MEMBRANE STRUCTURE AND FUNCTION. Membrane phospholipids form a bilayer. In membranes, phospholipids form a bilayer

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1 MEMBRANE STRUCTURE 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 forms a boundary between a living cell and its surroundings Exhibits selective permeability Controls traffic of s in and out LE 5-10 Outside of cell Membrane phospholipids form a bilayer Phospholipids are the main structural components of s Two nonpolar hydrophobic fatty acid "tails" One phosphate group attached to the hydrophilic glycerol "head" Hydrophilic head Phosphate group Symbol Hydrophobic tails In s, phospholipids form a bilayer Two-layer sheet with Phospholipid heads facing outward and tails facing inward Selectively permeable Polar lipid-soluble s pass through Nonpolar s not soluble in lipids do not pass through Hydrophilic heads Hydrophobic tails Water Water LE 5-12 The is a fluid mosaic of phospholipids and proteins A is a mosaic s and other s are embedded in a framework of phospholipids A is fluid Most protein and phospholipid s can move laterally Membrane glycoproteins and glycolipids function in cell identification Plasma Extracellular matrix Glycoprotein Carbohydrate Phospholipid s Microfilaments Cholesterol of cytoskeleton Glycolipid 1

2 LE 5-13a s make the a mosaic of function s perform most functions Identification tags Junctions between adjacent cells Enzymes Receptors of chemical messages from other cells (signal transduction) ers of substances across the Enzyme activity LE 5-13b LE 5-13c Messenger Receptor Activated ATP Signal transduction LE 5-14a Passive transport is diffusion across a 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 s occurs when a diffuses down a concentration gradient Small nonpolar s such as O 2 and CO 2 diffuse easily across the phospholipid bilayer of a Molecules of dye Membrane Equilibrium Same concentration on both sides 2

3 LE 5-14b proteins may facilitate diffusion across s Equilibrium In facilitated diffusion proteins that span the 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 Osmosis is the diffusion of water across a protein In osmosis, water s diffuse across a selectively permeable 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 The direction of movement is determined by the difference in total solute concentration Not by the nature of the solutes LE 5-16 Lower concentration of solute Higher concentration of solute Equal concentration of solute Water balance between cells and their surroundings is crucial to organisms Selectively permeable Water Osmoregulation is the control of water balance Tonicity is the tendency of a cell to lose or gain water in solution with cluster of water s Net flow of water 3

4 Hypotonic solution: solute concentration is lower outside the cell than inside. Cell gains water through osmosis Animal cell lyses; plant cell becomes turgid Hypertonic solution: solute concentration is higher outside the cell than inside Cell loses water through osmosis Animal cell shrivels; plant cell plasmolyzes Isotonic solution: solute concentration is the same in the cell and in the solution No net movement of water Animal cell volume remains constant; plant cell becomes flaccid Animal cell Plant cell LE 5-17 Isotonic solution Hypotonic solution Hypertonic solution Normal Lysed Shriveled Plasma Flaccid Turgid Shriveled (plasmolyzed) Plasmolysis Turgid Elodea Video: Turgid Elodea LE 5-18 Cells expend energy for active transport Active transport requires energy to move solutes against a concentration gradient ATP supplies the energy proteins move solute s across the protein binding ATP P ADP Phosphorylation P changes shape PhosphateP detaches reversion 4

5 Exocytosis and Endocytosis transport large s LE 5-19a Fluid outside cell Exocytosis To move large s or particles through a cell A vesicle may fuse with the and expel its contents outside the cell (exocytosis) Membranes may fold inward, enclosing material from the outside (endocytosis) Vesicle LE 5-19b LE 5-19c Endocytosis can occur in three ways Pseudopodium of amoeba Food being ingested Vesicle forming Phagocytosis LM 230 Plasma Material bound to receptor proteins Pinocytosis TEM 54,000 PIT Receptor-mediated endocytosis TEM 96,500 Faulty s can overload the blood with cholesterol LE 5-20 Phospholipid outer layer Cholesterol is carried in the blood by lowdensity lipoprotein (LDL) particles Normally, body cells take up LDLs by receptormediated endocytosis Harmful levels of cholesterol can accumulate in the blood if s lack cholesterol receptors People with hypercholesterolemia have more than twice the normal level of blood cholesterol LDL particle Cholesterol Plasma Receptor protein Vesicle 5

6 Animation: Membrane Selectivity Animation: Diffusion Animation: Osmosis Animation: Active Animation: Exocytosis and Endocytosis Introduction Animation: Exocytosis Animation: Receptor-Mediated Endocytosis Animation: Pinocytosis Animation: Phagocytosis 6