Membranes

Outline Phospholipid Bilayer Fluid Mosaic Model Membrane Proteins Diffusion Facilitated Diffusion Osmosis Osmotic Balance Bulk Transport Active Transport

Nonpolar (hydrophobic) region Phospholipid Bilayer Phospholipid has two fatty-acid chains attached to its backbone. One end is strongly nonpolar while the other end is strongly polar. polar head oriented toward water and nonpolar tails oriented away from water bilayer is stable because water’s affinity for hydrogen bonding never stops Fatty acid Phosphorylated alcohol Polar (hydrophilic) region Nonpolar (hydrophobic) region G L Y C E R O

Fluid Mosaic Model Plasma membrane is composed of both lipids and globular proteins. Membrane proteins are not very soluble in water.

Cell Membrane Cell membrane components phospholipid bilayer transmembrane proteins interior protein network cell surface markers extra cellular matrix (ECM)

Membrane Proteins transporters enzymes cell surface identity markers cell adhesion proteins attachments to cytoskeleton

Plasma Membrane Proteins

Membrane Protein Structure Anchoring proteins in the bilayer Transmembrane proteins Cytoplasmic side of cell membrane Cytoskeletal proteins Junctional complex 100 nm Actin Spectrin Linker protein

Membrane Protein Structure Transmembrane proteins single-pass anchors multiple-pass channels and carriers pores Phospholipids Polar areas of protein Cholesterol Nonpolar areas of protein NH2 H+ COOH Cytoplasm Retinal chromophore Nonpolar (hydrophobic) a-helices in the cell membrane

Solutions Intra cellular fluid (ICF) - within cells Extra cellular Fluid (ECF) - outside cells Inter cellular = tissue fluid = interstitial fluid Plasma = fluid portion of blood Composition of fluids change as substances move between compartments nutrients, oxygen, ions and wastes move in both directions across capillary walls and cell membranes

Selective Permeability of Membrane Lipid bilayer permeable to nonpolar, uncharged molecules -- oxygen, CO2, steroids Transmembrane proteins act as specific channels small and medium polar & charged particles Macromolecules unable to pass through the membrane vesicular transport

Passive Transport Diffusion Random motion causes a net movement of substances from regions of high concentration to regions of low concentration – down their concentration gradient. Animation

Diffusion Rates Factors affecting diffusion rate through a membrane temperature -  temp.,  motion of particles molecular weight - larger molecules move slower steepness of concentrated gradient - difference,  rate membrane surface area -  area,  rate membrane permeability -  permeability,  rate

Selective Membrane Transport Cell transport channels are selectively permeable, as only certain molecules are allowed to enter. Ions cannot move between the cytoplasm of a cell and the extracellular fluid without the assistance of membrane transport proteins. ion channels

Facilitated Diffusion Carriers transport ions and other solutes across the plasma membrane. Facilitate movement by physically binding molecules on one side of the membrane, and releasing them on the other down their concentration gradient. animation essential characteristics specific passive (no ATP) transport saturation Outside of cell Inside of cell

Osmosis In an aqueous solution, both water and solutes diffuse down concentration gradients. Net movement of water through a selectively permeable membrane from an area of high water concentration to an area of lower water concentration Only occurs if membrane is permeable to water but not to certain solutes Aquaporins = channel proteins specialized for osmosis

Osmotic Pressure Amount of hydrostatic pressure required to stop osmosis Osmosis slows due to filtration of water back across membrane due to increased hydrostatic pressure Animation

Tonicity Tonicity - ability of a solution to affect fluid volume and pressure within a cell depends on concentration and permeability of solute Hypotonic (hypoosmotic) solution low concentration of nonpermeating solutes (high water concentration) cells absorb water, swell and may burst (lyse) Hypertonic solution (hyperosmotic) has high concentration of nonpermeating solutes (low water concentration) cells lose water + shrink (crenate) Isotonic (isoosmotic) solution = normal saline

Maintaining Osmotic Balance

Osmotic Pressure Hydrostatic pressure - pressure of the cytoplasm pushing out against the cell membrane Osmotic pressure - pressure that must be applied to stop the osmotic movement of water across a membrane

Bulk Transport Endocytosis - enveloping food phagocytosis - material taken in is in particulate form pinocytosis - material taken in is in liquid form receptor-mediated - transport of specific molecules Exocytosis - discharge of material from vesicles at the cell surface

Active Transport Active transport involves the expenditure of energy to move substance against their concentration gradient. involves highly selective protein carriers within the membrane sodium-potassium pump Secondary active transport coupled transport - using energy stored in a gradient of a different molecule

Secondary Active Transport Uniporter carries only one solute at a time Symporter carries 2 or more solutes simultaneously in same direction (cotransport) Antiporter carries 2 or more solutes in opposite directions (countertransport) (animation) sodium-potassium pump brings in K+ and removes Na+ from cell Any carrier type can use either facilitated diffusion or active transport (primary or secondary)

Digitalis Slows the sodium pump, which lets more Na+ accumulate in the heart muscle cells. Less Na+ concentration gradient across the membrane Na+/Ca+2 antiporters slow down so more Ca+2 remains inside the cardiac cells Strengthening the force of contraction Balance between concentration of Na+ and Ca+2 in cytosol & extracellular fluid is important

Summary Phospholipid Bilayer Fluid Mosaic Model Membrane Proteins Diffusion Facilitated Diffusion Osmosis Osmotic Balance Bulk Transport Active Transport