The cell wall was first observed by Robert Hooke in 1665 in a thin slice of cork. Cell wall is a characteristics feature of prkaryotes (except mycoplasma) and plant cells. It is absent in animal cells. The cell wall is usually composed of cellulose. It may consist of other substances like lignin, pectin, hemi-cellulose etc. The cell wall has three layers called primary cell wall, secondary cell wall and middle lamellae. It determines the shape of the cell and provides structural support to the cell.
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.