Objectives:Explain the process of diffusion.Describe how osmosis occurs.Compare active and passive transport.Explain how large particles get into and out of cells.

Exchange with the Environment

The exchange of materials between a cell and its environment takes place at the cell’s membrane.

What is Diffusion?Remember that particles of a

substance are always moving and colliding with each other. Particles naturally travel from areas where they are crowded to areas where they are less crowded.

What is Diffusion?• Diffusion is the movement of these

particles from area where their concentration is high to an area where their concentration is low.

• This movement can occur across the cell membrane or outside of cells, and it does not require energy.

IMPORTANT FACT:

Remember all organisms need water to live. Their cells are surrounded by and filled with fluids made mostly of water.

Osmosis is the diffusion of water through the cell membrane

(selectively permeable)

Moving Small Particles Many particles can diffuse directly

through the cell membrane. Others larger particles must travel through protein “doorways” in the cell membrane in order to enter or leave the cell.

Particles can travel through these proteins either by passive transport or by active transport.

Passive vs. ActivePassive Transport

The diffusion of particles through the proteins. The particles move from an area of high concentration to an area of low concentration.

The cell does not need to use energy to make this happen.

Active Transport The movement of

particles through proteins against the normal direction of diffusion. The particles move from an area of low concentration to an area of high concentration.

The cell uses energy to make this happen.

Moving Large Particles The cell membrane has two ways

of moving large particles: endocytosis and exocytosis.

Endocytosis Means inside the

cell The cell membrane

surrounds a particle and encloses it in a vesicle.

Exocytosis:means outside the cellWhen a large particle must be removed from the cell, the cell uses a different process.Vesicles are formed at the ER or Golgi complex and carry the particles to the cell membrane.