In which type of organisms is mechanical strength provided by cell walls essential to counteract internal osmotic pressure?

Mechanical strength provided by cell walls is crucial in certain organisms like plants, fungi, and bacteria because these structures help maintain their shape and integrity under internal osmotic pressure. When these organisms absorb water, osmotic pressure increases, which could lead to cell swelling or even bursting if not properly countered.

In plants, the rigid cell wall, primarily composed of cellulose, provides structural support and prevents excessive expansion under high turgor pressure, allowing them to maintain their upright position. Similarly, fungal cell walls, made up of chitin, and bacterial cell walls, often composed of peptidoglycan, serve similar protective roles that help withstand osmotic forces.

In contrast, animals lack cell walls, relying on a different structural framework (such as the cytoskeleton and extracellular matrix) to handle osmotic pressure. Consequently, cell wall presence is a defining characteristic of those specific organisms that require additional support against osmotic stress, which is why the answer highlights plants, fungi, and bacteria.