The cell walls of diatoms are embedded with silica and form two thin overlapping shells, which fit together as in a soap box. Here I’ll be explaining it in detail and also highlighting their general characteristics.
It’s an important topic in Class 11 Biology NCERT and comes under Chrysophytes in the second chapter “Biological Classification”. This article will provide a detailed to-the-point answer as to what is the nature of cell walls in diatoms. It will surely help you in your NEET exam or boards preparation.
As we know, Kingdom Protista consists of single-celled eukaryotes. Protists are classified into 5 subdivisions:
- Slime moulds
Chrysophytes include diatoms and golden algae (desmids).
Let’s go through the nature and structure of the cell wall in diatoms and then discuss some general features of Chrysophytes.
Nature of Cell Wall in Diatoms
The cell walls of diatoms are called frustules or shells and are chiefly composed of cellulose impregnated with glass-like silica in large quantities. The nature of the structure is such that it forms two thin overlapping halves (or theca) that fit together like two parts of a soap box. The upper half (lid) is called epitheca and the lower half (case) is called hypotheca.
Because the walls are embedded with silica, they are hard and indestructible in nature. Thus, diatoms leave behind large amounts of cell wall deposits in their habitat and this accumulates over billions of years.
The cell walls do not get decomposed even after the death of the diatoms therefore at the bottom of the water reservoir, huge rocks of dead diatoms are formed which is known as diatomite or diatomaceous earth or kieselguhr. Being gritty this soil is used in polishing, filtration of oils, and syrups.
- They are found in freshwater (mainly desmids) as well as in marine (mainly diatoms) environments. A few forms occur as benthos in the bottom of water reservoirs.
- They are microscopic in nature.
- They are immotile (because flagella are absent in them). Hence they float freely and passively in water currents (plankton) due to the presence of lightweight lipids. They show a gliding type of movement with the help of mucilage secretion. Diatoms constitute a major part of the phytoplankton of the oceans.
- Exhibit mainly two types of symmetry – radial symmetry as in Centrales (Cyclotella, Biddulphia, Triceratium, Melosira) and isobilateral symmetry as in Pennales (Pinnularia, Synedra, Adinella, Navicula).
- The reserve food material is oil and a polysaccharide – chrysolaminarin (or leucosin).
- The most common method of multiplication is binary fission (cell division) that occurs at night.
- Sexual reproduction takes place by the fusion of gametes. Meiosis is gametic (takes place during the formation of gametes). In diatoms, during reproduction, special types of spores are formed which are known as auxospores.
- They are found in different colours and shapes (diverse forms) such as circular, rectangular, triangular, elongated, spindle-shaped, half-moon shaped, boat-shaped, or filamentous (in Melosira).
- They are mostly photosynthetic. Some species of diatoms are devoid of chromatophores (Nitzschia alba) which are saprotrophic in nutrition.
- Diatoms are the chief producers in the oceans.
- Diatoms have pigments Chlorophyll a, Chlorophyll c, and xanthophyll (fucoxanthin).
- Each cell has a large central vacuole in which a prominent nucleus is suspended by means of cytoplasmic strands. The cells are diploid (2N). In the case of Centrales, the nucleus lies in the peripheral region.
Examples: Triceratium, Pleurosigma, Navicula, Cymbella, Amphipleura, Nitzschia, Melosira, Pinnularia.
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