Introduction to Cell Biology

Cell is the structural and functional unit of life. It is often called as the “building block of life”. The cell was discovered by Robert Hook in 1665. An organism consisting of
single cell is called unicellular while an organism consisting of more than one cell is called multi-cellular.

Simply, the study of cells is called cell biology or cytology. The term "cytology" is derived from Greek word "kytos" and suffix "-logia" which means "a hollow" and "study of" respectively. 

Cell theory:

The cell theory was proposed by M.J. Schleiden and T. Schwann. According to this theory:

1.       Cells are the structural unit of life: The body of an individual is always made up of cells and cell products.

2.       Cells are the functional units of life: Cells carry out all important functions of an organism, such as respiration, nutrition, growth etc.

3.       Life begins with a single cell: A single celled embryo which is formed after fertilization is the primary structural unit of life. From this embryo a full grown body of an individual is formed.

4.       Every cell originates from pre-existing cell: Cells divide to form daughter cells which in turn divides again and again.

Types of cell:

An organism may be unicellular or multicellular which may contain any one of the following types of cells:

                     i.            Prokaryotic cell

                   ii.            Eukaryotic cell

Prokaryotic cells: Prokaryotic cells are the most primitive type of cell. The characteristic of prokaryotic cells are as follows:

1.       Prokaryotic cells lack true nucleus.

2.       Prokaryotic cells divide by binary fission, fragmentation.

3.       Mitosis and meiosis do not occur in prokaryotic cells.

4.       Mitochondria, Golgi complex, endoplasmic reticulum etc. are absent in prokaryotic cells.

5.       Ribosomes in prokaryotic cells are smaller than eukaryotic cells.

6.       Prokaryotes are haploid organism.

7.       Prokaryotes bear sex pili in addition to flagella.

Eukaryotic cells: Eukaryotes are more advance than prokaryotes. The characteristic of eukaryotic cells are as follows:

1.       Eukaryotic cells contains true nuclei and the nucleus is always enclosed by a membrane.

2.       The nucleus contains a nucleolus and a definite number of chromosomes.

3.       Mitochondria, Golgi complex, endoplasmic reticulum etc. are present in eukaryotic cells.

4.       Mitosis and meiosis occur in eukaryotic cells.

5.       Ribosomes in eukaryotic cells are larger than prokaryotic cells.

6.       Eukaryotes are typically diploid organisms.

7.       Replication of DNA occurs only in S-phase.

8.       In eukaryotes flagella and cilia bear 9+2 pattern.

9.       In eukaryotes there is no plasmid and pilli.