membrane structure and function. membrane structure plasma membrane is a boundary that separates the...
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Membrane Structure and
Function
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Membrane Structure
Plasma membrane is a boundary that separates the living cell from it’s non-living surroundings. @ 8 nm thick Controls chemical traffic Selectively permeable. Unique structure.
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Singer and Nicolson Fluid Mosaic Model
Proteins imbedded in membraneHydrophilic and Hydrophobic regions will be
stable.Membrane is a mosaic of proteins bobbing in
a fluid bilayer of phospholipids.Freeze fracture gives clear evidence.
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Freeze Fracture
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Fluid Mosaic Model
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Fluid Mosaic Model
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Fluidity of Membranes
1970 Mouse/Human Hybrid cellsLabeled with two different fluorescent dyes.After a couple of hours they were evenly
distributed.
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Human Mouse
Hybrids
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Fluidity of Membranes
Unsaturated tails enhance fluidityMore saturated the membrane
phospholipids the easier it is to solidify.Cholesterol in eukaryotes modulates
fluidity less fluid in warmer temp by restraining
phospholipid movement More fluid in colder temp by preventing
close packing of phospholipids.
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Membrane Viscosity
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Mosaic of Structure
Integral proteins Generally transmembrane
Peripheral proteins not embedded but attached to the membrane
surface. May be attached to integral proteins or held
by fibers of ECM (Extra Cellular Matrix). On cytoplasmic side may be held by
cytoskeleton.
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Integral vs. Peripheral
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Bifacial Nature
Carbohydrates when present are on the outer surface.
ER, Golgi and vesicles contribute to membrane
Six major kinds of functions by proteins of plasma membrane.
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Function of Membrane Proteins
TransportEnzymatic activitySignal TransductionIntercellular joiningCell-Cell recognitionAttachment to cytoskeleton
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Membrane Proteins Functions
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Membrane Carbohydrates
Cell Cell Recognition Sorting embryonic cells into tissues and organs Rejecting foreign cells Usually branched Oligosaccharides Covalently bonded to lipids (Glycolipids) Most covalently bonded to proteins
(Glycoproteins) Lots of variety
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Glycoprotein
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Glycolipids
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Movement Across Membranes
Selectively Permeable Types of membrane proteins Nature of the substance
Non polar Molecules Dissolve in the membrane smaller move faster than larger
Polar molecules small polar uncharged go right between the
phospholipids
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Polar vs Non Polar Entry
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Movement Across Membranes
Polar molecules Larger molecules need a transport protein Ions also need transport help.
Protein transport Transport Proteins – Specific molecules or
ions can pass through integral proteins May have a tunnel ( due to hydrophilic amino
acids ) May bind and physically move it across membrane Are specific for the substance they transport.
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Passive Transport
Concentration Gradient Change of concentration over a given
distance.Net directional movement
overall movement away from center of concentration
Diffusion- net movement from high to low concentration. Due to kinetic energy, random molecular movement Continues until dynamic equilibrium.
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Passive Transport
Diffusion of a substance across a biological membrane. No energy required. Concentration gradient drives it Rate regulated by permeability, concentration
How do you get the most efficient diffusion? Steep gradient Short Distance
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Counter Current Exchange
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Counter Current Exchange
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Osmosis
Osmosis is the passive transport of water across a membrane. (from high to low concentration) Hypertonic – A solution with a greater
concentration of solute. Hypotonic – A solution with a lower
concentration of solute Isotonic – A solution with an equal amount of
solute.
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Solute and Water
Solute- What is dissolved in the solventSolvent- the medium in which molecules are
being distributed. Hypertonic- More solute as compared to another
location. Hypotonic- Less solute as compared to another
location. Isotonic- equal solute as compared to another
location
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Water movement due to solute concentration
100% water
30% solute
Hypotonic
Hypertonic
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Water movement due to solute concentration
100% water
30% solute
Direction Osmosis Will Occur
Direction Diffusion of Solute Will Occur
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Cells with Walls
Water Moves into vacuole by osmosisvacuole expands
and exerts turgor pressure
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Plasmolysis
Add Salt Water
Water moves out and Plasmolyzes cells (color gets more concentrated)
Plasma membrane
Cell Wall
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Lysis
When water rushes in, cells that don’t have cell walls can Lyse.
Bursting due to too much osmotic pressure.
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Protein and Facilitated Diffusion
Facilitated Diffusion: Diffusion across a membrane with the help of transport proteins. Passive Helps polar molecules and ions that are
slowed down by the membranes lipid nature. They are like enzymes because
have active sites Max rate can be reached. Can be inhibited.
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Transport
ATP powered pump helps transport another substance as it moves back with the gradient.
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Active Transport
Energy is required to go against the gradient.+GRequires energyHelps maintain steep gradientsTransport proteins work with ATP to provide
energy necessary.
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Sodium Potassium Pump
Three sodium are pumped out for every two potassium pumped in. Each is being pumped against the concentration gradient.
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Na/K Pump
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Electrogenic Pumps
Na/K ATPase: Main electrogenic pump in animal cells
Proton pumps: plants, bacteria, and fungi and Chloroplasts, Mitochondria.
Pumps are used to do work in organisms. contract muscles Make ATP Conduct Nerve Impulses
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Cotransport
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Exocytosis
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Endocytosis
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Phagocytosis
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Pinocytosis
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Receptor Mediated Endocytosis