Big Idea 2B Growth, reproduction, and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments 2B1 – Cell membranes are selectively permeable due to their structure. 2B2 – Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes.

Membrane structure, I (Ch7) Selective permeability – only some substances can cross – separates the internal/external environments Amphipathic – has both hydrophobic & hydrophilic regions Singer-Nicolson: fluid mosaic model Fluid structure w/ various proteins embedded

Membrane structure, II Phospholipids – membrane fluidity Lipids move along membrane - lateral May move in/out – rare – flip-flop Unsaturated – kinked tails – increase fluidity Cholesterol – membrane stabilization “Mosaic” Structure – collection of different proteins – unique to each cell Integral proteins – transmembrane proteins – cross entire membrane Peripheral proteins – surface of membrane Membrane carbohydrates – cell to cell recognition Oligosaccharides (cell markers); glycolipids; glycoproteins Immune system, blood types

Membrane structure, III Membrane protein function: Transport – channel, shuttle Enzymatic activity – active site exposed Signal transduction (receptor) – binds to chem messenger – sends message inside Intercellular joining – gap/tight junctions Cell-cell recognition ECM attachment – maintain cell shape and stability

Membrane traffic, I Passive transport~ diffusion of a substance across a biological membrane – no energy Diffusion~ tendency of any molecule to spread out into available space Moves down its concentration gradient Small, uncharged polar molecules and small nonpolar molecules may pass Osmosis~ the diffusion of water across a selectively permeable membrane via aquaporins

Membrane traffic, II Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient Channel proteins: ion channels, gated proteins, carrier proteins Allows movement of hydrophilic substances – large polar molecules and ions

Water Balance, I Osmoregulation – control of water balance Hypertonic solution – higher concentration of solutes – cell is hypotonic – cell shrinks Hypotonic solution – lower concentration of solutes – cell is hypertonic – cell swells Isotonic – equal concentrations of solutes Cells with Walls: Turgid (very firm) - hypo Flaccid (limp) - iso Plasmolysis – plasma membrane pulls away from cell wall - hyper

Water Balance, II Water potential (Ψ) - movement of water from an area of high concen. to low concen. Ψ = ΨP + ΨS Water Potential = Pressure Potential + Solute Potential At atmospheric pressure - Ψp = 0 Water potential of pure water in an open beaker is zero (ψ = 0) Water will move from an area of high Ψ to an area of low Ψ Tonic refers to solute concen., potential refers to water concen. The Solute Potential of the Solution Ψs = – iCRT i = ionization constant (For sucrose this is 1.0 because sucrose does not ionize in water.) C = molar concentration R = pressure constant (R = 0.0831 liter bars/mole K) T = temperature in Kelvin (273 + °C)

Active Transport, I Movement of a substance against its concentration gradient with the help of cellular energy Sodium-potassium pump – electrochemical gradient – nerve cells Cotransport – solute pumped out can do work as it diffuses back

Active Transport, II Exocytosis~ secretion of macromolecules by the fusion of transport vesicles with the plasma membrane Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane Phagocytosis – solids Pinocytosis – liquids Receptor-mediated Ligands – bind to receptor