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 distributedo 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 soluteso Solvents are usually liquido Solutes are usually solid

Once the solvent and solute are evenly distributed, they result in a solution

Diffusion in Cells

• Diffusion does not require energyo It is known as passive transport

• Only certain things can move freely through the cell membraneo 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 sideo Water will moves because it fits through the

membraneo 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