biology

Cells-Biological Units  

The living organisms,all bacteria,plants and animals,have cellular organization; their body is composed of  tiny microscopic units of the protoplasm,called cells. Their number varies one to many in the body of the protoplasm, called cells . Their number varies one to many in the body of the organisms ; the body composed of just one cell is called the unicellular body as it happens in bacteria,cyanobacteria and protozoans. The body composed of many cells is designated as the multicellular body which is characteristic of the metazoan animals and all eukaryotic plants which have a thlloid body or a body differentiated to shoot and root systems. The cells are as important as the body because these maintains the body and enable it to perform the biological activities. Loewy and siekevitz[1963] have defined cell as an unit of the biological activities , democrated by a semipermeable membrane and capable of self multiplication in an organism-free medium.                                                                                                                                                                                                                                                                                        

Discovery of Cells

    The discovery of the cells dates back to the year 1665 when Robert hooke ,an English scientist ,published a report titled ‘Micrographia’. In this, he described  his microscopic ex- animation of a thin piece of cork sheet . his observation showed that the cor piece was made of a mass of tiny cavities . Each cavities was enclosed  by walls , reminding  him of honey comb cells . these  tiny cavities were designated by him as cells meaning hollow spaces [l.cella =hollow space ] quite soon N.Grew and Malpighi made repeated observation on thin slice of the cork and reported the cellulose cell walls which separated one cell from another . in 1672,  grew presented  illustrations of a cellular structures of many plant matertials . Leeuwenhoek[1723] studied and described cells of microscopic organisms such as protozoans and bacteria and rbc. After a gap of about  100years, dutrochet[1824] deascribed that mimosa leaves are made of globular cells . in 1831, Robert brown noticed a spherical body in the cell  for which he coined the name nucleus[=kkaryon].quite soon dujardin[1835],a german botanist  noticed and described  the nucleolus in plant cells Three years later, schleiden[1838] again that all plants  are made of cells, ayears later, theoder scwann[1839], a german zoologist, declared that all plants are composed of cell.Considering their observations,Mathias J.Shleiden[1838] and theodor Schwann[1839] separately formulated the cell theory according to which,the cells are structural and functional units of the body of all living objects or organisms. This theory is a generalization about the architectural organization of the body of the living organisms membrane on the surface, usually one nucleos in the centre and cytoplasm in between them. The nucleus is,usually spherical and bounded by a nuclear membrane. It is filled with the nucleoplasm which has chromatin and nucleolus. The cyto plasm contains specialized living parts ,called cell organellesor cell organoids,and non living substances as particles called cell illusions . the organelles include mitochondria,golgi body and centrosome and some vacuoles as hollow spaces. The inclusions mostly occur as glycogen granules.