CHAPTER 6 INTRODUCTION

CORE CONCEPTS

6.1 Metabolism is the set of biochemical reactions that transforms biomolecules and transfers energy.

6.2 The energy of a system is its capacity to do work.

6.3 The laws of thermodynamics govern energy flow in biological systems.

6.4 Chemical reactions are subject to the laws of thermodynamics.

6.5 The rate of biochemical reactions is controlled by protein catalysts called enzymes.

We have seen that cells require a way to encode and transmit information and a membrane to separate inside from out. The third requirement of a cell is energy, which cells need to do work. Among other things, they must grow and divide, move, change shape, pump ions in and out of the cell, transport vesicles, and synthesize macromolecules such as DNA, RNA, proteins, and complex carbohydrates. All these activities are considered work, and they require energy.

We are all familiar with different forms of energy—the sun and wind provide a source of energy, as do fossil fuels such as oil and natural gas. We have learned to harness the energy from these sources and convert it into other forms, such as electricity, to provide needed power to our homes and cities.

Cells are faced with similar challenges. They must harness energy from the environment and convert it to a form that allows them to do work necessary to sustain life. Cells harness energy from the sun and from chemical compounds, including carbohydrates, lipids, and proteins. Although the source of energy may differ among cells, all cells convert energy to a form that can be easily used for cellular processes. All cells use energy in the form of a molecule called adenosine triphosphate, or ATP.

ATP is often called the universal “currency” of cellular energy to indicate that ATP provides energy in a form that all cells can readily use to perform the work of the cell. Although the term “currency” is a useful shorthand, keep in mind an important distinction between actual currency, such as a dollar bill, and ATP. A dollar bill represents a certain value but does not in fact have any value in itself. By contrast, ATP does not represent energy; it actually contains energy in its chemical bonds. Nevertheless, the analogy is useful because ATP, like a dollar bill, engages in a broad range of energy “transactions” in the cell, as we discuss below.

In this chapter, we consider energy in the context of cells. What exactly is energy? What principles govern its flow in biological systems? And how do cells make use of it?