Q.What is the nature of cell walls in diatoms?

Diatom Cell Wall Composition

Imagine a tiny glass box — that is the most direct way to picture a diatom. These are single-celled algae that live in water, and their most striking feature is the cell wall, which is not made of the cellulose you find in plants. Instead, it is made of silica (silicon dioxide, SiO2), the same material that makes up sand and glass.

Why silica? Diatoms live in aquatic environments where silicon is naturally dissolved in the water. They actively take up dissolved silicic acid (Si(OH)4) and polymerise it into solid, hydrated silica inside their cells. The result is a rigid, transparent and intricately patterned shell called a frustule.

The Frustule: A Two-Part Box

The frustule is not a single piece. It consists of two overlapping halves, like a Petri dish or a soap box:

• The larger, upper half is the epitheca (the lid).
• The smaller, lower half is the hypotheca (the box).

The epitheca overlaps the hypotheca like the two halves of a soap dish. This overlapping structure matters for how diatoms reproduce — when the cell divides, each daughter cell keeps one half and builds a new, smaller half to complete it, which is why diatom cell size tends to shrink across generations.

The Organic Coating

Silica alone would be brittle, so the frustule carries a thin coating of organic material — mostly polysaccharides and proteins. This organic coating:

• Protects the silica from dissolving in water.
• Provides a template for the precise, ornate pattern of pores and ridges unique to each diatom species.

These pores are functional, not decorative — they allow nutrients, gases and waste to pass through the otherwise impermeable silica wall.

The Precise Statement