Membrane Structure and Dynamics
Membrane functions - physical barrier from entry and exit form cell
and organelles
Membranes - Li pid, protein and carbohydrate
Membrane
% Protein
% Lipid
Plasma membrane
46
54
Mitochondria
76
24
% Carbohydrate
2-4
1-2
What are membranes - Lipid bilayers with proteins imbedded or
associated on either side of the membrane
Ions and polar molecules basically impermeable to membrane -
energy costs too high
Membrane components • 60 to 70% of mammalian lipids are phospholipids
• Bacteria have almost no PC and are mostly PE
• Neuronal tissue (myelin) PI > PC
Lipid
P-Choline
P-Ethanolamine
P-Insositol
P-Serine
Sphingosine
plasma membrane
35
19
7
9
18
golgi
45
17
9
4
12
mito
50
23
13
5
3
• Alterations in lipid composition - permeability, fluidity,
exocytosis, neural transmission and signaling potential
nuclei
62
23
9
4
3
• Membrane Asymmetry
– P-ethanolamine and P-serine predominately faces inside of
cell
– P-choline faces outside of membrane and inside of
organelles
– carbohydrates of glycoproteins facing outside
• During apoptosis there is a re-arraignment of lipids where
phosphatidyl serine moves to the exterior face of the membrane.
One of the key signals of cell death
• Membrane Fluidity - Singer and Nickolson fluid mosaic model
- allows for dynamic nature of membrane
- little transition of lipids can take place without specific enzymes
to mediate transfer - flipase
• Proteins - Add function and structure to membrane
• Extrinsic proteins (peripheral)
– Loosely attached to membrane
– ionic bonds with polar head groups and carbohydrates
– hydrophobic bonds with lipid
– proteins have lipids tails
– easily displaced from membrane
– salt, pH, sonication
Transmembrane portion often a helix

takes about 20 aa to cross membrane

many proteins cross many times
 odd # of transmembrane regions, why
 -COOH terminal usually cytosolic
+
 -NH
3 terminal extracellular

can be predicted by amino acid sequence

high % of side chains will be hydrophobic

Hydropathy scale used to predict
 free energy change - from organic to water
 long regions unusual in soluble proteins
• Non membrane sections often modified
 lipid, carbohydrate
Intrinsic proteins - tightly bound to membrane - span both sides
Protein has both polar and hydrophobic sections removed only through
disrupting membrane with detergents
detergents disrupt lipid bilayer and incorporate proteins and some lipids into
detergent micelles
 allows for purification of membrane proteins
 reconstitute into specific vesicles for study
Membrane associated proteins
N or C terminal modifications
Tightly associates protein to membrane
Isoprenylated at C Terminus
-Geranylgeranyl and farnesyl groups - from cholesterol
biosynthesis
- Lovastatin inhibits post-translational modification deterimined for Ras and pancreatic cancer.
-CAAX box
- C = Cys A = aliphatic and X = various
Last 4 aas are removed and new C-term is esterified with isoprenyl
Other fatty acids can be modified at N terminus - Modification
on amine or other amino acid residues
- Myristoylation or Palmitoylation - usually occurs on Cys
residues - highly reversible
Permeability - charged substances do not
cross without help
 measured by ability of small molecules
to cross membranes
• Synthetic lipid vesicles formed by sonication
• Measure trapped ions that cross back out into
solution
• Only charged molecule that can cross easily is
water
• Movement slowed by transport though two
environments
• Shed layers of hydration
Summary of membrane
transport
• Three types of membrane transporters enhance the movement of
solutes across plant cell membranes
– Channels – passive transport
– Carriers – passive transport
– Pumps- active transport
Channels
• Transmembrane proteins that
work as selective pores
– Transport through these passive
• The size of the pore determines
its transport specifity
• Movement down the gradient in
electrochemical potential
• Unidirectional
• Very fast transport
• Limited to ions and water
Channels
• Sometimes channel transport
involves transient binding of the
solute to the channel protein
• Channel proteins have structures
called gates.
– Open and close pore in response
to signals
• Light
• Hormone binding
• Only potassium can diffuse
either inward or outward
– All others must be expelled by
active transport.
Remember the aquaporin
channel protein?
• There is some diffusion of
water directly across the bilipid membrane.
• Aquaporins: Integral
membrane proteins that form
water selective channels –
allows water to diffuse faster
– Facilitates water movement in
plants
• Alters the rate of water flow
across the plant cell
membrane – NOT direction
Carriers
• Do not have pores that extend
completely across membrane
• Substance being transported is
initially bound to a specific site
on the carrier protein
– Carriers are specialized to carry a
specific organic compound
• Binding of a molecule causes the
carrier protein to change shape
– This exposes the molecule to the
solution on the other side of the
membrane
• Transport complete after dissociation of
molecule and carrier protein
• Moderate speed
Carriers
– Slower than in a channel
• Binding to carrier protein is like
enzyme binding site action
• Can be either active or passive
• Passive action is sometimes
called facilitated diffusion
• Unidirectional
Active transport
• To carry out active transport:
– The membrane transporter must couple the uphill
transport of a molecule with an energy releasing event
• This is called Primary active transport
– Energy source can be
• The electron transport chain of mitochondria
• The electron transport chain of chloroplasts
• Absorption of light by the membrane transporter
• Such membrane transporters are called PUMPS
Primary active transportPumps
• Movement against the
electrochemical gradient
• Unidirectional
• Very slow
• Significant interaction with
solute
• Direct energy expenditure
pump-mediated transport against the
gradient (secondary active transport)
• Involves the coupling of the
uphill transport of a
molecule with the downhill
transport of another
• (A) the initial conformation
allows a proton from outside
to bind to pump protein
• (B) Proton binding alters the
shape of the protein to allow
the molecule [S] to bind
pump-mediated transport against the
gradient (secondary active transport)
• (C) The binding of the
molecule [S] again alters
the shape of the pump
protein. This exposes the
both binding sites, and the
proton and molecule [S] to
the inside of the cell
• (D) This release restores
borh pump proteins to their
original conformation and
the cycle begins again
pump-mediated transport against the
gradient (secondary active transport)
• Two types:
• (A) Symport:
– Both substances move in the
same direction across
membrane
• (B) Antiport:
– Coupled transport in which the
downhill movement of a
proton drives the active
(uphill) movement of a
molecule
– In both cases this is against
the concentration gradient of
the molecule (active)
pump-mediated transport against the
gradient (secondary active transport)
• The proton gradient required for secondary active
transport is provided by the activity of the
electrogenic pumps
• Membrane potential contributes to secondary active
transport
• Passive transport with respect to H+ (proton)
The end