If a cell in mitosis is cooled to 0 o C, the microtubules in the spindle depolym

Question

If a cell in mitosis is cooled to 0oC, the microtubules in the spindle depolymerize into tubulin subunits. The same is true for microtubules made from pure tubulin in a test tube; they assemble readily at 37oC, but disassemble at low temperature. In fact, many protein assemblies that are held together by noncovalent bonds show the same behavior: they disassemble when cooled. This behavior is governed by the basic thermodynamic equation for the forward reaction (polymerization):

a. The change in free energy must be negative for the reaction (tubulin subunits à microtubules) to proceed at high temperature. At low temperature, DG must be positive to permit disassembly; that is, to favor the reverse reaction. Decide what signs (positive or negative) of DH and DS must be, and show how your choices account for polymerization of tubulin at high temperature and its depolymerization at low temperature. (Assume that the DH and DS values themselves don’t change with temperature.)

b. Polymerization of tubulin subunits into microtubules at body temperature intuitively occurs with an increase in the orderliness of the subunits (Figure below). Yet tubulin polymerization occurs with an increase in entropy (decrease in order). Given what you’ve learned about the relationship between entropy and order, how can that be?

Explanation / Answer

a) Polymerization

dG = dH - TdS

If T is high, then:

-TdS < dH , in order to get dG< 0 and this to be spontaneous

b) Depolimerization

dG = dH - TdS

dG must be still negative, but

dH - Tds < 0

if T is low, you got to havee then

dH < 0, also negative

b)

Entropy increases in the UNIVERSE will be spontaneous

Entropy decreas ein the universe will NOT be spontaneous.

Entropy increase/decrease in a SYSTEM (in this case the tubulins) will not be enough to know if the system may or may not be spontaneous!

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