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

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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