Carotenoids in a chloroplast serve to
Answer: A
They broaden the range of wavelengths harvested and quench damaging excited states that would otherwise harm the reaction centres. Their colours appear in autumn as chlorophyll breaks down. Autumn colour is therefore a revealing of pigments that were there all along.
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A leaf absorbs 1,000 units of light energy and converts 40 units into chemical energy. What is its efficiency?
Answer: B
Forty out of a thousand is four per cent. Real photosynthetic efficiency is of this order, which is well below the theoretical maximum and a long-standing target for crop improvement. Most of the incident energy is reflected, transmitted or lost as heat.
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State whether the following is true or false.
Mitochondria are found only in animal cells.
Answer: C
Plants respire just as animals do and possess mitochondria in every living cell, chloroplasts being an addition rather than a substitute. Assuming photosynthesis replaces respiration is a persistent misconception. A plant in darkness respires exactly as an animal does.
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A cell placed in a medium lacking oxygen but supplied with glucose will
Answer: D
Glycolysis in the cytosol needs no oxygen and yields a small amount of ATP per glucose. The much larger yield from the mitochondrion is lost, so the cell must consume glucose far faster to keep up. This is why yeast ferments large quantities of sugar for little energy.
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Consider the following statements about organelle genomes.
1. Mitochondrial DNA is usually circular.
2. Most mitochondrial proteins are encoded in the nucleus.
3. Chloroplasts have no genome of their own.
Answer: A
The first two are correct. The third is wrong, since chloroplasts carry a circular genome of their own along with the ribosomes to read it, just as mitochondria do. Both organelles retain a reduced but functioning genetic system.
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The endosymbiotic origin of mitochondria is thought to have involved an ancestral host cell that
Answer: B
The engulfed cell survived and both partners came to depend on the arrangement. Over time most of its genes migrated to the host nucleus, leaving the reduced genome seen today. The transfer was gradual and is still detectable in nuclear sequences.
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State whether the following is true or false.
Chloroplasts are believed to have arisen from an endosymbiotic cyanobacterium.
Answer: C
Their pigments, their thylakoid membranes and their genome all point that way, and the event is thought to have occurred after mitochondria were already established. Some algae show evidence of further engulfments beyond the first. Secondary endosymbiosis explains the four membranes seen in some algal plastids.
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