CDS (I) 2022 General Knowledge · Question 29 of 120

Correct — B, Fungi. Baker's yeast is Saccharomyces cerevisiae, a single-celled fungus. It is a eukaryote, so it has a true nucleus enclosed in a membrane; its cell wall is built largely of chitin rather than cellulose; and it feeds by secreting enzymes onto its food and absorbing the products, which is the fungal way of eating. What it does in dough is fermentation — deprived of oxygen it breaks sugar down to ethanol and carbon dioxide, and it is the bubbles of carbon dioxide trapped in the gluten that make bread rise, the alcohol being driven off in the oven. Yeast is unusual among fungi in being single-celled rather than filamentous, and that is exactly what makes this question worth asking.

• (a)Protista — Tempting, because Protista is where a great many single-celled eukaryotes are placed. But yeast has the defining fungal characters — a chitin cell wall and absorptive nutrition — and being unicellular does not by itself put an organism in Protista.
• (c)Monera — The kingdom of prokaryotes, the bacteria and the cyanobacteria, whose cells have no membrane-bound nucleus. Yeast is a eukaryote with a proper nucleus, so it cannot belong there.
• (d)Protozoa — The animal-like protists — Amoeba, Paramecium, Plasmodium — which take food in and digest it internally. Yeast does the opposite, digesting outside itself and absorbing what results.

Fungi are eukaryotic, non-photosynthetic organisms with cell walls made largely of chitin, and they feed by absorption — secreting enzymes onto their food and taking up the small molecules released. Most fungi are filamentous, built of fine threads called hyphae that mat together into a mycelium, and the group includes moulds, mushrooms and the rusts and smuts of crops. Yeasts are the exception, living as single cells and reproducing mainly by budding, in which a small outgrowth on the parent cell swells and separates. Saccharomyces cerevisiae is the same species used in baking and in brewing, and it is one of the most studied organisms in all of biology.

The single-celled body is what makes candidates hesitate, so the way through is to classify by characters rather than by size. A chitin wall, a true nucleus and food digested outside the body point at Fungi and at nothing else. It is worth noticing that the yeast in bread and the yeast in beer are doing the identical chemistry — sugar to ethanol and carbon dioxide — and that only the product being kept differs: the baker wants the gas and lets the alcohol evaporate, while the brewer wants the alcohol and lets the gas escape.

• Baker's yeast is Saccharomyces cerevisiae, a single-celled fungus and a eukaryote with a true nucleus.
• Fungal cell walls are built largely of chitin, unlike the cellulose walls of plants.
• Fungi feed by absorption, secreting enzymes onto food and taking up the products.
• Yeast ferments sugar to ethanol and carbon dioxide; the carbon dioxide is what makes dough rise.
• Yeast reproduces mainly by budding, in which an outgrowth on the parent cell enlarges and separates.

• Placing yeast in Protista because it is single-celled.
• Assuming any microorganism must be a bacterium and therefore in Monera.
• Attributing a cellulose cell wall to fungi, when theirs is chitin.

As a classification question like this one, or through what yeast does — the products of fermentation, or why bread rises.

• practice — not a real PYQ

  Yeast reproduces asexually mainly by

  • (a)binary fission
  • (b)budding
  • (c)spore formation in the soil
  • (d)fragmentation of a mycelium

  Answer(b) budding — a small outgrowth appears on the parent cell, enlarges, and eventually separates as a new individual.

• practice — not a real PYQ

  The cell wall of a fungus is made mainly of

  • (a)cellulose
  • (b)chitin
  • (c)peptidoglycan
  • (d)lignin

  Answer(b) chitin — cellulose belongs to plant walls and peptidoglycan to bacterial ones, which is one of the characters used to place fungi in a kingdom of their own.