CDS (I) 2022 General Knowledge · Question 26 of 120

Correct — C, Lichen. Litmus, the purple dye of every school laboratory, is obtained from lichens. Several species have been used over the centuries — Roccella tinctoria, Roccella fuciformis, Lecanora tartarea, Ochrolechia parella and others — with Roccella montagnei and Dendrographa leucophoea the main commercial sources today. In neutral solution the dye is purple; it turns red in acid and blue in base, and the change spans a pH range of about 4·5 to 8·3 at 25 °C. A lichen is not a single organism but a partnership: a fungus supplying the structure and the anchorage, and an alga or a cyanobacterium supplying food by photosynthesis. That partnership is what lets lichens grow on bare rock where nothing else can, which is why they are also the classic pioneer species in ecological succession.

• (a)Protozoa — Single-celled organisms with animal-like nutrition, living in water and in the bodies of hosts. They produce no dye of this kind, and nothing in the group has an industrial use of this sort.
• (b)Virus — Not a free-living organism at all. A virus has no metabolism of its own outside a host cell, so it manufactures nothing, dyes included.
• (d)Saccharomyces — The genus of yeast, used in baking and brewing because it ferments sugar to ethanol and carbon dioxide. It is a genuine and useful organism, but it has no part in making litmus.

An indicator is a substance that changes colour according to whether it is in acid or in base, and litmus is the oldest and most familiar of them. Blue litmus paper turns red in acid, red litmus paper turns blue in base, and neutral litmus is purple. Others are used where a sharper or a differently placed change is wanted — methyl orange, phenolphthalein, and the universal indicator that gives a whole spectrum against pH. Litmus itself is a natural indicator, and so are turmeric, which turns red in base, and the red cabbage extract used in classroom demonstrations. Lichens are the source organism, and the older school phrasing describes them as belonging to the division Thallophyta.

The question is one line of recall, but it opens onto two subjects worth having. The first is what a lichen actually is: an association between a fungus and an alga or cyanobacterium, which is why it appears in ecology as a pioneer on bare rock and in environmental studies as an indicator of clean air, since most lichens are killed by sulphur dioxide pollution. The second is the family of indicators. Because litmus changes over a broad pH span it tells you only which side of neutral a solution lies, and a titration needs an indicator with a narrower range — phenolphthalein or methyl orange — chosen to match the pH at which that particular titration ends.

• Litmus is obtained from lichens, among them Roccella tinctoria, Roccella fuciformis and Lecanora tartarea.
• Neutral litmus is purple; blue litmus turns red in acid and red litmus turns blue in base.
• The colour change spans a pH range of about 4·5 to 8·3 at 25 °C.
• A lichen is a partnership between a fungus and an alga or cyanobacterium, able to colonise bare rock.
• Most lichens are sensitive to sulphur dioxide, which makes them useful indicators of air quality.

• Attributing litmus to algae alone or to a fungus alone, when a lichen is both together.
• Confusing the direction of the colour change, so that blue litmus is expected to turn red in a base.
• Assuming litmus can measure pH; it only tells you which side of neutral a solution is on.

As a source-organism question like this one, as a colour-change question, or through the ecological role of lichens in succession and in monitoring air quality.

• practice — not a real PYQ

  A solution turns red litmus paper blue. The solution is

  • (a)acidic
  • (b)basic
  • (c)neutral
  • (d)a saturated salt solution

  Answer(b) basic — red litmus turning blue is the standard test for a base, just as blue turning red is the test for an acid.

• practice — not a real PYQ

  Lichens are often the first organisms to colonise bare rock. This is possible because

  • (a)they are the only organisms that can photosynthesise without light
  • (b)a fungal partner anchors and protects while an algal partner makes food
  • (c)they obtain all their nutrients from the rock by digesting it whole
  • (d)they reproduce faster than any other group of organisms

  Answer(b) a fungal partner anchors and protects while an algal partner makes food — the partnership supplies between them what neither could manage alone on bare rock.