Chapter 19, Problem 30A

Summary Introduction

To determine:

The reason for occasional sexual reproduction in diatoms.

Introduction:

Diatoms are unicellular marine algae. They are found as solitary cells as well as in colonies. They are photosynthetic autotrophs that produce their food by the process of photosynthesis. A unique characteristic of diatoms is that they have a cell wall composed of silica (hydrated silicon dioxide) which is known as a frustule. The frustule (cell wall) consists of overlapping halves that are similar to the pillbox.

Explanation of Solution

Diatoms reproduce by both the methods of reproduction, asexual and sexual. They reproduce asexually by the process of mitosis cell division to produce two new daughter cells. In asexual reproduction two halves of a diatom get separated and each halves forms a new diatom cell which is half in the size of the old one. This process of reproduction produces increasingly smaller cells of the diatoms. When a diatom cell is get reduced to one-fourth of the original size then diatom triggers sexual reproduction. In sexual reproduction diploid diatom cells produce haploid gametes. After that fusion of the gametes take place and a zygote is formed that develops into a full original sized diatom. This cycle of reproduction then repeats. The ability of diatoms to reproduce sexually is closely related to the cell size of the diatoms. Due to asexual reproduction the cell size of diatoms gets reduced and to restore the original cell size diatoms perform sexual reproduction. The sexual reproduction in diatoms is usually triggered when the cell size of diatom is reduced to a critical level (smaller than the original size). Sexual reproduction in the diatoms is the most usual method of restoring the cell size.

Conclusion

Diatoms must reproduce sexually occasionally in their life cycle because they have to restore their cell size when it reaches less than a critical size.