Structure of a cell can be studied under light microscope as well as electron microscope. The modern technology enables us to isolate various components of cells including its organelles by a process of cell fractionation and study their structure and function in detail.
During cell fractionation the tissues are homogenized or disrupted with special instruments and the various parts of the cells are separated by density gradient centrifugation.
This separation is achieved by spinning he homogenized or disrupted cells in a special medium in a centrifuge at medium speed. The various cellular parts separate out in different layers depending upon their size and weight, and density of the medium.
Some cellular components require very high speeds for separation from other parts of the cells. This is achieved through ultracentrifugation.
A cell consists of the following basic components:
- Plasma membrane, also a cell wall in plant cell.
- Cytoplasm, containing cell organelles.
- Nucleus, with nuclear or chromatin material.
In the traditional system of classification all organisms are divided into plants and animals. The cells of plants and animals can be distinguished by the presence or
Absence of cell wall. Cells of animals and plants are complex and have a distinct nucleus (chromatin material is bounded by a membrane) and are called eukaryotic. On the other hand, the primitive type of cells, such as bacteria. Lack a definite nucleus and are said to be prokaryotic. In prokaryotes the nuclear material is directly submerged in the cytoplasm and is not separated from it by membranes.
The eukaryotic cells vary greatly in size. They could be as big as an ostrich’s egg. Most of the cells are microscopic and are not visible to the naked eye. Their size is measured in micrometer (µm). one µm is 0.000,001 meter or 
of a metre.
The use of modern technology has made it possible to study the following components of the cell in detail (fig. 4.2 and 4.3).
Further Reading: Introduction to Cell (Video)