the book is divided into EXAMPLES that illustrate the topics covered in the main text, explanations of the MEDICAL RELEVANCE of the material, and IN-DEPTH sections that extend the coverage beyond the content of the main text. We include BRAINBOXES highlighting a number of the scientists who have made significant contributions to cell biology, either by making critical discoveries, or by creating the tools that made those discoveries possible. REVIEW QUESTIONS in extended matching‐set format at the end of each chapter help the reader assess how well they have assimilated and understood the material, while each chapter also poses a “THOUGHT QUESTION” that tests concepts rather than facts.
A comprehensive website accompanies the book at www.wiley.com/go/bolsover/cellbiology4. This includes additional examples, in‐depth explanations, and medical discussions for which there was no room in the printed book. Students who would like to test their understanding of the subject will find additional review questions, while teachers can find suggestions of essay titles. The website gives links to other internet resources together with references to primary research publications to allow readers to trace the origin of statements in the text. Lastly, the website allows download of all the figures from the book as slides for use by teachers.
ACKNOWLEDGMENTS
We are very grateful to Professors Jean‐Claude Labbé (IRIC, Université de Montréal) and Stephanie Schorge (University College London) for critical reading of the manuscript.
ABOUT THE COMPANION WEBSITE
This book is accompanied by a companion website.
www.wiley.com/go/bolsover/cellbiology4
This website includes:
Figures from the book in PowerPoint
Additional examples and revision resources
SECTION 1
THE STRUCTURE OF THE CELL
The cell is the fundamental unit of life. A cell comprises a complex and ordered mass of protein, nucleic acid, and many biochemical species separated from the world outside by a limiting membrane. Cells expend energy to maintain a highly ordered state, and this expenditure of energy and the ability to repair themselves distinguishes living cells from lifeless packets of biological material such as viruses. In the first two chapters we will describe the basic structure of cells and how they can be observed with a microscope. We will describe how, in animals, cells containing the same DNA database assume very different shapes and functions and organize themselves into tissues.
Chapter 1: A Look at Cells and Tissues
Chapter 2: Membranes and Organelles
1 A LOOK AT CELLS AND TISSUES
With very few exceptions, all living things are either a single cell or an assembly of cells. This chapter will begin to describe what a cell is, and further chapters will say much more. However, to begin with, we can briefly describe a cell as an aqueous (watery) droplet enclosed by a lipid (fatty) membrane. Cells are, with a few notable exceptions, small (Figure 1.1), with dimensions measured in micrometers (μm, 1 μm = 1/1000 mm). They are more or less self‐sufficient: a single cell taken from a human being can survive for many days in a dish of nutrient broth, and many human cells can grow and divide in such an environment. In 1838 the botanist Matthias Schleiden and the zoologist Theodor Schwann formally proposed that all living organisms are composed of cells. Their “cell theory,” which nowadays seems so obvious, was a milestone in the development of modern biology. Nevertheless, general acceptance took many years, in large part because the plasma membrane (Figure 1.2), the membrane surrounding the cell that divides the living inside from the nonliving extracellular medium, is too thin to be seen using a light microscope. Microorganisms such as bacteria, yeast, and protozoa exist as single cells. In contrast, the adult human is made up of about 30 trillion cells (1 trillion = 1012), which are mostly organized into collectives called tissues.
Superficially at least, cells exhibit a staggering diversity. Some have defined, geometric shapes; others have flexible boundaries; some lead a solitary existence; others live in communities; some swim, some crawl, and some are sedentary. Given these differences, it is perhaps surprising that there are only two types of cell (Figure 1.2). Prokaryotic (Greek for “before nucleus”) cells have very little visible internal organization so that, for instance, the genetic material, stored in the molecule deoxyribonucleic acid (DNA), is free within the cell. These cells are especially small, the vast majority being 1–2 μm in length. The prokaryotes are made up of two broad groups of organisms, the bacteria and the archaea (Figure 1.3). The archaea were originally thought to be an unusual group of bacteria but we now know that they are a distinct group of prokaryotes with an independent evolutionary history. The cells of all other organisms, from yeasts to plants to worms to humans, are eukaryotic (Greek for “with a nucleus”). These are generally larger (5–100 μm, although some eukaryotic cells are large enough to be seen with the naked eye; Figure 1.1) and structurally more complex. Eukaryotic cells contain a variety of specialized structures known collectively as organelles, embedded within a viscous substance called cytosol. Their DNA is held within the largest organelle, the nucleus. The structure and function of organelles will be described in detail in subsequent chapters. Table 1.1 summarizes the differences between prokaryotic and eukaryotic cells.
Cell Division
One of the major distinctions between prokaryotic and eukaryotic cells is their mode of division. In prokaryotes the circular chromosome is duplicated from a single replication origin by a group of proteins that reside on the inside of the plasma membrane. At the completion of replication the old and new copies of the chromosome lie side by side on the plasma membrane which then pinches inwards between them. This process, which generates two equal, or roughly equal, daughter cells is described as binary fission. In eukaryotes the large, linear chromosomes, housed in the nucleus, are duplicated from multiple origins of replication by enzymes located in the nucleus. Sometime later the nuclear envelope