The second main chemical constituent of the plasma membrane is protein. Integral proteins are encapsulated in the plasma membrane and might stretch a part or all of the membrane and act as pumps or channels that help in the movement of particles inside and outside of the cell. Peripheral proteins are present on the internal areas or exterior of the plasma membrane, attached to either phospholipid molecules or to integral proteins. Integral and peripheral proteins both acts as enzymes, as constructural bonds for cytoskeleton fires, or as a component of recognition sites of the cell.

Receptors are the recognized sites on the membrane that are sites for attachments for materials that collaborate with the cell, and each receptor is formed to attach to a particular material. The attachment of a particular material to its receptor on the plasma membrane accelerates activities inside the cell, like stimulating enzymes engaged in metabolic pathways. These metabolic pathways are important for giving energy to the cell, preparing substances for the cell or toxins for discarding or breakdown of the cell's waste. In addition, neurotransmitters and extracellular hormones attach to receptors of the plasma membrane that transfer a signal into the cells to molecules inside the cell. Viruses use some recognition sites as attachment points. Howbeit they are largely specified, disease-causing organisms like the virus may emerge to misuse the recognition sites to enter inside the cell by imitating the particular substance that is meant to be bound with the receptor. This particularity aids in the explanation of why HIV (Human Immunodeficiency Virus) or the hepatitis virus conquers only particular cells.