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Catalysts#

  • Page ID
    21265
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    Catalysts

    For a chemical reaction to happen, the reactants must first find one another in space. Chemicals in solution don't "plan" these collisions; they happen at random. In fact, most times, it's even more complicated. Not only do the reactants need to run into one another, but they also need to come into contact in a specific orientation. If reactants are very dilute, the rate of the reaction will be slowcollisions will happen infrequently. Increasing the concentrations will increase the rate of productive collisions. Another way to change the rate of reaction is to increase the rate of collisions by increasing the rate at which the reactants explore the reaction spaceby increasing the velocity of the molecules or their kinetic energy. This can be accomplished by transferring heat into the system or raising the temperature. Those two strategies are often suitable for increasing the rates of chemical reactions that happen in a tube. However, in the cell, transferring heat may not be practical, as it may damage cellular components and lead to death. Cells sometimes use mechanisms to increase concentrations of reactants (we'll see some examples below), but this is rarely enough to drive reaction rates in a biologically relevant regime. This is where catalysts come in.

    A catalyst is something that helps increase the rate of a chemical reaction undergoing no change itself. You can think of a catalyst as a chemical change agent.

    The most important catalysts in biology are called enzymes. An enzyme is a protein catalyst. Other cellular catalysts include molecules called ribozymes. A ribozyme is a catalyst composed of a ribonucleic acid (RNA). Both will be discussed in more detail later in the course. Like all catalysts, enzymes work by lowering the level of energy that needs to be transferred into a chemical reaction to make it happen. A chemical reaction’s activation energy is the “threshold” level of energy needed to start the reaction.

    catalyzed_reaction_txtopt.png

    Figure 1. Enzymes and other catalysts decrease the activation energy required to start a given chemical reaction. Without an enzyme (left), the energy input needed for a reaction to begin is high. With the help of an enzyme (right), la reaction needs less energy  to begin. Attribution: Marc T. Facciotti (original work)

     

     In the figure above, What do you think the units are on the x-axis? Time would be one guess. However, if you compare the figures, it appears that the products are formed at the same time whether the activation energy barrier is high or low. Wasn't the point of this figure to illustrate that reactions with high activation energy barriers are slower than those with low activation energy barriers? What's going on?


    This page titled Catalysts# is shared under a not declared license and was authored, remixed, and/or curated by Marc Facciotti.

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