11th Class Biology Cell - The Unit Of Life Cell As A Unit Of Life

Cytology : (Gk Kyios = cell ; logas = study) It is the branch of biology. Which comprises the study of cell structure and function. “Cell is the structural and functional unit of all living beings”. Study of metabolic aspects of cell components is called cell biology.

Robert Hooke (1665) discovered hollow cavities (empty boxes) like compartments in a very thin slice of cork (cell wall) under his microscope. He wrote a book “Micrographia” and coined the term cellula, which was later changed into cell. Grew and Malpighi also observed small structures in slice of plants and animals. Leeuwenhoek was the first to see free cells and called them “wild animalcules” and published a book “The secret of nature”. He observed bacteria, protozoa, RBCs, sperms, etc. under his microscope.

Cell theory : H.J. Dutrochet (1824) a French worker gave the idea of cell theory.

The actual credit for cell theory goes to two German scientists, a Botanist M.J. Schleiden (1838) and a Zoologist T. Schwann (1839). They gave the concept “all living organisms are composed of cell”. Schleiden and Schwann both supported the theory of “spontaneous generation”. They also mentioned that “the new cell arises from nucleus by budding”.

Exceptions to the cell theory : Viruses, viroids and prions are an exception to the cell theory as they are obligate parasites (sub–cellular in nature).

Modification of cell theory : Modification of cell theory was done by Rudolf Virchow (1855). He proposed the “law of cell lineage” which states that cell originates from pre-existing cells. i.e., (omnis cellula-e-cellula). It is also called “cell principle” or “cell doctrine”. It states :  

(1) Life exists only in cells.

(2) Membrane bound cell organelles of the protoplasm do not survive alone or outside the protoplasm.

(3) Cells never arise de novo. The new cells are like the parent cell in all respect.

(4) All cells have similar fundamental structure and metabolic reactions.

(5) Cells display homeostasis and remain alive.

(6) Genetic information is stored in DNA and expressed within the cells.

(7) DNA controls structure and working of a cell.

The cell as a self contained unit : Autonomy of a cell is believed due to presence of DNA and its expressibility, otherwise, cell components have different shape and function. It has two positions.

(1) Autonomy in unicellular organisms : Unicellular organisms leads to a totally independent life due to different shape, size and role of different organelles shows division of labour. All these display homeostasis. Unicellular organisms are more active due to large surface volume ratio.

(2) Autonomy in multicellular organisms : In multicellular organisms life activities are displayed by each of the cells independently. Multicellular organisms have one thing advantage over unicellular organisms is division of labour.

Cellular totipotency : Totipotency was suggested by Haberlandt (1902). When cells have tendency or ability to divide and redivide the condition of the cell is called totipotent and this phenomenon is called totipotency. Steward et.al. showed the phenomenon of cellular totipotency in carrot culture.

Surface volume ratio : Metabolically active cells are small, as small cells have higher nucleocytoplasmic ratio for better control and higher surface volume ratio for quicker exchange of materials between the cell and its outside environment.  Larger cells have lower surface volume ratio as well as lower nucleocytoplasmic ratio. Surface volume ratio decreases if cell size increases.

Differences between plant and animal cell