In keeping with the goals of this revision, you will find significant changes throughout the text. In reviewing the table of contents, you will see that we've expanded the text from 55 chapters to 60 chapters. This change was in direct response to reviewers who said that we shortchanged some key areas in the fourth edition. We expanded the coverage of the viruses and simple organisms (Part VIII) from four to five chapters. We expanded the coverage of plants from five to eight chapters. We expanded the coverage of animal diversity, devoting entire chapters to both arthropods and echinoderms. We also expanded the coverage of animal form and function, devoting entire chapters to both respiration and circulation.

We changed the format and organization of the text to follow a "learning modules" approach. The material is presented according to key concepts that are numbered for ease of identification. The student can get an overview of the chapter content by reading the key concept statements and the headings that follow each statement. A walkthrough of the learning modules is presented on the next page.

Other significant changes to the content are listed below:

Ribosomal RNA now includes the E site; the treatment of transcription is greatly expanded, covering a two-page spread; and the discussion and illustrations of translation have been updated to include the action of the ribosomal E site.

New additions to this chapter include: resource partitioning, including character displacement in Darwin's finches; a two-page spread on biodiversity and community structure, including species richness and species diversity discussions and trends; biogeographic patterns of species diversity; island biogeography; an expanded and revised section on succession; and preserving biodiversity.

The lytic and lysogenic cycles are discussed in the text; the new finding of the mechanism of how cholera-causing bacteria are transformed by a bacteriophage has been added; the HIV infection cycle has been updated to include the latest findings on co-receptors and the latency period; a new section has been added on the future of HIV treatment, including art and discussions of the variety of methods that are being researched; and the section on prions and viroids has been updated to include prion formation and mad-cow disease.

This entire chapter is new and includes up-to-date discussions, art, and photos covering the size, organization, and mapping of plant genomes; plant tissue culture methods and applications, and genetic engineering in plants.

New additions to this chapter include a new phylogeny of the animals, and an illustrated table of the major animal phyla, a section discussing and illustrating four key transitions in body plan, an updated classification of the cnidarians, and a discussion with a photo of the newly discovered animal phylum Cycliophora.

A new section on the evolution of the immune system discusses and illustrates the progression from invertebrates to vertebrates and the types of immune cells that have evolved; an illustrated table of the cells of the immune system has been added; the discussion of B cells has been revised and includes a clear layout of the humoral immune response, a discussion of how the diversity of antibodies is produced by somatic rearrangement, and the uses of the antibodies in medical diagnoses; an up-to-date AIDS epidemic chart has been added; and a new finding of how DNA vaccines may get around antigen shifting is included.

A new section on the reversal of determination discusses the recent animal cloning experiments, and a new section on aging discusses and illustrates recent theories of aging.

You will find the chapters in this edition organized in conceptual units. Each chapter presents three or four general concepts that together explain the key ideas that the chapter addresses. Each of these concepts is presented as a numbered statement, and is explored in a series of learning modules; each module is typically a one or two page spread (some use three or four pages) ending with a statement summarizing that module. The numbered general concepts explored in specific supporting modules provide the student with a clear outline of how the various topics covered in the chapter relate to one another, revealing in a direct way the conceptual skeleton of the chapter, so students don't miss the "big picture."

Each chapter opening page contains a Concept Outline that presents an overview of the conceptual units in the chapter. A portion of the Concept Outline for chapter 6 is reproduced here. The Key Concepts are numbered and the supporting topics are listed under the key concepts and provide the student with an overview of contents discussed in the chapter.

The numbered concepts begin the discussion of a general concept with a conceptual unit covering usually one page or a two-page spread. The first topic under the concept 6.1, "Biological membranes are fluid layers of lipid," is reproduced here.

The concept module "The Phospholipid Bilayer" ends with a summary statement shown below, as do all of the topics, whether they cover one page or two or more pages. This makes it very clear to the student where a topic begins and ends.

The organization of the material into conceptual units is carried through to the end of the chapter. The chapter summaries at the end of the chapter are organized as a "Summary of Concepts" according to the "Concept Outline" found at the beginning of the chapter. Each numbered concept is followed by summary statements of the material found in the conceptual unit.

A section entitled "Discussing Key Terms" is far more than a list of key terms in the chapter. This section discusses the significance of the term within the context of the material covered in the chapter.