NDA (I) 2017 General Ability Test · Question 62 of 149

Correct — B, chitin. Both plants and fungi have a cell wall outside the membrane, which is one reason fungi were once grouped with plants, but the material of that wall is different. A plant wall is built mainly of cellulose, a polymer of glucose. A fungal wall is built largely of chitin, a polymer of a nitrogen-containing sugar, the same tough material that stiffens the outer covering of insects and crustaceans. Chitin is therefore the feature that sets the fungal wall apart from the plant wall, which is exactly what the question asks for. Fungi differ from plants in another basic way as well — they have no chlorophyll and cannot make their own food, so they live as saprophytes or parasites or in symbiosis.

• (a)cellulose — Cellulose is the main structural material of the plant cell wall, so it is what fungi and plants would have in common rather than what tells them apart. A few fungus-like organisms do contain cellulose, but the wall of a true fungus is characteristically chitinous.
• (c)cholesterol — Cholesterol is a sterol of animal cell membranes, and it is not a wall material at all. The characteristic sterol in a fungal membrane is ergosterol, which is why several antifungal drugs are aimed at ergosterol rather than at cholesterol.
• (d)glycogen — Glycogen is a storage carbohydrate, and fungi do store it, as animals do. But it is held inside the cell as a food reserve and is not built into the wall, so it cannot be the structural difference the question is after.

Fungi form a kingdom of their own in the five-kingdom classification. They are eukaryotic and non-green, their body is usually a network of thread-like hyphae called a mycelium, their cell wall contains chitin, they store food as glycogen, and they reproduce by spores. Plants, by contrast, have cellulose walls, contain chlorophyll, make their own food and store it as starch.

Build the comparison as a short table and most fungal questions answer themselves — wall of chitin against wall of cellulose, glycogen against starch, no chlorophyll against chlorophyll, spores against seeds. The trap in this item is that cellulose is the first carbohydrate that comes to mind for any cell wall, and cholesterol and glycogen are both genuinely present in living things, just not in a fungal wall. Chitin is also worth remembering outside biology of fungi, since it is the material of the exoskeleton of insects and crustaceans and the target of the enzyme chitinase.

• The fungal cell wall is made largely of chitin, while the plant cell wall is made largely of cellulose.
• Chitin is a polymer of a nitrogen-containing sugar and also forms the exoskeleton of insects and crustaceans.
• Fungi lack chlorophyll and cannot photosynthesise, so they live as saprophytes, parasites or symbionts.
• Fungi store carbohydrate as glycogen, whereas plants store it as starch.
• Ergosterol, not cholesterol, is the characteristic sterol of the fungal cell membrane.

• Choosing cellulose because it is the best-known cell-wall material — that is the plant wall, not the fungal one.
• Confusing the storage product with the wall material; fungi store glycogen but build with chitin.
• Treating fungi as plants because both are non-motile and walled; fungi have no chlorophyll.

NDA asks for the fungal wall material directly, or hides it in a statement set on cell-wall composition across bacteria, fungi, plants and animals.

• practice — not a real PYQ

  The cell wall of a bacterium is made mainly of

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

  Answer(c) peptidoglycan — bacteria build their walls from peptidoglycan, fungi from chitin and plants from cellulose.

• practice — not a real PYQ

  Which one of the following is not a characteristic of fungi?

  • (a)Cell wall containing chitin
  • (b)Storage of food as glycogen
  • (c)Presence of chlorophyll
  • (d)Reproduction by spores

  Answer(c) Presence of chlorophyll — fungi have no chlorophyll and cannot make their own food.