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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
2
Phospholipds are amphipathic molecules (contain both hydrophilic and hydrophobic parts)
Hydrophilic head
Hydrophobic tails
Hydrophobic interior is an impermeable barrier to passage of hydrophilic molecules, but not to hydrophobic molecules
~30 Å (3 nm)
~45 Å (4.5 nm)
Hydrophobic Interior
Phospholipids form Membrane Bilayers
Bilayer consisting of two inverted phospholipid
layers (leaflets)
Cholesterol has profound effects on membrane fluidity
Fig 7.8: Membrane Fluidity
(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)
Membrane Proteins can Move Laterally Within the Lipid Bilayer
Supports fluid-mosaic model of a dynamic membrane
structure
Membrane proteins labeled with
different color fluorescent dyes
Three Types of Membrane Proteins
Transmembrane domain
1. Integral membrane proteins (transmembrane proteins)
• span the bilayer • transmembrane domain has
hydrophobic surface • cytosolic and extracellular
domains have hydrophilic surfaces
Extracellular domain
Cytosolic domain
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
δ-
δ+ δ-
δ+
How do proteins cross lipid bilayer membranes?
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
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
EXTRACELLULAR SIDE
CYTOPLASMIC SIDE
Fig 7.12: Membrane Synthesis & Sidedness
Enzymes Signal
Receptor ATP
Transport Enzymatic activity Signal transduction
Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extra-cellular matrix (ECM)
Glyco- protein
Fig. 7.9: Functions of Membrane Proteins
Fibers of extracellular matrix (ECM) bind to some membrane proteins
Glyco- protein
Microfilaments of cytoskeleton linked to some membrane proteins
Cholesterol
Peripheral proteins
Integral protein
CYTOPLASMIC SIDE OF MEMBRANE
Glycolipid EXTRACELLULAR SIDE OF MEMBRANE
Carbohydrate
Fig 7.7: Overview of the Plasma Membrane
Fig. 7.22: Endocytosis
Solutes
Pseudopodium
“Food” or���other particle
Food ���vacuole
CYTOPLASM
Plasma���membrane
Vesicle
Receptor Ligand
Coat proteins
Coated ���pit
Coated ���vesicle
EXTRACELLULAR ���FLUID
Phagocytosis Pinocytosis Receptor-Mediated Endocytosis
Membrane Transport
Why do molecules diffuse? A difference in concentration contains chemical potential energy. Molecules diffuse to
try to equalize concentrations.
Energetics of Diffusion
[A] [A]
[A] [A]
Molecules of dye Membrane (cross section)
WATER
Net diffusion Net diffusion Equilibrium
(a) Diffusion of one solute
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
(b) Diffusion of two solutes
Fig. 7.13: Diffusion of Solutes Across A Membrane
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.
Water Movement
Membrane permeable to water but impermeable to solute
Fig. 7.15: Water Balance of Living Cells
Hypotonic solution
Osmosis
Isotonic solution
Hypertonic solution
(a) Animal cell
(b) Plant cell
H2O H2O H2O H2O
H2O H2O H2O H2O Cell wall
Lysed Normal Shriveled
Turgid (normal) Flaccid Plasmolyzed
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