Across Biological Membranes

Review for Transport

1. What are the two parts of a solution?

2. In cells, what is normally the solvent?

3. How would you describe the activity of solute particles in solutions?

Passive Transport

Concentration Gradient• A difference in concentrations across a membrane

Simple Diffusion

Particles diffuse through lipid bi-layers to reach equilibrium

Mainly small, nonpolar

Facilitated Diffusion Membrane proteins carry materials

through the lipid bilayer.1. Particle binds to membrane protein.

2. Membrane protein changes shape.

3. Particle is released on the other side.

Channel Proteins Membrane protein opens a passage for

materials through the lipid bilayer.

Osmosis Diffusion of water

• When solute cannot cross membranes to reach equilibrium, the solvent (water) does

• Water moves from where it is purest (hi conc.) to where it is least pure (lo conc.)

H2

O

H2

O

H2

OH2

OH2

O

H2

O

H2

O

H2

OH2O

H2O

H2O

H2OH2OH2O

H2O

H2O

Transport Review What two things are always true of

passive transport? What are the examples of passive

transport we discussed? Why shouldn’t we drink salt water? How do we regulate the amount of

water (and concentration of solutes) in our bodies?

ADP + P

Pump Similar to facilitated diffusion except it’s AT

ATP

Exocytosis• Pre-packaged materials are released from

the cell.

Active Transport

Endocytosis• Cells engulf large materials (nutrients,

enemies) with cell membrane.

Active Transport

Endocytosis Cells engulf large materials (nutrients, enemies)

with cell membrane.

Active Transport

Review Questions1. What’s the difference between active

and passive transport?

2. In what direction will particles always diffuse?

3. What happens to a wall-less cell in a hypotonic solution?

4. Why might a cell want to build up a concentration gradient?