Campbell Chapter 7: Membrane Structure and Function PDF

MEMBRANE  STRUCTURE  AND   FUNCTION    Life  at  the  Edge   The  plasma  membrane  is  the  boundary  that   separates  the  living  cell  from  its  surroundings   The  plasma  membrane  exhibits  selec4ve   permeabi...

MEMBRANE  STRUCTURE  AND   FUNCTION    Life  at  the  Edge   The  plasma  membrane  is  the  boundary  that   separates  the  living  cell  from  its  surroundings   The  plasma  membrane  exhibits  selec4ve   permeability,  allowing  some  substances  to   cross  it  more  easily  than  others    Cellular  membranes  are  fluid  mosaics   of  lipids  and  proteins   Phospholipids  are  the  most  abundant  lipid  in   the  plasma  membrane   Phospholipids  are  amphipathic  molecules,   containing  hydrophobic  and  hydrophilic   regions   A  phospholipid  bilayer  can  exist  as  a  stable   boundary  between  two  aqueous   compartments   Figure 7.2 Hydrophilic head WATER WATER Hydrophobic tail    The  fluid  mosaic  model  states  that  a   membrane  is  a  fluid  structure  with  a  “mosaic”   of  various  proteins  embedded  in  it   Proteins  are  not  randomly  distributed  in  the   membrane   Figure 7.3 Fibers of extra- cellular matrix (ECM) Glyco- Carbohydrate Glycolipid protein EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments Peripheral of cytoskeleton proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE  The  Fluidity  of  Membranes   Phospholipids  in  the  plasma  membrane  can   move  within  the  bilayer   Most  of  the  lipids,  and  some  proteins,  driC   laterally   Rarely,  a  lipid  may  flip-­‐flop  transversely  across   the  membrane   Figure 7.4-3 Membrane proteins Mixed proteins Mouse cell after 1 hour Human cell Hybrid cell    As  temperatures  cool,  membranes  switch   from  a  fluid  state  to  a  solid  state   The  temperature  at  which  a  membrane   solidifies  depends  on  the  types  of  lipids   Membranes  rich  in  unsaturated  faGy  acids  are   more  fluid  than  those  rich  in  saturated  faGy   acids   Membranes  must  be  fluid  to  work  properly;   they  are  usually  about  as  fluid  as  salad  oil      The  steroid  cholesterol  has  different  effects   on  membrane  fluidity  at  different   temperatures   At  warm  temperatures  (such  as  37°C),   cholesterol  restrains  movement  of   phospholipids   At  cool  temperatures,  it  maintains  fluidity  by   prevenPng  Pght  packing   Figure 7.5 (a) Unsaturated versus saturated hydrocarbon tails Fluid Viscous Unsaturated tails Saturated tails pack prevent packing. together. (b) Cholesterol within the animal cell membrane Cholesterol reduces membrane fluidity at moderate temperatures, but at low temperatures hinders solidification. Cholesterol EvoluPon  of  Differences  in  Membrane   Lipid  ComposiPon   VariaPons  in  lipid  composiPon  of  cell   membranes  of  many  species  appear  to  be   adaptaPons  to  specific  environmental   condiPons   Ability  to  change  the  lipid  composiPons  in   response  to  temperature  changes  has  evolved   in  organisms  that  live  where  temperatures   vary    Membrane  Proteins  and  Their   FuncPons   A  membrane  is  a  collage  of  different  proteins,   oCen  grouped  together,  embedded  in  the   fluid  matrix  of  the  lipid  bilayer   Proteins  determine  most  of  the  membrane’s   specific  funcPons      Peripheral  proteins  are  bound  to  the  surface   of  the  membrane   Integral  proteins  penetrate  the  hydrophobic   core     Integral  proteins  that  span  the  membrane  are   called  transmembrane  proteins   The  hydrophobic  regions  of  an  integral  protein   consist  of  one  or  more  stretches  of  nonpolar   amino  acids,  oCen  coiled  into  alpha  helices   Figure 7.6 N-terminus EXTRACELLULAR SIDE α helix CYTOPLASMIC C-terminus SIDE    Six  major  funcPons  of  membrane  proteins   – Transport   – EnzymaPc  acPvity   – Signal  transducPon   – Cell-­‐cell  recogniPon   – Intercellular  joining   – AGachment  to  the  cytoskeleton  and  extracellular   matrix  (ECM)   Figure 7.7 Signaling molecule Receptor Enzymes ATP Signal transduction (a) Transport (b) Enzymatic (c) Signal activity transduction Glyco- protein (d) Cell-cell (e) Intercellular (f) Attachment to recognition joining the cytoskeleton and extracellular matrix (ECM)    HIV  must  bind  to  the  immune  cell  surface   protein  CD4  and  a  “co-­‐receptor”  CCR5  in   order  to  infect  a  cell     HIV  cannot  enter  the  cells  of  resistant   individuals  that  lack  CCR5   Figure 7.8 HIV Receptor Receptor (CD4) (CD4) Co-receptor but no CCR5 Plasma (CCR5) membrane (a) (b)  The  Role  of  Membrane  Carbohydrates  in   Cell-­‐Cell  RecogniPon   Cells  recognize  each  other  by  binding  to   molecules,  oCen  containing  carbohydrates,  on   the  extracellular  surface  of  the  plasma   membrane   Membrane  carbohydrates  may  be  covalently   bonded  to  lipids  (forming  glycolipids)  or  more   commonly  to  proteins  (forming  glycoproteins)   Carbohydrates  on  the  external  side  of  the  plasma   membrane  vary  among  species,  individuals,  and   even  cell  types  in  an  individual    Synthesis  and  Sidedness  of   Membranes   Membranes  have  disPnct  inside  and  outside   faces   The  asymmetrical  distribuPon  of  proteins,   lipids,  and  associated  carbohydrates  in  the   plasma  membrane  is  determined  when  the   membrane  is  built  by  the  ER  and  Golgi   apparatus   Figure 7.9 Transmembrane glycoproteins Secretory protein Golgi apparatus Vesicle Attached carbohydrate Glycolipid ER lumen Plasma membrane: Cytoplasmic face Transmembrane Extracellular face glycoprotein Secreted protein Membrane glycolipid  Membrane  structure  results  in   selecPve  permeability   A  cell  must  exchange  materials  with  its   surroundings,  a  process  controlled  by  the   plasma  membrane   Plasma  membranes  are  selecPvely  permeable,   regulaPng  the  cell’s  molecular  traffic    The  Permeability  of  the  Lipid  Bilayer   Hydrophobic  (nonpolar)  molecules,  such  as   hydrocarbons,  can  dissolve  in  the  lipid  bilayer   and  pass  through  the  membrane  rapidly   Hydrophilic  molecules  including  ions  and  polar   molecules  do  not  cross  the  membrane  easily    Transport  Proteins   Transport  proteins  allow  passage  of   hydrophilic  substances  across  the  membrane   Some  transport  proteins,  called  channel   proteins,  have  a  hydrophilic  channel  that   certain  molecules  or  ions  can  use  as  a  tunnel   Channel  proteins  called  aquaporins  facilitate   the  passage  of  water      Other  transport  proteins,  called  carrier   proteins,  bind  to  molecules  and  change  shape   to  shuGle  them  across  the  membrane   A  transport  protein  is  specific  for  the   substance  it  moves    Passive  transport  is  diffusion  of  a   substance  across  a  membrane  with  no   energy  investment   Diffusion  is  the  tendency  for  molecules  to  spread   out  evenly  into  the  available  space.  It  involves  the   movement  of  molecules  from  an  area  of  high   concentraPon  to  an  area  of  low  concentraPon.   Although  each  molecule  moves  randomly,  diffusion   of  a  populaPon  of  molecules  may  be  direcPonal   At  dynamic  equilibrium,  as  many  molecules  cross   the  membrane  in  one  direcPon  as  in  the  other   Figure 7.10 Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes    Substances  diffuse  down  their  concentra4on   gradient,  the  region  along  which  the  density  of  a   chemical  substance  increases  or  decreases   No  work  must  be  done  to  move  substances  down   the  concentraPon  gradient   The  diffusion  of  a  substance  across  a  biological   membrane  is  passive  transport  because  no   energy  is  expended  by  the  cell  to  make  it  happen    Factors  that  Affect  the  Rate  of   Diffusion   Temperature   Molecular  Weight   Solubility   Viscosity    Effects  of  Osmosis  on  Water  Balance   Osmosis  is  the  diffusion  of  water  across  a   selecPvely  permeable  membrane   Water  diffuses  across  a  membrane  from  the   region  of  lower  solute  concentraPon  to  the   region  of  higher  solute  concentraPon  unPl  the   solute  concentraPon  is  equal  on  both  sides   Another  way  to  say  this  is  that  water  diffuses   from  where  its  more  concentrated  to  where   it’s  less  concentrated   Figure 7.11 Lower concentration Higher concentration More similar of solute (sugar) of solute concentrations of solute Sugar H 2O molecule Selectively permeable membrane Osmosis Figure 7.11a Selectively Water permeable molecules can membrane pass through pores, but sugar Water molecules molecules cluster around cannot. sugar molecules. This side has This side has fewer solute more solute molecules, molecules, more free fewer free water molecules. Osmosis water molecules.  Water  Balance  of  Cells  Without  Cell   Walls   Tonicity  is  the  ability  of  a  surrounding  soluPon  to   cause  a  cell  to  gain  or  lose  water   Isotonic  soluPon:  Solute  concentraPon  is  the   same  as  that  inside  the  cell;  no  net  water   movement  across  the  plasma  membrane   Hypertonic  soluPon:  Solute  concentraPon  is   greater  than  that  inside  the  cell;  cell  loses  water   Hypotonic  soluPon:  Solute  concentraPon  is  less   than  that  inside  the  cell;  cell  gains  water   Figure 7.12 Hypotonic Isotonic Hypertonic (a) Animal cell H 2O H 2O H 2O H 2O Lysed Normal Shriveled Cell Plasma wall membrane H 2O Plasma H 2O membrane H 2O H 2O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed Video:  Plasmolysis      Hypertonic  or  hypotonic  environments  create   osmoPc  problems  for  organisms   Osmoregula4on,  the  control  of  solute   concentraPons  and  water  balance,  is  a   necessary  adaptaPon  for  life  in  such   environments    Water  Balance  of  Cells  with  Cell  Walls   Cell  walls  help  maintain  water  balance   A  plant  cell  in  a  hypotonic  soluPon  swells  unPl   the  wall  opposes  uptake;  the  cell  is  now  turgid   (firm)   If  a  plant  cell  and  its  surroundings  are  isotonic,   there  is  no  net  movement  of  water  into  the   cell;  the  cell  becomes  flaccid  (limp)      In  a  hypertonic  environment,  plant  cells  lose   water     The  membrane  pulls  away  from  the  cell  wall   causing  the  plant  to  wilt,  a  usually  lethal  effect   called  plasmolysis   Facilitated  Diffusion:  Passive  Transport   Aided  by  Proteins   In  facilitated  diffusion,  transport  proteins   speed  the  passive  movement  of  molecules   across  the  plasma  membrane   Transport  proteins  include  channel  proteins   and  carrier  proteins      Channel  proteins  provide  corridors  that  allow   a  specific  molecule  or  ion  to  cross  the   membrane   Aquaporins  facilitate  the  diffusion  of  water   Ion  channels  facilitate  the  diffusion  of  ions     – Some  ion  channels,  called  gated  channels,  open   or  close  in  response  to  a  sPmulus   Figure 7.14 EXTRACELLULAR FLUID (a) A channel protein Channel protein Solute CYTOPLASM Carrier protein Solute (b) A carrier protein    Carrier  proteins  undergo  a  subtle  change  in   shape  that  translocates  the  solute-­‐binding  site   across  the  membrane    AcPve  transport  uses  energy  to  move   solutes  against  their  gradients   Facilitated  diffusion  is  sPll  passive  because  the   solute  moves  down  its  concentraPon  gradient,   and  the  transport  requires  no  energy   Some  transport  proteins,  however,  can  move   solutes  against  their  concentraPon  gradients    The  Need  for  Energy  in  AcPve   Transport   Ac4ve  transport  moves  substances  against   their  concentraPon  gradients   AcPve  transport  requires  energy,  usually  in   the  form  of  ATP   AcPve  transport  is  performed  by  specific   proteins  embedded  in  the  membranes      AcPve  transport  allows  cells  to  maintain   concentraPon  gradients  that  differ  from  their   surroundings   The  sodium-­‐potassium  pump  is  one  type  of   acPve  transport  system   Figure 7.15 EXTRACELLULAR [Na+] high FLUID [K+] low Na+ Na+ Na+ Na+ Na+ [Na+] low P ATP Na+ CYTOPLASM [K+] high ADP K+ 1 2 K+ Na+ 6 Na+ Na+ K+ K+ P K+ K+ 3 P Pi 5 4  How  Ion  Pumps  Maintain  Membrane   PotenPal   Membrane  poten4al  is  the  voltage  difference   across  a  membrane   Voltage  is  created  by  differences  in  the   distribuPon  of  posiPve  and  negaPve  ions   across  a  membrane      Two  combined  forces,  collecPvely  called  the   electrochemical  gradient,  drive  the  diffusion   of  ions  across  a  membrane   – A  chemical  force  (the  ion’s  concentraPon   gradient)   – An  electrical  force  (the  effect  of  the  membrane   potenPal  on  the  ion’s  movement)      An  electrogenic  pump  is  a  transport  protein   that  generates  voltage  across  a  membrane   The  sodium-­‐potassium  pump  is  the  major   electrogenic  pump  of  animal  cells   The  main  electrogenic  pump  of  plants,  fungi,   and  bacteria  is  a  proton  pump   Electrogenic  pumps  help  store  energy  that  can   be  used  for  cellular  work   Figure 7.17 ATP − + EXTRACELLULAR − FLUID + H + H+ H+ Proton pump H+ − + H+ CYTOPLASM − + H+  Cotransport:  Coupled  Transport  by  a   Membrane  Protein   Cotransport  occurs  when  acPve  transport  of  a   solute  indirectly  drives  transport  of  other   substances     Plants  commonly  use  the  gradient  of   hydrogen  ions  generated  by  proton  pumps  to   drive  acPve  transport  of  nutrients  into  the  cell   Figure 7.18 + − Sucrose Sucrose Sucrose-H+ cotransporter Diffusion of H+ H+ + H+ − H+ + − H+ H+ H+ H+ Proton pump H+ ATP − + H+  Bulk  transport  across  the   plasma  membrane  occurs  by   exocytosis  and  endocytosis   Small  molecules  and  water  enter  or  leave  the   cell  through  the  lipid  bilayer  or  via  transport   proteins   Large  molecules,  such  as  polysaccharides  and   proteins,  cross  the  membrane  in  bulk  via   vesicles   Bulk  transport  requires  energy    Exocytosis   In  exocytosis,  transport  vesicles  migrate  to   the  membrane,  fuse  with  it,  and  release  their   contents  outside  the  cell   Many  secretory  cells  use  exocytosis  to  export   their  products    Endocytosis   In  endocytosis,  the  cell  takes  in   macromolecules  by  forming  vesicles  from  the   plasma  membrane   Endocytosis  is  a  reversal  of  exocytosis,   involving  different  proteins   There  are  three  types  of  endocytosis   – Phagocytosis  (“cellular  eaPng”)   – Pinocytosis  (“cellular  drinking”)   – Receptor-­‐mediated  endocytosis   Figure 7.19 Receptor-Mediated Phagocytosis Pinocytosis Endocytosis EXTRACELLULAR FLUID Solutes Pseudopodium Receptor Plasma membrane Coat protein “Food” Coated or pit other particle Coated vesicle Food vacuole CYTOPLASM    In  phagocytosis  a  cell  engulfs  a  parPcle  in  a   vacuole   The  vacuole  fuses  with  a  lysosome  to  digest   the  parPcle      In  pinocytosis,  molecules  dissolved  in  droplets   are  taken  up  when  extracellular  fluid  is   “gulped”  into  Pny  vesicles      In  receptor-­‐mediated  endocytosis,  binding  of   ligands  to  receptors  triggers  vesicle  formaPon   A  ligand  is  any  molecule  that  binds  specifically   to  a  receptor  site  of  another  molecule