membrane structure and function - weebly...membrane structure and function what you must know: why...
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Membrane Structure and Function
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What You Must Know: Why membranes are selectively permeable.
The role of phospholipids, proteins, and carbohydrates in membranes.
How water will move if a cell is placed in an isotonic, hypertonic, or hypotonic solution.
How electrochemical gradients are formed.
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Cell Membrane A. Plasma membrane is selectively permeable
Allows some substances to cross more easily than others
B. Fluid Mosaic Model
Fluid: membrane held together by weak interactions
Mosaic: phospholipids, proteins, carbs
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Early membrane model (1935) Davson/Danielli –
Sandwich model
phospholipid bilayer between 2 protein layers
Problems: varying chemical composition of membrane, hydrophobic protein parts
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The freeze-fracture method: revealed the structure of membrane’s interior
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Fluid Mosaic Model
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Phospholipids Bilayer
amphipathic = hydrophilic head, hydrophobic tail
Hydrophobic barrier: keeps hydrophilic molecules out
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Membrane fluidity Low temps: phospholipids
w/unsaturated tails (kinks prevent close packing)
Cholesterol resists changes by:
limit fluidity at high temps
hinder close packing at low temps
Adaptations: bacteria in hot springs (unusual lipids); winter wheat ( unsaturated phospholipids)
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Membrane Proteins Integral Proteins Peripheral Proteins Embedded in membrane
Determined by freeze fracture
Transmembrane with hydrophilic heads/tails and hydrophobic middles
Located on the top and bottom of the membrane
NOT embedded
Held in place by the cytoskeleton and the ECM
Provides stronger framework
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Integral & Peripheral proteins
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Some
functions of
membrane
proteins
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Carbohydrates Function: cell-cell recognition; developing organisms
Glycolipids, glycoproteins
Eg. blood transfusions are type-specific
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Synthesis and sidedness of membranes
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Selective Permeability Nonpolar, small molecules cross easily (hydrocarbons,
hydrophobic molecules, CO2, O2)
Hydrophobic core prevents passage of ions, polar molecules
Transport proteins: allow passage of hydrophilic substances
Aquaporin: transport protein that allows passage of H2O
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Passive Transport NO ENERGY needed!
Diffusion down concentration gradient (high low concentration)
Eg. hydrocarbons, CO2, O2, H2O
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Osmosis: diffusion of H2O
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Hypotonic / Hypertonic / Isotonic
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Osmoregulation Control solute & water balance
Contractile vacuole: “bilge pump” forces out fresh water as it enters by osmosis
Eg. paramecium caudatum – freshwater protist
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Facilitated Diffusion Transport proteins (ion
channels, carrier proteins) help hydrophilic substance cross
(1) Provide hydrophilic channel or (2) loosely bind/carry molecule across
Eg. ions, polar molecules
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Active Transport Requires ENERGY (ATP)
Transport substances against concentration gradient (low high conc.)
Eg. sodium-potassium pump, proton pump
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Electrogenic Pumps: generate voltage across membrane
Na+/K+ Pump Proton Pump
Pump Na+ out, K+ into cell
Nerve transmission
Transport protons (H+) out of cell
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Cotransport: membrane protein enables “downhill” diffusion of one solute to drive “uphill” transport of other
Eg. sucrose-H+ cotransporter
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Passive vs. Active Transport
Little or no Energy
Moves from high to low concentrations
Moves down the concentration gradient
eg. diffusion, osmosis, facilitated diffusion (with a transport protein)
Requires Energy (ATP)
Moves from a low concentration to high
Moves against the concentration gradient
eg. pumps, exo/endocytosis
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Bulk Transport Transport of proteins, polysaccharides, large
molecules
Endocytosis: take in
macromolecules, form new
vesicles
Exocytosis: vesicles fuse
with cell membrane, expel
contents
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Types of Endocytosis Phagocytosis:
“cellular eating” - solids
Pinocytosis:
“cellular drinking” - fluids
Receptor-Mediated
Endocytosis:
Ligands bind to specific
receptors on cell surface
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Membrane Transport