CDS (I) 2021 GK Q91: Cell wall is not present in cells

Q: What is the answer to CDS (I) 2021 GK Q91?

D. Correct — D, Humans. A cell wall is a rigid layer laid down outside the plasma membrane, and animal cells never build one. A human cell is bounded by its plasma membrane and nothing else, which is why our tissues bend, why a white blood cell can crawl between the cells of a vessel wall, and why a red cell can fold itself through a capillary narrower than its own diameter. The other three organisms in the list all wall themselves in, and each uses a different material: bacteria use peptidoglycan, plants use cellulose, fungi use chitin. That difference in chemistry is exactly what medicine exploits — penicillin blocks the cross-linking of bacterial peptidoglycan, so it wrecks the bacterium and leaves our own cells untouched, because there is no wall in us for it to attack.

CDS (I) 2021 General Knowledge · Question 91 of 120

Question text verified against the official CDS question paper.

Cell wall is not present in cells of

(a)Bacteria
(b)Plants
(c)Fungi
(d)Humans

Answer

Why

Correct — D, Humans. A cell wall is a rigid layer laid down outside the plasma membrane, and animal cells never build one. A human cell is bounded by its plasma membrane and nothing else, which is why our tissues bend, why a white blood cell can crawl between the cells of a vessel wall, and why a red cell can fold itself through a capillary narrower than its own diameter. The other three organisms in the list all wall themselves in, and each uses a different material: bacteria use peptidoglycan, plants use cellulose, fungi use chitin. That difference in chemistry is exactly what medicine exploits — penicillin blocks the cross-linking of bacterial peptidoglycan, so it wrecks the bacterium and leaves our own cells untouched, because there is no wall in us for it to attack.

Why the others are wrong

(a)Bacteria — Almost every true bacterium carries a peptidoglycan wall, and the Gram stain works precisely because that wall is thick in some groups and thin in others. Mycoplasma is the textbook wall-less exception, and the stem does not name it.
(b)Plants — The plant cell wall of cellulose is the reason a plant cell placed in pure water swells to a firm, turgid state instead of bursting, and the reason a wilted plant stiffens again after watering.
(c)Fungi — Fungal walls are built of chitin, the same polymer that stiffens an insect's exoskeleton. It is one of the features that separates fungi from plants.

Concept

Every living cell has a plasma membrane; only some have a wall outside it. The wall is a rigid, freely permeable jacket that resists stretching, so a walled cell can sit in dilute surroundings without bursting. Animal cells manage without one, holding their shape instead through an internal cytoskeleton and an outside network of proteins called the extracellular matrix.

Read the option list as four different wall chemistries plus one blank. Bacteria, plants and fungi each answer 'yes, and it is made of this'; humans, like all animals, answer 'no wall at all'. The trap for a hurried candidate is reading the stem as 'which of these is not a plant', in which case bacteria looks attractive. The word being tested is 'wall', and the fastest safe check is the burst test: a human red blood cell put into pure water swells and ruptures, while an onion cell in the same water does not. On the printed page the word 'not' is set in bold italics, which is the paper's own signal that this is an exclusion item.

Key facts

Bacterial cell walls are made of peptidoglycan; plant walls of cellulose; fungal walls of chitin.
Animal cells, including all human cells, have only a plasma membrane and no cell wall.
Penicillin works by blocking bacterial cell-wall synthesis, which is why it harms bacteria and not human cells.
Mycoplasma is the standard example of a bacterium with no cell wall.
Put in pure water, a walled plant cell becomes turgid and stops; a wall-less red blood cell keeps taking in water and bursts.

Same membrane, four different outer jackets

Bacteria→Plasma membrane plus a peptidoglycan wall. Thick in Gram-positive groups, thin and sandwiched under an outer membrane in Gram-negative ones.

Plants→Plasma membrane plus a cellulose wall, with pectin cementing neighbouring cells together.

Fungi→Plasma membrane plus a chitin wall — the polymer of insect exoskeletons, not cellulose.

Humans→Plasma membrane and nothing outside it. Shape comes from the cytoskeleton inside and the extracellular matrix around.

Three of the four rows name a wall material. The fourth names none, and that is the answer.

Study next

Prokaryotic and eukaryotic cells — what a bacterium has that a human cell lacks, and the other way roundPlasma membrane structure and the fluid mosaic modelPlasmolysis, turgidity and what happens to walled and unwalled cells in strong salt solutionHow penicillin and other cell-wall antibiotics achieve selective toxicityExtracellular matrix and cell junctions — how animal tissues hold together without walls

Common traps

Assuming a cell wall and a cell membrane are two names for one thing — every cell has the membrane, only some have the wall.
Reading the item as 'which one is an animal' rather than 'which one lacks a wall'.
Forgetting that fungal walls are chitin, not cellulose, and grouping fungi with plants.

Either as an odd-one-out on wall presence, or as a match of organism to wall material — peptidoglycan, cellulose, chitin.

Related PYQs

NDA_GAT_2023_I_Q722023Open & attempt →

Which one of the following structures or components is not always present in living cells ?

(a) Cell wall
(b) Plasma membrane
(c) Cytoplasm
(d) Genetic material

Answer(a) Cell wall

The same fact stated as a universal rule rather than a species list. Membrane, cytoplasm and genetic material are in every living cell; the wall is the one item on that list that comes and goes, which is what makes the human cell the odd one out here.

CDS_GK_2023_II_Q452023Open & attempt →

Cell wall is absent in which one of the following organisms?

(a) Bacteria
(b) Diatom
(c) Mushroom
(d) Tapeworm

Answer(d) Tapeworm

The same test, one paper later and with harder examples. That item hides the wall-less organism among a bacterium, a diatom and a mushroom, so a candidate has to know that diatoms build silica walls and mushrooms chitin ones before spotting the animal.

Practice

practice — not a real PYQ
The cell wall of a fungus is made mainly of which one of the following?
- (a)Cellulose
- (b)Chitin
- (c)Peptidoglycan
- (d)Lignin
Answer(b) Chitin — the same polymer that stiffens the exoskeleton of insects and crustaceans, and one of the features separating fungi from plants.
practice — not a real PYQ
A human red blood cell placed in distilled water bursts, while an onion cell in the same water does not. The reason is that the onion cell has
- (a)a thicker plasma membrane
- (b)a rigid cell wall
- (c)no nucleus
- (d)a contractile vacuole
Answer(b) a rigid cell wall — the cellulose wall resists further stretching once the cell is turgid, so water stops entering.