ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Six Outline INTRODUCTION Cell Survival Requires Interactions With the Environment fig 6.1 Every Cell Is Encased in an Interactive Plasma Membrane (Plasmalemma) THE LIPID FOUNDATION OF MEMBRANES Membranes Are Composed of Protein Collections Within a Lipid Framework Phospholipids Form the foundation of cell membranes fig 6.2 Backbone is a three-carbon glycerol molecule Attach to three fatty acid chains in a fat molecule Attach to two fatty acid chains in a phospholipid molecule Phosphate group attaches a polar organic alcohol to the third carbon One end of the molecule is strongly nonpolar and water insoluble The other end is strongly polar and water soluble Phospholipids are diagrammed as a polar head with two nonpolar tails Phospholipids Form Bilayer Sheets Interactions between phospholipids and water Nonpolar tails are pushed away from water molecules Nonpolar tails cannot form hydrogen bonds with water Water molecules form bonds with each other excluding nonpolar tails Spontaneously form a lipid bilayer fig 6.3 Polar heads face water on either side Nonpolar tails face inward toward each other Lipid bilayer sheets are the foundation of biological membranes Nonpolar interior repels water-soluble molecules Proteins in the lipid bilayer allow passage of polar molecules The Lipid Bilayer Is Fluid Lipid molecules move within the stable bilayer Closely aligned tails create less fluid membranes Less closely aligned tails create more fluid membranes Associated with double-bonded carbons in the tail chain May contain short lipids that prevent contact between tails fig 6.4 ARCHITECTURE OF THE PLASMA MEMBRANE Cell Membranes Are Assembled From Four Components tbl 6.1 Lipid bilayer foundation fig 6.5 Other components distributed within foundation Provides a flexible matrix which is a barrier to permeability Transmembrane proteins fig 6.6 Move within the lipid bilayer, not located in fixed positions Provide channels through which molecules and information pass Network of supporting fibers fig 6.7 Structurally supported by proteins like spectrin Connects membrane proteins to cell's actin filament cytoskeleton Control lateral motion of key membrane proteins fig 6.8 Exterior proteins and glycolipids Membranes assembled in the ER, transferred to the Golgi Golgi adds glycocalyx, chains of sugars, to membrane proteins and lipids fig 6.9 Sugar molecules function as cell identity markers Some Proteins Traverse the Lipid Bilayer Via single spiral helix of nonpolar amino acids fig 6.10 Include receptor proteins Portion of receptor that sticks outward binds with molecules Binding induces changes in part of protein on the inside Channel proteins wind back and forth through the membrane Create a hole in the membrane like that in a donut Locked into shape by several nonpolar helical segments Water-soluble molecules pass through these channels Example: photosynthetic transmembrane protein Non-polar beta-pleated sheet transmembrane proteins Characteristic motif where sheets fold back over themselves Form a pore called a beta-barrel Examples include porins of bacterial outer membranes fig 6.11 HOW A CELL'S PLASMA MEMBRANE REGULATES INTERACTIONS WITH ITS ENVIRONMENT Structure of the Membrane Enables a Broad Range of Interactions fig 6.12 Interactions With the Environment Include Passage of water Passage of bulk material Selective transport of molecules Reception of information Expression of cell identity Physical connection with other cells THE PASSAGE OF WATER INTO AND OUT OF CELLS Molecules Dissolved in Water Are in Constant Random Motion Diffusion fig 6.13 Causes net movement from higher to lower concentration Equilibrium when there is uniform concentration Aqueous solution: a mixture of water and molecules Solvent: water, most common molecules in the solution Solute: other molecules dissolved in the water Both water and molecules diffuse down their concentration gradient Osmosis Membrane prevents equal motion of solvent and solute Many solutes cannot pass through biological membranes Water can freely pass through membrane Osmosis: diffusion with net movement of water across a membrane fig 6.14 Concentration of all solutes establishes osmotic concentration Solution with higher concentration is hyperosmotic Solution with lower concentration is hypoosmotic Solutions with equal concentrations are isosmotic Cellular changes fig 6.15 Shrinks (looses water) when hypoosmotic to environment Swells (gains water) when hyperosmotic to environment Hydrostatic pressure of cytoplasm pushes against cell membrane Osmotic pressure: force required to stop osmosis across membrane fig 6.16 Equilibrium between osmotic concentration difference and pressure When pressure is too high most unsupported cells burst Cells with cell walls can withstand pressure and will not burst fig 6.17 Maintaining Osmotic Balance Many cells adjust internal solute concentration to match environment Cells are isosmotic with environment Cell is in osmotic balance with environment Multicellular organisms similarly regulate composition of body fluids Water removal Gaining water is a dilemma of eukaryotes in fresh water Hyperosmotic with respect to environment Water removal, extrusion, requires expenditure of energy Example: contractile vacuole of Paramecium Plant cell walls Plant cells do not circulate in isosmotic solution Cells are hyperosmotic with respect to their immediate environment Possess a high solute concentration within the central vacuole Osmotic pressure pushes cytoplasm against cell wall, causes rigidity Turgor pressure: internal pressure of plant cells BULK PASSAGE INTO AND OUT OF THE CELL Phagocytosis and Pinocytosis Mechanism to get large polar molecules through cell membrane Called endocytosis fig 6.18 Membrane encircles and engulfs food particle Part of exterior medium captured within a vesicle Phagocytosis: material brought in is particulate Pinocytosis: material is liquid, contains dissolved molecules Receptor-mediated endocytosis Associated with transport of specific macromolecules Cytoplasmic side of plasma membrane is covered with clathrin Indentations in plasma membrane called clathrin-coated pits fig 6.19 Pit closes over when proper molecule enters Process is highly specific, very fast but transient Fluid-phase endocytosis is same process with fluids Exocytosis: Reverse of Endocytosis Materials extruded from cell by discharge from surface vesicles fig 6.20 Utilized by plants to construct cell wall Includes protist contractile vacuole discharge Used by animal cells to secrete chemical materials SELECTIVE TRANSPORT OF SUBSTANCES ACROSS MEMBRANES Disadvantages of Endocytosis and Exocytosis Requires expenditure of large amounts of energy Not usually selective to materials brought inward Selective permeability gained through use of channels or carriers Diffusion of Ions Through Channels Review definitions of ion, cation and anion Due to charge, ions are repelled by non-polar lipid bilayer interior Movement of ions requires membrane transport proteins Water-filled pore spans membrane No interaction between channel and ion Net movement dependent on concentration and voltage Facilitated Diffusion fig 6.21 Selective carriers allow passage of certain molecules in both directions Facilitate movement with physical binding Rate of movement can become saturated Increasing concentration affects movement only to a certain point When all carriers are occupied diffusion reaches its limit Capacity of the transport system is at maximum Example: transport of Cl- and HCO3- in red blood cells Prevents buildup of unwanted materials Essential characteristics Specific to certain molecules with a given carrier Passive process driven by internal and external concentrations System may become saturated when all carriers are in use Active Transport Transport of molecules against concentration gradient Expends energy Involves highly selective protein carriers Molecules moved may be ions, sugars, amino acids or nucleotides fig 6.22 Enables cell to concentrate materials inside itself Allows cell to export materials even if concentrated on outside The sodium-potassium pump Cells maintain low internal concentration of sodium: pump it out fig 6.23 Cells maintain high internal concentration of potassium: pump it in Energy provided by adenosine triphosphate (ATP) Associated with conformational changes in transmembrane protein fig 6.24 Three molecules of Na+ bind to cytoplasmic subunits Complex binds, cleaves one ATP; ADP released, Pi remains bound Three Na+ molecules move across channel are released on outside Complex binds two K+ molecules Pi released, complex disassociates K+, released to the inside Process removes three Na+ and brings in two K+ Cotransport and countertransport Accumulate amino acids and sugars against concentration gradient Cotransport moves molecules and Na+ together fig 6.25 Na+ moves down its concentration gradient Molecule moves up its concentration gradient Countertransport couples Na+ movement with Ca++ or H+ Na+ and molecule bind to same transport protein Bind on opposite sides of membrane Na+ moves down its gradient Molecule extruded against its concentration gradient The proton pump Involves two special transmembrane protein channels One pumps protons (H+) across membrane, expends energy Creates proton gradient with more H+ on outside of membrane Diffusion drives protons back down concentration gradient Protons return by other channel coupled to ATP production Process called chemiosmosis THE IMPORTANCE OF THE PLASMA MEMBRANE tbl 6.2 Lipid Membrane Separates Cell From Its Environment Membrane Embedded Proteins Enable Cell to Communicate With Environment