Chapter 4 - Chapter 4 Cells: The Working Units of Life What...

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Chapter 4 Cells: The Working Units of Life What Features of Cells Make Them the Fundamental Unit of Life? • The cell theory states that: Cells are the fundamental units of life. All organisms are composed of cells. All cells come from preexisting cells. • The study of cell biology is the basis for studying all life, whether single-celled or multicellular. Cell size is limited by the surface area-to-volume ratio Most cells are tiny, with volumes in the range of 1 to 1000 m m 3 . Some cells are relatively large (e.g., birds’ eggs, some nerve cells, and several types of bacteria and algae). The surface of a cell is the area that interfaces with the cell’s environment. The larger the surface area of a cell, the faster a cell can take in substances and remove waste products. The volume of a cell is a measure of the space inside a cell. The larger the volume of a cell, the more chemical activity it can have. Surface area-to-volume ratio is defined as the surface area divided by the volume. For any given shape, increasing volume decreases the surface area-to-volume ratio. Small size benefits cells in many ways: As a cell grows larger its need for resources and generation of waste increase faster than its surface area. Cells must often distribute substances within themselves, which is made much easier if the cell is small. This explains why large organisms must consist of many small cells to maintain sufficient surface area- to-volume ratios and to maintain an ideal internal volume. Microscopes are needed to visualize cells The small size of cells makes the use of microscopes necessary to view them. If two objects are too close together, they start to look like one object. With normal human vision the smallest objects that can be resolved (i.e., distinguished from one another) are about 200 m m (0.2 mm) in size. (resolution) There are two basic types of microscopes: Light microscopes use glass lenses and visible light and typically have a resolving power of 0.2 m m. Electron microscopes use electromagnets, rather than glass lenses, to focus an electron beam. • Resolution is about 0.2 nm or 1,000,000 times finer than that of the human eye. • Subcellular features can be distinguished at this resolution. Cells are surrounded by a plasma membrane Every cell has a plasma membrane, a continuous membrane that surrounds the fluids and other structures of a cell. The membrane is composed of a lipid bilayer with proteins floating within it and protruding from it. The membranes allow the cell to maintain a rather constant internal environment. This process is called homeostasis. It acts as a selectively permeable barrier. Some substances can diffuse in and out; others cannot.
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Chapter 4 - Chapter 4 Cells: The Working Units of Life What...

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