While light microscopy shows many types of cells and even some structure inside them, the cell membrane is best observed with an electron microscope. This very thin structure forms the outer boundary of the cytoplasm of every cell, and it has similar thickness and molecular structure in all cells. You will learn about membranes in more detail in Chapter 6. For now, keep in mind that (with the exception of some species of archaea discussed in Chapter 26) the cell membrane consists of a phospholipid bilayer (see Key Concept 3.4), and that a variety of proteins are embedded within the bilayer.
The membrane has several important roles:
The cell membrane acts as a selectively permeable barrier, preventing some substances from crossing it while permitting other substances to enter and leave the cell.
By regulating transport across it, the cell membrane allows the cell to maintain a more or less constant internal environment. The maintenance of a constant internal environment (known as homeostasis) is a key characteristic of life and will be discussed in detail in Chapter 40.
As the cell’s boundary with the outside environment, the cell membrane is important in communicating with adjacent cells and receiving signals from the environment. You will learn about this in Chapter 7.
The cell membrane often has proteins protruding from it that are responsible for binding and adhering to adjacent cells. Thus the cell membrane plays an important structural role and contributes to cell shape.
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