Membrane Structure and Cellular Transportation Ch. 7 AP Biology Ms. Haut

Membrane Structure Made of phospholipid bilayer Polar (hydrophilic) heads of phospholipids oriented towards protein layers amphipathic Nonpolar (hydrophobic) tails of phospholipids are oriented between polar heads

Membrane Structure Proteins are individually embedded in the bilayer Hydrophilic portions exposed to water Hydrophobic portions in nonaqueous environment inside the bilayer Phospholipid bilayer

Fluid-Mosaic Model Membranes held together by weak hydrophobic interactions Lipids and some proteins can drift laterally within the membrane

Mosaic of Different Molecules Integral proteins—transmembrane proteins that span the hydrophobic interior of the membrane Transport proteins

Mosaic of Different Molecules Integral proteins—transmembrane proteins that span the hydrophobic interior of the membrane Peripheral proteins—attached t the membrane’s surface Transport proteins Carbohydrates—function in cell-to-cell recognition (cell markers) Glycolipids, glycoproteins

Traffic across Membranes Membrane’s molecular organization results in selective permeability Permits exchange of nutrients, waste products, oxygen, and inorganic ions. Allows some substances to cross 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: Diffusion Diffusion—movement of a substance down its concentration gradient due to random thermal motion Spontaneous process that decreases free energy and increases entropy ⁂ NO ENERGY EXPENDED Diffusion of a gas➞

Free energy Less stable Free energy More stable A substance will diffuse from where it is more concentrated to where it is less concentrated

←Water Selectively Permeable Membrane Solute Molecules

Passive Transport: Osmosis Osmosis—diffusion of water across a selectively permeable membrane Water diffuses down its own concentration gradient (from hypotonic solution to hypertonic solution) Hypotonic—lower concentration of solutes Hypertonic—higher concentration of solutes Isotonic—equal solute concentration

AP LAB: EXERCISE 1A: Diffusion H2O/Lugol’s Solution (IKI) 15% glucose/ 1% starch Initial contents Initial Color Bag 15%glucose and 1% starch clear Let stand 30 minutes Beaker H2O + IKI yellow

AP LAB: EXERCISE 1A: Diffusion H2O/Lugol’s Solution (IKI) 15% glucose/ 1% starch yellow H2O + IKI Beaker clear 15%glucose and 1% starch Bag Initial Color Initial contents blue Final Color - + Initial Glucose Final Glucose

Alternative Methods of Cellular Transportation Ch. 8 AP Biology

Facilitated Diffusion Diffusion of solutes across a membrane, with the help of transport proteins Is passive transport because solute is transported down its concentration gradient Aides transport of many polar molecules and ions that are inhibited by phospholipid bilayer

Facilitated Diffusion Transport proteins share similar properties with enzymes: They are specific for the solutes they transport They 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 Transport Energy-requiring process during which a transport protein pumps a molecule across a membrane, against its concentration gradient 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 Potential Gererated by some ion pumps Membrane potential—voltage across membranes Ranges from –50 to –200 mv (the inside of the cell is negatively charged relative to outside Affects traffic of charged substances across membrane Favors diffusion of cations into cell; anions out of cell (due to electrostatic attractions—cytoplasm is negatively charged)

Electrochemical Gradient Two forces drive the diffusion of ions across a membrane Chemical force—concentration gradient Electrical force—effect of membrane potential

Electrochemical Gradient Electrogenic pump—transport protein that generates voltage across 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

Cotransport Process where a single ATP-powered pump actively transport one solute and indirectly drives the transport of other solutes against their concentration gradient ATP-powered pump actively transports one solute and creates potential energy in the gradient it creates 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

Endocytosis Exocytosis Process of importing macromolecules into a cell by forming vesicles derived from the cell membrane Process of exporting macromolecules from a cell by fusion of vesicles with the cell membrane Endocytosis Exocytosis Vesicle forms from a localized region of cell membrane that sinks inward; pinches off into cytoplasm Vesicle usually budded from the ER or Golgi and migrates to cell membrane Used by cells to incorporate extracellular substances Used by secretory cells to export products (insulin in pancreas; neurotransmitter from neuron)

Phagocytosis—endocytosis of solid particles Forms food vacuoles that fuse with lysozome 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