Diffusion Through Cell Boundaries Every living cell exists in a liquid environment that it needs to survive. One of the most important functions of the cell membrane is to regulate the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side.

What is diffusion? Diffusion is the movement of molecules from an area where they are highly concentrated to an area where they are less concentrated The molecules will keep moving until they are evenly distributed o Evenly distributed molecules are said to be in equilibrium

Where can diffusion occur? The most common place for diffusion to happen is in a solution Solutions are made of solvents and solutes o Solvents are usually liquid o Solutes are usually solid  Once the solvent and solute are evenly distributed, they result in a solution

Diffusion in Cells Diffusion does not require energy o It is known as passive transport Only certain things can move freely through the cell membrane o The membrane is like a screen  Small molecules and gases can move through the bi-layer  Larger molecules can not pass through

A Special Kind of Diffusion: Osmosis Osmosis is the diffusion of water across a semi- permeable membrane, like a cell membrane Osmosis occurs when there is more solute on one side of a membrane then there is on the other side o Water will moves because it fits through the membrane o The solute does not move because it is too big to fit through the membrane

Facilitated Diffusion Facilitated diffusion uses carrier proteins to bind to molecules on one side of the membrane The protein then changes its shape and transports the molecule onto the other side of the membrane Facilitated diffusion moves molecules with the concentration gradient

Facilitated Diffusion

Ion Channels Ion channels are proteins that provide small passageways across the cell membrane through which specific ions can diffuse