Nucleus is the control unit of cell. It was discovered by Robert Brown in 1831. The nucleus is found in all eukaryotic cells exceptin mature RBCs and sieve tubes. It is covered by a nuclear membrane which consists of two parallel membrane, The space between the two membrane is called perinuclear space. The nuclear membrane has pores and controls the transfer of material in and out of the nucleus. The nucleoplasm or the matrix inside the nucleus consists of dense spherical bodies called nucleolus. The nucleplus houses the chromatin. nucleolus is the site for active protein synthesis. The chromatin contains the DNA, histone and non-histone protein and RNA. The chromatin becomes dense during interphase of the cell division.
Receptor molecules are specific proteins in the cell's plasma membrane that receive chemical signals from outside the cell. When such chemical signals bind to a receptor, they cause some form of cellular response.