Cellular Membranes
A.
Membrane Structure
1.
Overview of Membrane Structure
A phospholipid bilayer with associated proteins
2.
Structure of the Phospholipid bilayer
Phospholipids are amphipathic, having both
hydrophobic regions and hydrophilic “polar
head groups”
Glycerol-based Phospholipids
 Glycerol Molecule
 Two Fatty Acid Chains
 Polar Head Group, attached via phosphate
 The fatty acid chains may be saturated or
unsaturated
 Double bonds in unsaturated fatty acids may
be cis or trans
Other Lipids that contribute to the lipid bilayer
are sphingosine-based lipids and cholesterol
Liposomes: Models for Studying Bilayer Structure.
 These are artificial bilayers, generally composed
of pure phospholipid and water.
 In aqueous suspension, amphipathic lipids
will spontaneously arrange a bilayer
structure, forming the boundary of a sphere
that has water on both its inside and outside.
3.
Membrane Proteins
 Peripheral Membrane Proteins
 Integral Membrane Proteins
4.
Properties of Membranes
a.
Fluidity
The lipid molecules and some proteins
have the ability to move laterally (side-toside) within the membrane. They do not
have the ability to randomly “flip-flop”
(transverse mobility)
The degree of fluidity is influenced by the
polar head group sizes, the fatty acid
composition (especially by the length of
the chains and the amount of unsaturation)
and by the presence of cholesterol.
b.
Compositional Asymmetry
The inner leaflet of a naturally occurring
biological membrane will have a different
composition than the outer leaflet.
Both the lipid composition and the protein
composition of the outer and inner
membranes are different.
Example:
Erythrocyte Membrane
(numbers are % of total lipid)
Outside Inside
Sphingomyelin
20%
5%
P. choline
25%
5%
P. ethanolamine 5%
25%
P. serine
0%
5%
c.
Semipermeability
Biological membranes are semipermeable,
permitting some substances to pass but
not others.
B.
Transport Processes across Membranes
1.
Diffusion and Osmosis
Diffusion: The movement of molecules due to
kinetic energy
Net Diffusion: The diffusion of a substance from
a region of high concentration to a region of low
concentration until an equilibrium is reached
Osmosis: The diffusion of water molecules
across a semipermeable membrane
2.
Simple Diffusion across a Membrane
 Movement of substances directly across a
phospholipid bilayer or through an open
protein channel
 Movement from high to low concentration
 No energy expenditure (e.g. ATP) from cell
3.
Facilitated Diffusion
 Movement of substances across a membrane
with the assistance of a carrier protein
 Movement from high > low concentration
 No energy expenditure (e.g. ATP) from cell
4.
Active Transport
 Movement of substances across a membrane
with the assistance of a transport protein
 Movement from low > high concentration
 Energy expenditure (e.g. ATP or ion
gradients) from cell
 Active transport pumps are usually carrier
proteins
5.
Endocytosis
 The ingestion of large particles or
macromolecules by the plasma membrane of
a cell through the formation of an endocytotic
vesicle
 The endocytotic vesicle is a membraneenclosed packet coated by a protein called
clathrin
 Typically mediated by cell-surface receptors
that recognize the target being engulfed
 “Phagocytosis” refers to the ingestion of
relatively large particles (for example,
bacteria by a red blood cell)
 “Pinocytosis” refers to the ingestion of
dissolved substances and fluid
6.
Exocytosis
 The process by which secretory vesicles
(from the Golgi apparatus) fuse with the
plasma membrane to secrete large particles
or macromolecules
 Secretory vesicles are also coated with
clathrin
 Exocytosis and Endocytosis provide for a
dynamic recycling of membrane components
C.
Functions of Membranes
 Defining the outside of the cell and the organelle
compartments (a barrier)
 Regulating the contents of the cell and the
organelles
 Processing information: Signal Transduction
 Processing Energy