86 The movement of substances across cell membranes The plasma membrane has a dual responsibility as a selectively permeable barrier and transductor. The plasma membrane regulares the traffic of molecules into and out of the cell. Gases and small hydrophobic molecules by passive diffusion transport; Ions, sugars, amino acids, and sometimes water must be transported by a group of integral membrane proteins including channels, transporter and energy-powered on or molecule pumps§6 The movement of substances across cell membranes The plasma membrane has a dual responsibility as a selectively permeable barrier and transductor. The plasma membrane regulares the traffic of molecules into and out of the cell. Gases and small hydrophobic molecules by passive diffusion transport; Ions, sugars, amino acids, and sometimes water must be transported by a group of integral membrane proteins including channels, transporter, and energy-powered ion or molecule pumps