Rigid cell walls across domains: Which group is least likely to possess a rigid polysaccharide cell wall as a normal feature?

Introduction:
Cell walls confer shape and protection. Bacteria, archaea, fungi, and plants commonly possess rigid walls with distinct chemistries. Protozoa, in contrast, are usually wall-less and rely on flexible membranes and cytoskeleton for locomotion and feeding.

Given Data / Assumptions:

• We compare five broad groups with regard to the presence of a rigid cell wall.
• “Least likely” highlights the group where a wall is not standard.
• Special cases (e.g., cyst walls) do not change the typical trophozoite condition.

Concept / Approach:
Protozoa are characterized by motility and phagocytosis; both favor a flexible plasma membrane. Many do form temporary cyst walls under stress, but the active stage lacks a rigid polysaccharide wall. By contrast, bacteria (peptidoglycan), many archaea (S-layers/pseudomurein), fungi (chitin, glucans), and plants (cellulose) routinely maintain rigid walls.

Step-by-Step Solution:
List typical wall compositions across groups. Note trophic protozoa are wall-less and amoeboid/ciliated/flagellated. Apply “least likely”: choose the group where a wall is not the norm. Confirm exceptions (cysts) are life-cycle specific, not continuous.

Verification / Alternative check:
Light microscopy of living protozoa shows pliable shape changes and endocytosis; wall-bearing cells do not exhibit such dynamic membrane deformation.

Why Other Options Are Wrong:
Bacterium – peptidoglycan wall is standard.

Archaean – many have rigid surface layers or pseudomurein walls.

Fungus – chitinous wall is defining.

Plant cell – cellulose wall is defining.

Common Pitfalls:
Confusing cyst walls (temporary) with constant trophic stage structure.

Final Answer:
Protozoan.