Explain the difference between passive and active transport across the cell membrane.

Passive transport is the movement of substances across the cell membrane down their concentration gradient (from high to low concentration) without the expenditure of cellular energy (ATP). Examples include simple diffusion, facilitated diffusion, and osmosis. Active transport is the movement of substances against their concentration gradient (from low to high concentration), requiring the expenditure of cellular energy (ATP) and specific transport proteins. Examples include the sodium-potassium pump and endocytosis/exocytosis.