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Chapter 2 : The Cell: the Basic Unit of Life

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Abstract:

This chapter discusses essential functions, common features, chemical basis of cell organization and the two major classes of cells. Biologists examining the microscopic structures of microbes, plants, and animals found that every tissue or organism was made of cells, each enclosed by a membrane separating it from surrounding cells and the external environment. Chemical analysis revealed that even though various cell types looked very different physically, their chemical makeups were remarkably similar. Over time, cells increase in size, and this growth requires building materials and energy. Conveniently, cells get both of these supplies from the same source, the large molecules that make up food: proteins, carbohydrates, fats, and nucleic acids. Just as new animals and plants are generated by the reproduction of animals and plants, new cells are generated by the reproduction of cells. Carrying out activities, building and rebuilding molecules and structures, and maintaining an internal environment that differs from its surroundings all require energy, and cells obtain it from a variety of sources. All cells share certain basic features: they have molecular machinery for duplicating DNA and breaking down and synthesizing molecules, they reproduce by dividing in two, they use the same molecular building blocks, and they are enclosed by a hydrophobic membrane that separates the cell from its surroundings.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2

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Hydrophilic Molecules
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Figures

Image of Figure 2.1
Figure 2.1

Hooke’s drawing of cork cells.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.2
Figure 2.2

Different cell types. Liver cell. Goldfish skin. Neutrophil (a type of white blood cell). Neurons. (Photographs copyright Dennis Kunkel Microscopy, Inc.)

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.3
Figure 2.3

Cell metabolism. The biological molecules in food are broken down for energy and building block molecules and then reassembled according to the needs of the cell.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.4
Figure 2.4

Dividing cells. (Photograph copyright Dennis Kunkel Microscopy, Inc.)

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.5
Figure 2.5

The cell cycle. Cells go through cyclic periods of growth, DNA replication, and cell division as they reproduce.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.6
Figure 2.6

The cytoplasm of this amoeba is much more concentrated than the water in which it is living. (Photograph copyright Micrographia, Inc.)

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.7
Figure 2.7

The sensitive plant responds to being touched by curling up its leaves. Before stimulus. After stimulus. (Photographs courtesy of Kent Schwaegerle, University of Alaska.)

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.8
Figure 2.8

Egg and sperm exchange chemical signals during the process of fertilization. (Photograph courtesy of Gerald Schatten, University of Pittsburgh.)

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.9
Figure 2.9

A single fertilized egg divides thousands of times, and the descendent cells differentiate into many different cell types in the process of becoming a baby. Four-cell human embryo. (Photograph courtesy of Michael Vernon, West Virginia Center for Reproductive Medicine.) A newborn. (Photograph courtesy of Anastasia Lott.)

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.10
Figure 2.10

An enzyme catalyzes one specific chemical reaction, during which molecules fit together precisely.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.11
Figure 2.11

Hydrophobic and hydrophilic molecules tend to congregate and form compartments. Oil, a hydrophobic substance, and water form two separate compartments. A hydrophilic substance like salt will dissolve in the water but not in the hydrophobic oil. A hydrophobic substance will dissolve in the oil but not in the water.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.12
Figure 2.12

Some molecules have both hydrophobic and hydrophilic parts. The most favorable arrangement for them is to keep the hydrophobic parts in hydrophobic environments and the hydrophilic parts in hydrophilic environments. Here, the molecules form a sphere with the hydrophobic parts on the inside. You are looking at a cross section.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.13
Figure 2.13

Three-dimensional structure of the protein flavodoxin. (Structure courtesy of Antonio Romero and Javier Sancho.)

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.14
Figure 2.14

Cell membranes are a two-molecule-thick layer with a hydrophobic core and hydrophilic faces. The cell membrane surrounds the cell like the membrane of a balloon. A cross section is shown.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.15
Figure 2.15

Micrograph of cell membrane. (Photograph copyright Dennis Kunkel Microscopy, Inc.)

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.16
Figure 2.16

A protein embedded in a membrane. The portion of the protein inside the hydrophobic core of the membrane is also hydrophobic, while the external portions are hydrophilic.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Image of Figure 2.17
Figure 2.17

Typical prokaryotic and eukaryotic cells. These simplified representations incorporate features from typical cells, but there is great variety in cell shape and content (for examples, see Figure 2.2 ). A prokaryotic cell has no internal membranes. Animal and other eukaryotic cells contain many membrane-bound organelles that are the sites at which specific processes occur. Here, we show some of them. Plant cells are surrounded by supportive walls. In addition to the organelles found in animal cells, they contain membrane-bound chloroplasts, where solar energy is converted to chemical energy, and a storage compartment called a vacuole.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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Untitled

virus, in its simplest form, consists of genetic material enclosed in a protein capsid.

Citation: Kreuzer H, Massey A. 2005. The Cell: the Basic Unit of Life, p 35-49. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch2
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