Chapter 11

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Cell Comm KEY

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CONCEPTS

11.1 External signals are converted to responses within the cell 11.2 Reception: A signaling molecule hinds to a receptor protein, causing it to change shape 11.3 Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell 11.4 Response: Cell signaling leads to regulation of transcription or cytoplasmic activities 11.5 Apoptosis (programmed cell death) integrates

multiple cell-signaling pathways

hiker slips and falls down a steep ravine, injuring her leg in the fall. Tragedy is averted when she is able to pull out a cell phone and call for help. Cell phones, the Internet, e-mail, instant messaging-no one would deny the importance of communication in our lives. The role of communication in life at the cellular level is equally critical. Cell-to-cell communication is absolutely essential for multicellular organisms such as humans and oak trees. The trillions of cells in a multicellular organism must communicate with each other to coordinate their activities in a way that enables the organism to develop from a fertilized egg, then survive and reproduce in turn. Communication between cells is also important for many unicellular organisms. Networks of communication between cells can be even more complicated than the World Wide Web. In studying how cells signal to each other and how they interpret the signals they receive, biologists have discovered some universal mechanisms ofcellular regulation, additional evidence for the evolutionary relatedness ofall life. The same small set of cell-signaling mechanisms shows up again and again in many

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.... Figure 11.1 How do the effects of Viagra (multicolored) result from its inhibition of a signaling-pathway enzyme (purple)? lines of biological research-from embryonic development to hormone action to cancer. In one example, a common cell-tocell signaling pathway leads to dilation ofblood vessels. Once the signal subsides, the response is shut down by the enzyme shown in purple in Figure 11.1. Also shown isa multicolored molecule that blocks the action ofthis enzyme and keeps blood vessels dilated. Enzyme-inhibiting compounds like this one are often prescribed for treatment of medical conditions. The action of the multicolored compound, known as Viagra, will be discussed later in the chapter. The signals received by cells, whether originating from other cells or from changes in the physical environment, take various forms, including light and touch. However, cells most often communicate with each other by chemical signals. In this chapter, we focus on the main mechanisms by which cells receive, process, and respond to chemical signals sent from other cells. At the end, we will take a look at apoptosis, a type of programmed cell death that integrates input from multiple signaling pathways.

r;:~:tr~'a7 s~:~~ls are converted to responses within the cell

What does a "talking" cell say to a ~listeningn cell, and how does the latter cell respond to the message? Let's approach these questions by first looking at communication among microorganisms, for modern microbes are a window on the role of cell signaling in the evolution oflife on Earth.

Evolution of Cell Signaling One topic of cell "conversation" is sex-at least for the yeast Saccharomycescerevisiae, which people have used for millennia to make bread, wine, and beer. Researchers have learned


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