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6 Your ultimate goal is to understand human consciousness however your advisor wants you to understand some basic facts about actin assembly first He tells you that ATP binds to actin monomers and is required for assembly but ATP hydrolysis is not necessary for polymerization since ADP can under certain circumstances substitute for the ATP requirement However ADP filaments are much less stable than ATP filaments thus supporting your secret suspicion that the free energy of ATP hydrolysis really is used to drive actin assembly ATP hydrolyzed mol 30 20 10 0 light scattering ATP hydrolysis 200 100 time seconde Your advisor suggests that you make careful measurements of the quantitative relationship between the number of ATP molecules 300 hydrolyzed and the number of actin monomers linked into polymer The experiments are straightforward To measure ATP hydrolysis you add ATPg 2P to a solution of polymerizing actin take samples at short intervals and determine how much radioactive phosphate has been produced To measure polymerization you follow in a spectrophotometer the increase in light scattering that is caused by formation of the actin filaments Your results are shown in Figure 5 Your light scattering measurements indicate that 20 micromoles of actin monomers were polymerized Since the number of polymerized actin monomers matches exactly the number of ATP molecules hydrolyzed you conclude that one ATP is hydrolyzed as each new monomer is added to an actin filament When you show your advisor the data and tell him your conclusions he smiles and very gently tells you to look more closely at the graph He says your data prove that actin can polymerize without ATP hydrolysis A What does your advisor see in the data that you have overlooked B What do your data imply about the distribution of ATP and ADP in

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