What is Cell
Cytology(Gk Kytos=cell; logos=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, sperm,etc under his microscope.
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).The 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'.
Modification of cell theory:
Modification of cell theory was done by Rudolf Virchow(1885).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.
Types of cell
Chatton gave the term prokaryote and eukaryote.Depending upon the nature of nucleus, cells are classified .Incipient nucleus is present in prokaryotes,where as in eukaryotes well organised nucleus is present.
It is a single membrane system. Cell wall surrounds the plasma membrane. Cell wall is composed of peptidoglycans.Strengthening material is murein. Cell membrane bears respiratory enzymes. Cytoplasm lacks cell organelles e.g.,Mitochondria,ER ,Golgi body etc. Ribosome are only 70s type. There are no streaming movements of cytoplasm. Nuclear material is not enclosed by nuclear envelope and lies directly in cytoplasm. It is called nucleiod. DNA is circular and not associated with histone proteins. Sexual reproduction absent but parasexuality present . Cell division mostly amitotic.
It is a double membrane system. Cell wall surrounds the plasma membrane in some protists,most fungi and all plant cell. Animal cell lacks it. It is composed of polysaccharide.Strengthening material is chitin in fungi and cellulose in other plants. It lacks respiratory enzymes. Cytoplasm contains various cell organelles. Ribosome are both 80s and 70s type. Cytoplasm show streaming movements. It is enveloped by nuclear envelope.Nucleus is distinct from cytoplasm. Nuclear DNA is linear and associated with histone proteins extranuclear DNA is circular and histone protein free. Sexual reproduction is present. Cell division is typically mitotic.