All living organisms are constituted of structural and functional units called cell.
• Cytology is the study of form and structure of cells as well as their organelles with the help of microscope. These days cytology has been replaced by cell biology.
• Cell biology is the branch of biology that deals with the study of structure, biochemistry, physiology, reproduction, evolution and genetics of cells. A number of biochemical techniques are used in the study of cell biology.
• Robert Hooke in 1665 coined the term ‘cell’ and saw dead cell of cork.
• Some organisms are composed of a single cell known as unicellular organisms while others, like us, composed of many cells, are called multicellular organisms.
• All life begins as a single cell.
• Cells are grouped into tissue, tissues into organ and organs into organ system, resulting in the division of labour.
• All cells interact and cooperate with each other and remain partly dependent on each other.
• Anton von LLeeeeuweenhoeek fifi rst saw and described a live cell.
• Robert Brown later had discovered the nucleus.
• Every cell has its own life span. The old & worn out cells are continuously replaced by new cells.
CELL THEORY
• Cell theory was proposed by Schleiden and Schwann in 1855 to explain the concept of the cellular nature of living organism.
• But they did not explain as how new cells are formed. Rudolf Virchow in 1858 made a generalization “omnis cellula e cellula” i.e., all cells comes from pre-existing cells.
The three fundamental facts of the cell theory are as follows:
• All living organisms (animals, plants and microbes) are made up of cells and cell products i.e., cell is the structural unit of life.
• All metabolic reactions in unicellular and multicellular organisms take place in cell i.e., cell is the functional unit of life.
• Cells originate only from pre-existing cells, i.e., no cell can originate spontaneously or de novo Exceptions to cell theory :
• Cells originate only from pre-existing cells, i.e., no cell can originate spontaneously
Exceptions to cell theory :
i. Viruses are acellular and do not have a cellular components. But posses genetic material and show multiplication.
ii. In some organisms, the body is not differentiated into cells through it may have numerous nuclei (coenocytes, e.g., Rhizopus).
iii. Protozoans and some thallophytes have a uninucleate body (e.g., Acetabularia) which cannot be divided into cells. They are acellular.
iv. Bacteria and cyanobacteria do not have nucleus and membrane bound organelles.
v. RBCs and sieve tube cells are the cells without nucleus.
vi. Protoplasm is replaced by nonliving materials in the surface cells of skin and cork.
• Signifi cance of cell theory : Cell theory is one of the fundamental generalisation of biology.
