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.
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