Osmosis, diffusion, and active transport across the phospholipid bilayer
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Diffusion: Solute molecules move from high → low concentration (passive, no ATP). Osmosis: Water moves from low solute → high solute across a semi-permeable membrane. Active transport: Molecules pumped against their gradient using ATP energy (e.g., Na⁺/K⁺ pump). The membrane's phospholipid bilayer selectively controls what passes through.