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Membrane Structure Membrane Structure and and
Cellular TransportationCellular TransportationCh. 7Ch. 7
AP BiologyAP Biology
Ms. HautMs. Haut
Membrane Membrane StructureStructure
Made of phospholipid Made of phospholipid bilayer bilayer
Polar (hydrophilic) heads Polar (hydrophilic) heads of phospholipids of phospholipids oriented towards protein oriented towards protein layerslayers• Nonpolar (hydrophobic) tails of phospholipids are oriented between polar heads
am
ph
ipath
ic
Membrane Membrane StructureStructure
Proteins are individually Proteins are individually embedded in the bilayerembedded in the bilayer Hydrophilic portions Hydrophilic portions
exposed to waterexposed to water Hydrophobic portions in Hydrophobic portions in
nonaqueous environment nonaqueous environment inside the bilayerinside the bilayer
Phospholipid bilayer
Fluid-Mosaic ModelFluid-Mosaic ModelMembranes held together by weak Membranes held together by weak hydrophobic interactionshydrophobic interactions
Lipids and some proteins can drift Lipids and some proteins can drift laterally within the membranelaterally within the membrane
Mosaic of Different Mosaic of Different MoleculesMolecules
Integral proteins—transmembrane Integral proteins—transmembrane proteins that span the hydrophobic proteins that span the hydrophobic interior of the membraneinterior of the membrane Transport proteinsTransport proteins
Integral proteinsIntegral proteins—transmembrane —transmembrane proteins that span the hydrophobic proteins that span the hydrophobic interior of the membraneinterior of the membranePeripheral proteins—attached t the membrane’s surface Transport proteinsTransport proteins
Carbohydrates—function in cell-to-cell recognition (cell markers) Glycolipids, glycoproteins
Mosaic of Different MoleculesMosaic of Different Molecules
Traffic across MembranesTraffic across Membranes
Membrane’s molecular Membrane’s molecular organization results in organization results in selective selective permeabilitypermeability Permits exchange of nutrients, waste Permits exchange of nutrients, waste
products, oxygen, and inorganic ions.products, oxygen, and inorganic ions. Allows some substances to cross Allows some substances to cross
more easily than others:more easily than others:Hydrophobic molecules—hydrocarbons, CO2, and O2 dissolve in and cross membrane
Very small polar molecules, including H2O can cross easily
Passive Transport: DiffusionPassive Transport: Diffusion
DiffusionDiffusion—movement of a substance —movement of a substance down its concentration gradient due down its concentration gradient due to random thermal motionto random thermal motion Spontaneous process that decreases Spontaneous process that decreases
free energy and increases entropyfree energy and increases entropy⁂ ⁂ NO ENERGY EXPENDEDNO ENERGY EXPENDED
Diffusion of a gas➞
A substance will diffuse from where it is more concentrated to where it is less concentrated
Free energyLess stable
Free energyMore stable
←Water
Selectively PermeableMembrane
Solute Molecules
Passive Transport: Passive Transport: OsmosisOsmosis
OsmosisOsmosis—diffusion of water across a —diffusion of water across a selectively permeable membraneselectively permeable membrane Water diffuses down its own concentration Water diffuses down its own concentration
gradient (from hypotonic solution to gradient (from hypotonic solution to hypertonic solution)hypertonic solution)
Hypotonic—lower concentration of solutesHypertonic—higher concentration of solutesIsotonic—equal solute concentration
Alternative Methods of Alternative Methods of Cellular TransportationCellular Transportation
Ch. 8Ch. 8
AP BiologyAP Biology
Facilitated DiffusionFacilitated DiffusionDiffusion of solutes across a membrane, Diffusion of solutes across a membrane, with the help of transport proteinswith the help of transport proteins
Is passive transport because solute is transported down its concentration gradientAides transport of many polar molecules and ions that are inhibited by phospholipid bilayer
Facilitated DiffusionFacilitated Diffusion
Transport proteins share similar Transport proteins share similar properties with enzymes:properties with enzymes:They are specific for the solutes they
transportThey can be saturated with solute—
maximum rate occurs when all binding sites are occupied
They can be inhibited by molecules that resemble the solute (similar to competitive inhibition)
Active TransportActive TransportEnergy-requiring process during which a Energy-requiring process during which a transport protein pumps a molecule across transport protein pumps a molecule across a membrane, a membrane, againstagainst its concentration its concentration gradientgradient
Is energetically uphill (+G) and requires the cell to expend energy
Helps cells maintain steep ionic gradients across cell membrane (e.g., Na+, K+, Mg 2+, Ca 2+ , and Cl-)
Transport proteins involved get energy from ATP to pump molecules against their concentration gradients
Sodium/Potassium Pump
Membrane PotentialMembrane PotentialGererated by some ion pumpsGererated by some ion pumpsMembrane potential—voltage Membrane potential—voltage across membranesacross membranes Ranges from –50 to –200 mv (the Ranges from –50 to –200 mv (the
inside of the cell is negatively inside of the cell is negatively charged relative to outsidecharged relative to outside
Affects traffic of charged substances Affects traffic of charged substances across membraneacross membrane
Favors diffusion of cations into cell; Favors diffusion of cations into cell; anions out of cell (due to anions out of cell (due to electrostatic attractions—cytoplasm is electrostatic attractions—cytoplasm is negatively charged)negatively charged)
Electrochemical GradientElectrochemical Gradient
Two forces drive the diffusion of ions Two forces drive the diffusion of ions across a membraneacross a membrane
Chemical force—concentration gradientChemical force—concentration gradient
Electrical force—effect of membrane Electrical force—effect of membrane potentialpotential
Electrochemical GradientElectrochemical GradientElectrogenic pump—transport Electrogenic pump—transport protein that generates voltage protein that generates voltage across membranesacross membranes–Example is Na+/K+ Pump
•3 Na+ ions out/ 2 K+ ions in equals a net transfer of one positive charge from the cytoplasm to the extracellular fluid (a net loss of one positive charge), a process that stores energy in the form of voltage
–Stored energy can be trapped for cellular work
CotransportCotransportProcess where a single ATP-powered Process where a single ATP-powered pump actively transport one solute and pump actively transport one solute and indirectly drives the transport of other indirectly drives the transport of other solutes against their concentration solutes against their concentration gradientgradient
1. ATP-powered pump actively transports one solute and creates potential energy in the gradient it creates
2. Another transport protein couples the solute’s downhill diffusion as it leaks back across the membrane with a second solute’s uphill transport against its concentration gradient
Vesicle forms from a Vesicle forms from a localized region of cell localized region of cell membrane that sinks inward; membrane that sinks inward; pinches off into cytoplasmpinches off into cytoplasm
Vesicle usually budded from Vesicle usually budded from the ER or Golgi and migrates the ER or Golgi and migrates to cell membraneto cell membrane
Used by cells to incorporate Used by cells to incorporate extracellular substancesextracellular substances
Used by secretory cells to Used by secretory cells to export products (insulin in export products (insulin in pancreas; neurotransmitter pancreas; neurotransmitter from neuron)from neuron)
Process of importing Process of importing macromolecules into a cell by macromolecules into a cell by forming vesicles derived from forming vesicles derived from the cell membranethe cell membrane
Process of exporting Process of exporting macromolecules from a cell macromolecules from a cell by fusion of vesicles with the by fusion of vesicles with the cell membranecell membrane
EndocytosisEndocytosisExocytosisExocytosis
PhagocytosisPhagocytosis—endocytosis of solid —endocytosis of solid particles particles Forms food vacuoles that fuse with Forms food vacuoles that fuse with
lysozome to be digestedlysozome to be digested
• Pinocytosis—endocytosis of fluid droplets•Takes in solutes dissolved in the droplet
• Receptor-mediated endocytosis—process of importing specific macromolecules into the cell by inward budding of vesicles formed from coated pits
•Occurs in response to binding specific ligands to receptors on cell’s surface