Chapter  1:  The Dynamic Cell

An artist’s rendition of the interior of a eukaryotic cell. Depicting a cell’s interior is difficult because electron micrographs provide detailed pictures of only a thin slice of a cell, while the cell itself is a three-dimensional object with a very complex interior structure. Thus an artist can create a special sense of the cell’s inner workings by using color and shading. Here the artist rendered the organelles inside the cell as he imagined them rather than as a faithful reconstruction from electron micrographs. The blue object is the cell’s nucleus with the DNA visible inside as a coil. The red strands emerging from the nucleus are RNA molecules. In the rest of the cell is the cytoplasm, which contains many organelles like the red, kidney-shaped mitochondria and the sectioned orange vesicles. The green stack of flattened vesicles near the nucleus is the Golgi apparatus, and the other flat vesicles represent the cell’s endoplasmic reticulum. All of these cellular elements are described in later chapters. [This picture, drawn by Tomo Narashima, originally appeared on the cover of the second edition of this book.]

At first glance, the biological universe appears amazingly diverse — from tall palm trees to tiny ferns, from single-cell bacteria and protozoans visible only under a microscope to multicellular animals of all kinds. Yet the bewildering array of outward biological form overlies a powerful uniformity: all biological systems are composed of the same types of chemical molecules and employ similar principles of organization at the cellular level. Although the basic kinds of biological molecules have been conserved during the billion years of evolution, the ways in which they are assembled with one another to form functioning cells and organisms have undergone considerable change.

To study the properties of the molecules of life and the innumerable variations on basic themes that are found in different organisms, modern researchers employ concepts and experimental techniques drawn from biochemistry, molecular biology, genetics, and cell biology. The resulting discipline of molecular cell biology investigates how cells develop, operate, communicate, and control their activities, and on occasion go awry. This book contains the authors’ attempt to describe systematically the current state of knowledge about cells and to present many of the key experiments that have led to our current understanding of cellular life. It may seem to you, our new reader, a daunting challenge. Our hope is that the overwhelming inventiveness and sheer beauty of construction of biological systems will intrigue you and amply reward your efforts to understand the story we tell.

Living systems, including the human body, consist of such closely interrelated elements that no single element can be fully appreciated in isolation from the others. Organisms contain organs; organs are composed of tissues; tissues consist of cells; and cells are formed from molecules (Figure 1-1). The unity of living systems is coordinated by many levels of interrelationship: molecules carry messages from organ to organ and cell to cell; tissues are delineated and integrated with other tissues by noncellular membranes secreted by cells; and cells gain identity from contact with other cells. Generally all the levels into which we fragment biological systems interconnect. To learn about biological systems, however, we must take a segment at a time. The biology of cells is a logical starting point because an organism can be viewed as consisting of interacting cells, which are the closest thing to an autonomous biological unit that exists. The integration of cellular activity into tissues, the development of organisms by growth and specialization of cells, and the metabolic events fueling the dynamism of living systems are all topics on which we will touch, but they are all topics that fall within the province of other subdisciplines of biological science.

In this chapter, we provide a framework for understanding the primacy of cells in biological systems and review several fundamental concepts that recur throughout our more detailed discussions in subsequent chapters. We begin with a brief look at the role of evolution and then discuss the general properties of the molecules found in biological systems. Next, we review the main features of cellular architecture, noting the similarities and differences between the three main cell lineages that have emerged over evolutionary time. The remaining topics covered in this chapter focus on the assemblage of cells into organized structures and their dynamic nature, in preparation for later chapters dealing with various processes critical to cellular growth, differentiation, and adaptation to changing circumstances.