CDS (II) 2023 General Knowledge · Question 45 of 120

Correct — D, Tapeworm. A cell wall is a rigid layer outside the plasma membrane, and it is found in every major group of living things except the animals. The three rejected options all have one, and each has a different chemistry. Bacteria have a wall built of peptidoglycan, which is what penicillin attacks and what the Gram stain distinguishes. A diatom is a single-celled alga whose wall is a frustule made of silica — hydrated silicon dioxide — which is why diatom shells survive as diatomaceous earth long after the organism dies. A mushroom is the fruiting body of a fungus, and fungal walls are made of chitin, the same material as the exoskeleton of an insect. A tapeworm is none of these: it is a flatworm, an animal, and animal cells are bounded only by the plasma membrane. That absence is not incidental — it is what allows animal cells to change shape, to move and to form the flexible tissues that a parasite living inside a host's intestine depends on.

• (a)Bacteria — Bacteria have a cell wall made of peptidoglycan, a mesh of sugars and amino acids. It is the target of penicillin, which blocks wall formation and so kills growing bacteria while leaving human cells, which have no wall, untouched.
• (b)Diatom — Diatoms are unicellular algae surrounded by a cell wall of silica, called a frustule. The two-part glassy shell is the feature they are best known for, and it is a wall by any definition.
• (c)Mushroom — A mushroom is a fungus, and fungal cell walls are made of chitin rather than cellulose. The material differs from a plant's, but the wall is certainly present.

Every cell is bounded by a plasma membrane, which is thin, flexible and selectively permeable. Some cells add a rigid cell wall outside that membrane, and the material varies by kingdom: cellulose in plants, chitin in fungi, peptidoglycan in bacteria and silica in diatoms. A wall gives the cell its shape, resists bursting when water enters by osmosis, and provides mechanical support. Animal cells have no wall at all, which is why an animal cell placed in pure water can swell and burst while a plant cell in the same water merely becomes turgid.

The reliable move is to identify the kingdom of each organism first and let the wall follow from that, rather than trying to recall each organism individually. Bacteria are monerans, diatoms are protists, mushrooms are fungi and a tapeworm is an animal — and only the animals lack a wall. The design of the item rewards exactly that habit, because the three walled organisms are drawn from three different kingdoms with three different wall materials, so no single chemical formula will get you there. The trap is that a tapeworm is a parasite and looks nothing like a familiar animal, so a candidate scanning for something plant-like or microbial may pass over it. Flatworm means animal, and animal means no cell wall.

• Animal cells have only a plasma membrane; a cell wall is absent throughout the animal kingdom, and a tapeworm is a flatworm and therefore an animal.
• Bacterial cell walls are made of peptidoglycan, which is why penicillin, which blocks wall formation, harms bacteria and not human cells.
• Diatoms 'are surrounded by a cell wall made of silica (hydrated silicon dioxide), called a frustule'.
• Fungal cell walls are made of chitin, unlike the cellulose walls of plants.
• Because they lack a wall, animal cells cannot resist osmotic swelling the way walled plant cells can.

• Assuming a cell wall must be made of cellulose. Cellulose is the plant material; fungi use chitin, bacteria peptidoglycan and diatoms silica.
• Overlooking the tapeworm because a parasite seems an unusual thing to call an animal. Flatworms are animals and their cells have no wall.
• Confusing the cell wall with the plasma membrane; every cell has a membrane, and only some cells add a wall outside it.

As an odd-one-out over organisms from different kingdoms, or as a match-the-organism-to-its-wall-material list.

• practice — not a real PYQ

  The cell wall of a fungus differs from that of a plant in being made mainly of

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

  Answer(b) chitin — fungal walls are chitinous, plants use cellulose, bacteria peptidoglycan and diatoms silica.

• practice — not a real PYQ

  An animal cell placed in distilled water is more likely to burst than a plant cell in the same water because the animal cell

  • (a)has a larger vacuole
  • (b)lacks a rigid cell wall
  • (c)has no plasma membrane
  • (d)contains no cytoplasm

  Answer(b) lacks a rigid cell wall — the wall of a plant cell resists the inward pressure of water and the cell merely becomes turgid, while an unwalled animal cell keeps swelling.