Cell Membrane, Part 2

• flexibility: ability to change shape without losing integrity or becoming leaky • basis for this are non covalent interactions among lipids in the bilayer (thus, mobility) • structure and flexibility of bilayer depend on kinds of lipids present • changes with temperature

• intracellular structures consisting of liquid enclosed in lipid bilayer • form naturally during: • secretion (exocytosis) • uptake (endocytosis) • membrane transport

• synthetic spherical bilayers vesicles - model membranes in experimental studies

• planar bilayers • formed across a hole in a partition btwn 2 aqueous compartments • used to measure permeability properties of synthetic membranes

• movement of phospholipids within lipid bilayer

• flip-flop - happens rarely

• happens readily and rapidly

• need ATP

• does not need ATP - special transport proteins move phospholipid and other lipids in the membrane

• fluidity must be precisely regulated • depends on its composition and temperature • stabilized by hydrophobic interactions btwn lipids’ fatty acid chains

• change of a lipid bilayer from a liquid to a 2-D rigid crystalline state (gel) at a characteristic temperature • phase transition and temp at which this occurs is lower if phospholipid chains are short or have double bonds

• lipid composition • more cis-double bonds -> more fluid • double bonds make it difficult to pack chains together • more spread apart -> form a thinner membrane • shorter chain: reduces tendency of hydrocarbon tails to interact with one another -> membrane remains fluid at lower temps • cholesterol content • temperature • at low temps, lipids show less movement • lipid is in paracrystalline state (more rigid state) • at higher temps (20-40C), plasma membrane becomes more fluid

• tend to form paracrystalline structures (less space btwn phospholipid tails • the more saturated fatty acids a lipid bilayer contains, the higher the paracrystalline-to-fluid transition temp of a membrane

• yields to formation of kinks -> inhibits paracrystalline conformation (more space btwn tails)

• also contains cholesterol and glycolipids • eukaryotic plasma membranes contain large amounts of cholesterol • cholesterol molecules improve permeability-barrier properties of lipid bilayer • orient themselves with hydroxyl groups close to polar heads of phospholipid molecules • rigid steroid ring can support hydrocarbon chains and stabilize them • inhibits possible phase transitions

• orients itself in bilayer with its small hydroxyl groups close to the polar head groups of adjacent phospholipids and the hydrophobic chain btwn phospholipid tails (hydrophobic interaction) • helps keep membrane more stable, stiffening bilayer and making it less fluid and less permeable • stops tails from clumping and interacting • helps membrane remain fluid

• passive transport - transporters or permeases

• simple diffusion • facilitated diffusion • electrochemical gradient • channels

• carriers • active transporters • pumps • symport systems

• complex and intricate network of macromolecules • provides structural and biochemical support to surrounding cells • common functions: • cell adhesion, cell-cell communication and differentiation • animal ECM includes: interstitial matrix and basement membrane/basal lamina

• present btwn diff animal cells (in intercellular spaces) - gels of polysaccharides and fibrous proteins fill intercellular spaces -> act as compression buffer to ECM

• sheet-like depositions of ECM with epithelial cells

• bone tissue: collagen fibers and bone mineral • loose connective tissue: reticular fibers and ground substance • blood: ECM is blood plasma

• macromolecules in ECM produced locally by cells in the matrix and secreted via exocytosis