Hi
As we know that proteins molecules are an integral part of plasma membranes. When we talk about cell membrane, there are two main components in its definition: 1: Lipid bilayer, 2: Immersed protein molecule.
Now these proteins perform vital functions in plasma membrane for example: help the cell in its interaction with external environment, receive chemical signals from outside and trigger appropriate responses, sometimes they help in proper positioning or anchoring of a cell to a particular place.
Therefore the role of protein in cell membrane is vital and its absence will cause disruption and instability of overall cell structure and the cell (and ultimately whole organism) will not be able to respond to any environmental change or signal (such as heat, cold, pain senses, hearing in case of humans etc).
Hope it helps!
