Diatoms: Silica Cell Wall and Diatomaceous Earth Explained
Biology · Biological Classification · NEET
Diatoms are microscopic algae (chrysophytes) that live in fresh and marine water and float as plankton. Their cell wall is made of silica and forms two thin shells that fit together like a soap box, so the walls are indestructible. When diatoms die, these silica walls pile up over billions of years to form "diatomaceous earth". Memory hook: "Diatom = silica soap box that never rots."
The diatom cell wall is made of silica in two overlapping shells that fit like a soap box. Because silica does not rot, dead diatom walls pile up over billions of years to form gritty diatomaceous earth.
Your doubts, answered
Why is the diatom cell wall indestructible?
Because the wall is made of silica (the same hard material as glass and sand). Silica does not rot or break down easily, so when a diatom dies, its wall stays behind. NCERT says the walls are 'embedded with silica and thus the walls are indestructible'. NEET often turns this into a trap: any option saying diatom walls are 'easily destructible' is WRONG.
What is 'diatomaceous earth' and how does it form?
When diatoms die, their silica walls do not decay. Over billions of years these walls collect in huge amounts in the same place. This deposit of old diatom cell walls is called diatomaceous earth. NCERT says it is 'gritty', so it is used in polishing, filtering oils and syrups. Remember: diatomaceous earth = piled-up silica walls, NOT living diatoms.
What does the 'soap box' cell wall of diatoms mean?
The diatom wall is in two thin halves (called shells or valves). One half is slightly bigger and fits over the smaller half, exactly like the lid and base of a soap box or a Petri dish. The two shells overlap. This is a favourite NEET fill-in, so learn the exact phrase: 'two thin overlapping shells, which fit together as in a soap box.'
Why are diatoms called the chief producers in the oceans?
Diatoms are photosynthetic (they make their own food using sunlight). They float everywhere in ocean water as plankton in huge numbers, so they make more food (organic matter) than any other ocean organism. That is why NCERT calls them the 'chief producers in the oceans'. In NEET, if a question asks the chief producer of the ocean, the answer is Diatoms, not cyanobacteria or dinoflagellates.
Are diatoms plants, algae or Protista?
For NEET, diatoms belong to Kingdom Protista, inside the group Chrysophytes. They are single-celled eukaryotes. People loosely call them 'golden algae', but in Whittaker's Five Kingdom system they are placed in Protista, not in Plantae. This is a common trap: 'Chrysophytes are included in the kingdom ___' → Protista.
⚠️ The NEET trap ✗ The walls of diatoms are easily destructible. ✓ Diatom walls are embedded with silica, so they are indestructible; their deposits form diatomaceous earth. 🧠 Silica = glass = never rots. If an option says diatom walls break down easily, it is the WRONG statement NEET wants you to pick.
Real NEET questions
NEET 2016 Phase 2 · 2018
Select the wrong statement.
A · The walls of diatoms are easily destructible ✓
B · 'Diatomaceous earth' is formed by the cell walls of diatoms
C · Diatoms are chief producers in the oceans
D · Diatoms are microscopic and float passively in water
Solution: Diatom walls are embedded with silica, so they are indestructible (NOT easily destructible) — this is the wrong statement, so option A is the answer. The other three are correct: their silica walls form diatomaceous earth, they are the chief producers in oceans, and they are microscopic plankton floating passively in water.
NEET 2018
Which of the following organisms are known as chief producers in the oceans?
A · Cyanobacteria
B · Diatoms ✓
C · Dinoflagellates
D · Euglenoids
Solution: Diatoms are photosynthetic and float as plankton in huge numbers, so NCERT calls them the chief 'producers' in the oceans. Cyanobacteria, dinoflagellates and euglenoids are not given this title.
NEET 2016 Phase 1
Chrysophytes, Euglenoids, Dinoflagellates and Slime moulds are included in the kingdom:
A · Monera
B · Protista ✓
C · Fungi
D · Animalia
Solution: NCERT places Chrysophytes (which include diatoms), Dinoflagellates, Euglenoids, Slime moulds and Protozoans under Kingdom Protista — single-celled eukaryotes. So the answer is Protista.
Solved Biological Classification NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. It is only the leftover silica cell walls of dead diatoms piled up over a very long time. It contains no living cells.
What is the diatom silica wall called?
The two-part silica wall is called a frustule. Its two overlapping halves fit like a soap box or Petri dish. For NEET, the key phrase to remember is 'two overlapping shells like a soap box'.
What is diatomaceous earth used for?
NCERT says it is gritty, so it is used in polishing, and in filtration (filtering oils and syrups). It is a common exam fact.
Are diatoms fresh water or marine?
Both. Diatoms are found in fresh water as well as marine (sea) water, floating as plankton.
Do diatoms make food?
Yes. Most diatoms are photosynthetic, which is why they are the chief producers in the oceans.
