong because Lactobacillus species produce lactic acid and are used in curd and other fermented milk products, not citric acid. Option C is wrong because Clostridium tetani is the bacterium of tetanus, an anaerobe that enters through wounds; it is a pathogen, not an industrial organism, although a related species yields butyric acid. Option D is wrong because Anabaena is a nitrogen-fixing cyanobacterium, used with the water fern Azolla as a biofertiliser in paddy fields. Other industrial pairs to remember are Saccharomyces for ethanol and Monascus purpureus for statins. Remembering a product with its organism, acid with Aspergillus, alcohol with yeast and antibiotic with Streptomyces, answers a whole set of questions on industrial microbiology.

The correct answer is B, 70 degrees Celsius for 15 to 30 seconds, followed by rapid chilling and storage. The short, sharp heating kills the disease-causing and most spoilage microbes while leaving the taste and most of the food value of the milk unchanged, which is exactly what boiling does not do. Option A is wrong because forty degrees is close to the temperature at which microbes grow best and would preserve nothing. Option C is wrong because a hundred degrees for ten minutes is ordinary boiling, which alters taste and destroys some vitamins. Option D is wrong because a hundred and twenty-one degrees under pressure for twenty minutes describes autoclaving, used to sterilise instruments and culture media, not milk. The method is named after Louis Pasteur.

The correct answer is B. A virus has heredity and mutates like a living thing, yet it has no cell, no metabolism and no means of reproduction of its own; outside a host it is an inert particle that can even be crystallised, as W. M. Stanley showed with the tobacco mosaic virus in 1935, while inside a host cell it directs that cell to make copies of itself. Option A is wrong because small size alone says nothing about being alive; many bacteria are tiny and are fully living. Option C is wrong because a virus carries either DNA or RNA and never both, a favourite point of examiners. Option D is wrong because antibiotics act on bacterial structures and are useless against viruses, which are controlled by vaccines and antiviral drugs.

The correct answer is B, the bacterium Mycobacterium tuberculosis, identified by Robert Koch in 1882, which is why the organism is also called Koch's bacillus. It spreads through droplets in the air, chiefly attacks the lungs, and is treated with a long and uninterrupted course of several antibiotics. Option A is wrong because tuberculosis is bacterial; treating it as viral is the error that leads candidates to rule out antibiotics. Option C is wrong because Aspergillus causes other lung conditions in people with weak immunity and is used industrially for citric acid, but it does not cause tuberculosis. Option D is wrong because Entamoeba histolytica causes amoebic dysentery, a disease of the intestine spread by contaminated food and water. Leprosy is caused by a related species, Mycobacterium leprae.

The correct answer is B, yeast producing carbon dioxide. The yeast Saccharomyces cerevisiae ferments the sugars of the dough into ethanol and carbon dioxide, and the gas, trapped in the gluten, makes the dough rise and leaves the bread spongy; the alcohol evaporates during baking. Option A is wrong because lactic acid bacteria sour milk into curd and are what make idli and dosa batter tangy, though their gas plays a smaller part than yeast's. Option C is wrong because a virus cannot multiply except inside a living host cell and has no role in dough. Option D is wrong because protozoa are not used in baking at all. The same fermentation by yeast, carried further, yields alcoholic drinks and industrial ethanol. The gluten of wheat flour is what traps the gas, which is why bread made from wheat rises better than bread of flours poor in gluten.