<P> The lipid bilayer ( or phospholipid bilayer ) is a thin polar membrane made of two layers of lipid molecules . These membranes are flat sheets that form a continuous barrier around all cells . The cell membranes of almost all organisms and many viruses are made of a lipid bilayer , as are the nuclear membrane surrounding the cell nucleus , and other membranes surrounding sub-cellular structures . The lipid bilayer is the barrier that keeps ions , proteins and other molecules where they are needed and prevents them from diffusing into areas where they should not be . Lipid bilayers are ideally suited to this role , even though they are only a few nanometers in width , they are impermeable to most water - soluble ( hydrophilic ) molecules . Bilayers are particularly impermeable to ions , which allows cells to regulate salt concentrations and pH by transporting ions across their membranes using proteins called ion pumps . </P>
<P> The lipid bilayer ( or phospholipid bilayer ) is a thin polar membrane made of two layers of lipid molecules . These membranes are flat sheets that form a continuous barrier around all cells . The cell membranes of almost all organisms and many viruses are made of a lipid bilayer , as are the nuclear membrane surrounding the cell nucleus , and other membranes surrounding sub-cellular structures . The lipid bilayer is the barrier that keeps ions , proteins and other molecules where they are needed and prevents them from diffusing into areas where they should not be . Lipid bilayers are ideally suited to this role , even though they are only a few nanometers in width , they are impermeable to most water - soluble ( hydrophilic ) molecules . Bilayers are particularly impermeable to ions , which allows cells to regulate salt concentrations and pH by transporting ions across their membranes using proteins called ion pumps . </P>
what is composed of two layers of phospholipids associated with proteins
what is composed of two layers of phospholipids associated with proteins
a thin polar membrane
a thin polar membrane