To put it simply, the cell is what differentiates a living organism from a non-living object. An entity can be considered living if it possesses the basic unit of life – the cell. All living organisms you see around you (and even those you cannot see by the naked eye) are composed of cells. Some of them consist of a single cell and are known as unicellular organisms. Others are composed of numerous cells and are known as multicellular organisms.
However, it is worth mentioning that unicellular organisms are perfectly capable of both independent existences and performing all the essential functions of life. In other words, the capability to live independently does not exist for anything less than the complete structure of a cell.
The cell was first discovered in 1665 by Robert Hooke, an English scientist who first observed a slice of bottle cork under a primitive microscope. Although he only observed the dead cell walls, in this case, he laid down the foundations of cell biology. Rightfully, he is now known as the father of cytology.
Anton von Leeuwenhoek was the first scientist to observe and describe live cells such as bacteria, red blood cells, protozoa, sperms, and so on. Later, Robert Browndiscovered the nucleus of the cell. The invention of the electron microscope by Knoll and Ruska in 1932 led to the discovery of even smaller, never-before-seen cell organelles and paved the way for a colossal revolution in the field of biology.
Plant Cells vs Animal Cells
In typical plant cells such as onion cells, a clear and rigid cell wall serves as the outer boundary and the cell membrane lies just within its confines.
On the other hand, the cell membrane is the delimiting structure of typical animal cells such as human cheek cells. A dense membrane-bound structure known as the nucleus lies within each cell. The nucleus, in turn, contains chromosomes that harbor the genetic material of the cell – namely, the DNA.
Cells whose nuclei are bound by a distinct nuclear membrane are known as eukaryotic cells, whereas those that lack a membrane-bound nucleus are known as prokaryotic cells.