The cell membrane forms an outer boundary of every cell

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:

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