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Cryoelectron microscopy uses quickly frozen samples to reduce aberrations that are seen when samples are treated chemically. Computer analysis of thick sections can reconstruct a sample in three dimensions.

4.4 Looking at Cells The top six panels show some techniques used in light microscopy. The lower three images were created using electron microscopes.

Cells are surrounded by a plasma membrane

As we have noted, a plasma membrane separates each cell from its environment, creating a segregated (but not isolated) compartment. The plasma membrane is composed of a phos-pholipid bilayer, with the hydrophilic "heads" of the lipids facing the cell's aqueous interior on one side of the membrane and the extracellular environment on the other (see Figure 3.21). Proteins are embedded in the lipids. In many cases, these proteins protrude into the cytoplasm and into the extracellular environment. We will devote most of Chapter 5 to detailing the structure and functions of the plasma membrane, but summarize its roles here.

► The plasma membrane allows the cell to maintain a more or less constant internal environment. A self-maintaining, constant internal environment is a key characteristic of life that will be discussed in detail in Chapter 41.

► The plasma membrane acts as a selectively permeable barrier, preventing some substances from crossing while permitting other substances to enter and leave the cell.

► As the cell's boundary with the outside environment, the plasma membrane is important in communicating with adjacent cells and receiving extracellular signals. We will describe this function in Chapter 15.

► The plasma membrane often has molecules protruding from it that are responsible for binding and adhering to adjacent cells.

Cells show two organizational patterns

Prokaryotic cell organization is characteristic of the domains Bacteria and Archaea. Organisms in these domains are called prokaryotes. Their cells do not have membrane-enclosed internal compartments. The first cells ever to form were undoubtedly similar in organization to modern prokaryotes.

Eukaryotic cell organization is found in the domain Eu-karya, which includes the protists, plants, fungi, and animals. The genetic material (DNA) of eukaryotic cells is contained in a special membrane-enclosed compartment called the nucleus. Eukaryotic cells also contain other membrane-enclosed compartments in which specific chemical reactions take place. Organisms with this type of cell organization are known as eukaryotes.

Both prokaryotes and eukaryotes have prospered for many hundreds of millions of years of evolution, and both are great success stories. Let's look first at prokaryotic cells.

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Essentials of Human Physiology

Essentials of Human Physiology

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