CH 5: MEMBRANE STRUCTURE AND FUNCTION

I.  Functions of cell/plasma membrane

            A.  Enclose the cell

            B.  Protects: barrier to surrounding environment

            C.  Regulate: what goes into and out of the cell

            D.  Marks the cell to the organism

            E.  May have cell junctions: connect cells and facilitates communication

                 with neighboring cells

II.  Membrane models

            A.  1925: Gorter and Grendel, phospholipid bilayer

                        1.  Hydrophilic: polar, phosphate outside

                        2.  Hydrophobic: nonpolar, lipid tails inside

            B.  1940's: Davson and Danielli, sandwich model (later rejected)

                        1.  Proteins are also part of the membrane

                        2.  Phospholipid bilayer is a filling between 2 layers of protein

            C.  1950's: Robertson and electron microscope: unit membrane model

                 (rejected)

                        1.  Outer layers contain protein and hydrophilic heads

                        2.  Interior hydrophobic tails

                        3.  All membranes have basically the same structure in all cells

            D.  1972: Singer and Nicolson: Fluid-mosaic model (current model)

                        1.  Fluid (flows, pliable) phospholipid bilayer

                                    a.  Cholesterol: embedded in double membrane, reduces

                                         permeability

                                    b.  Glycolipids protect and identify the cell

                        2.  Mosaic: proteins partially or wholly embedded

                                    a.  Channels: space where a substance (water) can move

                                        across the membrane

                                    b.  Carriers: combine with a substance and help it move

                                         across the membrane

                                    c.  Receptors: specific shape that allows a molecule to bind

                                         to it, ex) hormone

                                    d.  Enzymes: carry out metabolism

                                    e.  Stabilize and give shape to the plasma membrane

                                    f.  Glycoproteins: for cell to cell identification

                                    g.  Asymmetrical: inside and outside are not the same

III.  Selectively permeable plasma membrane: allows some molecules to pass

      through

            A.  Diffusion: movement of molecules from high to low concentration

                        1.  Small noncharged, lipid-soluble molecules, gases, water pass

                             through easily, large molecules don't

                        2.  Solute: usually a solid; solvent: usually a liquid

            B.  Osmosis: movement of water from high to low concentration across a

                 semipermeable membrane

                        1.  Hydrostatic pressure: measure of water pressure

                        2.  Osmotic pressure: greater where there is greater tonicity

                        3.  Tonicity: osmolarity, based on concentrations of solutes

                        4.  Isotonic solutions: same concentration of solutes on either side

                             of the membrane

                        5.  Hypotonic solutions: low concentrations of solutes in solution,

                            water enters the cell, hemolysis

                        6.  Hypertonic solutions: high concentrations of solutes in solution,

                            causes water to leave the cell (water follows salt), crenation

                        7.  Turgor pressure: plant cells swell in response to a hypotonic

                            solution

            C.  Carrier proteins: specific for certain molecules

                        1.  Facilitated transport: proteins assist movement down

                             concentration gradient, doesn't require energy

                             eg., glucose, amino acids

                        2.  Active transport: requires ATP to function

                                    a.  Small molecules move against their concentration

                                         gradient

                                    b.  Na-K pump: more Na⁺ outside, K⁺ inside cell membranes;

                                          causes interior of cell to become relatively negative

            D.  Membrane-assisted transport (takes ATP)

                        1.  Exocytosis: move materials outside the cell, secretion, wastes

                        2.  Endocytosis: move materials inside the cell

                        3.  Phagocytosis: "cell eating", white blood cells

                        4.  Pinocytosis: "cell drinking", root cells get water

                        5.  Receptor-mediated encocytosis: a form of pinocytosis, coated its

                            attract specific molecules

IV.  The cell surface is modified

            A.  Plant: cell walls

                        1.  Primary cell wall: cellulose

                        2.  Pectin, allows the cell to stretch when growing

                        3.  Secondary cell wall: greater concentration of cellulose, lignin

                             adds strength in woody plants, common ingredient

                        4.  Plasmodesma: connect neighboring cells by narrow channels,

                            Cytoplasm can flow from one cell to the next

            B.  Animal: extracellular matrix: supports and fills the spaces between

                  cells

                        1.  Fibers:

                                    a.  Collagen: gives strength

                                    b.  Elastin: gives resilience

                                    c.  Fibronectins and Laminins-proteins that direct movement

                                         of cells and facilitate cellular communication

                        2.  Glycoproteins: packing gel of matrix

                        3.  Junctions: for coordination of tissues

                                    a.  Adhesion junction: (desmosomes) intercellular filaments

                                          that hold cells together

                                    b.  Tight junction: plasma membrane proteins attach in

                                          zipper-like fashion

                                    c.  Gap junction: plasma membrane channels join,

                                         allows small molecules and ions to pass