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Membrane Structure and
Function
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Membrane Composition Membrane fluidity
Fatty acids, cholesterol Membrane Proteins Membrane Permeability
Diffusion, osmosis, tonicity, osmoregulation Active transport, membrane potential
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Boundary that separates living cell from itsenvironment
Selective permeability- allowing some substances to
cross it more easily than others
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Composed of lipids,proteins, carbohydrates
Phospholipids- most abundantlipid in the plasma membrane
Amphipathicmolecules-
containing hydrophobic &hydrophilic regions
Hydrophilic
Hydrophilic
Hydrophobic
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Membrane is a fluid structure with a mosaic ofvarious proteins embedded in it
Hydrophilic region
of protein
Hydrophobic region of protein
Phospholipidbilayer
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Phospholipids in the plasma membrane can move within
the bilayer (fluid as salad oil)
Held in place by hydrophobic interactions (weak)
Lipids, (some proteins), drift laterally Rarely- molecule flip-flop across the membrane
Lateral movement
(~107times per second)
Flip-flop
(~ once per month)
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As temperatures cool, membranes switch from afluid state to a solid state
Temperature where membrane solidifies dependson types of lipids (fatty acids)
Membranes rich in unsaturated fatty acids aremore fluid at lower temperature than saturatedfatty acids
*IMPORTANT CONCEPT!
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ViscousFluid
Unsaturated hydrocarbontails with kinks
Saturated hydro-carbon tails
Cannot pack together
due to double bondsbetween carbons in the
hydrocarbon chain
No double bonds
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Cholesterol (steroid) - effects membrane fluiditydepending on the temperature
Warm temperatures- restrains movement ofphospholipids, making them less fluid
Cool temperatures- maintains fluidity bypreventing tight packing Lowers the temperature required for a membrane to
solidify
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Cholesterol
Cholesterol within the animal cell membrane
Fluidity must be maintained to work properly- balance
-too fluid or not fluid enough will not function
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Membrane Proteins
Integral or peripheral
Functions
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Proteins in the plasma membrane can move Different types of cells have different types of
proteins in their plasma membrane
Integral protein- penetrate the hydrophobic core of thelipid bilayer
Transmembrane protein- spans the entire membrane
Peripheral protein- not embedded in the membrane,bound to it
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(type of integral protein)
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EXTRACELLULARSIDEN-terminus
C-terminusCYTOPLASMIC
SIDEHelix
Integral protein- hydrophobic regions consist ofnonpolar AAs, often coiled into a-helices
Transmembrane Integral Protein
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Transport
Enzymatic activity
Signal transduction Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and ECM
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ATP
Enzymes
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Signal
Receptor
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Cells recognize each other by binding to surfacemolecules
Recognize self vs non-self (pathogens, transplant tissue)
Surface molecules usually carbohydrates Covalently bonded to lipids- glycolipids
Covalently bonded to proteins- glycoproteins
Markers on your blood cells
Type A, B, AB or O
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Permeability of lipid bilayer
Polar vs Nonpolar
Transport proteins
Types of transport (sec 4)
Diffusion Facilitated diffusion
Active transport
Bulk transport
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Hydrophobic (nonpolar) molecules- dissolve in thelipid bilayer & pass through the membrane rapidly
O2, CO2, hydrocarbons
Hydrophilic (polar) molecules- do not cross the
membrane easily Sugars (glucose), ions, water
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Allow passage of hydrophilic substances- veryspecific
Channel proteins- hydrophilic channel that specificmolecules or ions can use as a tunnel
Aquaporins- facilitate the passage of water
Carrier proteins- bind to molecules and changeshape to shuttle them across the membrane
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ATP
Channel
Protein CarrierProtein
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Types of transport
Passive (diffusion): high to low
Channel proteins Aquaporin or ion channel
Carrier proteins
Active: low to high ONLY carrier proteins
Hydrophobic
substances diffuse
right across
membrane. Dontneed transport
proteins.
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Thermal motion (heat)- movement of molecules
Diffusion- molecules spread out evenly into the
available space
Each molecule moves randomly (Brownian Motion)
All the of molecules- movement in one direction
Molecules move HIGH concentration to LOW concentration
Equally spread throughout
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Heat: increase heat, increase molecular collisions,increase diffusion rate
Direct relationship
Size: smaller molecules move faster than largermolecules, smaller molecule will diffuse faster
Inverse relationship
Concentration gradient: steeper gradient, fasterdiffusion
Direct relationship
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Molecules of dye Membrane
WATER
Net diffusion Net diffusion Equilibrium
Diffusion
Dye is diffusing down its own concentration gradient.
NO work- no energy is added to the system.
Spontaneous process.
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Concentration Gradient- the difference in concentration of
a substance from one area to another
Moves HIGH to LOW
Passive transport- requires no energy from the cell tomake it happen
Only happens in a cell if the substance can readily cross
the plasma membrane
Water can diffuse across the cell membrane through
aquaporins (hydrophilic channels)
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Diffusionrefers to the movement of
SOLUTE (molecules) down its OWN
concentration gradient
Do NOT confuse this with osmosis- thediffusion of WATER (solution)
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Osmosis-diffusion of water across a selectivelypermeable membrane
Water is diffusing on its OWN concentrationgradient From a high to low concentration
Water diffuses across a membrane- from an area oflow solute concentration (high water) to an area ofhigh solute concentration (low water)
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Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Selectively
permeable mem-
brane: sugar mole-
cules cannot pass
through pores, but
water molecules can
H2O
Osmosis- diffusion
of water (not
solute- sugar)
High
H2O Low
H2O
Solutes Suck
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Ability of a solutionto cause a cell to gain or lose water
Isotonic solution: solute concentration is the same as that
inside the cell; no net water movement across the plasmamembrane
Hypertonic solution: solute concentration is greater than
that inside the cell; cell loses water (cell shrivel up)
Hypotonic solution: solute concentration is less than that
inside the cell; cell gains water (cell burst)
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To maintain their internal environment-osmoregulate
Control the balance of water within themselvesand the outside environment
Paramecium lives in a hypotonic environment(pond water)
Contractile vacuole that acts as a pump
50
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Filling vacuole50 m
50 mContracting vacuole
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Hypotonicenvironment- plantcell swells, cell wallexerts pressure, opposes uptake- turgid (firm)
Isotonic environment- no net movement of water;flaccid (wilt)
Hypertonicenvironment- plant cells lose water Plasmolysis- plasma membrane pulls away from the wall
***Plants, bacteria, fungi***
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Tonicity- ability of a solutionto cause a cell togain or lose water
Osmoregulation- animals/plants control thebalance of water within themselves and theoutside environment
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Animalcell
Lysed
H2O H2O H2O
Normal
Hypotonic solution Isotonic solution Hypertonic solution
H2O
Shriveled
H2OH2OH2OH2OPlantcell
Turgid (normal) Flaccid Plasmolyzed
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Transport proteins speed movement of polar moleculesacross the plasma membrane Very specific, water or small ions
Channel proteins- provide corridors that allow a specificmolecule or ion to cross the membrane
Ion channels- gated channels, stimulus causes them to
open/close Stimulus- chemical or electrical
Chemical- not the substance that will be transported
Neurotransmitter can open Na+ ion channels
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Carrier proteins undergo change in shape thattranslocates the solute-binding site across themembrane EXTRACELLULAR
FLUID
Channel protein SoluteCYTOPLASM
Carrier protein Solute
Passive diffusion-
Moving down its
concentration gradient
(high to low)
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Moving solutes from LOWconcentration to aHIGHconcentration
Active transport- requires energy, usually ATP
Performed by carrier proteins only Channels are just open passage ways
Sodium-potassium pump- higher concentration ofK+ and lower concentration of Na+ inside the cellcompared to environment
EXTRACELLULAR [Na+] high Na+
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Cytoplasmic Na+bonds to
the sodium-potassium pump
CYTOPLASMNa+
[Na+] low
[K+] high
Na+
Na+
FLUID[Na ] high
[K+] low
Na+
Na+
Na+
ATP
ADP
P
Na+binding stimulates
phosphorylation by ATP.
Na
Na+
Na+
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
P
Extracellular K+binds
to the protein, triggering
release of the phosphategroup.
P
P
Loss of the phosphate
restores the proteins
original conformation.
K+is released and Na+
sites are receptive again;
the cycle repeats.
add a phosphate group
Passive transport Active transport
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Diffusion Facilitated diffusion
Passive transport
ATP
Active transport
Nonpolar PolarPolar
hydrophobic hydrophilic hydrophilic
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All cells have voltage across their plasma membranes
Voltage-electrical potential energy, separation of opposite
charges
Cytoplasm has a negativecharge compared to
extracellular fluid
Unequal distribution of anions (- ions) & cations (+ ions)
Membrane potential- voltage difference across a
membrane
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Range -50 to -200 mV Minus sign indicated negative inside the cell compared to
outside
Acts like a battery- energy source that affects thetrafficking of all charged substances across themembrane
Inside cell is negative- membrane potential favorsthe passive transport of cations (+) in to the cell &anions (-)outof the cell
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Two combined forces drive the diffusion of ionsacross a membrane: Ions concentration gradient- chemical force
Effect of the membrane potential on the ionsmovement-electrical force
Passive diffusion- ion moves down itselectrochemical gradient Includes concentration, as well as charge across
membrane
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Resting nerve cell: [Na+] lower inside cellthan out
Stimulated- gated Na+ channels open up Na+ ions fall down electrochemical gradient
1. Driven by concentrationgradient of Na+
2. Driven by the attraction of cations to thenegative interior of the cell (charge)
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Membrane proteins contribute to membranepotential (difference of voltage across the membrane)
Sodium-potassium pump- pumps 3 Na+ ions outfor every 2 K+ ions pumps in
Net transfer of +1 charge to extracellular environment
Stores energy in form of voltage
Electrogenic pump - transport protein thatgenerates the voltage across a membrane
El t i P t
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H+
ATP
CYTOPLASM
EXTRACELLULAR
FLUID
Proton pump
H+
H+
H+
H+
H+
+
+
+
+
+
Electrogenic Pump- proton pump
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H+
ATP
Proton pump
Sucrose-H+
cotransporter
Diffusion
of H+
Sucrose
H+
H+
H+
H+
H+H+
+
+
+
+
+
+
Downhilldiffusion of
H+ coupled
to uphill
transport ofsucrose
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Large molecules cross the membrane via vesicles
Exocytosis-transport vesicles migrate to
membrane, fuse with it, & release their contents Many secretory cells use exocytosis to export their
products
Endocytosis-cell takes in macromolecules byforming vesicles from the plasma membrane
Reversal of exocytosis, involving different proteins
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Phagocytosis (eating)- cell engulfs particles in avacuole
Pinocytosis (drinking)- cell creates vesicle aroundfluid
Receptor-mediated endocytosis- binding ofligands to receptors triggers vesicle formation
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Receptor
RECEPTOR-MEDIATED ENDOCYTOSIS
Ligand
Coated
pit
Coated
vesicle
Coat protein
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1. Passive- NO energy High to LowA. Diffusion across membrane
Small or hydrophobic (nonpolar) molecules only
B. Facilitated diffusion (through a transportprotein)
1. Channel protein- specific, some gated2. Carrier protein- specific
2. Active- ENERGY (ATP!) Low to HighA. Molecule must go through transport protein
CARRIER proteins only!
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Pump- Active ATP- Active
Diffusion- Passive
Explain the concept of voltage, electrochemical
gradient, membrane potential in your ownwords.
A. Plasma membrane content
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1. Phospholipids
2. Proteins
B. Membrane Fluidity
1. Unsaturated/saturated fats- cold temp
2. Cholesterol- cold/hot temp
C. Membrane Proteins
1. Types
A. Integral (transmembrane)
B. Peripheral
2. Functions
D. Types of Transport
Passive (diffusion): high to low Channel proteins- aquaporin or ion channel
Carrier proteins
Active: low to high
ONLY carrier proteins
A. Tonicity
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1. Plants & animal cells
2. Isotonic, hypertonic, hypotonic
B. Na/K+ Pump (electrogenic pump)
1. How does it function?
2. What are the consequences on the cell?
C. Membrane Potential
1. Electrochemical gradient- gradient of ions based on concentration &
charge
2. Voltage
3. Electrogenic pump- Na/K+ pump and proton pump (establish
volatage)
4. Co-transportD. Bulk Transport
1. Endocytosis: pinocytosis, phagocytosis, receptor-mediated
2. Exocytosis
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