Proteins

• Transport proteins: Most transport of small molecules across the membrane take place through integral proteins. This transport includes facilitated diffusion and active transport.
• Receptor proteins: Receptor proteins must be on the outside surface of cell membranes and have a specific binding site where hormones or other chemicals can bind to form a hormone-receptor complex (like an enzyme substrate complex) This binding then triggers other events in the cell membrane or inside the cell.
• Enzymes: Enzyme proteins catalyse reactions in the cytoplasm or outside the cell, such as maltase in the small intestine. 
• Recognition proteins: some proteins are involved in cell recognition. These are often glycoproteins such as the A and B antigens on red blood cell membranes. 
• Structural proteins: Structural proteins on the inside surface of the cell membranes are attached to the cytoskeleton. They are involved in maintaining the cells shape, or in changing the cells shape for cell motility. Structural proteins on the outside surface can be used in cell adhesion - sticking cells together temporarily or permanently.