artificiallipid bilayer

High permeability

Low permeability

HYDROPHOBIC MOLECULES

O2

N2

SMALL UNCHARGEDPOLAR MOLECULES

H2OCO2UreaGlycerol

LARGE UNCHARGEDPOLAR MOLECULES

GlucoseSucrose

IONSNa+ K+

Cl- Ca2+

Mg2+ HCO3-

Specific membrane proteins areresponsible for transferring large

uncharged polar molecules and ionsacross cell membranes

PERMEABILITY OF THE LIPID BILAYER

Transport across the plasma membrane

1. Diffusion (passive)2. Osmosis (passive)3. Active transport (active)4. Endocytosis (in..e.g.phagocytosis) and

Exocytosis (out..e.g.secretion –lysosomes fusing with membrane and releasing contents to the outside of the cell)

Diffusion

• The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower.

• Dynamic equilibrium: the particles are still changing position but the concentration remains evenly distributed.

Rate of diffusion

• Concentration gradient• Area over which diffusion takes place• Thickness of exchange surface

Surface area x difference in concentration/ length of diffusion path

Facilitated diffusion (1)

• Protein channels• They are water filled so glucose and amino

acids (water soluble) can diffuse through.• Channels only open in the presence of specific

molecules so that there is control over the entry and exit of substances

Facilitated diffusion (2)

• Carrier proteins• When a particular molecule that is specific to

the protein is present, it binds with the protein. This causes it to change shape in such a way that the molecule is released to the inside of the membrane.