Transport across cell membranes

Diffusion is a passive movement of particles - it requires no energy to occur. Diffusion is the net movement of particles from an area of high concentration to an area of low concentration down a concentration gradient Molecules can diffuse both way, but the majority of the time they diffuse into an area of lower concentration. The concentration gradient is the pathway from an area of higher concentration to an area of lower concentration down a concentration gradient.

The rate of diffusion depends on: 1.) The concentration gradient - the higher it is, the faster the rate of diffusion 2.) The thickness of exchange surfaces - the thinner the exchange surface, the shorter the diffusion pathway, and therefore the faster the rate of diffusion 3.) The surface area - the larger the surface area, for example, of a cell membrane, the faster the rate of diffusion

Osmosis is the net movement of free water molecules through a partially permeable membrane from a area of high water potential to an area of low water potential down a water potential gradient. Water potential is the likelihood of water molecules to diffuse out of or into a solution. Pure water has the highest water potential - everything else will have a negative water potential (lower water potential than pure water.) Water potential of a solution can be described in 3 ways - hypertonic solution, hypotonic solution and isotonic solution. 1.) Hypertonic solution - more solutes - less free water molecules - lower water potential - free water molecules move into the solution. 2.) Hypotonic solution - less solutes - more free water molecules - higher water potential - free water molecules move out of the solution. 3.) Isotonic solution - the net movement of free water molecules into and out of the solution is the same.