58 Chapter Summary
6.1 Energy and Metabolism
Cells perform the functions of life through various chemical reactions. A cell’s metabolism refers to the chemical reactions that take place within it. Scientists refer to this process as catabolism, and we associate such reactions an energy release. On the other end of the spectrum, anabolism refers to metabolic processes that build complex molecules out of simpler ones, such as macromolecule synthesis. Anabolic processes require energy. Glucose synthesis and glucose breakdown are examples of anabolic and catabolic pathways, respectively.
6.2 Potential, Kinetic, Free, and Activation Energy
Energy comes in many different forms. Objects in motion do physical work, and kinetic energy is the energy of objects in motion. Objects that are not in motion may have the potential to do work, and thus have potential energy. Molecules also have potential energy because breaking molecular bonds has the potential to release energy. Living cells depend on harvesting potential energy from molecular bonds to perform work. Free energy is a measure of energy that is available to do work. A system’s free energy changes during energy transfers such as chemical reactions, and scientists refer to this change as ∆G.
A reaction’s ∆G can be negative or positive, meaning that the reaction releases energy or consumes energy, respectively. A reaction with a negative ∆G that gives off energy is an exergonic reaction. One with a positive ∆G that requires energy input is an endergonic reaction. Exergonic reactions are spontaneous because their products have less energy than their reactants. Endergonic reactions’ products have a higher energy state than the reactants, and so these are nonspontaneous reactions. However, all reactions (including spontaneous -∆G reactions) require an initial energy input in order to reach the transition state, at which they will proceed. This initial input of energy is the activation energy.
6.3 The Laws of Thermodynamics
In studying energy, scientists use the term “system” to refer to the matter and its environment involved in energy transfers. Everything outside of the system is the surroundings. Single cells are biological systems. We can think of systems as having a certain amount of order. It takes energy to make a system more ordered. The more ordered a system, the lower its entropy. Entropy is a measure of a system’s disorder. As a system becomes more disordered, the lower its energy and the higher its entropy.
The laws of thermodynamics are a series of laws that describe the properties and processes of energy transfer. The first law states that the total amount of energy in the universe is constant. This means that energy cannot be created or destroyed, only transferred or transformed. The second law of thermodynamics states that every energy transfer involves some loss of energy in an unusable form, such as heat energy, resulting in a more disordered system. In other words, no energy transfer i