Transport Across Membrane Passive Transport Simple Diffusion - water, oxygen and other molecules move from areas of high concentration to areas of low concentration, down a concentration gradient Facilitation Diffusion - diffusion that is assisted by proteins (channel or carrier proteins) Osmosis - diffusion of water from a high concentration to low concentration. Salt Sucks!!!! Types of Solutions: 1. Isotonic: The amount of water (solvent) and dissolved substances (solutes) are the same. Cell is in balance 2. Hypotonic: Causes cell to swell because the concentration of water is greater outside the cell than in. This is because the solute concentration is greater in the cell than outside. 3. Hypertonic: Causes cell to shrink because the concentration of water is less inside the cell than outside. This is because the solute concentration is greater outside the cell than inside.