Transport with the Help of Membrane Protein

Even though, the lipid bilayers do not permit the entry of polar molecules such as ions, sugars, amino acids, nucleotides and cell metabolities, these molecules enter the cell through

• Aqueous protein channels (pores),
• Carrier proteins that rotate and move across the membrane and
• Transmembrane proteins that transport solute by undergoing changes in shape

Figure:  Three different possible modes of transport of ions and other molecules across me plasma membrane