CH 5 The working cell

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					                                             The Working Cell ch 5
HOW CELLS USE ENERGY, ENZYMES and MEMBRANES
Fire flies

Glow: light energy

attract mates
attract prey

Luciferase + ATP + Oxygen converts Luciferin into a chemical that emits energy.

Enzymes control a cells chemical reactions by reducing the energy needed [activation energy EA] for a reaction to occur.
         SO FIREFLIES NEED ENERGY AND ENZYMES TO PRODUCE LIGHT.
Energy
   capacity to do work
      Kinetic (motion)
         heat (molecular movement)
         light (powers photosynthesis
      Potential (position)
         chemical energy (arrangement of nuclei and electrons)
         ex. sugar

E Transformations



                                                          ------------>   sugar
                                                                            protein
                                                                            nucleic acids
                                                                            lipids


                                   kinetic                 to             potential
                                                          --OR--
                                   potential              to              kinetic




Laws of Thermodynamics
  1) 1st law _________E-transferred or transformed (but not created or destroyed)
        nd
  2) 2 law __________E transfer or transformation makes universe more disordered (raises entropy)
        closed system--isolated from surrounding
        open system--E transferred between system and surroundings
     evolution of complex life forms from simple forms does not violate the second law.

E taken from surroundings( raises entropy of universe)
ex.
   *maintain highly ordered structure by raising entropy of environment
   *take in complex high energy molecules as food--extract energy (create, maintain order)
   *return to the environment simpler low energy molecules (CO      + H O) and heat
                                                                              2        2
Free Energy (Delta G)



                         +                    --------->                     + E -- Delta G




                        + E ----------->                     +Delta G




   exergonic--products have less free energy than reactants
   endergonic--products have more free energy than reactants

ATP and Cellular Work
  ATP--immediate source of energy
     mechanical work
        chromosome movement, mitosis/meiosis

      transport work




      chemical work




Structure of ATP
                                                 O-       O-         O-
                                                  I         I        I
         adenine             ribose        --P--O~~~P--O~~~P--O--
                                                 II      II          II
                                                 O        O           O

                                                    unstable bonds

       H O + ATP ---------> ADP + P -                 G    7.3 K/cal / mol
        2
How ATP Works
  ATP is hydrolyzed, the phosphate group is transferred to another molecule WHICH BECOMES MORE REACTIVE




            A--P---P---P                                    A---P---P
        Glu + NH       --------> Gln
                     3
        Glu + ATP ---------> Glu--           + ADP
        Glu--     + NH        ---------> Gln + P

Making new ATP

      ADP +            + E ---------> ATP
                          from cell respiration)

   ATP used and regenerated continually by cells
        7
     10 molecules / sec / cell


                   -------->                            -------->
                               I
                               I                   ENERGY           I
                                                                         I
                           -Delta G                                           Delta G=
                          7.3 K/cal / mol                               + 7.3 K/cal / mol

ATP and Cellular Work
  ATP--immediate source of energy
     mechanical work
        chromosome movement, mitosis/meiosis

      transport work




      chemical work




Structure of ATP
                                                  O-       O-                O-
                                                   I         I               I
          adenine              ribose       --P--O~~~P--O~~~P--O--
                                                  II      II              II
                                                  O        O               O

                                                   unstable bonds

        H O + ATP ---------> ADP + P -              G        7.3 K/cal / mol
         2

Enzymes
  free E change indicates a reaction that is spontaneous from one that is not               --Delta G
   spontaneous reactions may be too s l       o       w
   enzymes speed up and control rates

catalyst--accelerates reaction without being permanently changed in the process, and can therefore be used over
            and over

Enzymes---lower activation energy
             usually proteins
             very specific

            I
            I                                          Transition state
            I
Free        I
Energy      I      reactants
            I
            I                                                                                --Delta G
            I                                           products
            I__________________________________________________
               progress of reaction------------------>



Free         I                                            Transition state
Energy       I
              I                                            products
              I
              I                                                                                +Delta G
              I
              I
              I      reactants
              I__________________________________________________
                 progress of reaction------------------->
Specificity of Enzymes
  substrate-the substance an enzyme acts on


      Enzyme + Substrate --------> Enzyme substrate complex----------> Product + Enzyme



Active Site
   place on the enzyme where the substrate fits
      *pocket or groove
      *changes shape with contact by substrates
      *determines enzyme specificity

   induced fit-change in shape of active site induced by substrate [old “lock and key hypothesis”]


Catalytic Cycle of Enzymes
  Step 1) substrate binds to active site
                H-bonds, ionic bonds
  Step 2) induced fit of active site around substrate
                side chains of a few A.A. catalyze conversion of substrate ----> product
  Step 3) product departs active site


Lowering activation E / Speeding up reaction rate
  1) active site properly aligns reactants so they may react (two or more reactants)
  2) induced fit may distort the substrates’ chemical bonds; therefore, less E needed to form / break bonds
  3) active site produces a micro environment conductive to a particular reaction (A.A. side chains)
initial substrate concentration partly determines rate of reaction
          higher concentration--faster reaction (up to a limit)
          temp may affect the speed of a reaction


if the substrate concentration is high enough enzyme is saturated (all sites filled)



saturated--speed depends upon individual enzyme



unsaturated--slower




Factors affecting enzyme activity
 1) enzymes have optimal conditions
      *Temperature
         greatest number of collisions without denaturing enzyme (35-40 most humans)
                      37 C                        40 C                    45 C
      *pH
         optimal pH for most enzymes is 6-8 (some exist for extremes--pepsin in stomach pH 2)



                                pH 7                             pH 3




2)       Ionic concentration
          ions can interfere with ionic bonds within the enzyme (most enzymes can’t tolerate high salt)

             I                          I                          I                      I
             I                          I                          I                     I
             I                          I                          I                      I
             I                          I                          I                      I
             I                          I                          I                     I
             I                          I                          I                      I
             I_______________          _I                          I___      _____________I


2) Cofactors
     non-protein molecules required by many enzymes
       complete active site
       some inorganic Zn, Fe, Cu
       some organic--coenzymes (most vitamins)
                     +                     ----------->



                vitamin C

       unable to catalyze                                 able to catalyze


 3) Inhibitors
      chemicals that selectively inhibit enzyme activity
          irreversible--inhibitor attaches with covalent bonds
          reversible--inhibitor attaches with weak bonds

      competitive inhibitors--chemicals that resemble the enzyme’s normal substrate, therefore compare for the
        active site
            *block active site
            *if reversible, can be overcome by a raise in substrate concentration
                 antifreeze / ethanol
      non-competitive inhibitors--inhibitor does not enter active site, binds to another location
         *causes enzyme to change shape---active site altered


                             <--active site (correct)                                    <--active site (damaged)


                                                                                   I
                                                                                   I
                                                                             inhibitor

                                                  DDT / many antibiotics
                                                  penicillin--cell wall of bacteria


   selective activation



                              substrates (won’t fit)                                          substrates fit—reaction occurs

selective inhibition and activation essential mechanisms for metabolic control


Allosteric Regulation
    allosteric--specific receptor other than active site
        *two or more peptide chains
        *2 conformations         active / inactive
          bind an activator--locks enzyme in the active shape

          active sites                                                                                              activator
                              <=====>


            active                          inactive                                                                inhibitor

   concentrations of activators and inhibitors can control enzyme activity ex. ATP /ADP
      presence of ADP changes enzyme to ATP SYNTHESIS activity
      presence of ATP changes enzyme to inactivate further ATP SYNTHESIS

   cooperativity--substrate locks enzyme in active shape
Control of Metabolism
metabolism regulated by controlling enzyme activity

feedback inhibition
   end product inhibits an enzyme within a pathway
            enzyme
                 1       2     3       4      5
   threonine---> A ---> B ---> C ---> D---> isoleucine (end product--allosteric inhibitor of enzyme)
                  /\ ______________________I


   prevents the cell from wasting chemical resources

   Multi-enzyme complex
     enzymes assembled for steps of a metabolic pathway
         within or along cell structures
             fixed on a membrane, dissolved within an organelle
                                                                       high salt

Membrane Structure
  artificial membranes
  fluid mosaic model
     phospholipid bilayer
      embedded proteins
          some drift, some tethered
          unsaturated fatty acids and embedded cholesterol enhance fluidity
          proteins determine membrane function
      surface carbohydrates
          cell--cell recognition
          sorting cells into tissues
          bonded to protein / lipids
      membrane protein functions
          transport (ATP)
          enzymes
          cell-cell adhesion


Traffic of small molecules
   selective permeability

                                                             non-polar + O
      lipid                                                                   2
      permeability                                       small polar--H O, CO
                                                                            2         2
                                                       large polar-glucose
                                                  ions       Na+, H+
      transport proteins
         integral proteins, specific, receptor site
         avoids contact with lipid bilayer
             tunnels, physical movement

                                   anti port     2 solutes opposite direction
                                      Na+      Ca++
                                   symport--2 solutes same time, same direction
                                   uniport single solute



  1. diffusion and passive transport
     *concentration gradient
      *net movement
      *diffusion--movement down a concentration gradient
          random molecular motion, no ATP energy required
      *passive transport
          diffusion across a membrane

   2. osmosis
       *diffusion-H O---across selectively permeable membrane
                     2




hyperosmotic solution

hypoosomotic              selectively permeable membrane
solution


osmotic pressure--amount of (energy) force needs to prevent H O movement
                                                                                     2


                 H O-------> until equilibrium reached

      water balance           maintaining correct solute / H O concentrations
                                                 2
   3. facilitated diffusion
       proteins help solutes cross a cell membrane
           transport proteins
               specific
               saturation possible
               inhibited by similar solutes
               do not catalyze reactions

      facilitated diffusion

   4. active transport
      energy requiting
      against concentration gradient
      maintains strong gradients
         Na+, K+, Mg+, Ca+, Cl-
      sodium potassium pump

_____________________________________


   ion transport
      anions and cations unequally distributed across plasma membrane; voltages across the membrane
      membrane potential
          measured voltage range-50mv to -200mv (inside--compared to outside cell)
                                           this difference represents potential E
____________________________________


   cotransport
      ATP powered pump transports one solute
         indirectly drives transport of two solutes against concentration gradient

Transport of LARGE molecules
   exocytosis
   endocytosis

				
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posted:11/17/2011
language:English
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