Unit two. The Living Cell


5. Energy and Life


5.3. Chemical Reactions


In a chemical reaction, the original molecules before the chemical reaction occurs are called reactants, or sometimes substrates, whereas the molecules that result after the reaction has taken place are called the products of the reaction. Not all chemical reactions are equally likely to occur. Just as a boulder is more likely to roll downhill than uphill, so a reaction is more likely to occur if it releases energy than if it needs to have energy supplied. Consider how the chemical reaction proceeds in figure 5.4 1. Like when rolling a boulder uphill, energy needs to be supplied. This is because the product of the reaction contains more energy than the reactant. This type of chemical reaction, called endergonic, does not occur spontaneously. By contrast, an exergonic reaction, shown in 2, tends to occur spontaneously because the product has less energy than the reactant, like a boulder that has rolled downhill.



Figure 5.4. Chemical reactions and catalysis.

1 The products of endergonic reactions contain more energy than the reactants. 2 The products of exergonic reactions contain less energy than the reactants, but exergonic reactions do not necessarily proceed rapidly because it takes energy to get them going. The "hill" in this energy diagram represents energy that must be supplied to destabilize existing chemical bonds. 3 Catalyzed reactions occur faster because the amount of activation energy required to initiate the reaction—the height of the energy hill that must be overcome—is lowered, and the reaction proceeds to its end faster.


Activation Energy

If all chemical reactions that release energy tend to occur spontaneously, it is fair to ask, “Why haven’t all exergonic reactions occurred already?” Clearly they have not. If you ignite gasoline, it burns with a release of energy. So why doesn’t all the gasoline in all the automobiles in the world just burn up right now? It doesn’t because the burning of gasoline, and almost all other chemical reactions, requires an input of energy to get it started—a kick in the pants such as a match or spark plug. Even in exergonic reactions where the product contains or stores less energy than the reactants, it is first necessary to break existing chemical bonds in the reactants, and this takes energy. The extra energy required to destabilize existing chemical bonds and so initiate a chemical reaction is called activation energy, indicated by brackets in figure 5.4 2 and 3. You must first nudge a boulder out of the hole it sits in before it can roll downhill. Activation energy is simply a chemical nudge.



One way to make an exergonic reaction more likely to happen is to lower the necessary activation energy. Like digging away the ground below your boulder, lowering activation energy reduces the nudge needed to get things started. The process of lowering the activation energy of a reaction is called catalysis. Catalysis cannot make an endergonic reaction occur spontaneously—you cannot avoid the need to supply energy— but it can make a reaction, endergonic or exergonic, proceed much faster. Compare the activation energy levels (the red arched arrows) in 2 and 3 below: The catalyzed reaction has a lower barrier to overcome.


Key Learning Outcome 5.3. Endergonic reactions require an input of energy. Exergonic reactions release energy. Activation energy that initiates chemical reactions can be lowered by catalysis.