Composition

• Frustules are made up of silica, which is a chemical compound made up of silicon and oxygen (one molecule of silica is made up of one silicion atom and two oxygen atoms, SiO2). The frustule itself is actually made up of two halves called thecae or valves that slide on top of one another; these valves are held together by more silica. Silica is used commercially by human beings in insulation and detergent.
  
  

Purpose

• The primary purpose of a cell wall in algae (and any organism that has a cell wall) is to be simultaneously rigid enough to retain the cell's structure, while also being water permeable. Water is necessary for photosynthesis to occur, thus the frustule must let it in, however, the frustule keeps the cell's structure intact, lest the water cause it to burst. The frustule also allows waste products to pass out of the cell.
  
  

Diatoms

• Diatoms are the only organism that have frustules, as opposed to a different type of cell wall. Diatoms are eukaryotes, which means they are single-celled organisms, but they have a cellular nucleus; they are classified in the Kingdom Protista. A large majority of phytoplankton -- microscopic photosynthetic organisms found in water, commonly called algae, though not all algae are diatoms -- make up the diatom population. According to the University of Colorado, diatoms are believed to be responsible for around 40% of the Earth's oxygen supply (oxygen is a byproduct of photosynthesis).
  
  

Types of Diatoms

• Diatoms are classified based on the type of symmetry of the frustule. Organisms in the class Bacillariophyceae (also called pennate diatoms) have bilateral symmetry, meaning the frustule is symmetrical along a line down the middle (similar to human beings). Pennate diatoms cannot move on independently and are carried along with the water. Diatoms in the class Coscinodiscophyceae (also called centric diatoms) have radial symmetry -- they are symmetrical based on a point in the middle of the frustule (similar to a sponge).