The CM determines what gets in and out of cells.

Some molecules pass through easily including lipids that dissolve in the lipid layer of the CM.

Small molecules such as water, glucose and amino acids also pass through easily.

Some molecules do not pass through easily including large molecules (starch, protein) and those that carry charges (attract to opposite charge on CM).

How cells move substances across the cell membrane involves a number of different processes.

Molecules of liquids and gases are always in motion. These molecules collide with each other,and thus undergo diffusion.

Diffusion: Movement of molecules from an area of high concentration, to one of low concentration(Ex. Perfume,coffee)

The difference in concentration between a region of greater concentration and a region of lesser concentration is called a concentration gradient (shown below).

Low O2 level

High CO2 level

High O2 level

Low CO2 level

Movement of gases in cellular respirationinvolves concentration gradients.Note the directionof movement from high to low.

Click here for animation.

This is a “fast” form of diffusion. Molecules get some help crossing the

membrane. Evidence shows that there are proteins in

the cell membrane with hollow channels. Molecules pass through these channels

along a concentration gradient.

Animation

This is another way to move materials into and out of cells.

In this case the cell must use energy to move substances against a concentration gradient.

This allows the cell to maintain internal conditions that are chemically different from the external environment.

Animation

Osmosis is a type of diffusion. In this case, water moves across a

semipermeable membrane from an area of high water concentration to one of low water concentration.

Animation

Name Type of Transport

Direction of

Movement

Conditions Examples

Diffusion  Passive

towards lower

concentration

Concentration gradient

Water, gases (02and CO2), and steroid hormones.

Facilitated Diffusion Passive

towards lower

concentration

Concentration gradient, plus channel or carrier proteins

Water, glucose, and amino acids.

Osmosis passivetowards lower

concentration

Concentration gradient, channel proteins optional

Water 0nly.

Active Transport active

towards higher

concentration

Carrier protein and ATP energy

Ions, sugars, and amino acids.