A. Membrane Functions
Biological Membranes are composed of…
Membrane
Lipid Protein
Myelin Sheath
80% 20%
Plasma Membrane 50% 50%
Mitochondrial
Inner Membrane
25% 75%
Fig. 5.12:
Phospholipids
Hydrophilic
head
2
Hydrophobic
tails
Phospholipds are amphipathic molecules (contain both
hydrophilic and hydrophobic parts)
Phospholipids form Membrane Bilayers
Bilayer consisting of
two inverted phospholipid
layers (leaflets)
Hydrophobic
Interior
~30 Å
(3 nm)
~45 Å
(4.5 nm)
Hydrophobic interior is an impermeable barrier to passage of
hydrophilic molecules, but not to hydrophobic molecules
Cholesterol has profound effects on membrane fluidity
Fig 7.8: Membrane
(a) Phospholipid molecules move
side-to-side within leaflet easily
(lateral diffusion) but do not
“flip-flop” across bilayer
(transverse diffusion)
(b) Phospholipids containing
unsaturated acyl chains increase
membrane fluidity by reducing
packing efficiency
(c) Cholesterol reduces membrane
fluidity at normal temperatures
(reduces phospholipid movement)
At low temperatures it keeps
membrane fluid (disrupts packing)
Fluidity
Membrane Proteins can Move Laterally
Within the Lipid Bilayer
Membrane proteins
labeled with
different color
fluorescent dyes
Supports fluid-mosaic model
of a dynamic membrane
structure
Three Types of Membrane Proteins
1. Integral membrane proteins
(transmembrane proteins)
•  span the bilayer
•  transmembrane domain has
hydrophobic surface
•  cytosolic and extracellular
domains have hydrophilic surfaces
2.  Lipid-anchored membrane proteins
- anchored via a covalently attached lipid
3.  Peripheral membrane proteins
- interact with hydrophilic lipid
head groups
or with integral membrane proteins
Extracellular
domain
Transmembrane
domain
Cytosolic
domain
How do proteins cross lipid bilayer membranes?
δδ+
δ-
δ+
Even if the R-groups are hydrophobic, the peptide bond
atoms are hydrophilic (polar) and will want to form Hydrogen
Bonds; there are no H-bond donors or acceptors in the middle
of a lipid bilayer.
Fig 7.9:
α-Helices Are Commonly Found in Membrane Proteins
EXTRACELLULAR
SIDE
Polar peptide
bond atoms
H-bond with
each other.
α-helix of 20
amino acids is
long enough to
cross the
bilayer.
N-terminus
α helix
C-terminus
CYTOPLASMIC
SIDE
Fig 7.12: Membrane
Synthesis & Sidedness
Fig. 7.9:
Functions of Membrane Proteins
Enzymes
Signal
Receptor
ATP
Transport
Enzymatic activity
Signal transduction
Glycoprotein
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton
and extra-cellular matrix (ECM)
Fig 7.7: Overview
of the Plasma Membrane
Fibers of extracellular matrix (ECM)
bind to some membrane proteins
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments of cytoskeleton
linked to some membrane proteins
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
Fig. 7.22: Endocytosis
Phagocytosis
Pinocytosis
Receptor-Mediated Endocytosis
EXTRACELLULAR
FLUID
Solutes
Pseudopodium
Receptor
Ligand
Plasma
membrane
Coat proteins
Coated
pit
“Food” or
other particle
Coated
vesicle
Vesicle
Food
vacuole
CYTOPLASM
Membrane Transport
Energetics of Diffusion
Why do molecules diffuse? A difference in concentration
contains chemical potential energy. Molecules diffuse to
try to equalize concentrations.
[A]
[A]
[A]
[A]
Fig. 7.13: Diffusion
of Solutes Across A Membrane
Membrane (cross
Molecules of dye
section)
WATER
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Equilibrium
(a) Diffusion of one solute
Net diffusion
Net diffusion
(b) Diffusion of two solutes
Net diffusion
Equilibrium
Water Movement
If a membrane is permeable to water but impermeable to a solute with different
concentrations in two compartments, water will move to try to equalize the
concentrations on the two sides of the membrane.
Membrane permeable to water but
impermeable to solute
Fig. 7.15:
Water Balance of Living Cells
Hypotonic
solution
Isotonic
solution
Hypertonic
solution
(a) Animal cell
H 2O
Lysed
(b) Plant cell
H2O Cell wall
Turgid (normal)
H 2O
H 2O
Normal
H 2O
Flaccid
Osmosis
H 2O
Shriveled
H 2O
H 2O
Plasmolyzed