Question

A student observed a water drop under a microscope. He found a photosynthetic organism. Its cell wall forms two overlapping shells which fit together as in a soap box. Which of the following organisms it is
A. Euglenoid
B. Dinoflagellate
C. Sporozoans
D. Diatoms

Answer 1

Hint: These photosynthetic organisms are microscopic, single celled forms. Cell is called a frustule or shell. They are present in both fresh and salt water and comprise an important food for the aquatic animals. However out of 5,500 species most are marine

Complete answer: Diatoms, included in phylum Chrysophyta of algae are also known by the name golden algae. They have been grouped under two categories namely pennate types and centric types. Pennate types have bilateral symmetry whereas centric types have radial symmetry. They show the following characters:
They lack flagella and float mainly due to light storage lipids present in them.
Diatoms have cell walls containing silica, constructed in two overlapping halves, which fit together like two parts of a soap box are a pair of Petri dishes.
Due to their silica impregnations, the walls of diatoms are indestructible. As a result diatomaceous earth formation has occurred due to remains of cell walls if diatoms in the form of fossils.
Silica wall of diatoms is designed with fine ridges, lines, pores etc. These ornamentations are either radially symmetrical or bilaterally symmetrical on either side of the long axis of the cell. The outer wall is called epitheca and the inner wall is called hypotheca. The cell wall is made up of two units and thus the name diatom.
So, the correct answer is “Option C”.

Note: Diatoms may exhibit slow gliding movements produced by the streaming of cytoplasm through grooves on the surface of the cell wall. Due to various designs over the cell wall they have been regarded as the jewels of the plant kingdom.