Chapter 7: Membrane Structure and Function

Chapter 7: Membrane Structure and FunctionSelectively Permeable membranes allow some materials to cross them more easily than other which enables the cell to maintain a unique internal environmentPlasma membrane7.1 Cellular membranes are fluid mosaics of lipids and proteinsFluid mosaic model the structure of biological membranes consists of proteins that are attached to or embedded in a bilayer of amphipathic phospholipids

Models for cell membranesTwo dutch scientists (1925)- cell membranes must be a phospholipid bilayer with hydrophobic hydrocarbon tais and hydrophillic headsDavison and Danielli (1935) A bilayer of phospholipids is coveered with a coat of hydrophillic proteinsSinger & Nicolson (1972) Fluid mosaic modelMembranes are held together by weak hydrophobic interactions that allow the lipids and some proteins(directed by cytoskeletal fibers) to drift laterallySome proteins held rigidly by cytoskeletonPhospholipids with unsaturated tails maintain fluidity at lower temperaturesCholesterol (animals) restricts movement of phospholipids and reduces fluidity at warmer temperatures but by preventing the close packing of lipids enhances fluidity at lower temperatures Integral proteins often extend through the membrane (transmembrane) with two hydrophilic ends. The middle is alpha-helical hydrophobic amino acid(s)Peripheral proteins are attached to the surface of the membrane or integral proteins.When these attach to the cytoskeleton while in the cytoplasm and extracellular matrix on the exterior a supportive framework for the plasma membrane is madeGlycolipids and glycoproteins attached to the outside of plasma membranes vary from species to species, from individual to individual and even among cell types.These aide with the ability of a cell to distinguish other cells based on recognition of membrane carbohydrates

7.2 Membrane Structure results in selective permeabilityThe plasma membrane permits a regular exchange of nutrients, waste products, oxygen and inorganic ions. Biological membranes are selectively permeable.Hydrophobic nonpolar molecules can dissolve in and cross a membraneIons and polar molecules may move across the plasma membrane with the aid of transport proteinsHydrophillic passageways are provided for specific molecules by channel proteins (aquaporins water passage)Carrier proteins physically bind and transport a specific molecule(s)

7.3 Passive transport is diffusion of a substance across a membrane with no energy investmentDiffusion is the movement of a substance down its concentration gradient due to random thermal motion.The cell does not expend energy when substances diffuese across membranes down their concentration gradientsPassive transportOsmosis is the diffusion of water across a selectively permeable membraneTonicity is the tendency of a cell to gain and lose water in a given solutionAffected by the relative concentration of solutes that cannot cross the membrane in the solution and the cell.Isotonic an animal cell will neither gain nor loseHypertonic An animal cell has more nonpenetrating solutes so loses water and shrivelsHypotonic An animal cell will gain water swell and possibly lyse (burst)Cells without rigid walls must live in an isotonic environment or have adaptions in osmoregulation the control of water balanceAnimal cell

In plantsTurgid cells provide mechanical support for nonwoody plants (the normal state)Plant cells in an isotonic surrounding are flaccidPlant cells undergo plasmolysis (pulling away of the plasma membrane from the cell wall as water leaves and the cell shrivels) in a hypertonic medium

Facilitated diffusion involves the diffusion of polar molecules and ions across a membrane with the aid of transport proteins (channel or carrier)Many ion channels are gated channels which will open or close in response to electrical or chemical stimuli7.4 Active transport uses energy to move solutes against their gradientsActive transport requires the expenditure of energy to transport a solute against its concentration cradientEssential for a cell to maintain internal concentrations.Sodium-potassium pump exchanges N+ and K+ across animal cell membranesCells have a membrane potential* a voltage across the plasma membrane due to the unequal distribution of ions.Cytoplasm is negative relative to the extracellular fluidMembrane potential favors the diffusion of cations into the cell and anions out of the cellElectrochemical gradient what an ion diffuses downElectrogenic pumps membrane proteins that generate voltage across a membrane by the active transport of ionsA proton pump that transport H+ outside of the cell generates voltage across membranes in plants, fungi and bacteria.Cotransport is a mechanism through which the active transport of a solute is indirectly driven by an ATP-powered pump that transports another substance across its gradient. As that transported substance then diffuses back down its concentration gradient through a cotransporter, the solute is carried against its concentration gradient across the membrane

7.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosisRequires energyExocytosis the cell secretes large molecules by the fusion of vesicles with the plasma membraneEndocytosis a region of the plasma membrane sinks inward and pinches off to forma vesicle containing material that had been outside the cellPhagocytosis psuedopodia wrap around food particlesPinocytosis droplets of extracellular fluid are taken into the cell in small vesiclesReceptor mediated endocytosis allows a cell to acquire specific substances from extracellular fluidLigands molecules that bind specifically to receptor sites