The plasma membrane is a phospholipid bilayer that isolates the cell's contents from the external environment and regulates what enters and exits the cell. It is composed of around 50% lipids and 50% proteins. The phospholipid bilayer is impermeable to water-soluble molecules but permeable to small nonpolar molecules. Passive transport mechanisms like simple diffusion, facilitated diffusion, and osmosis allow for movement of substances based on concentration gradients without energy input. Active transport mechanisms like the sodium-potassium pump and endocytosis/exocytosis require energy to move substances against gradients.