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Membrane Structure and Function
Chapter 7
• The plasma membrane selectively permeable
Overview: Life at the Edge
• Phospholipids most abundant lipids in plasma membrane
• amphipathic = hydrophobic and hydrophilic regions– Polar head– Hydrocarbon tails
Cell membranes
• Phospholipid
Hydrophilichead
WATER
Hydrophobictail
WATER
Membrane Models: Scientific Inquiry
• 1935Davson and Danielli - bilayer model
• 1972 Singer and Nicolson - fluid mosaic model– membrane mosaic of proteins dispersed within bilayer
• Current model: mosaicism
TECHNIQUE
Extracellularlayer
KnifeProteins Inside of extracellular layer
RESULTS
Inside of cytoplasmic layer
Cytoplasmic layerPlasma membrane
• Freeze-fracture splits membrane along middle of bilayer (lipid layer is weak)
proteins
Phospholipidbilayer
Hydrophobic regionsof protein
Hydrophilicregions of protein
Proteins embedded in bilayer
Phospholipids in membrane move laterally within bilayer
rarely flip flop• Lipids, proteins, may move laterally
The Fluidity of Membranes
(a) Movement of phospholipids
Lateral movement(107 times per second)
Flip-flop( once per month)
RESULTS
Membrane proteins
Mouse cellHuman cell
Hybrid cell
Mixed proteinsafter 1 hour
1. Type of phospholipid
Membrane fluidity affected by:
Fluid
Unsaturated hydrocarbontails with kinks
Viscous
Saturated hydro-carbon tails
2. Temperature
• cool gel– Tightly packed tails
• warm fluid
3. Cholesterol • Stabilizes membrane fluidity with changing
temperature
FYICholesterol can compose ½ of the membrane
Bacterial cell membranes do not contain cholesterol
Plant cells do not contain much
Cholesterol
(c) Cholesterol within the animal cell membrane
• Mosaic of proteins embedded in lipid bilayer• Proteins determine most of membrane’s
functions
Membrane Proteins and Their Functions
Membrane Proteins
Peripheral proteins – bound to _____________ of membrane
Integral proteins – penetrate hydrophobic region– Transmembrane proteins
N-terminus
C-terminus
• ex. Insulin receptor
1. Receptor proteins for signal transduction
• Hydrophilic core• Ex. Aquaporins
2. Channel proteins for passage of molecules hydrophilic core
Ex. Glucose transporter shuttles glucose across membrane
3. Transport proteins
(a) Transport (b) Enzymatic activity (c) Signal transduction
ATP
Enzymes
Signal transduction
Signaling molecule
Receptor
(d) Cell-cell recognition
Glyco-protein
(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)
• Glycoproteins – Carbohydrates attached to proteins
mucin
4. Cell-cell recognition
Ex. gap junctions allow passage of ions and small molecules from cell to cell
5. Intercellular joining proteins
6. Extracellular matrix proteins
7. Membrane enzymes
– Transport– Enzymatic activity– Signal transduction– Cell-cell recognition– Intercellular joining– Attachment to the cytoskeleton and extracellular matrix
(ECM)
Six major functions of membrane proteins:
• Plasma membrane regulates cell molecular traffic
Selective permeability
• Hydrophobic molecules dissolve in bilayer and pass through membrane rapidly– O2, CO2, NO, steroids, nanoparticles
O2
CO2
Permeability of Lipid Bilayer
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• U-M scientists used a manmade nanoparticle less than 5 nm. They are small enough to cross cell membranes. One nanometer equals one-billionth of a meter, which means it would take 100,000 nanometers lined up side-by-side to equal the diameter of a human hair.
• Folate molecules on the nanoparticle bind to receptors on cancer tumor cell membranes and the cell immediately internalizes it, because it thinks it's getting folate, which it needs to grow. Folate is a vitamin
• But while it's bringing folate across the cell membrane, the cell also draws in the methotrexate, a poison that will kill it.
Nanoparticles can cross cell membranes!
• Hydrophilic (polar) molecules do not cross easily– sugar, water, ions
molecules move randomly
A. Diffusion
• Molecules diffuse down their concentration gradient from high to lower concentration until equilibrium
Passive transport = no energy used
(b) Diffusion of two solutes
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
What molecules can diffuse across the cell membrane?
Answer: O2, CO2, urea
• B. Osmosis is diffusion of water across a selectively permeable membrane
• Water diffuses across membrane from region of higher water concentration to the region of lower water concentration until equilbrium
Lowerconcentrationof sugar)
H2O
Higher Concentrationof sugar
Selectivelypermeablemembrane
Same concentrationof sugar
Osmosis
• Tonicity =ability of solution to cause cell to gain or lose water
• Isotonic solution: Solute concentration same as in cell– no net water movement across membrane
• Hypertonic solution: Solute concentration greater than inside cell– cell loses water
• Hypotonic solution: Solute concentration is less than inside cell– cell gains water
Water Balance of Cells Without Walls
Hypotonic solution
(a) Animal cell
H2O
Lysed
H2O H2O
Normal
Isotonic solution
H2O
Shriveled
Hypertonic solution
Solution type? Isotonic, hypotonic, hypertonic?
• Osmoregulation= control of water balance
• Ex. Paramecium video – pond water is _____________ to the protista– contractile vacuole
How do cells deal with changing external water concentrations?
Filling vacuole 50 µm
(a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm.
Contracting vacuole
(b) When full, the vacuole and canals contract, expelling fluid from the cell.
• Isotonic solution no net movement of water into cell– flaccid (limp)
Water Balance in plants (cell wall)
• Hypotonic solution cell (vacuole) swells – cell wall opposes uptake turgid (firm)
• Hypertonic lose water; membrane pulls away from wall – plasmolysis (lethal)
Hypotonic solution
(b) Plant cell
H2O
Turgid (normal)
H2O H2O
Isotonic solution
Flaccid
H2O
Plasmolyzed
Hypertonic solution
C. Facilitated Diffusion: Passive Transport aided by proteins
• Channel proteins• Carrier proteins
What molecules use facilitated diffusion to cross membrane?
Answer: glucose, sodium ions, chloride ions, water
EXTRACELLULAR FLUID
Channel protein
(a) A channel protein
Solute CYTOPLASM
Solute Carrier protein
(b) A carrier protein
• Energy (ATP) required to move solutes against their gradients (from lower to higher conc. !)
Pumps are membrane proteins Ex. sodium-potassium pump (an enzyme)
FYI:All animalsNobel prize 1997 (Jens Skou)Uses 1/3 of cells total energy productionProvides driving force for other cell processes (secondary
transport, volume, gradients)
Active transport
EXTRACELLULAR
FLUID [Na+] high [K+] low
Na+
Na+
Na+ [Na+] low[K+] high CYTOPLASM
Cytoplasmic Na+ binds to
the sodium-potassium pump. 1
Examine the figure:
Na+ high outside cellK+ low
Na+ low inside cellK+ high
According to diffusion?
1. Cytoplasmic Na+ binds to pump
Na+ binding stimulatesphosphorylation by ATP.
Na+
Na+
Na+
ATP P
ADP
2
2. ADP phosphorylated to ATP
What is ATP?
Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside.
Na+
P
Na+ Na+
3 Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside.
Na+
P
Na+ Na+
3
3. Na+ out of cell
shape change ofpump
Against conc. grad.
K+ binds on theextracellular side andtriggers release of thephosphate group.
P P
K+
K+
4
4. Extracellular K+ binds to pump
ATP used
Shape change
K+
K+
5 + 6 K+ inside cell Pump animation
Step by step
Passive transport
Diffusion Facilitated diffusion
Active transport
ATP
Review the differencepassive vsactive transport
NO ATP IS USED
WHICH ONE REPRESENTSTHE CHANNEL PROTEIN?
a. Maintain membrane potential = voltage difference across membrane
Inside of cell more electronegative than out
= negative membrane potential
Why do cells need pumps?
EXTRACELLULARFLUID
H+
H+
H+
H+
Proton pump
+
+
+
H+
H+
+
+
H+
–
–
–
–
ATP
CYTOPLASM
–
– chemical = concentration gradient
– electrical = membrane potential
b. Maintain electrochemical gradients
• c. Cotransport
• Ex. Sucrose transporter sodium driven
Proton pump
–
–
–
–
–
–
+
+
+
+
+
+
ATP
H+
H+
H+
H+
H+
H+
H+
H+
Diffusionof H+
Sucrose-H+
cotransporter
Sucrose
Sucrose
• Exocytosis – To secrete products from cell– Vesicles fuse with membrane
Bulk transport
2. Endocytosis cell takes in macromolecules by forming vesicles from membrane
a. Phagocytosis – for large particle
Vesicle fuses with lysosome to digest particle
PHAGOCYTOSIS
CYTOPLASM EXTRACELLULARFLUID
Pseudopodium
“Food” orother particle
Foodvacuole Food vacuole
Bacterium
An amoeba engulfing a bacteriumvia phagocytosis (TEM)
Pseudopodiumof amoeba
1 µm
b. Pinocytosis – for fluids/small molecules
PINOCYTOSIS
Plasmamembrane
Vesicle
0.5 µm
Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)
RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor Coat protein
Coatedpit
Ligand
Coatedvesicle
c. receptor-mediated endocytosis, ligand binds to receptor vesicle