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Membrane Structure and Function
What is the Function of The Plasma Membrane?
• Boundary
• Must be selectively permeable
• What, then is the structure that allows the membrane to perform this function successfully?…..
Phospholipids• Recall that phsopholipids are
amphipathic (both hydrophilic and hydrophobic).
• Artificial membranes showed phsopholipids will form a layer in water:
The Bilayer• Why are membranes organized into a
bilayer?
• There are two “watery” areas that interact with the membrane, outside of the cell and inside the cell.
Science as a Process:• 1935 Davson-Danielli model
“The Fat Sandwich”. The evidence:
• Both protein and phsopholipids were isolated from membranes.
• Thickness measured equals the bilayer
• Phspholipids alone are not as attracted to water as the real membrane surfaces. Therefore coat with proteins!
Problems with the model:
• In what way are membrane proteins a problem for this model?
• Hint: Membrane proteins are amphipathic.
• Hydrophobic parts of the proteins are in hydrophilic zones, resulting in an unstable structure.
• What other problem was there with this model?
• The model suggests that all membranes are identical with regard to thickness, they’re not!
Fluid Mosaic Model - 1972• In what way does this
model solve the problems?
• Hydrophobic parts of proteins are embedded within the membrane.
• Thickness between different membranes is a function of the proteins
What evidence is there to support this model?
Fluid Mosaic Model - 1972• In what way does this
model solve the problems?
• Hydrophobic parts of proteins are embedded within the membrane.
• Thickness between different membranes is a function of the proteins
Freeze-fracture technique with electron
microscope
Membrane Fluidity
• Why is it that membrane phospholipids drift laterally, and rarely flip?
How is this fluidity maintained?
• Kinks in unsaturated fatty acid tails of phospholipids.
• Cholesterol
How are proteins arranged to contribute to membrane function?
• Membrane proteins contribute to the mosaic quality of the structure.
• Different proteins convey different properties to each membrane.
• Integral proteins are inserted within the membrane.• Peripheral proteins are attached to membrane
surface• Proteins attach to cytoskeleton or to extracellular
fibers to help give animal cells a stronger framework
Membrane Carbohydrates• Found only on the outside of the membrane.• What is their function?• Cell to cell recognition. • Sorting cells into tissues. • Immune defense.• Usually oligosaccharides (15 or less sugar
units)• glycolipids or glycoproteins
How do ions and other polar molecules pass into and out of cells?
• Transport proteins:
• Provide hydrophilic tunnel for ions.
• They are specific for the substances they transport.
What determines the direction of traffic across a membrane?
• Diffusion.• What causes diffusion? Why is it
spontaneous?• Concentration gradient represents
potential energy! • Since the direction of movement
decreases the free energy of the system it is spontaneous.
Does the diffusion of more than one kind of particle work
together or separately?
Do the particles stop moving once equilibrium
is reached?
Does the diffusion of more than one kind of particle work
together or separately?
NO!
If a molecule can move freely through the phospholipid bilayer what
always controls the direction of its movement?
• Concentration gradient.
• Remember that the concentration gradient represents potential energy.
Osmosis
• What is osmosis?
• The diffusion of water across a semi-permeable membrane.
Since water passes freely across the membrane, how can the cell control the direction of osmosis?
• The cell can concentrate solutes that are not permeable to the phospholipid bilayer on one side of the membrane.
• Which way will water move?• Water will follow the solutes!• What do the terms hypotonic, hypertonic
and isotonic mean?• Hypotonic = lower solute concentration• Hypertonic = greater solute concentration• Isotonic = equal solute concentration
??
Which way will the water
move?
WHY?
Do Water Molecules Stop Moving in Isotonic Conditions?• No.
• They continue to diffuse, however there is no net movement!
• In general, which way does water move?
• From hypotonic to hypertonic!
Facilitated Diffusion• What is facilitated diffusion?
• Diffusion of solutes with the help of transport proteins.
• Is this a passive or an active process?
• Passive.
• Why do these solutes need a protein to facilitate their diffusion?
• They are too polar to pass through the lipid bilayer.
Active Transport
• What is active transport?• Pumps molecules across the membrane
against their concentration gradients.• Requires energy, in the form of ATP• Used to help maintain ionic gradients
across membranes.• What do these ionic gradients represent?• Potential energy.
Membrane Potential
• Membrane potential is the voltage across a membrane
• Usually around -70 mV
• How is it maintained?
• An unequal distribution of anions inside the cell to cations outside the cell
What 2 forces drives the diffusion of ions?
• Concentration gradient of the ion• Effect of membrane potential (charge) on the ion• This is called the electrochemical gradient.• Ions diffuse down their electrochemical gradient!• A transport protein that generates voltage across
a membrane is called an electrogenic pump.• One example is the sodium potassium pump
Proton pumps are the main electrogenic pumps of bacteria, fungi and plants.
Review of passive and active transport:
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Review of passive and active transport:
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Review of passive and active transport:
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Review of passive and active transport:
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Review of passive and active transport:
What is cotransport ?
Transport of large molecules:
• Endocytosis – – Phagocytosis – endocytosis of large
particulate substances– Pinocytosis – endocytosis of fluid and
disolved solutes– Receptor mediated
• Exocytosis -
