Chapter 7: Membrane Structure and Function
Selective permeability: Allows some substances to cross more easily than others
I. Membrane Structure
Amphipathic molecule: A molecule with both a hydrophilic and hydrophobic region
(phospholipid)
Fluid mosaic model: Membrane is a fluid structure with various proteins embedded in or
attached to a phospholipid bilayer
A. Membrane models
1. Charles Overton (1895): Membranes are made of lipids
a. Substances that dissolve in lipids enter cells faster than substances that are
insoluble in lipids
b. 20 years later: RBC membranes isolated and chemically analyzed
1.Composed of lipids and proteins
2. Irving Langmuir (1917): Made artificial membranes
a. Added phospholipids dissolved in benzene (organic solvent) to water
b. Benzene evaporated
1. Film of lipids covered surface of water
2. Hydrophilic heads immersed in water
3. Gorter and Grendel (1925): Phospholipid bilayers
a. Molecular arrangement shelters hydrophobic tails from water
b. Exposes hydrophilic heads
c. Measured phospholipid content of membranes isolated from RBC
1.Just enough to cover cells with 2 layers
4. Davson and Danielli (1935): Phospholipid bilayer between 2 layers of globular
proteins
a. Sandwich model
b. By end of 1960s: 2 problems
1. Not all membranes are alike
a. Differences in thickness and appearance
b. Different number of proteins
c. Differences in types of lipids and phospholipids
2.Hydrophobic parts of proteins exposed to aqueous environment
5.Singer and Nicolson: Fluid mosaic model
a. Placed proteins in a location compatible with their amphipathic character
b. Dispersed and individually inserted
c. Hydrophilic regions protruding
6.Freeze-fracture:
a. Phospholipid bi-layer is split
b. Halves viewed (EM)
c. Protein particles interspersed in a smooth matrix
B. Membranes are fluid
1. Membranes held together by hydrophobic attractions
2. Most lipids and some of the proteins drift laterally
a. Rare for molecules to flip-flop from one layer to the other
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1. Hydrophilic part of molecule would have to cross the hydrophobic
core
3. Phospholipids move rapidly (2 um/sec)
a. Proteins: Much larger, move more slowly
1. Some drift
a. Human cell fused to mouse cell (less than an hour)
2.Some move in a highly directed manner
a. Driven along cytoskeletal fibers by motor proteins
3. Some proteins are immobile: Attached to cytoskeleton
5. Membrane remains fluid as temp decreases
a. At some critical temp it solidifies 1.Phospholipids become tightly
packed
b. Critical temp depends types of lipids membrane is made of
1.Rich in phospholipids with unsaturated hydrocarbon tails
a. Remains fluid to lower temps
b. Kinks where double bonds are located
c. Do not pack together as closely as saturated hydrocarbons
6. Cholesterol is wedged between the phospholipid molecules in the plasma
membranes of animal cells
a. Different effects on membrane fluidity at different temps
1. 37oC : Makes membrane less fluid by restraining movement of
phospholipids
2. Lowers the temp required for membrane to solidify
7. Membranes must be fluid to work properly
a. When it solidifies: Permeability changes, enzymatic proteins may become
inactive
b. Some cells alter lipid composition as an adjustment to changing temp
1. Plants that tolerate extreme cold
a. Winter wheat
b. % of unsaturated phospholipids increases in autumn
c. Keeps membrane from solidifying in winter
C. Membranes as Mosaics of Structure and Function
1. Proteins determine membranes specific functions
2. Plasma membrane and membranes of organelles have their own unique proteins
3. 2 Major populations of proteins
a. Integral proteins: Penetrate hydrophobic core of lipid bilayer
1. Transmembrane proteins: Completely span membrane
2. Some are unilateral: Reach partway across membrane
b. Peripheral proteins: Not imbedded in bilayer
1. Appendages attached to the surface
2. Cytoplasmic side: Peripheral proteins and their integral partners
may be held in place by attaching to the cytoskeleton
4.Exterior: Membrane proteins are attached to fibers of the ECM
(Integrins)
4. Membranes are bifacial: Inside and outside faces
a. Each protein has directional orientation
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b. Carbohydrates restricted to exterior surface
c. Molecules that start on inside face of ER end up on outside face of plasma
membrane
d. Vesicle fusion with plasma membrane
1. Enlarges membranes, secretes products
2. Extra-cellular surface carbohydrates
1.Synthesized in ER
2.Modified in Golgi
D. Membrane Carbohydrates and Cell-Cell Recognition:
1. Sorting of cells into tissues and organs in an animal embryo
2. Basis for rejection of foreign cells by the immune system
3. Cells recognize each other by keying on surface molecules (often carbohydrates)
a. Branched oligosaccharides:
b. Some: Covalently bonded to lipids: Glycolipids
c. Most: Covalently bonded to a protein: Glycoprotein
1. Four blood groups
2. Variation in oligosaccharides on surfaces of RBCs
II. The Traffic of Small Molecules
A. Selective Permeability
1. Permeability of the Lipid Bilayer
a. Hydrophobic core impedes transport of ions and polar molecules
(Hydrophilic)
b. Hydrophobic molecules:
c. Hydrophobic core impedes transport of ions and polar molecules
1. Larger, uncharged polar molecules (glucose, other sugars)
2. All ions
3. H2O cannot pass through easily
2. Transport Proteins: A protein that spans the membrane providing hydrophilic
channel across the membrane, that is selective for a particular solute
a. Hydrophylic channel that certain molecules or atomic ions can tunnel
through
b. Some bind to their passengers and physically move them across the
membrane
into the cell
c. Specific for the substance it translocates
1. Glucose carried to liver by blood
a. Specific transport proteins
b. So selective: Rejects fructose
d. Diffusion determines the direction of the traffic
B. Passive Transport: Diffusion
1.Diffusion: The tendency for molecules of any substance to spread out into the
available space
a. KE: Thermal motion (heat)
b. Simple law: In the absence of other forces a substance will diffuse from
where it is more concentrated to where it is less concentrated
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c. Concentration gradient: Moving from high to low concentrations across
plasma membrane
1. Each substance diffuses down its own concentration gradient
a. Unaffected by concentrations of other substances
2. Example: Uptake of O2 by a cell during cellular respiration
a. Dissolved O2 diffuses into cell across plasma membrane
b. Cellular respiration consumes O2
c. Maintains concentration gradient in that direction
3. Passive Transport: Diffusion of a substance across a membrane
C. Osmosis; the passive transport of water
1. Hypertonic: Solution with higher concentration of solute
2. Hypotonic: Solution with lower solute concentration
a. Tap water is hypertonic to distilled water
3. Isotonic: Solutions of equal solute concentration
4. Osmosis: Diffusion of water across selectively permeable membrane
a. Sea water:
b. Isotonic solutions: Water moves across membrane at equal rates in both
directions
D. Balancing water uptake and loss
1. Water Balance of Cells without walls
a. Animal cell immersed in an environment that is isotonic to cell
1. No net movement across membrane
2. Volume is stable
b. Environment hypertonic to cell (salt)
1. Cell shrivels
c. Environment Hypotonic to cell
1. Cell bursts or lyses
d. No cell walls: Cannot tolerate excessive water uptake or loss of water
e. Isotonic environment
1. Seawater to many marine invertebrates
2. Extra-cellular fluid of terrestrial animals
f. Osmoregulation: The control of water balance
1. Paramecium:
a. Pond water is hypotonic to the cell
b. Plasma membrane that is much less permeable to water
than the membranes of most other cells
c. Contractile vacuole: Functions as a pump to force water
out of the cell as fast as it enters by osmosis
2. Water Balance of Cells with walls
a. Plant cell in hypotonic solution (rainwater)
1. Swells as water enters by osmosis
2. Wall expands until it exerts back pressure
3. Opposes further water uptake
4. Wall pressure exerts a force equal and opposite to the osmotic
pressure of the cell
a. Turgid: Very firm
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b. Mechanical support (non-woody)
c. Cells must be hypertonic to solution
d. If isototic: Water doesn’t enter
1. Flaccid: (limp)
2. Over fertilizing
b. Hypertonic environment: Cell wall is of no advantage
1. Plasmolysis: As plant cell shrivels; plasma membrane pulls away
from cell wall
E. Specific proteins facilitate passive transport of water and solutes
1. Facilitated diffusion: Diffusion with the help of transport proteins that span the
membrane
2. Transport protein: Many of the properties of an enzyme
a. Proteins are specialized for the solute it transports
1. Specific binding site
b. Transport proteins can be saturated
c. Can be inhibited by molecules that imitate
d. Do not catalyze chemical reactions
1. Catalyze the process of transporting molecules
3. Channel proteins: Provide corridors for specific molecule or ion to cross
1. Hydrophilic passage way
a. Water molecules, small ions
b. Aquaporins: Water channel protein
2. Gated channels: Stimulus causes them to open or close
a. Chemical:
4. Some helps molecule across by changing shape
1. Triggered by the binding and release of transported molecules
3. Inherited diseases: Specific transport systems are defective or missing
a. Cysitnuria: Absence of protein that carries cystine and other aa across
membranes of kidney cells
b. Reabsorb aa from urine and return them to blood
b. Painful stones accumulate and crystallize
F. Active Transport
1. Active transport: Moving a molecule against concentration gradient
a. Animal cells have a higher concentration of K+ than surroundings
b. Lower concentration of Na+
c. Pumps Na+ out and K+ IN
D. SPECIFIC PROTEINS, ATP SUPPLIES ENERGY
1. Transfers terminal phosphate group to transport protein
1. Induces conformation change
2. Translocates a solute across the membrane
3. Sodium-potassium pump: Transport system which exchanges
Na+ for K+ across animal cells
G. Ion Pumps Generate voltage (electrical PE) across membranes
1. Membrane potential: Voltage across membrane
1.-50 to –200 mvolts
2. Neg sign: Inside cell
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3. Favors passive transport of cations into the cell and anions out
2. 2 forces drive diffusion
1. Chemical force: Ion’s concentration gradient
2. Electrical force: Effect of membrane potential
3. Electrochemical gradient: Combination of chemical and electrical force
on movement of ions across membranes
4. Ex: Resting nerve cell
1. Na+ inside cell is much lower than outside
2. When cell is stimulated: Gated channels that facilitate Na+
diffusion open
+ +
f. Na K pump
1. Pumps 3 Na+ out and 2 K+ in
2. Electrogenic pump: Transport protein that generates voltage
across membrane
a. Stores energy
3.Main pump of animal cells
g. Proton pump: Main electrogenic pump of plants, bacteria and fungi
1. Actively transports H+ ions (protons) out
H. Cotransport: A single ATP powered pump that transports specific solute can indirectly
drive active transport of several other solutes
1. Protein couples downhill diffusion of one substance with uphill transport of a
second against its concentration gradient
a. Plant cell: Uses gradient of H+ to drive active transport of aa, sugars and
nutrients into cell
1. Transport protein couples return of H+ and transport of sucrose
a. Sucrose into cell against concentration gradient
b. Must travel with a H+
I. Exocytosis and endocytosis transport large molecules
1. Exocytosis: Cell secretes macromolecules by fusion of vesicles with plasma
membrane
2. Endocytosis: Cell takes in macromolecules and particulate matter by forming
new vesicles from plasma membrane
a. Small area of plasma membrane sinks inward to form pocket
b. Deepens, pinches; forming a vesicle
1.Contains material that had been outside cell
c. 3 types
1. Phagocytosis: (Cellular eating)
a. Cell engulfs particle by wrapping pseudopodia around it
b. Packaging it within a membrane-enclosed sac (food
vacuole)
c. Particle digested after food vacuole fuses with lysosome
1.Hydrolytic enzymes
2.Pinocytosis: (Cellular drinking)
a. Cell takes in droplets of extracellular fluid into tiny
vesicles
b. Dissolved solutes: Unspecific transport
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e. Receptor-mediated endocytosis: Specific endocytosis
1. Proteins imbedded in membrane exposed to extracellular fluid
2. Ligands: Any molecule that binds specifically to a receptor site of
another molecule
a. Ligare: to bind
3. Receptor proteins clustered in coated pits
a. Lined on cytoplasmic side with a fuzzy layer of protein
b. Help deepen pit and form vesicle
4. Enables cell to acquire bulk quantities of specific substances
a. Human cells take in cholesterol for synthesis of
membranes and precursor for synthesis of other steroids
1.Travels in blood in particles: Low-density
lipoproteins (LDLs)
a. Complexes of lipids and proteins
b. Bind to LDL receptors on membranes
1.Enter by endocytosis
2.Hypercholesterolemia: High level of cholesterol in
blood
a. Defective LDL receptor proteins
b. LDL particles cannot enter cells
c. Cholesterol accumulates in blood:
atherosclerosis
1.Build up of fats on blood vessel
linings
f. Exo and endocytosis occur continually
1.Amount of plasma membrane in non-growing cell remains
constant
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