BSC 114 test number 1 notes
BSC 114 test number 1 notes BSC 114
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This 22 page Bundle was uploaded by Ashley Bartolomeo on Thursday January 21, 2016. The Bundle belongs to BSC 114 at University of Alabama - Tuscaloosa taught by Edwin Stephenson in Winter 2016. Since its upload, it has received 79 views. For similar materials see Principles Of Biology I in Biological Sciences at University of Alabama - Tuscaloosa.
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Date Created: 01/21/16
Chapter 3 Water Hydrogen bonds in water Properties of water o Cohesion and adhesion o Specific heat/ heat of vaporization o Properties of ice o Solubility of compounds in water Water ionization Covalent bonds Polar and nonpolar covalent bonds More from topic 2: electron distribution for various bonds o Non-polar covalent bond: the two atoms are similar/same in electronegativity C-C C-H o Polar covalent bond: the two atoms are very different in electronegativity O-H Hydrogen Bonds In a polar covalent bond, each atom has a partial charge due to unequal sharing of electron pair o Example: O-H bond in water Opposite partial charges attract. Result: water molecules are bound in a hydrogen bond lattice Properties of Water Cohesion and adhesion H-bonds cause water molecules to cohere to each other, ie., stick together, and to adhere to other polar molecules o Cohesion: attraction of the same type of molecule, eg., water-to- water o Adhesion: attraction of two different molecules, eg., water-to- another polar molecule Relevance to living organisms: cohesion accounts for water transport in plants (a column of water is “pulled” through vascular tissue), surface tension, etc. Specific Heat & Heat of Vaporization Water has a high specific heat (amount of energy absorbed per rise in temperature). Heat= molecule movement. Because water molecules are bound together, large amounts of energy required to disrupt lattice. o 1 calorie = energy required to raise 1 gram of water 1 degree C o Relevance to living organisms: buffering from temperature fluctuation High heat of vaporization (amount of energy to go from liquid to gas phase). Water has high HoV because lattice of bound molecules in liquid water resists “escape” into gas phase o Relevance: evaporative cooling. Large amounts of heat removed by evaporation Ice Solid H-bond lattice is fixed, with defined crystalline structure Consequence: Ice is less dense than water, so ice floats and insulates bodies of water Solubility Polar and charge compounds dissolve in water (hydrophilic) o Surrounded by a shell of water molecules, the hydration shell Non-polar molecules (no hydrogen bonds) are insoluble in water (hydrophobic) o Excluded from hydrogen bond lattice, eg., oils. o Relevance: membranes Chemistry Digression Most chemical reactions have an equilibrium, indicated by double arrows A + B C o Compounds A and B combine to make C, and C breaks down to make A and B o At equilibrium you find a fixed ratio of (A and B) : C When emphasizing a particular reaction sometimes a single arrow is used, usually when reaction is essentially complete at equilibrium o E + F G Brackets indicate concentration of the substance enclosed Water Ionization Ionization via gain or loss of electrons, eg., Na and Cl Ionization can also happen when molecules release H+ Water ionization: H2O H+ +OH- o (A rare event: the concentrations of H+ and OH- ions are 10^-7 M each = about 1 ionized molecule vs. 500 billion unionized water molecules) o H+ = hydrogen ion, or a proton. May join a water molecule to form a hydronium ion, it., H2O + H+ H3O+ o OH- = hydroxide ion In pure water H+ = OH- In solutions the concentrations of H+ and OH- can differ o If H+ = OH-, solution is neutral o If H+ > OH-, solution is acidic o If H+ < OH-,solution is alkaline (AKA basic) Acids and Bases Measure of acidity/ alkalinity: pH pH = -(loghydrogen ion) o pH of pure water = 7, eg., hydrogen ion = 10^-7 o 10x increase in H+ to 10^-6 M would be pH = ^, etc. An acid is a compound that ionizes to release H+ & increases H+ (pH < 7): HCl H+ + Cl- A base is a compound that ionizes to release OH-, or directly combines with H+; both decrease H+ (pH > 7): NaOH Na+ + OH- NH3 + H+ NH4+ Buffers Buffers: chemicals that resist changes in pH. A buffer absorbs excess H+ when H+ is high, and donates H+ when H+ is low. H2CO3 HCO3- + H+ Relevance: pH within cells stay within a narrow range (slightly basic). Learning goals Hydrogen bonding of water Properties of water relevant to living organisms: o High cohesion/ adhesion o High specific heat and heat of vaporization o Structure of ice o Differential solubility of compounds in water as a solvent Water ionization o pH = measure of acidity / alkalinity ~ hydrogen ion concentration o acids and bases o buffers Chapter 4 Carbon Major Subtopics models and the depiction of carbon – contains molecules function groups isomers Carbon Chemistry = Organic Chemistry C has a valence of 4. Makes 4 covalent bonds. Does not ionize Organic chemistry = chemistry of C- containing compounds Carbon Containing Molecules Models C- containing molecules are often three-dimensional although drawn as two- dimensional Organic chem shorthand: carbons and hydrogens often not shown o Assume C at junction of two bonds o Assume # of hydrogens to complete 4 bonds C makes single, double or triple bonds Hydrocarbons: consist of carbon and hydrogen only Compounds can be linear, branched or ring molecules Functional Groups Reactivity of organic molecules is determined by functional groups = small molecules attached to C 6 groups: hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate 1 more: methyl (not a function group, but important in biology) Hydroxyl -O-H polar, form hydrogen bonds characteristic of alcohol Carbonyl C=O Electronegativity of C < O, so C=O bonds are polar forms hydrogen bonds Carboxyl (ie., a hydroxyl plus a carbonyl) hydroxyl ionizes to –O-, releasing H+ because it increases the conc of H+ it is an acid, eg., acetic acid Amino NH2 Ionizes: accepts a free H+ to become NH3+. Decreases H+ conc, so it’s a base Sulfhydryl -S-H important in protein structure: two sulfhydryl groups make a covalent bond to crosslink proteins Phosphate PO4- Valance of phosphorous = 5 Non ionized structure An acid: -OH ionizes to O- Methyl -CH3 not reactive, so not really a functional group often added to biological molecules to modify structure/ function Isomers isomer: compound with the same chemical formula, but different structures 3 types o structural isomer o geometric isomer o enantiomer Structural Isomer different in arrangement of covalent bonds Cis- trans Isomer different atom arrangements around a double bond. Inflexibility of the double bonds make these different (Cis: on the same side. Trans: on opposite sides) (Cis-trans isomer is also known as geometric isomer) Enantiomers different arrangement of atoms on groups attached to an asymmetric carbon asymmetric carbon has 4 different functional groups attached chemical mirror images. ~identical chemical properties biological relevance: usually only one enantiomer is found in biological molecules. Eg., all amino acids in proteins are L amino acids Learning Goals the properties of carbon and the diversity of carbon-containing compounds functional groups. Recognize, identify, know structures and properties of o 6 true functional groups o methyl isomers. Differences in structure defined by: o structural isomers o cis-trans isomers o enantiomers Chapter 5 Biological Molecules A. polymer formation, carbohydrates and fats Major topics Polymers Carbohydrates Lipids Proteins Nucleic acids Polymers Macromolecules (=large molecules) are polymers: long chains of similar subunits o Each unit = monomer Dehydration Polymers are assembled by the same chemical mechanism, a dehydration reaction o Dehydration reaction: a hydroxyl group and hydrogen are replaced by a covalent bond, producing a polymer, releasing water All macromolecules are assembled by this basic reaction Hydrolysis Hydrolysis is opposite of dehydration: o Water molecule replaces a covalent bond, adding a hydroxyl to one and a hydrogen to the other Macromolecules are broken down through hydrolysis, in the cell of elsewhere (eg., digestion) Carbohydrates Carbohydrates = sugars and polysaccharides o Name derived from carbon-hydrate, eg., equal amounts of carbon and water Types of carbohydrates o Sugars Monosaccharides – monomer that others are based on (AKA simple sugar) Disaccharides o Polysaccharides o Examples: starch, cellulose, chitin Monosaccharides Commonly 3, 5 or 6 carbons Structure: 1 carbonyl group, remaining C bonds to –OH and –H Sugar families named according to # of carbons o 3 C = triose o 5 C = pentose o 6 C = hexose 2 families based on carbonyl position. o Aldoses: carbonyl is terminal C o Ketoses: carbonyl is internal C Sugar Isomers Arrangement of –H and –OH at a single asymmetric carbon causes different properties o Eg., glucose and galactose are enantiomers Ring Sugars Pentoses and hexoses spontaneously circularize to rings When sugars form rings, the carbon atom in the reaction (C #1) is an asymmetric carbon and thus 2 different enantiomers are possible o Example: a-glucose and b-glucose Disaccharides Two monosaccharides joined by a dehydration reaction o Gylcosidic linkage = covalent bond between two sugars o Monosaccharide components and bond position determine disaccharide identity Sucrose = glucose + fructose Maltose = glucose + glucose Polysaccharides Generally not sweet Multiple monosaccharides joined in a polymer o 100s to 1000s of monomers o glycosidic linkages o unbranched or branched o properties determined by monosaccharide monomers and bond type polysaccharides with diff properties are made from a and b monomers examples: o starch, glycogen (carbohydrate storage in plants, animals) o cellulose (plant structural) o chitin (fungal cell wall, insect exoskeleton, dissolving surgical thread) Artificial Sweetener artificial sweeteners mimic the structure of sugars example: Splenda, a disaccharide like molecule. Note chlorine atoms in place of hydroxyls – these prevent breakdown by the body, so no calories chlorine can form covalent bonds (as in Splenda) or ionic bonds (as in NaCl) Lipids fats and oils phospholipids steroids not really polymers or macromolecules common property: hydrophobic Fats and Oils function: storage molecules structure: glycerol and fatty acids glycerol: 3-carbon alcohol fatty acid: linear hydrocarbon with terminal carboxyl o most common have 16 or 18 carbons Synthesis fatty acids are joined to glycerol by a dehydration reaction Triacylglycerol triacylglycerol: 3 fatty acids attached to glycerol o all fatty acids may be the same of may be different Saturated and Unsaturated fatty acids may have all single bonds or some double bonds o all single bonds: hydrocarbon chain is straight fats with all single bond fatty acids = saturated fats o some double bonds: hydrocarbon chain is bent o oils with some double bond fatty acids = unsaturated fats saturated fats pack together tightly o high melting point o solid at room temperature o animal and some plant storage fats unsaturated fats do not pack because of kink in hydrocarbon chain o low melting point o liquid at room temperature = “oil” o most plant storage fats Phospholipids function: major component of membranes. Not energy storage structure: o 2 fatty acids attached to glycerol. Fatty acids can be same of drdferent, saturated or unsaturated o 3 glycerol carbon attached to phosphate and to a small polar molecule o presence of both hydrophilic and hydrophobic regions is important for membrane structure Steroids structure: basic structure is four attached rings o examples: cholesterol, estrogen, testosterone, anabolic steroids o different steroids differ in chemical groups attached to basic 4- ring structure Proteins ubiquitous and diverse. 50% mass of cells functions o enzymes o structure o transport o movement o DNA synthesis o Transcription, translation and regulation Monomers Amino Acids Polymer = protein (AKA “polypeptide”) Monomer = amino acid Amino acid structure: amino group – a carbon – carboxyl group A carbon also attached to –H and an “R” group o “R” (AKA “side chain”) = 20 different chemical groups The 20 common amino acids. R groups in colors according to chemical properties Non- polar amino acids The properties of amino acids are defined by the properties of their R groups Synthesis Peptide Bond Amino acids joined by dehydration reaction between amino and carboxyl groups Bond = peptide bond Polymer (polypeptide): o Unbranched o Backbone = repeating N-C-C - N-C-C - N-C-C o Side chains extend from backbone Structure Primary structure – sequence of amino acids Secondary structure - local folding; H bonds between polypeptide backbone Tertiary structure – overall 3D folding; interactions between amino acid R groups Quaternary structure – association of 2 or more polypeptides (not all proteins) Primary Primary structure = order of amino acids in polypeptides o Determined by instructions in a gene o Range of polypeptide lengths: tens to thousands of amino acids o Because of the amino- to – carboxyl nature of the peptide bond, one end of the peptide has an exposed amino group, and the other a carboxyl group = N-terminus and C-terminus Secondary Secondary structure = local interactions, due to H-bonds between N-H and C=O groups in the polypeptide backbone 2 common secondary structures, present in many different proteins: o a-helix: backbone makes a spiral o b-pleated sheets: backbone makes waves, or pleats a-helix & b-pleated sheets a-helix held togetthr by hydrogen bonds between carbonyl and amino groups of every 4 amino acid in the helix b-pleated sheets lie adjacent and form a sheet. Adjacent strands aligned by H-bonds Tertiary Tertiary structure = overall large scale 3D structure. Interactions between amino acid R groups o Examples: lysozyme (antibacterial component of egg whites) Compact, roughly spherical proteins (eg., lysozyme) = globular Extended, roughly linear proteins (eg., collagen) = fibrous Protein Folding Secondary and tertiary structures: what determines how proteins fold? o Chemical interactions between amino acids: Hydrogen bonds between C=O and N-H groups in the backbone and Various interactions between R groups Ionic and hydrogen bonds and Van der Waals interactions Hydrophobic interactions Disulfide bridges o Hydrophobic interactions Water makes H-bonds with polar and charged amino acid side chains. Non-polar (hydrophobic) R groups fold to the internal, non-aqueous portion of the protein o Disulfide bridges: two cysteine amino acids adjacent through folding form a covalent bond o Cysteine R groups is –CH2-S-H o Two cysteines –CH2-S-H H-S-CH2- are linked by a covalent bond –CH2-S –S-CH2- + H2 = disulfide bond The function of some proteins is to help other proteins fold Helper proteins = chaperonins Complex mechanism (omit) Quaternary Quaternary structure = functional protein composed of >1 polypeptide. Many but not all proteins o Examples: collagen (3 polypeptides intertwined). Hemoglobin: 2 molecules of a-globin + 2 molecules of b-globin o Hydrogen bonds, ionic bonds, van der Waals, hydrophobic interactions, disulfide bonds Genetic Diseases Most genetic diseases are caused by mutations in genes that result in an altered amino acid sequence o Example: sickle cell disease o Causes change in protein folding, assembly into fibers, misshapen red blood cells with lower O2 capacity Nucleic Acids Monomer = nucleotide o Components of a nucleotide: Phosphate Sugar Base Polymer = polynucleotide o Examples: DNA and RNA Nucleotide Structure Phosphate and ribose sugars Phosphate: -PO4 Sugar: pentose o Ribose in RNA Typical pentose sugar o Deoxyribose in DNA Atypical – has 2 H’s on C #2 rather than 1 H and 1 OH Nitrogenous Bases Nitrogenous bases. Two families: o Pyrimidines. Single ring. 4Cs and 2Ns in ring. Different functional groups o Purines. Double ring. 5Cs and 4Ns in rings. Different functional groups Nucleotides Nucleotides are the monomers for DNA and RNA, AND Nucleotides are the energy containing and regulatory molecules ATP and GTP Polymers DNA and RNA Nucleotides linked together in polymers: o Sugar – phosphate – sugar – phosphate backbone with bases sticking out Learning Objectives Dehydration and hydrolysis reactions. Mechanisms and identification Carbohydrates o Generic formulas and structures of sugars in linear and ring form. Isomers o The formation of disaccharides and polysaccharides o Biological roles of carbohydrates o How the artificial sweetener Splenda resembles and differs from sugars Lipids o Fats and oils. The structure of glycerol and fatty acids. How joined to form fats and oils. Structures of fats and oils o Saturated and unsaturated fatty acids, and the structural differences between them o Structures of phospholipids and steroids, and their biological functions The structure of amino acids The formation of peptide bonds Primary, secondary, tertiary and quaternary structure of proteins Forces responsible for protein folding The structure of nucleotides and their components o Sugar: ribose and deoxyribose o Phosphate o Bases: purine and pyrimidine The structure of polynucleotides Chapter 6 Tour of the Cell A. Membrane-bound organelles Major Topics Classification of living organisms based on cell type Membrane bound organelles Methods used to study cell structure Basis of Cell Structure All cells have membranes Membrane: barrier to passage of most molecules. Selective, regulated permeability. Plasma membrane: encloses the entire cell. All cells have plasma membranes Cytoplasm: everything inside the plasma membrane Organismal Classification based on cell type Fundamental division of living organisms, eukaryotes, prokaryotes and archaea, is based on cell structure o Eukaryotes: “true kernel”. Have a nucleus and other organelles “kernel” = nucleus euks = animals, plants, fungi, true algae, protists o prokaryotes: “pre-kernel” o bacteria (includes blue-green algae) o archaea Eukaryotic Cells All cells have an external membrane, the plasma membrane Eukaryotic cells also contain internal membrane-bound organelles Organelle: membrane-enclosed structures Membrane-Bound Organelles Nucleus The endomembrane system o Smooth endoplasmic reticulum o Rough endoplasmic reticulum o Transport vesicles o Golgi apparatus o Lysosomes o Vacuoles Mitochondria, chloroplasts and peroxisomes Nucleus Function: contains genes and necessary enzymes and regulatory molecules, etc. Structure: membrane = Nuclear envelope o Double membrane. Outer membrane continuous with endoplasmic reticulum o Inner and outer membranes are fused in some locations, forming “holes” or nuclear pores o Nuclear lamina: network of fibers on inside of inner membrane Inside the nucleus: o Chromosomes: structures that contain genes o Chromosomes are made of chromatin: DNA plus protein Endomembrane System Endomembrane system: network of vesicles. (vesicle: small membrane “sac”) Functions: protein synthesis and metabolic functions Components o Smooth endoplasmic reticulum o Rough endoplasmic reticulum o Golgi apparatus o Transport vesicles o Lysosomes o Vacuoles Smooth Endoplasmic Reticulum Structure: network of vesicles & tubes, usually close to nucleus o Lumen: interior of vesicle Function: metabolic. Enzymes are concentrated in lumen o Lipid synthesis: membrane lipids, steroid hormones, etc. o Carbohydrate metabolism o Detoxification of drugs and other “foreign” chemicals Rough Endoplasmic Reticulum Structure o Layers of flattened sacs o Quantity varies depending on cell type o Ribosomes attached to outer surface. (roughness = ribosomes) Digression: ribosomes o Function: protein synthesis o Complex of protein and RNA o Location: cytoplasm “bound” ribosomes: attached to outer surface of rER “free” ribosomes: not attached to ER Protein synthesis in rER: o Ribosomes on rER membrane synthesize proteins that pass thru a pore into rER lumen o In rER lumen, proteins fold in 3D shapes and carbohydrates are added rER is 1 stage of secretory pathway and related pathways Goal: synthesize, process and sort proteins for export from cell (secretion) and other destinations within cell (lysosomes, vacuoles) The secretory pathway o 1. rER: synthesis, processing o 2. Golgi apparatus: processing, sorting o 3. Plamsa membrane: secretion Transitions in stages above: transport vesicles carry proteins from rER to Golgi, between Golgi stacks, from Golgi to plasma membrane, etc. Proteins synthesized in rER: o Secreted proteins (outside the cell) o Plasma membrane proteins o Proteins destined for various endomembrane compartments, eg., sER, rER, Golgi, lysosomes rER quantity varies by cell type: o cells in glands have lots of rER to produce secreted enzymes/ hormones (pancreas, salivary, adrenal, etc.) Golgi Apparatus Structure: flattened stacks of vesicles, more compact, less extensive then ER o Transport vesicles bud from rER, fuse with cis face of Golgi stack o Transport vesicles carry form one level of Golgi to next o Transport vesicles leave from trans face to plasma membrane, lysosomes & vacuoles o Cis: near. Trans: away from Functions o Processing More carbohydrates added to proteins o Sorting Contents are sorted and marked for destinations: Cell surface (secretion/ exocytosis) Lysosomes (digestive enzymes) Various vacuoles Lysosomes Structure: ~Spherical organelles Function: digestion of endocytosed particles and work-out organelles o Some cells engulf other cells via phagocytosis (cell eating). Internalized food vacuoles fuse with lysosomes for digestion o Damaged organelles are enclosed by vesicles, which fuse with lysosomes for digestion (autophagy) Lysosomal digestive enzymes are routed to lysosomes via rER and Golgi Vacuoles Various functions and structures o Contractile vacuoles pump excess water from cell (freshwater protozoans) o Food vacuole: product of phagocytosis o Central vacuole: mostly storage (plant cells) o Fat storage vacuoles in adipose tissues Peroxisomes Structure: roughly spherical with granular/ crystalline core Function: o Detoxification of alcohol & poisons, breakdown of drugs, etc (liver). o Breakdown of fatty acids o Detox reactions remove hydrogens from drugs to produce H2O2 (hydrogen peroxide) Methods Light Microscopy Advantages o Many specimen prep methods are simple o Can be used on living cells o Methods to detect specific molecules o Variations to enhance contrast Disadvantages o Most organelles too small o Resolution is limited to ~0.2 um (physical property of light), although computer processing can partially circumvent this limit Electron Microscopy Advantages o High resolution (~2nm), enough to see organelles Disadvantages o Not for living specimens o Complex specimen preparation o Difficult methods to detect positions of specific molecules Cell Fractionation Goal: purify organelles based on size or density o 1. Homogenize (gently grind up) cells o 2. Centrifuge at low speed, to produce a pellet and supernatant o 3. Centrifuge at higher speed o 4. Centrifuge at even higher speed Tour of the Cell B. the cytoskeleton, extracellular components and cell junctions Major Topics The cytoskeleton Extracellular Components Cell Junctions The Cytoskeleton “Skeleton” and “muscles” of the cell. Necessary for strength/ rigidity and force/ motility Three components: o Microtubules o Microfilaments (AKA actin filaments) o Intermediate filaments Microtubules Composition: hollow tube composed of protein tubulin o Tubulin: dimer of a-tubulin and b-tubulin Position within cell: internal, appear to radiate from a ~central point, the centrosome Function: rigidity, strength, organelle movement Motor proteins move along microtubules using energy from ATP hydrolysis Can move attached organelles or slide microtubules past each other Cilia and Flagella Microtubule based Structures: o Cilia: short and many o Flagella: long and few (usually 1 or 2) Structure: cytoplasmic extension containing complex arrangement of microtubules Bending results from microtubules sliding past each other, driven by motor protein dynein Microfilaments Composition: rod composed of globular protein actin (AKA “actin filaments”) Position within the cell: usually peripheral Function: cell motility and structural Motor: myosin Functions: o Reinforce microvilli o Amoeboid movement o Cytoplasmic streaming o Muscle contraction Intermediate Filaments Composition: rope-like filaments composed of fibrous proteins Position within the cell: internal, throughout Function: strictly structural Motors: none. No function in movement Also extracellular intermediate filaments: skin, hair, nails, feathers, etc composed of keratin Extracellular Components Extracellular components: structures formed outside the cell, ie., external to the plasma membrane o Cell wall (plants, fungi, algae) o Extracellular matric (animals, protozoans) Cell Wall Provides the main structural feature of plants Components: cellulose and other polysaccharides, proteins and glycoproteins Glycoproteins = proteins with attached polysaccharides Components secreted by cells into extracellular space via the secretory pathway Extracellular Matrix Animal cells Structure: interlocked extracellular fibers made of proteins, polysaccharides, glycoproteins, etc. o Collagen (most abundant extracellular matric protein) also fibronectin and proteoglycans o Cells anchor to extracellular matrix proteins through integrin proteins Synthesis: components secreted via secretory pathway Function: mechanical strength. Cell-to-cell communication In some tissues, extracellular matrix >> cells. Examples: o Tendons and ligaments o Bone and cartilage o Teeth Cell Junction Cell junctions: connections between cells o Cell-cell connections in plants o Cell-cell connections in animals Plants: o Plasmodesmata: holes in cell wall. Membranes of adjacent cells are continuous and cytoplasm is in contact Animals: o Tight junctions: specialization connects membranes of adjacent cells o Acts a barrier to passage of fluid between adjacent cells Example: intestine, blood vessels. Contents in the lumen cannot ooze between cells Desmosomes Animals: o Desmosomes: mechanical cell-cell attachments, reinforced with intermediate filaments Examples: skin Gap Junctions Animals: o Gap junctions: cytoplasmic continuity between adjacent cells, passage of molecules Equivalent in function to plasmodesmata Learning Objectives Evolutionary relationships of organisms are reflected in shared cellular structures Nuclear envelope Components of endomembrane system and their structures and positions in the cell Routing of proteins through the endomembrane system and the function of each component in this process Methods used to study cell structure The cytoskeleton o Microtubules, microfilaments, intermediate filaments Composition, function, typical positions in the cell, motor molecules Structure of cilia and flagella Structure and function of plant cell walls Structure and function of the animal extracellular matrix Functions of plant and animal cellular junctions Chapter 7 Membranes Major Topics Membrane Structure: The Fluid Mosaic Model o Lipid bilayer o Membrane proteins o Membrane fluidity Membrane function o Permeability of the lipid bilayer o Osmosis o Membrane transport o Bulk transport Membrane Structure Membranes separate external from internal, and subdivide cytoplasm into organelles o Cell contents and external environment are aqueous. Membranes form a hydrophobic barrier between inside and outside o Purpose: concentration and regulation of biological chemistry, and compartmentalization of function Fluid Mosaic Model Positions of two components are accounted for by the Fluid Mosaic Model: o 1. The lipid bilayer o 2. Proteins A. integral membrane proteins B. peripheral membrane proteins Lipid Bilayer Phospholipid structure: o Fatty acid tails (hydrophobic) o Polar “head” (hydrophilic) o Amphipathic: molecules with both hydrophobic and hydrophilic regions Lipid bilayer: double layer of lipids o Polar head groups on the periphery make H-bonds & ionic bonds with water & other hydrophilic molecules o Fatty acid tails internal form an internal hydrophobic “core” Membrane Proteins Integral membrane proteins Integral Membrane Proteins “float” like icebergs in a 2D sea of lipids o Most are transmembrane proteins that span the lipid bilayer o Hydrophobic regions of proteins are embedded in the core of the bilayer o Hydrophilic parts are exposed on either side Most integral membrane proteins span the bilayer multiple times o Membrane spanning parts are usually a-helices with hydrophobic amino acids o Hydrophilic amino acids on the cytoplasmic and extracellular faces Integral membrane proteins are more or less free to move in the lipid bilayer Experiment: membrane proteins are labeled with dyes, one color for each cell Cells are fused. Membrane proteins move in the hybrid cell bilayer, eventually become mixed and uniform Peripheral Membrane Proteins Peripheral membrane proteins are not embedded in the membrane. Attached by covalent or non-covalent bonds to: o Lipid head groups o Integral proteins, or o Other peripheral proteins Membrane Fluidity Membrane fluidity: degree to which lipids and proteins move in the plane of the membrane Fluidity varies with temperature (higher fluidity at higher temp) Fluidity varies with lipid composition: o Many unsaturated phospholipids = high fluidity o Many saturated phospholipids = viscous (low fluidity) Organisms adjust to temp changes by varying membrane lipid composition Membrane Function Permeability of the Lipid Bilayer Movement of molecules across the lipid bilayer (no proteins): o Cross easily: non-polar molecules (CO2, O2, steroid hormones) o Cross slowly: small polar molecules (H2O) o Do not cross: larger polar, or charged molecules of any size (ions, sugars, amino acids, etc). Diffusion Diffusion: spreading of a molecule in available space Molecules move spontaneously from high to low concentration, ie., down a concentration gradient Equilibrium: no net movement = equal concentration everywhere Example: sugar dissolved in a cup of water: o Sugar concentration is high at the bottom of the cup, lower at the top. Water concentration is high in the top part of the cup, lower at the bottom o Eventually the sugar and water concentrations become uniform, through diffusion Osmosis Osmosis: diffusion of water across a membrane. A special case of diffusion Relevance to biology: water can cross lipid bilayers but dissolved chemicals (salts, sugars, etc) cannot Illustration of osmosis: two sugar solutions separated by a membrane. Membrane allows water to cross, but not sugar The plasma membrane of cells (and other membranes) is a semi- permeable membrane o Allows the passage of water o Prevents the passage of most solutes Solvent: liquid in which solute is dissolved (in biology = water) Solute: molecule dissolved in a solvent Compare external solute concentration, relative to internal (inside the cell) Hypotonic o External solute concentration is lower than that of cell. external water concentration is higher o Result: water rushes into cell (high conc to low conc). Animal cell swells and explodes o Rigid cell walls limit cell expansion (plants, fungi, bacteria) o Osmoregulation example: Paramecium (fresh water protozoan, no cell wall). Water is collected in contractile vacuole, which expels water by contraction. Hypertonic o External solute conc is higher than internal. external water conc is lower than internal o Result: cells lose water and become shriveled o Osmoregulation example: ocean fish and birds must excrete salt to avoid becoming hypertonic Isotonic: solute conc same as that inside cell o Result: no movement of water. Red blood cell remains same size o Animals maintain internal fluids that are isotonic with cells Membrane Transport Cells move molecules across membranes… o …from higher to lower concentration; “down” their concentration gradient; passive transport = diffusion or facilitated diffusion… or o …from lower to higher concentration; “against” their concentration gradient = active transport Passive Transport passive transport: molecule moves from high to lower concentration o Diffusion: molecule crosses membrane without assistance. Eg., O2, CO2 into lung cells, water, etc. o Facilitated diffusion: large, polar or charged molecules cannot cross lipid bilayers. Examples: ions, sugars, amino acids, etc. Facilitated diffusion Facilitated diffusion requires transporter proteins: o Channels: hydrophilic channel on inside of protein o Carriers: hydrophilic space changes shape, alternately exposed to inside and outside of membrane Facilitated diffusion is always gated, ie., pores can be closed and opened Water crosses the plasma membrane by diffusion and sometimes by facilitated diffusion o Diffusion: leaks through the lipid bilayer o Facilitated diffusion: through channels called aquaporins Examples: plants (to maintain turgor), kidney cell (reclaiming water from urine) Active Transport Active transport: movement of a solute across a membrane against its concentration gradient (from lower to higher con) Two forms of active transport: o ATP-driven o Co-transport ATP-driven Example: sodium-potassium pump. Maintains low intracellular sodium, high intracellular potassium o Uses energy obtained by breaking bonds of ATP (= “ATP hydrolysis”) to cycle pump o Pumps Na+ out of and K+ into cell Co-transport Co-transport: movement of one solute down its concentration gradient provides the energy to move another up its concentration gradient o Example: sucrose transporter. Movement of H+ ions down its concentration gradient drives movement of sucrose against its conc. Gradient Bulk Transport Exocytosis Exocytosis and endocytosis: specialized processes for bulk movement into and out of cells, respectively Exocytosis: small membrane-bound vesicles fuse with the plasma membrane. Contents of vesicles are delivered to outside of cell o Last step in the secretory pathway Endocytosis Endocytosis: cell “engulfs” nearby parts of environment, forming a new internal vesicle from part of the plasma membrane o Phagocytosis: cell engulfs a particle (“cell eating”). Example: amoeba or white blood cell “eats” a bacterium o Pinocytosis: cell engulfs small volumes of liquid (“cell drinking”) o Receptor-mediated endocytosis: cell internalizes tiny sections of membrane, where ligands have bound to their receptors. Example: uptake of cholesterol from blood stream o Endocytic vesicles usually fuse with lysosomes for digestion & use contents Learning Objectives Membrane structure: arrangement and structure of lipid bilayer, integral and peripheral membrane proteins Functions of membrane proteins Membrane fluidity Differential permeability of membranes Cellular behaviors in iso-, hypo-, and hyper-tonic solutions Diffusion and facilitated diffusion Transport against concentration gradient by active transport Bulk transfer process
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