How do molecules diffuse?

Movement of solutes across a lipid bilayer plasma membrane can occur in many ways:

1. Osmosis: diffusion of water across membranes to balance solute concentrations.

- No energy req

- Water passes through membrane via aquaporins, pores in the membrane permeable only to water.

2. Simple diffusion: diffusion of only small, hydrophobic (nonpolar) molecules across a membrane down their concentration gradient.

- No energy req

- Only moves small nonpolar molecules down their concentration gradient

3. Facilitated diffusion (passive transport): uses transmembrane protein transporters (channels and carriers) to move polar molecules (ions) across a membrane

- No energy required

- Channel proteins: Allow for rapid diffusion of specific ions down their concentration gradient

- Can be activated by:

- Voltage

- Ligand

- Mechanical force

- Carrier proteins: Allow for selective diffusion of specific ions down their concentration gradient.

- Uniport carriers: only transport one specific ion down its concentration gradient

- Symport carriers: move two different types of ions in the same direction.

- Antiport carriers: moves two different ions in opposite directions

-Can use the energy created moving one molecule down its concentration gradient to move an ion against its concentration gradient

- Carrier proteins exhibit saturation, competition, and specificity.

4. Active transport: transport of solutes across a membrane against their concentration gradient.

- ATP req

- Uses carrier proteins

- Leads to creation of potential energy stored in electrochemical gradients