The Eukaryotic Cell

Eukaryotic cells are generally much larger than prokaryotic cells, and their internal structures are far more complex (figure 3.48). One of their most distinguishing characteristics is the abundance of membrane-enclosed compartments or organelles. The most important of these is the nucleus, which contains the DNA. The organelles can take up half the total cell volume and enable the cell to perform complex functions in spatially separated regions. For example, degradative enzymes are contained within an organelle, isolated from other cell contents, where they digest food and other material without posing a threat to the integrity of the cell itself.

Organelles are functionally beneficial, but they also create transit problems. Each organelle contains a variety of proteins and other molecules, many of which are synthesized at other locations. To deliver these to the lumen, or interior, of the

Golgi apparatus

Cytoskeleton

Smooth endoplasmic reticulum

Nuclear envelope

Nucleolus

Nucleus

Microvilli

Nucleus

Microvilli

Plasma membrane

Centriole

Plasma membrane

Lysosome Ribosomes

Mitochondrion

Rough endoplasmic reticulum Cytoplasm

Plasma membrane

Lysosome Ribosomes

Mitochondrion

Rough endoplasmic reticulum Cytoplasm

Mitochondrion Ribosomes

Chloroplasts

Lysosome

Nuclear envelope

Rough endoplasmic reticulum

Cytoplasm

Smooth endoplasmic reticulum

Mitochondrion Ribosomes

Smooth endoplasmic reticulum

Figure 3.48 Eukaryotic Cells (a) Animal cell. (b) Plant cell. (c) Micrograph of animal cell shows several membrane-bound structures including mitochondria and a nucleus.

Cell membrane

Mitochondrion Nucleus

Nuclear membrane

Chloroplasts

Lysosome

Nuclear envelope

Rough endoplasmic reticulum

Cytoplasm

Cell membrane

Nuclear membrane

70 Chapter 3 Microscopy and Cell Structure appropriate organelle, an elaborate transportation system is required. To transfer material, a section of an organelle will bud or pinch off, forming a small membrane-enclosed vesicle (figure 3.49). This mobile vesicle, containing a sampling of the contents of the organelle, can move to other parts of the cell. When that vesicle encounters the lipid membrane of the appropriate organelle, the two membranes will fuse to become one contiguous unit. By doing so, the vesicle introduces its contents to the lumen of that organelle. In this way, the contents of one membrane-enclosed compartment can be transferred to another. A similar process is used to export molecules synthesized within an organelle to the external environment.

As a group, eukaryotic cells are highly variable in many aspects. For example, protozoa, which are single-celled organisms, must function exclusively as a self-contained unit that seeks and ingests food. These cells must be mobile and flexible to take in food particles, including bacteria. Consequently, they lack a cell wall that would otherwise provide rigidity. Animal cells also lack a cell wall, because they too must be flexible to accommodate movement. Fungi, on the other hand, are stationary and benefit from the protection provided by a rigid cell wall. Compounds that make up their cell walls include glucan and mannan, which are polysaccharides, and chitin, a polymer of N-acetylglucosamine that is also found in crustaceans and insects. Plant cells, which are also stationary, have cell walls composed of cellulose, a polymer of glucose. ■ polysaccharides, p. 29

The individual cells of a multicellular organism may be distinctly different from one another. Mammals, for example, are composed of several hundred different types of cells, and it is obvious that a liver cell is quite different from a bone cell. Cells of plants and animals function in cooperative associations called tissues. The tissues in your body include muscle, connective, nerve, epithelial, blood, and lymphoid. Each of these provides a different function. Connective tissue, for example, which includes bone and cartilage, provides structure and support. Muscle provides movement. Combinations of various tissues function together to make up larger units, organs. These include skin, heart, and liver. Organs and the tissues that constitute them will be covered in detail in chapters 22 through 29 on infectious diseases. At this point, however, it is helpful to recognize that a

Figure 3.49 Vesicle Formation and Fusion A vesicle forms when a section of an organelle buds off.The mobile vesicle can then move to other parts of the cell, ultimately fusing with the membrane of another organelle.

generic discussion of eukaryotic cells encompasses many functional varieties. ■ types of epithelium, p. 374 ■ lymphoid tissue, p. 396

A comprehensive coverage of all aspects of eukaryotic cells is beyond the scope of this textbook. Instead, this section will focus on key characteristics, particularly those that directly affect the interaction of a microbe with a human host. These characteristics are summarized in table 3.6. A comparison of functional aspects of prokaryotic and eukaryotic cells is presented in table 3.7.

Figure 3.49 Vesicle Formation and Fusion A vesicle forms when a section of an organelle buds off.The mobile vesicle can then move to other parts of the cell, ultimately fusing with the membrane of another organelle.

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