The cell membrane, or phospholipid bilayer, encompasses the contents of a cell and allows for organelles to be contained within the cytoplasm. The cell membrane is semi-permeable, thus is allows the movement of certain molecules and ions in and out of the cell by means of simple diffusion, facilitated diffusion (through integral proteins), osmosis for maintaining an isotonic water pressure, or active transport through pump proteins. The phospholipid bilayer is comprised of hydrophilic phosphate heads and hydrophobic fatty acid tails. Integrated within the bilayer, cholesterol is a key molecule that allows the membrane to maintain its fluidity and rigidity.
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