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Lecture Cell Chapters 5 and 6 Biological Membranes and Cell Communication Membrane structure, I Selective permeability Amphipathic~ hydrophobic & hydrophilic regions Singer-Nicolson: fluid mosaic model Carbohydrate chains Glycoprotein Carbohydrate chain Extracellular fluid Hydrophobic Hydrophilic Glycolipid Cholesterol Hydrophilic Cytosol α-helix Peripheral protein Integral proteins Fig. 5-6, p. 111 Membrane structure, II Phospholipids~ membrane fluidity Cholesterol~ membrane stabilization “Mosaic” Structure~ Integral proteins~ transmembrane proteins Peripheral proteins~ surface of membrane Membrane carbohydrates ~ cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins Hydrophilic region of protein Hydrophobic region of protein Phospholipid bilayer Peripheral protein Integral (transmembrane) protein (b) Fluid mosaic model. According to this model, a cell membrane is a fluid lipid bilayer with a constantly changing “mosaic pattern“ of associated proteins. Fig. 5-2b, p. 108 The Lipid Bilayer Lipids of the bilayer fluid or liquid-crystalline state Proteins move within the membrane Membrane Proteins Functions of Membrane Proteins Membrane traffic Diffusion~ tendency of any molecule to spread out into available space Concentration gradient Passive transport~ diffusion of a substance across a biological membrane without using energy Osmosis~ the diffusion of water across a selectively permeable membrane Diffusion Osmosis Pressure applied to piston to resist upward movement Water plus solute Pure water Selectively permeable membrane Molecule of solute Water molecule Fig. 5-12, p. 117 Water balance Osmoregulation~ control of water balance Hypertonic~ higher concentration of solutes Hypotonic~ lower concentration of solutes Isotonic~ equal concentrations of solutes Cells with Walls: Turgid (very firm) Flaccid (limp) Plasmolysis~ plasma membrane pulls away from cell wall Osmotic Pressure Outside cell Inside cell H20 molecules Solute No net water molecules movement (a) Isotonic solution. When a cell is placed in an isotonic solution, water molecules pass in and out of the cell, but the net movement of water molecules is zero. Fig. 5-13a, p. 118 Outside cell Inside cell Net water movement out of the cell (b) Hypertonic solution. When a cell is placed in a hypertonic solution, there is a net movement of water molecules out of the cell (blue arrow). The cell becomes dehydrated and shrunken. Fig. 5-13b, p. 118 Outside cell Inside cell Net water movement into the cell (c) Hypotonic solution. When a cell is placed in a hypotonic solution, the net movement of water molecules into the cell (blue arrow) causes the cell to swell or even burst. Fig. 5-13c, p. 118 Turgor and Plasmolysis Plasma membrane Nucleus Cytoplasm Plasma membrane Vacuole Vacuole Vacuolar membrane (tonoplast) Fig. 5-14, p. 119 Specialized Transport Transport proteins Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient Active transport~ movement of a substance against its concentration gradient with the help of cellular energy Facilitated Diffusion Outside cell Glucose High concentration of glucose Low concentration of glucose Glucose transporter (GLUT 1) Cytosol 1 Glucose binds to GLUT 1. Fig. 5-16a, p. 120 2 GLUT 1 changes shape and glucose is released inside cell. Fig. 5-16b, p. 120 3 GLUT 1 returns to its original shape. Fig. 5-16c, p. 120 Types of Active Transport Sodium-potassium pump Exocytosis~ secretion of macromolecules by the fusion of vesicles with the plasma membrane Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane •phagocytosis •pinocytosis •receptor-mediated endocytosis (ligands) Higher Outside cell Active transport channel Sodium concentration gradient Lower Lower Cytosol Higher (a) The sodium–potassium pump is a carrier protein that requires energy from ATP. In each complete pumping cycle, the energy of one molecule of ATP is used to export three sodium ions (Na+) and import two potassium ions (K+). Fig. 5-17a, p. 121 2. Phosphate group is transferred from ATP to transport protein. 3. Phosphorylation causes carrier protein to change shape, releasing 3 Na+ outside cell. 1. Three Na+ bind to transport protein. 4. Two K+ bind to transport protein. 6. Phosphate release causes 5. Phosphate is released. carrier protein to return to its original shape. Two K+ ions are released inside cell. Fig. 5-17b, p. 121 Phagocytosis Large particles enter cell Receptor-Mediated Endocytosis Intracellular junctions PLANTS: Plasmodesmata: cell wall perforations; water and solute passage in plants ANIMALS: Tight junctions~ fusion of neighboring cells; prevents leakage between cells Desmosomes~ riveted, anchoring junction; strong sheets of cells Gap junctions~ cytoplasmic channels; allows passage of materials or current between cells Tight Junctions Gap Junctions Cell Signaling 1. Synthesis, release, transport of signaling molecules - neurotransmitters, hormones, etc ligand binds to a specific receptor 2. Reception of information by target cells Cell Signaling 3. Signal transduction - - receptor converts extracellular signal into intracellular signal causes change in the cell 4. Response by the cell Cell Signaling 1 Cell sends signal Signaling molecules Receptor 2 Reception Signaling protein 3 Signal transduction Protein 4 Response Altered membrane permeability Enzyme Altered metabolism Protein that regulates a gene Altered gene activity Fig. 6-2, p. 136 Local Regulators Paracrine regulation diffuse through interstitial fluid act on nearby cells Local regulators histamine growth factors prostaglandins nitric oxide Local Signals Neurotransmitters Chemical signals released by neurons (nerve cells) Hormones Chemical messengers in plants and animals Secreted by endocrine glands in animals Transported by blood to target cells Hormones
