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Chapter 7Membrane Structure and Function
Plasma MembraneFlexible boundary separating living cell from
nonliving surroundingsSelectively permeable = choosy about what
enters and exitsControls traffic into and out of cell
Structure
Phospholipid BilayerHydrophilic heads are facing watery
environment (extracellular fluid and cytoplasm)Hydrophobic tails are inside bilayer
Fluid Mosaic ModelProteins embedded in bilayer of phospholipid
moleculesFreeze Fracture evidence supports
Freeze, cut, see that proteins are embedded
FluidityMembranes are held together by weak
hydrophobic interactionsAllows movement of phospholipidsCan move laterally (side to side) or flip (rare)
Need to be fluid to workUnsaturated tails prevent tight packingCholesterol restrains phospholipid movement
in warmer temps., and prevents close packing at lower temps.
Prot
eins
Integral proteinsTransmembrane (span entire membrane)
Peripheral proteinsNot embedded, just attatched to either ECM or
cytoskeleton
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Six Major Functions of Membrane Proteins
Figure 7.9
Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.
Enzymatic activity. A protein built into the membranemay be an enzyme with its active site exposed tosubstances in the adjacent solution. In some cases,several enzymes in a membrane are organized asa team that carries out sequential steps of ametabolic pathway.
Signal transduction. A membrane protein may havea binding site with a specific shape that fits the shapeof a chemical messenger, such as a hormone. Theexternal messenger (signal) may cause aconformational change in the protein (receptor) thatrelays the message to the inside of the cell.
(a)
(b)
(c)
ATP
Enzymes
Signal
Receptor
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Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells.
Intercellular joining. Membrane proteins of adjacent cellsmay hook together in various kinds of junctions, such asgap junctions or tight junctions
Attachment to the cytoskeleton and extracellular matrix(ECM). Microfilaments or other elements of thecytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes
(d)
(e)
(f)
Glyco-protein
Six Major Functions of Membrane Proteins
Synthesis of MembranesMade in ERTransport vesicle
to GolgiModified and
packaged in Golgi
Secretory vesicle to cell membrane
Cell-to-Cell RecognitionHelps cell recognize if a cell is alike or
differentImportant for immune responseCarbohydrates act as markers on cell
membraneOligosaccharides (<15 monomers)Can be found attached to proteins or lipids
Glycoprotein = protein + oligosaccharide Glycolipid = lipid + oligosaccharide
Moving through MembranesNonpolar (hydrophobic) molecules
Cross easilyHydrocarbons, Gases
Polar (hydrophilic) moleculesSmall can pass (water, ethanol)Large cannot (glucose)Ions have a hard time (Na+, H+)Can use transport proteins
Passive TransportNo energy required
- ∆GMoves down concentration gradient
Difference in solute concentration Net movement: High Low ConcentrationThrough Membrane = DiffusionThrough Transport Protein = Facilitated
DiffusionWater through Membrane = Osmosis
OsmosisIsotonic Solution = no net movement, equal
solute concentration on both sides of membrane, equilibrium, water moves equally in and out.
Hypotonic Solution = solution has LESS solute concentration than inside the cell, net movement into cell, cell swells, animal cells can burst (lyse).
Hypertonic Solution = solution has MORE solute concentration than inside the cell, net movement out of cell, cell shrinks.
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How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 100 molecules of fructose per ml
How will the water molecules move?
There will be no net movement of water since the concentration of solute in each solution is equal
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How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 75 molecules of fructose per ml
How will the water molecules move?
There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute
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How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 100 molecules of NaCl per ml How will the water molecules
move?
Each molecule of NaCl will dissociate to form a Na+ ion and a Cl- ion, making the final concentration of solutes 200 molecules per mil. Therefore, there will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute
Cell (compared to beaker) hypertonic or hypotonic
Beaker (compared to cell) hypertonic or hypotonicWhich way does the water flow? in or out of cell
.05 M .03 M
Do you understand Osmosis…
Differences in Animals + Plants
AdaptationsContractile Vacuole in parameciumSalt pumps in bony marine fish
Full
Empty
Facilitated DiffusionSpecific to soluteMax rate occurs at saturation of soluteCan be inhibited by molecules that resemble
solute
Channel Proteins – no shape changeCarrier Proteins – shape changeGated Channels – open only in response to
stimuli
Channel
Carrier
Gated
EXTRACELLULARFLUID
Channel protein SoluteCYTOPLASM
Active TransportEnergy requiringTransport protein pumps a molecule
AGAINST the concentration gradientNet movement: Low High+ ∆GImportant to maintain ion gradients (Na, K,
Cl, Ca)
Sodium-Potassium PumpNa+ binding sites towards cytoplasmK + binding sites towards exteriorATP becomes ADP and phosphorylates
proteinCauses shape change from Na + receptive to
K + receptiveSolutes travel across membrane3 Na + out of cell for every 2 K + into cellKeeps inside of cell negative compared to
outsideMajor pump in animal cells (esp. neurons!)
Proton PumpMajor pump in plants, bacteria, and fungiFound in Mitochondria and Chloroplasts
Cotransport1 ATP driven pump transports 1 solute and
indirectly drives the transport of other solutes against their concentration gradient
Large MoleculesEndocytosis = importing large
macromolecules by forming vesicles from plasma membranePhagocytosis: endocytosis of solid particles
Ex: Ameoba’s pseudopodiaPinocytosis: endocytosis of fluid dropletsReceptor Mediated Endcytosis: a ligand
(molecule that bonds to a receptor) binds to initiate endocytosis Cholesterol
In blood, cholesterol is bound to lipid and protein complexes called low-density lipoproteins (LDL). The LDLs bind to LDL receptors on cell membrane to initiate endocytosis of cholesterol. Defective LDL receptors mean build up of cholesterol in blood.
Large MoleculesExocytosis = exporting macromolecules
from a cell by fusion of vesicles with the cell membraneVesicles from ER or GolgiEx: Insulin from pancreatic cells