Biology XI Content
Cell: Cell can be defined as a unit of biological activity delimited by a semi-permeable membrane and capable of self-respiration. Calls are basic structural and functional unit of life. “Robert Hooke” (1665) discovered cell.
A German botanist MJ Schleiden (1838) and a German zoologist T. Schwann (1839) jointly gave a popular theory, as:
- All organisms are made up of cells and their products.
- Each cell is made of a small mass of protoplasm containing a nucleus in its inside and a plasma membrane with or without a cell wall.
- Each cell arises from pre-existing cells.
- All the cells are basically similar in chemical composition and metabolic activities.
- Function of an organism is the sum total of activities and interactions of constituent of cells.
Limitations of cell theory:
- Virus does not have cellular machinery i.e. Cytoplasm, cell organelles etc..
- Bacteria and cynobacteria do not pass a true nucleus, and lack important cell organelles.
- In some organisms suck as Rhizopus, Mucor and Vaucheria body is coenocytic i.e. not differentiated into cells, so they bear nuclei.
- Mature mammalian RBCs and sieve tube cells are without nucleus.
Cell is self-contained unit because every cell:
- Is capable of independent existence
- Contains whole genetic information
- Converts micro to macromolecules
- Can obtain/manufacture energy
- Can regulate flow of energy and information
- Has a definite life span
It is the ability of living nucleated vegetative cell of plant to regenerate into complete plant. The somatic cell of animal may from complete animal but it has yet to be experimentally proved.
The technique that when nucleus taken from a somatic cell of a somatic cell of animals if put into egg with no nucleus, forms a complete organism was successfully proved by WILMUT and CAMBEL when they cloned first sheep “DOLLY”.
- Concept proposed by : Haberlandt in 1902
- Proved by : Steward and co-workers of Cornell University in 1957.
Flow of information through a cell:
The conduction of information in or outside cell in order to carry out different life processes is known as flow of information through cell.
- Intrinsic Information: The unidirectional flow of information from DNA present in the same cell to MRNA and then to Protein is known as intrinsic information. Protein regulates the activities of the cell.
DNA→ MRNA →PROTEIN
- Extrinsic Information: The information that comes from outside the cell i.e. from certain stimuli, metal ions or hormones etc is known as extrinsic information. They provide the information to cells to modify their activities.
The nucleus which lacks nuclear membrane, nucleus and nucleoplasm is known as incipient nucleus. Prokaryotes have such nucleus in their cell.