Y khoa, y dược - Chapter 3: Cells: the living units

Chapter 3 : Part A Cells: The Living UnitsThe cell is the smallest structural and functional living unit Organismal functions depend on individual and collective cell functionsBiochemical activities of cells are dictated by their specific sub cellular structures called organelles Cell TheoryOver 200 different types of human cellsTypes differ in size, shape, subcellular components, and functionsCell DiversityFibroblastsErythrocytesEpithelial cells(d) Cell that fights diseaseNerve cellFat cellSperm(a) Cells that connect body parts, form linings, or transport gases(c) Cell that storesnutrients(b) Cells that move organs and body parts(e) Cell that gathers information and control body functions(f) Cell of reproductionSkeletalMusclecellSmoothmuscle cellsMacrophageFigure 3.1 All cells have some common structures and functions Human cells have four basic parts:Plasma membrane - flexible outer boundaryCytoplasm - intracellular fluidOrganelles - multiple functionsNucleus - control centerGeneralized CellFigure 3.2Secretion beingreleased from cellby exocytosisPeroxisomeRibosomesRoughendoplasmicreticulumNucleusNuclear envelopeChromatinGolgi apparatusNucleolusSmooth endoplasmicreticulum CytosolLysosomeMitochondrionCentriolesCentrosomematrixCytoskeletalelements• Microtubule• Intermediate filamentsPlasmamembraneBimolecular layer of lipids and proteins in a constantly changing fluid mosaic Plays a dynamic role in cellular activitySeparates intracellular fluid (ICF) from extracellular fluid (ECF)Interstitial fluid (IF) = ECF that surrounds cellsPlasma MembraneFigure 3.3Integralproteins Extracellular fluid(watery environment)Cytoplasm(watery environment) Polar head ofphospholipid moleculeGlycolipidCholesterolPeripheralproteins Bimolecularlipid layercontainingproteinsInward-facinglayer ofphospholipidsOutward-facinglayer ofphospholipidsCarbohydrate of glycocalyxGlycoproteinFilament of cytoskeletonNonpolar tail of phospholipid moleculePhospholipids (lipid bilayer) Phosphate heads: polar and hydrophilicFatty acid tails: nonpolar and hydrophobic (Review Fig. 2.16b) Membrane LipidsTransportReceptors for signal transductionAttachment to cytoskeleton and extracellular matrixFunctions of Membrane ProteinsFigure 3.4aA protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane.(a) TransportFigure 3.4bA membrane protein exposed to the outside of the cell may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external signal may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell.(b) Receptors for signal transductionSignalReceptorFigure 3.4cElements of the cytoskeleton (cell’s internal supports) and the extracellular matrix (fibers and other substances outside the cell) may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together.(c) Attachment to the cytoskeleton and extracellular matrix (ECM)Enzymatic activityIntercellular joiningCell-cell recognitionFunctions of Membrane ProteinsFigure 3.4dA protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane act as a team that catalyzes sequential steps of a metabolic pathway as indicated (left to right) here.(d) Enzymatic activityEnzymesFigure 3.4eMembrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. Some membrane proteins (CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions.CAMs(e) Intercellular joiningFigure 3.4fSome glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells.(f) Cell-cell recognitionGlycoproteinPlasma membranes are selectively permeableSome molecules easily pass through the membrane; others do notMembrane TransportPassive processesNo cellular energy (ATP) requiredSubstance moves down its concentration gradientActive processesEnergy (ATP) requiredOccurs only in living cell membranesTypes of Membrane TransportWhat determines whether or not a substance can passively permeate a membrane?Lipid solubility of substanceChannels of appropriate sizeCarrier proteinsPassive ProcessesPLAYAnimation: Membrane PermeabilitySimple diffusionCarrier-mediated facilitated diffusionChannel-mediated facilitated diffusionOsmosisPassive ProcessesNonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayerDiffusion is the movement of solutes from a solution of higher concentration to that of a lower concentrationPassive Processes: Simple DiffusionPLAYAnimation: DiffusionFigure 3.7aExtracellular fluidLipid-solublesolutesCytoplasm(a) Simple diffusion of fat-soluble molecules directly through the phospholipid bilayerCertain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which:Exhibit specificity (selectivity)Are saturable; rate is determined by number of carriers or channelsCan be regulated in terms of activity and quantity Passive Processes: Facilitated DiffusionTransmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids)Binding of substrate causes shape change in carrier Facilitated Diffusion Using Carrier ProteinsFigure 3.7bLipid-insoluble solutes (such as sugars or amino acids)(b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport proteinAqueous channels formed by transmembrane proteins selectively transport ions or water Two types:Leakage channelsAlways openGated channelsControlled by chemical or electrical signalsFacilitated Diffusion Using Channel ProteinsFigure 3.7cSmall lipid-insoluble solutes(c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and chargeMovement of solvent (water) from a solution of low concentration to that of a higher concentration, across a selectively permeable membrane Water diffuses through plasma membranes:Through the lipid bilayerThrough water channels called aquaporins (AQPs)Passive Processes: OsmosisFigure 3.7dWatermoleculesLipidbillayerAquaporin(d) Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayerWater concentration is determined by solute concentration because solute particles displace water moleculesOsmolarity: The measure of total concentration of solute particles When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reachedPassive Processes: OsmosisFigure 3.8a(a) Membrane permeable to both solutes and waterSolute and water molecules move down their concentration gradientsin opposite directions. Fluid volume remains the same in both compartments.Leftcompartment:Solution withlower osmolarityRightcompartment:Solution with greater osmolarityMembraneH2OSoluteSolutemolecules(sugar)Both solutions have thesame osmolarity: volumeunchangedFigure 3.8b(b) Membrane permeable to water, impermeable to solutesBoth solutions have identicalosmolarity, but volume of thesolution on the right is greaterbecause only water is free to moveSolute molecules are prevented from moving but water moves by osmosis.Volume increases in the compartment with the higher osmolarity.LeftcompartmentRightcompartmentMembraneSolutemolecules(sugar)H2OWhen osmosis occurs, water enters or leaves a cellChange in cell volume disrupts cell functionImportance of OsmosisPLAYAnimation: OsmosisTonicity: The ability of a solution to cause a cell to shrink or swell Isotonic: A solution with the same solute concentration as that of the cytosolHypertonic: A solution having greater solute concentration than that of the cytosolHypotonic: A solution having lesser solute concentration than that of the cytosolTonicityFigure 3.9Cells retain their normal size andshape in isotonic solutions (samesolute/water concentration as insidecells; water moves in and out).Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of solutes than are present inside the cells).(a) Isotonic solutions(b) Hypertonic solutions(c) Hypotonic solutionsCells take on water by osmosis untilthey become bloated and burst (lyse)in a hypotonic solution (contains alower concentration of solutes thanare present in cells).