Cell Membrane Proteins

Cell Membrane Proteins.

There are  globular  masses floating in the lipid bilayer. These are membrane  proteins, most of which are glycoproteins. Two types of proteins are present:

 Integral proteins:

 They  protrude  all  the  way through  the  membrane, Many of the integral proteins provide structural 

channels (or pores) through which water molecules  and water-soluble substances, especially ions, can  diffuse between the extracellular and intracellular  fluids. These protein channels also have selective properties  that allow preferential diffusion  of  some substances  over others. Integral membrane proteins can also serve as a  receptors  for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane. Still others act as enzymes.

Carrier proteins:

They  transport   substances  that otherwise could not penetrate  the lipid  bilayer . Sometimes these even transport  substances in the direction opposite to their  natural direction of diffusion, which is called “active  transport.”