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Chapter 7: The Plasma Membrane
Structure and Function
PLASMA MEMBRANE- surrounds ALL cells
• Cell membrane separates living cell from aqueous
environment
• Controls traffic in & out of the cell
– allows some substances to cross more easily than
others
• A membrane is a collage of different proteins embedded
in the fluid matrix of the lipid bilayer
MEMBRANE MODELS
• 1915: Scientists analyzed isolated red blood cell
membranes, found them made up of proteins and lipids
• 10 years later: E. Gorter and F. Grendel reasoned
membranes are made of a phospholipid bilayer
• 1935: Hugh Davson and James Danielli proposed membrane was a “sandwich” of proteins and phospholipid
bilayer with proteins on outside of phopholipids
• 1972- S.J. Singer and G. Nicolson propose membrane is a “mosaic” of proteins and phospholipids that are
constantly moving and changing
FLUID MOSAIC MODEL
• Current model for animal plasma membrane
• Held together by weak phobic interactions that make membrane fluid
• Components can move laterally
• Made of PROTEIN and PHOSPHOLIPIDS
SEMIPERMEABLE PHOSPHOLIPID BILAYER
FUNCTION OF PHOSPHOLIPIDS
• Phosphate head
– Polar
– Hydrophilic
• “water loving”
• Fatty acid tails
– Nonpolar
– Hydrophobic
• “water fearing”
• separate cell from liquid environment
PHOSPHOLIPIDS = AMPHIPATHIC = Have both philic and phobic regions
• Form a BILAYER with polar heads out/phobic tails in
• SELECTIVELY PERMEABLE- due to hydrophobic/hydrophilic arrangement of phospholipids
~ allows certain molecules to pass through- non-polar, hydrophobic, gases (hydrocarbons, CO2 & O2)
~ prevents other substances from passing through- ions, polar/hydrophilic, large
• phospholipid tails provide phobic barrier
MEMBRANE PROTEINS - STRUCTURE & FUNCTION
~ PERIPHERAL PROTEINS
• not embedded
• loosely bound to membrane surface
• ex: cell surface identity marker (antigens)
~ INTEGRAL PROTEINS- embedded in membrane
• penetrate the hydrophobic core
• often span the membrane = transmembrane proteins
• The hydrophobic regions of an integral protein consist of one or more stretches of
nonpolar amino acids, often coiled into alpha helices
PROTEIN STRUCTURE
 Proteins domains anchor molecule within membrane
 nonpolar & hydrophobic amino acids
 anchors protein into membrane
 outer surfaces
 polar & hydrophilic amino acids
 extend into extracellular fluid & into cytosol
TRANSPORT PROTEINS assist movement of substances across bilayer that can’t go on their own
 A channel proteins that allows water to pass = aquaporins
 A carrier proteins bind to molecules and change shape to shuttle them across the membrane
o EX: proton pump channel in photosynthetic bacteria
 Transport proteins function through conformational change = protein changes shape
PROTEIN FUNCTION
1. Transport
2. Enzymatic Activity
3. Signal Transduction
4. Cell-cell Recognition
5. Intercellular joining
6. Attachment to cytoskeleton & ECM
FLUIDITY of MEMBRANES
• Different species have different FATTY ACID tails in their phospholipids
• UNSATURATED FATTY ACIDS make “kinks” so phospholipids can’t pack as close together
(remain fluid @ colder temps)
CHOLESTEROL (in animal cells only)
makes membranes less fluid at higher temps (keep phospholipids from moving around)
makes membranes more fluid at lower temps (keep phospholipids from packing closely together)
OTHER MEMBRANE COMPONENTS
CARBOHYDRATES
 Carbohydrates on the membrane surface:
 Attached to proteins (glycoproteins) or lipids (glycolipids)
 Carbohydrates vary among species, individuals, and even cell types in an individual
~ important in cell-cell recognition/immune system function and tissue development/differentiation
Ex: four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cell
EX; important in blood transfusions/organ transplants/recognition of invaders by immune system
TYPES OF PASSIVE TRANSPORT
•
Remember: 2nd Law of Thermodynamics
– governs biological systems
– universe tends towards disorder
– trying to reach equilibrium
DIFFUSION
PASSIVE – Requires NO ENERGY
Automatic due to kinetic energy of molecules
Moves DOWN CONCENTRATION GRADIENT
from [HIGH] → [LOW] until reaches equilibrium
Ex: Oxygen/CO2 cross capillary cell membranes
DIFFUSION OF WATER = OSMOSIS
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Osmosis is the diffusion of water across a selectively permeable membrane
Cell survival depends on balancing water uptake & loss
Water diffuses from:
high concentration of water to low concentration of water
 BUT. . . the direction of water is determined only by a difference in total solute concentration
water will move until concentrations reach equilibrium
TONICITY
HYPOTONIC
[solute] outside ‹ inside
Net movement of water into cell
Animal cells:
swell & burst = CYTOLYSIS
Plant cells
increase TURGOR PRESSURE
*normal for plant cell
ISOTONIC
[solute] outside = inside
Net movement of water
is equal
No change in size
HYPERTONIC
[solute] outside > inside
Net movement of water out
Animal cells:
shrink = CRENATION
Plant cells:
can’t shrink due to cell wall
Cell membrane pulls away from cell
wall= PLASMOLYSIS
FACILITATED DIFFUSION
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passive transport = no cellular energy needed
Move from HIGH to LOW concentration
facilitated = with help
aided by membrane transport proteins
channels move specific molecules across cell membrane
FACILITATED DIFFUSION with AQUAPORINS
OSMOSIS= Diffusion of water across a semi-permeable membrane
AQUAPORIN proteins move POLAR WATER molecules past phobic tails
[HIGH] → [Low]
Transport proteins
speed movement of molecules across
Carrier proteins
Grab molecule, undergo a conformational change, that translocates the solute-binding site across the
membrane
FACILITATED DIFFUSION with CARRIER PROTEINS
PASSIVE- Requires NO ENERGY
Trans-membrane proteins assist in movement
Grab molecule, change shape, flip to other side
Moves from [HIGH] → [LOW]
Channel proteins
provide passage that allows a specific molecule or ion to cross the membrane
FACILITATED DIFFUSION with ION CHANNELS
transmembrane proteins form “tunnels” across membrane
Moves from [HIGH] → [LOW]
Moves charged ions (Na+ , K+, Ca++ Cl-) past hydrophobic tails in center
Can be “gated” or not
Gates can open/close in response to electrical/chemical signals
ACTIVE TRANSPORT
moves substances against their concentration gradient
movement from lower - higher concentration
requires cellular energy, usually in the form of ATP
performed by specific proteins embedded in the membranes
conformational shape change transports solute from one side of membrane to other
Uses protein pumps OR vesicles
diffusion of polar, hydrophilic molecules
CYTIC FIBROSIS
mutation in Cl- transport protein
MEMBRANE POTENTIAL
thick mucous builds up in lungs/body organs
tastepotential
“salty” due to salt imbalance
 All cells have voltages across their membranes = patients
membrane
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Two combined forces, collectively called the electrochemical gradient, drive the diffusion
of ions across a membrane:
1. Ions move DOWN the electrochemical gradient
o electrogenic pump = a transport protein that generates the voltage (electrical
force) across a membrane
2. Concentration gradient
 Cytoplasm inside cell is more negative than extracellular fluid outside
 Favors the passage of cations (+) into cells
TYPES OF ACTIVE TRANSPORT-requires energy
PROTON PUMP
Main electrogenic pump in PLANTS
ATP provides energy to pump H+ ions across a
membrane
Stored H+ = potential energy to do work
EX: COTRANSPORT (see below)
ATP PRODUCTION during cellular
respiration/photosynthesis
SODIUM-POTASSIUM (Na+-K+) PUMP
Moves 3 Na+ ions in and 2 K+ ions out
Main electro-genic pump in ANIMALS
EX: Na+-K+ pump sets up membrane potential
Nerve signal results when Na+ and K+ exchange places
Then pump resets membrane for next signal
COTRANSPORT
Na+-K+/ Proton pumps use ATP to create concentration gradient
Movement of substance is linked to return of Na+/H+
as it flows back down its concentration gradient
EX:
BULK TRANSPORT
ENDOCYTOSIS
cell membrane engulfs substance
brings it into cell in a VESICLE
PHAGOCYTOSIS- “phage” = cell eating
large molecules/ whole cells
PINOCYTOSIS- “pino” = cell drinking
small molecules, fluids
Ex: White blood cell eating bacteria
EX: sucrose is linked to H+ transport
EXOCYTOSIS
VESICLES fuse with cell
membrane and release
substances outside cell
Ex; Golgi export
RECEPTOR MEDIATED ENDOCYTOSIS
Substances (=LILGANDS) bind to specific
RECEPTORS in membrane
Vesicle forms from area with receptors
Often clustered in coated pits
EX: uptake of LDL-cholesterol carrier
requires receptor on cell surface