If you were to expose glucose to oxygen on your lab bench, why would you not expect to see it burn as described by the reaction in Figure 8.6?
a. The reaction is endergonic and requires an input of energy.
b. The reaction is not spontaneous unless an enzyme is added.
c. The sugar must first be phosphorylated to increase its potential energy.
d. Activation energy is required for the sugar and oxygen to reach their transition state.

Verified step by step guidance

1. Understand the chemical reaction: The reaction between glucose and oxygen is a redox reaction where glucose is oxidized to carbon dioxide and oxygen is reduced to water, releasing energy.
2. Identify the nature of the reaction: The reaction is exergonic, meaning it releases energy, as shown by the release of energy in the form of heat and light in the diagram.
3. Consider the role of activation energy: Even though the reaction is exergonic, it requires an initial input of energy to overcome the activation energy barrier, allowing the reactants to reach the transition state.
4. Recognize the need for a catalyst: In biological systems, enzymes act as catalysts to lower the activation energy, making the reaction proceed at a noticeable rate. Without an enzyme, the reaction would be too slow to observe under normal conditions.
5. Conclude why the reaction doesn't occur spontaneously on the lab bench: Without an enzyme or sufficient activation energy, the reaction between glucose and oxygen will not proceed spontaneously, which is why you wouldn't see it burn on the lab bench.

Key Concepts

Activation Energy

Activation energy is the minimum energy required for a chemical reaction to occur. In the case of glucose burning in the presence of oxygen, this energy is necessary to initiate the reaction, allowing the reactants to reach a transition state where they can transform into products. Without sufficient activation energy, the reaction will not proceed, even if the reactants are present.

Spontaneity of Reactions

A spontaneous reaction is one that occurs without the need for continuous external energy input. While the combustion of glucose is exothermic and releases energy, it is not spontaneous at room temperature without a catalyst or sufficient activation energy. Understanding the conditions under which a reaction becomes spontaneous is crucial for predicting its behavior in a lab setting.

Endergonic vs. Exergonic Reactions

Endergonic reactions require an input of energy to proceed, while exergonic reactions release energy. The combustion of glucose is typically exergonic, but the initial activation energy needed to start the reaction can make it seem endergonic until that threshold is met. Recognizing the energy dynamics of these reactions helps clarify why glucose does not spontaneously ignite in the presence of oxygen.