5.10 MEMBRANE STRUCT. AND FUNCTION Membranes organize the chemical activities of cells Membranes provide structural order for metabolism Form most of the cell's organelles Compartmentalize chemical reactions The plasma membrane forms a boundary between a living cell and its surroundings Exhibits selective permeability Controls traffic of molecules in and out

5.11: PHOSPHOLIPID BILAYER Membrane phospholipids form a bilayer Phospholipids are the main structural components of membranes Two nonpolar hydrophobic fatty acid "tails" One phosphate group attached to the hydrophilic glycerol "head"

LE 5-11a Hydrophilic head Phosphate group Symbol Hydrophobic tails

In membranes, phospholipids form a bilayer Two-layer sheet Phospholipid heads facing outward and tails facing inward Selectively permeable Polar lipid-soluble molecules pass through Nonpolar molecules not soluble in lipids do not pass through

LE 5-11b Water Hydrophilic heads Hydrophobic tails Water

5.12: FLUID MOSAIC MODEL (CELL MEMB) The membrane is a fluid mosaic of phospholipids and proteins A membrane is a mosaic Proteins and other molecules are embedded in a framework of phospholipids A membrane is fluid Most protein and phospholipid molecules can move laterally (more fluid than solid) Membrane glycoproteins and glycolipids function in cell identification

LE 5-12 Extracellular matrix Glycoprotein Carbohydrate Glycolipid Plasma membrane Phospholipid Proteins Microfilaments of cytoskeleton Cholesterol Cytoplasm

Proteins make the membrane a mosaic of function Proteins perform most membrane functions Identification tags (glycoproteins) Junctions between adjacent cells Enzymes Receptors of chemical messages from other cells (signal transduction) Transporters of substances across the membrane

5.14: PASSIVE TRANSPORT (NO ATP!) Passive transport is diffusion across a membrane Diffusion is the tendency for particles to spread out evenly in an available space From an area of high concentration to an area of low concentration Passive transport across membranes occurs when a molecule diffuses down a concentration gradient Small nonpolar molecules such as O2 and CO2 diffuse easily across the phospholipid bilayer of a membrane

LE 5-14a Molecules of dye Membrane Equilibrium

LE 5-14b Equilibrium

5.15: FACILITATED DIFFUSION Transport proteins may facilitate diffusion across membranes In facilitated diffusion Transport proteins that span the membrane bilayer help substances diffuse down a concentration gradient To transport the substance, a transport protein may Provide a pore for passage Bind the substance, change shape, and then release the substance

LE 5-15 Solute molecule Transport protein

Osmosis is the diffusion of water across a membrane In osmosis water, molecules diffuse across a selectively permeable membrane From an area of low solute concentration To an area of high solute concentration Until the solution is equally concentrated on both sides of the membrane The direction of movement is determined by the difference in total solute concentration Not by the nature of the solutes Animation: Osmosis

cluster of water molecules Lower concentration of solute Higher concentration of solute Equal concentration of solute H2O Solute molecule Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Net flow of water

SECTION 5.17 Water balance between cells and their surroundings is crucial to organisms Osmoregulation is the control of water balance Tonicity is the tendency of a cell to lose or gain water in solution Isotonic solution: solute concentration is the same in the cell and in the solution No osmosis occurs Animal cell volume remains constant; plant cell becomes flaccid

Hypotonic solution: solute concentration is greater in the cell than in the solution Cell gains water through osmosis Animal cell lyses; plant cell becomes turgid Hypertonic solution: solute concentration is lower in the cell than in the solution Cell loses water through osmosis Animal cell shrivels; plant cell plasmolyzes

LE 5-17 Isotonic solution Hypotonic solution Hypertonic solution H2O Animal cell (1) Normal (2) Lysed (3) Shriveled Plasma membrane H2O H2O H2O H2O Plant cell (4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed)

5.18: ACTIVE TRANSPORT USING PROTEINS Cells expend energy for active transport Active transport requires energy to move solutes against a concentration gradient ATP supplies the energy Transport proteins move solute molecules across the membrane Animation: Active Transport

LE 5-18 Transport protein P P Protein changes shape Phosphate detaches ATP Solute ADP Solute binding Phosphorylation Transport Protein reversion

5.19: ACTIVE TRANSPORT USING VESICLES Exocytosis and endocytosis transport large molecules To move large molecules or particles through a cell membrane A vesicle may fuse with the membrane and expel its contents outside the cell (exocytosis) Membranes may fold inward, enclosing material from the outside (endocytosis)

LE 5-19a Fluid outside cell Vesicle Protein Cytoplasm

Endocytosis can occur in three ways Phagocytosis ("cell eating") Pinocytosis ("cell drinking") Receptor-mediated endocytosis

Faulty membranes can overload the blood with cholesterol SECTION 5.20: CONNECTION Faulty membranes can overload the blood with cholesterol Cholesterol is carried in the blood by low-density lipoprotein (LDL) particles Normally, body cells take up LDLs by receptor-mediated endocytosis Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors People with hypercholesterolemia have more than twice the normal level of blood cholesterol