When yeast cells are O2 starved, fermentation serves as the source of energy. This results in the production of :
- A.ATP + CO2 + Ethanol
- B.ATP + O2 + Pyruvate
- C.ATP + CO2 + Lactic acid
- D.ATP + O2 + Acetaldehyde
Correct answer
Explanation
The correct answer is A, ATP + CO2 + Ethanol. Without oxygen, yeast carries out alcoholic fermentation, breaking glucose into ethanol and carbon dioxide and gaining a little ATP. Glucose is first split into two molecules of pyruvate by glycolysis, which yields a net gain of two ATP. In yeast the pyruvate then loses carbon dioxide to form acetaldehyde, which is reduced to ethanol. This is why yeast makes bread dough rise, through the carbon dioxide, and is used to brew beer and wine, for the alcohol. Option B is wrong because fermentation happens without oxygen and releases no oxygen; pyruvate is only an intermediate. Option C is wrong because lactic acid is the product of fermentation in human muscle cells and in lactic acid bacteria, which give out no carbon dioxide. Option D is wrong because no oxygen is formed and acetaldehyde is changed into ethanol before the process ends. Exam tip: yeast = ethanol + CO2; tired muscle = lactic acid.
Practice Questions
View allA recent movie titled The Man Who Knew Infinity is based on the biography of
- A.S. Ramanujan
- B.S. Chandrasekhar
- C.S. N. Bose
- D.C. V. Raman
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The 2015 film 'The Man Who Knew Infinity' tells the life of the Indian mathematician Srinivasa Ramanujan and is based on Robert Kanigel's 1991 biography of the same name.
Read the full explanation →The word 'Denisovan' is sometimes mentioned in media in reference to
- A.fossils of a kind of dinosaurs
- B.an early human species
- C.a cave system found in North-East India
- D.a geological period in the history of Indian subcontinent
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Denisovans were an archaic group of humans, close relatives of the Neanderthals, first identified in 2010 from a finger bone found in Denisova Cave in the Altai Mountains of Siberia.
Read the full explanation →Which one of the following statements best describes the role of B cells and T cells in the human body?
- A.They protect the body from environmental allergens.
- B.They alleviate the body's pain and inflammation.
- C.They act as immunosuppressants in the body.
- D.They protect the body from the diseases caused by pathogens.
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B cells and T cells are lymphocytes, the white blood cells of the adaptive immune system that recognise and fight specific bacteria, viruses and other pathogens.
Read the full explanation →Which one of the following is not used as fertilizer?
- A.Ammonium nitrate
- B.Ammonium sulphide
- C.Ammonium phosphate
- D.Ammonium sulphate
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Ammonium sulphide is an unstable, foul-smelling compound that breaks down to release ammonia and toxic hydrogen sulphide, so it is not used as a fertiliser.
Read the full explanation →The focal length of the objective lens of a telescope is 50 cm. If the magnification of the telescope is 25, then the focal length of the eye-piece is
- A.12.5 cm
- B.5 cm
- C.2 cm
- D.10 cm
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For an astronomical telescope in normal adjustment the magnifying power equals the focal length of the objective divided by that of the eyepiece, so the eyepiece focal length is 50 ÷ 25 = 2 cm.
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