Nucleolus is the round, naked and a slightly irregular structure, which is attached to the chromatin at a specific region called as Nucleolar Organizer Region (NOR) Nucleolus was first discovered by Fontana (1781).

(i) Nucleolus is the chief site for the ribosomal RNA synthesis.

(ii) It is the centre for the formation of ribosome components.

(iii) It is the colloidal complex that fills the nucleus.

(iv) It combines rRNA with proteins to produce ribosomal sub-units. After their formation, the ribosomes sub-units pass out and get established in the cytoplasm.

(v) It also receive and store ribosomal proteins formed in the cytoplasm.

(vi) These ribosomal proteins formed are the sites for protein synthesis in the cell.

(vii) Nucleolus is essential for spindle formation during nuclear division.

Sugar residues or carbohydrates attaches either with proteins or lipids usually making up less than $10 \%$ of the membrane weight, they can give rise to a wide variety of structures in relatively short chains. They give individual cell types a distinguishing features. Therefore, they may be involved in.

Cell Recognition e.g., surface of RBC have carbohydrates arranged in branched chains: differences in the arrangement give rise to different blood group antigens (i.e., AB and O ).

Cell surface differences are also responsible for the specificity of action of cells with hormones, drugs, viruses or bacteria. The cause of cell surface differences is related to characteristic surface due to carbohydrate component.

Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials both the centriole in centrosome lie perpendicular to each other in which each has an organisation like that of an 'cartwheel'.

A centriole possess a whorl of 9 peripheral fibrils. There fibrils are absent in the centre, hence the arrangement is called $9+0$. Each fibrils is made of 3 sub-fibres. Therefore, called triplet fibril.

The centrioles form the basal body of cilia and flagella. It also forms spindle fibres that gives rise to spindle apparatus during cell division in animals.

Schleiden and Schwann in 1838-39, formulated the cell theory, which originally contained following two statements

(i) All living beings are made up of cells and products formed by the cells.

(ii) Cells are the structural and functional units of life.

The cell theory stated by Schleiden and Schwann failed to explain the question of origin of cells. A major expansion of the cell theory was expressed by Virchow in 1855, in his statement 'Omnis cellula e cellula' (all cells arise from pre-existing cells). This concept, was the actual idea of Nagelli (1846), which later on was elaborated by Virchow, along with considerable evidences in its support. The work of Nagelli and Virchow established cell division as the central phenomenon in the continuity of life.

Thus, the modern cell theory is based on two facts

(i) all living organisms are composed of cells and products of cell.

(ii) cells are the bosic structural and functional unites of life.

(iii) all cells arise from pre-existing cells.

Viruses are exception to cell theory as they are not composed of cell. They consist of a nucleic acid (DNA or RNA) surrounded by a protein sheath and are incapable of independent existence, self regulation and self reproduction.

Difference between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER) are as below