When cells in multicellular organisms are badly damaged or infected with a virus, they die. Cell death resulting from such a traumatic event is messy and often releases potentially toxic cell constituents that can damage surrounding cells. Cells also may die when they fail to receive a life-maintaining signal or when they receive a death signal. In this type of programmed cell death, called apoptosis, a dying cell actually produces proteins necessary for self-destruction. Death by apoptosis avoids the release of potentially toxic cell constituents (Figure 1-19).
Programmed cell death is critical to the proper development and functioning of our bodies (Chapter 22). During fetal life, for instance, our hands initially develop with "webbing" between the fingers; the cells in the webbing subsequently die in an orderly and precise pattern that leaves the
▲ FIGURE 1-19 Apoptotic cells break apart without spewing forth cell constituents that might harm neighboring cells. White blood cells normally look like the cell on the left. Cells undergoing programmed cell death (apoptosis), like the cell on the right, form numerous surface blebs that eventually are released. The cell is dying because it lacks certain growth signals. Apoptosis is important to eliminate virus-infected cells, remove cells where they are not needed (like the webbing that disappears as fingers develop), and to destroy immune system cells that would react with our own bodies. [Gopal Murti/Visuals Unlimited, Inc.]
fingers and thumb free to play the piano. Nerve cells in the brain soon die if they do not make proper or useful electrical connections with other cells. Some developing lymphocytes, the immune-system cells intended to recognize foreign proteins and polysaccharides, have the ability to react against our own tissues. Such self-reactive lymphocytes become programmed to die before they fully mature. If these cells are not weeded out before reaching maturity, they can cause autoimmune diseases in which our immune system destroys the very tissues it is meant to protect.
T4| Investigating Cells and Their Parts
To build an integrated understanding of how the various molecular components that underlie cellular functions work together in a living cell, we must draw on various perspectives. Here, we look at how five disciplines—cell biology, biochemistry, genetics, genomics, and developmental biology—can contribute to our knowledge of cell structure and function. The experimental approaches of each field probe the cell's inner workings in different ways, allowing us to ask different types of questions about cells and what they do. Cell division provides a good example to illustrate the role of different perspectives in analyzing a complex cellular process.
The realm of biology ranges in scale more than a billion-fold (Figure 1-20). Beyond that, it's ecology and earth science
Red blood cell
▲ FIGURE 1-20 Biologists are interested in objects ranging in size from small molecules to the tallest trees. A sampling of biological objects aligned on a logarithmic scale. (a) The DNA double helix has a diameter of about 2 nm. (b) Eight-cell-stage human embryo three days after fertilization, about 200 ^m across. (c) A wolf spider, about 15 mm across. (d) Emperor penguins are about 1 m tall. [Part (a) Will and Deni McIntyre. Part (b) Yorgas Nikas/Photo Researchers, Inc. Part (c) Gary Gaugler/Visuals Unlimited, Inc. Part (d) Hugh S. Rose/Visuals Unlimited, Inc.]
at the "macro" end, chemistry and physics at the "micro" end. The visible plants and animals that surround us are measured in meters (100-102 m). By looking closely, we can see a biological world of millimeters (1 mm = 10~3 m) and even tenths of millimeters (10~4 m). Setting aside oddities like chicken eggs, most cells are 1-100 micrometers (1 ^m = 10~6 m) long and thus clearly visible only when magnified. To see the structures within cells, we must go farther down the size scale to 10-100 nanometers (1 nm = 10~9 m).
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