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Polymerisation of small subunit

I don't really understand why the monomer concentration on this graph will stay flat when the monomer concentration is above the Cc.

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    $\begingroup$ What do you think might be happening? What starts to happen at the Cc? What is the definition of Cc? $\endgroup$
    – tyersome
    Commented Nov 11, 2019 at 18:43
  • $\begingroup$ Cc is defined as the critical concentration which is a measure of a solution of G actin to polymerise. What I am thinking is that if the monomer concentration is above the Cc, the polymerisation will occur and forming the actin filament. But if the polymersiation occurs, suppose the monomer will decrease and the graph for monomer should drop. Thats why I don't get why the graph for monomer will be plateau. $\endgroup$
    – eric pang
    Commented Nov 11, 2019 at 19:01
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    $\begingroup$ Notice the vertical axis is not for monomers, it's for mass. $\endgroup$
    – mgkrebbs
    Commented Nov 11, 2019 at 20:51

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The critical concentration is better defined as the "equilibrium concentration of the pool of unassembled subunits"1.

An analogy that might be helpful is thinking about a NaCl (table salt) solution — once you add enough salt some will start to precipitate, but the amount in solution will remain the same.

You can think of the solution as having the capacity to support a certain concentration of molecules (or ions) — below that concentration subunits will separate from the polymerized form faster than it can accumulate new subunits, above that concentration the subunits will stick together faster than they "fall off" the polymer.

For more a detailed molecular understanding of why this is so, you probably need to read up on chemical equilibria — for example Chemistry or even Khan Academy might be helpful.

Reference:

1: Lodish, H., Berk, A., Zipursky, S. L., Matsudaira, P., Baltimore, D., & Darnell, J. (2000). Section 18.2 — The dynamics of actin assembly. Molecular Cell Biology.

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