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Cell Biology Exam 2 PDF Chapter 4 review study guide

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by: MelLem

Cell Biology Exam 2 PDF Chapter 4 review study guide BIOL 225

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Cell Biology Exam 2 PDF Chapter 4 review study guide - material for chapter 4 for exam on tuesday
Cell Biology
Dr. Lopilato
Study Guide
Cell, Biology, exam, Study Guide.
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Samuel Croteau

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This 6 page Study Guide was uploaded by MelLem on Thursday March 31, 2016. The Study Guide belongs to BIOL 225 at Simmons College taught by Dr. Lopilato in Fall 2016. Since its upload, it has received 58 views. For similar materials see Cell Biology in Biology at Simmons College.

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Date Created: 03/31/16
Exam  2  Review     Cell  Biology       Chapter  4  :  The  Structure  and  function  of  the  plasma  membrane     4.1  Overview  of  Membrane  Functions   1. Compartmentalization     • The  plasma  membrane  encloses  the  contents  of  the  entire  cell.   • The  nuclear  and  cytoplasmic  membranes  enclose  diverse  intracellular  spaces   • Membrane  compartmentalization  allows  specialized  activities  to  proceed   without  external  interference  and  enables  cellular  activities  to  be  regulated   independently  of  one  another     2. Scaffold  for  Biochemical  Activities   • Because  of  their  construction,  membrane  provide  the  cell  with  extensive  frame   work  in  which  can  be  ordered  for  effective  interaction.     3. Providing  a  selectively  permeable  barrier   • Membranes  prevent  the  unrestricted  exchange  of  molecules  from  one  side  to  the   other.   • Membranes  provide  the  means  of  communication  between  the  compartments   they  separate   • Gated  “Bridges”  that  promote  the  movement  of  select  elements  in  and  out  of  the   enclosed  space.     4. Transporting  Solutes   • Contains  the  machinery  for  physically  transporting  substances  from  one  side  of   the  membrane  to  the  other.   • *Genrally  from  a  region  where  the  solute  is  present  at  a  low  concentration  to  an   area  where  the  solute  is  at  a  higher  concentration  (NOT  DIFFUSION)   • Diffusion  –  going  from  the  higher  area  of  concentration  to  an  area  of  lower   concentration.   • The  plasma  membrane  is  able  to  transport  ions,  thereby  able  to  establish  ionic   gradients.  (Which  is  important  for  nerve  and  muscle  cells).     5. Responding  to  External  Stimuli   • Signal  Transduction  –  Response  of  a  cell  to  external  conditions  (external   Stimuli)   • Receptors  –  membranes  posses  receptors  –  they  combine  with  specific   molecules  (ligands)  or  respond  to  other  types  of  stimuli  like  light  and   mechanical  tension.     6. Intercellular  Interactions   • Plasma  membrane  allows  cells  to  recognize  and  signal  one  another  to  adhere   (when  appropriate)  and  exchange  materials  and  information.   • Proteins  within  the  plasma  membrane  may  also  facilitate  the  interaction   between    extracellular  materials  and  the  intracellular  cytoskeleton.     7. Energy  Transduction   • Energy  transduction  –  the  process  by  which  one  type  of  energy  is  converted  to   another   • The  most  fundamental  energy  transduction  occurs  during  photosynthesis  –   when  –  energy  in  the  sunlight  is  absorbed  by  membrane  bound  pigments,   converted  to  energy  and  stored  as  carbohydrates   • In  eukaryotes,  the  machinery  for  this  energy  is  housed  in  the  membrane  of  the   chloroplast  and  mitochondria.     4.3  The  Chemical  Composition  of  Membranes     • Membranes  are  lipid-­‐protein  assemblies  in  which  the  components  are  held  together   in  a  thin  sheet  by  non-­‐covalent  bonds.   • The  lipid  bilayer  serves  as  a  structural  backbone  and  to  provide  a  barrier  that   prevents  random  movements  of  water  soluble  molecules  over  the  membrane  (in   and  out  of  the  cell)     Membrane  Lipids   • Contains  a  wide  diversity  of  lipids  –  all  are  amphiapathic     • Amphiapathi  –  contain  both  hydrophobic  and  hydrophilic  regions.     • There  are  three    main  types  of  membrane  proteins   o Phosphoglycerides   o Sphingolipids   o Cholesterol   • Phosphoglycerides     o Most  membrane  lipids  have  phosphate  groups  and  are  deemed   phospholipids.     o Since  many  have  a  glycerol  backbone  –  theyre  phosphoglycerides   • Diglycerides   o Phosphoglycerides  –  only  have  two  of  the  hydroxyl  groups  of  the  glycerol   that  are  esterifies  to  fatty  acids.  The  third  is  esterified  to  a  hydrophilic   phosphate  group.   • Sphingolipids   o Less  abundant  class  of  membrane  lipids   o All  have  two  hydrophobic  hydrocarbon  chains  at  one  end  and  a  hydrophilic   region  at  the  other.   • Cholesterol   o Sterol  (cholesterol)  in  certain  animal  cells  that  may  constitute  up  to  50%  of   the  lipid  molecules  in  the  plasma  membrane.   o Plant  cells  contain  “cholesterol  like”  sterols     The  nature  and  important  of  the  lipid  bilayer   • Membrane  lipids  also  provide  the  precursors  for  highly  active  chemical  messengers   that  regulate  cellular  function.     The  asymmetry  of  membrane  lipids   • All  of  the  glycolipids  of  the  plasma  membrane  are  in  the  outer  leaflet   o They  serve  as  receptors  for  extracellilar  glands   • Membrane  Carbohydrates   o The  plasma  membrane  of  eukaryotes  also  has  carbohydrates  present   depending  on  the  cell  type,  it  can  be  2-­‐10%  carbohydrate  content  in  the   membrane.   o Most  (90%0  of  the  carbohydrates  in  the  membrane  are  covalently  linked  to   proteins  to  form  glycoproteins.   o The  remainder  of  the  carbohydrates  are  linked  to  lipids  to  form  glycolipids.     o All  of  the  carbohydrates  of  the  plasma  membrane  face  outward  into  the   extracellular  space.   o The  carbohydrates  of  the  glycoproteins  are  present  as  short,  branches   hydrophilic  oligiosaccharides  (typically  having  less  than  15  sugars)   o Carbohydrate  projections  play  an  important  role  in  mediating  the   interactions  of  a  cell  with  its  environment     4.4  The  Structures  And  Functions  of  Membrane  Proteins       • A  membrane  may  contain  hundreds  of  different  proteins   • Each  membrane  protein  has  a  defined  orientation  relative  to  the  cytoplasm  –  this   asymmetry  is  referred  as  “sidedness”     3  different  classes  distinguished  by  their  intimacy  with  the  lipid  bilayer:   • Integral  Proteins   o Penetrates  the  lipid  bilayer     o Are  known  as  transmembrane  proteins  –  they  pass  entirely  through  the  lipid   bilayer  –  have  domains  that  protrude  from  both  the  extracellular  and   cytoplasmic  sides  of  the  membranes.   • Peripheral  Proteins   o Located  entirely  outside  of  the  lipid  bilayer,  on  either  the  extracellular  or  the   cytoplasmic  side.  (associated  with  the  membrane  surface  by  noncovalent   bonds)   • Lipid  Anchored  Proteins   o Located  outside  the  lipid  bilayer  on  either  the  extracellular  side  or  the   cytoplasmic  side   o Covalently  linked  to  a  lipid  molecule  that  is  situated  within  the  bilayer.     • Integral  Membrane  Proteins   o Function  as  receptors  that  bind  specific  substances  at  the  membrane  surface,   as  channels  or  transporters  that  are  involved  in  the  movements  of  ions  and   solutes  across  the  membrane.   o They  are  also  amphiopathic   • Freeze  Fracture  Analysis   o The  concept  that  proteins  penetrate  through  membranes  rather  than   remaining  external  to  the  bilayer  was  derived  from  the  results  of  freeze   fracture  analysis   o Cracks  the  lipid  bilayer,  leaves  a  “pit”  in  the  bilayer  where  the  proteins  were.   (Figure  4.15a  represents  this  method  of  analysis)     • Peripheral  Membrane  Proteins   o Associated  with  the  membrane  by  weak  electrostatic  bonds   o These  proteins  provide  mechanical  support  for  the  membrane  and  function   as  an  anchor  for  integral  membrane  proteins.   o Other  peripheral  proteins  on  the  internal  plasma  membrane  surface  function   as  enzymes,  specialized  coats,  or  factors  that  transmit  transmembrane   signals.     • Lipid-­‐  Anchored  Membrane  Proteins   o Numerous  proteins  present  on  the  external  face  of  the  plasma  membrane  are   bound  to  the  membrane  by  a  small,  complex  oligiosaccharided  linked  to  a   molecule  of  phosphatidylinositol.   o Those  containing  this  type  of  glycosyl-­‐phosphatidylinositol  linkages  are   called  GPI-­‐Anchored  proteins.     4.7  The  movement  of  Substances  across  Cell  Membranes     • The  lipid  bilayer  of  the  membrane  is  ideally  suited  to  prevent  the  loss  of  charged   and  polar  solutes  from  the  cell.     o But  –  some  special  provisions  must  be  made  to  allow  the  movement  of   nutrients,  ions,  waste  products,  and  other  compounds  across  the  membrane.     Two  types  of  movement   • Substances  may  move  through  the  membrane  passively  by  diffusion,  OR  actively  by   an  d  energy-­‐couples  transport  process   • Both  movemenets  lead  to  the  net-­‐flux  of  a  particular  ion  or  compound.   • Net  Flux  –  indicates  that  the  movement  of  the  substance  into  the  cell  and  out  of  the   cell  is  NOT  balances,  but  exceeds  the  other.   • Substances  may  move  across  the  membrane  by:   o Simple  Diffusion  –  Through  the  lipid  bilayer   o Simple  Diffusion  -­‐    Through  an  aqueous  protein  lined  channel  (diffusion   that  is  facilitated    by  a  protein  transporter)   o Active  Transport  –  Requires  an  energy  driven  protein  pump  that  is  capable   of  moving  substances  against  a  concentration  gradient   The  energetics  of  solute  movement   • Diffusion    -­‐  The  spontaneous  process  in  which  a  substance  moves  from  a  region  of   higher  concentration  to  a  region  of  lowere  concentration     Diffusion  of  substances  through  membranes   • Substances  must  be  in  higher  concentration  on  one  region  and  lower  in  the  other.   • Membrane  must  be  permeable  to  the  solute.   • Must  satisfy  1/both:   o Solute  can  pass  directly  through  bilayer    -­‐or-­‐   o Solute  can  traverse  through  an  aqueous  pore  that  spans  the  membrane   • Partition  Coefficient  –  The  ration  of  its  solubility  in  a  non  polar  solvent.   • It  is  evident  that  the  greater  the  lipid  solubility,  the  faster  the  penetration.   • Another  factor  determining  the  rate  of  penetration  of  a  compound  through  a   membrane  is  size.   • IF  two  compounds  have  similar  partition  coefficients,  the  one  with  the  smaller  size   will  penetrate  the  membrane  faster  than  that  of  the  larger  molecule.   • Very  small  uncharged  molecules  penetrate  very  rapidly  through  the  cellular   membrane.   • Membranes  are  highly  permeable  to  small  inorganic  molecules  such  as  O2,  CO2,  NO,   and  H2O,  they  are  thought  to  slip  between  adjacent  phospholipids.     • Larger  polar  molecules,  sugar,  a.a’s  and  phosphoryared  intermediates  exhibit  poor   membrane  permeability.     • Materials  that  must  enter  from  the  blood  stream,  but  exhibit  poor  permeability  must   enter  through  special  mechanisms  AND  NOT  by  simple  diffusion.     The  diffusion  of  Water  through  membranes   • Membranes  are  said  to  be  semi-­‐permeable.   • Water  moves  readily  through  a  semi  permeable  membrane  from  a  region  of  lower   solute  concentration  to  an  area  of  higher  solute  concentration.   • This  process  is  known  as  osmosis    -­‐  the  diffusion  of  water.   • The  area  with  a  higher  solute  concentration  is  said  to  be  hypertonic  or   hyperosmotic.   • The  area  with  a  lower  solute  concentration  is  said  to  be  hypotonic  or   hypoosmotic.  (When  a  cell  is  palced  into  a  hypotonic  solution,  the  cell  gains  water   by  osmosis  and  swells  (lysis))   • The  cells  volume  is  controlled  by  the  difference  between  the  solute  concentration   inside    the  cell  and  that  in  the  extracellular  medium.   • Small  integral  proteins  called  aquaporins    allow  the  passive  movement  of  water   from  one  side  of  the  plasma  membrane  tot  the  other.       The  diffusion  of  ions  through  membranes   • The  lipid  bilayer  is  highly  impermeable  to  charged  substances  including  ins  such  as   Na+,  K+.  Ca+2  and  Cl-­‐   • The  rapid  movement  (conductance)  of  these  ions  across  membranes    play  a  critical   role  in  a  large  amount  of  cellular  activities.   • Ion  Channels  -­‐    openings  in  the  membrane  that  are  permeable  to  certain  (specific)   ions.   • Most  ions  channels  are  highly  selective  in  allowing  only  one  particular  type  of  ion  to   pass  through  the  pore.   • Most  ion  channels  that  have  been  identified  can  exist  in  open  or  closed   conformation  or  said  to  be  gated.   • The  opening/closing  of  the  gates  are  subject  to  complex  physiological  regulation.      


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