Question

Which one of the following is the smallest in size?
A. Bacteria
B. Mitochondrion
C. Mammalian cell
D. Virus

Answer 1

Hint: Various degrees of organization are exhibited by different organisms. Prokaryotic cells are much smaller than eukaryotic cells. The organism that has the smallest size is a submicroscopic infectious agent that replicates only inside the living cells of an organism.

Complete answer: . Viruses are microscopic organisms which cannot be seen with the naked eye. The average diameter of the virus ranges from 20 nanometers to 250-400 nanometers. Cell theory states that all living organisms are made up of cells. The cell is the basic, smallest and functional unit of life. The existing cells are produced from pre-existing cells. Cells can be further classified as prokaryotes and eukaryotes. Prokaryotes are considered as the smaller cells compared to the eukaryotes. Prokaryotic cells are surrounded by plasma membranes with a characteristic cell wall whose composition may differ depending on the organism. Eukaryotes have distinct nuclei bound by a nuclear membrane. They consist of membrane-bound organelles like endoplasmic reticulum and mitochondria. The shape and size of cells vary depending upon their function. The average diameter of bacteria which belongs to prokaryotes is 0.2 to 2 micrometre. Size of the mitochondrion is 0.5 to 10 micrometre. The mammalian cell is the largest of all cells from the above-mentioned option and its average diameter is 20 micrometres.
Hence, the correct answer is option D.

Additional information: Robert Hooke in 1665 discovered the first cell by observing very thin slices of bottle cork under a compound microscope. Robert Hooke observed a multitude of tiny pores that he named "cells". The word cell is derived from the Latin word “Cella” which means a small room.

Note: 1000 nanometer = 1 micrometre. The ovum found in the human body is the largest cell present in the human body and is 20 times bigger than the sperm cells. The diameter of the ovum is 0.1mm. The smallest cell in the human body is Cerebellum’s Granule Cell and its size ranges from 4 micrometres to 4.5 micrometres long.