Cell TransportChapter 4

Framing Question:How does a cell transport

materials across the cell membrane?

Passive TransportMovement of molecules

WITHOUT the use of energy by the cell.◦Diffusion◦Facilitated diffusion◦Osmosis

Diffusion

Molecules constantly move randomly.

Over time, molecules move from areas of high concentration to areas of low concentration.

Dynamic EquilibriumEventually the molecules are

evenly dispersed. This is known as equilibrium.

THE MOLECULES CONTINUE TO MOVE.

Diffusion Across MembranesParticles will move across membranes if there is a concentration difference.

Dynamic EquilibriumParticles continue to move across

the membrane in both directions even after equilibrium is reached. However, there is no net change in concentration.

PARTICLES STILL MOVE.

Checkpoint:How will the molecules move in

the following diagram?

Question: Why can’t the cell rely on diffusion for transport?

Facilitated Diffusion

Molecules that are small and uncharged can easily move across the membrane.◦E.g. oxygen, carbon dioxide

Facilitated Diffusion

Some molecules that cannot pass directly through the membrane can pass through using protein channels.

This is known as facilitated diffusion.

Facilitated Diffusion Moviehttps://

www.youtube.com/watch?v=vKGN_Zhz8AY

Two kinds of facilitated diffusion

Ion channels

◦This is a special kind of protein that has a polar pore in the middle

◦Ions have a charge, so they will be attracted to the polar pore and can diffuse through it.

Carrier ProteinsSome molecules are just too big

to get through the semi-solid membrane.

A protein carrier will help transport each across

Biggest example – GLUCOSE.

Checkpoint:How do diffusion and facilitated

diffusion differ?