Introduction Of Cytoplasm

Cells are the structural units of all living organisms. There are two fundamentally different types of cells, but so many biochemical similarities exist between them that some investigators have postulated that one group evolved from the other.

Prokaryotic (Gr. pro, before, + karyon, nucleus) cells are found only in bacteria. These cells are small (1–5 m long), typically have a cell wall outside the plasmalemma, and lack a nuclear envelope separating the genetic material (DNA) from other cellular constituents. In addition, prokaryotes have no histones (specific basic proteins) bound to their DNA and usually no membranous organelles.

In contrast, eukaryotic (Gr. eu, good, + karyon, nucleus) cells are larger and have a distinct nucleus surrounded by a nuclear envelope (Figure 2–1). Histones are associated with the genetic material, and numerous membrane-limited organelles are found in the cytoplasm. This book is concerned exclusively with eukaryotic cells.

Figure 2–1

The ultrastructure and molecular organization (right) of the cell membrane. The dark lines at the left represent the two dense layers observed in the electron microscope; these are caused by the deposit of osmium in the hydrophilic portions of the phospholipid molecules.

Cellular Differentiation

The human organism has about 200 different cell types, all derived from the zygote, a single cell formed by fertilization of an oocyte by a spermatozoon. The first cellular divisions of the zygote originate cells called blastomeres, which are able to form all cell types of the adult. Through this process, called cell differentiation, the cells synthesize specific proteins, change their shape, and become very efficient in specialized functions. For example, muscle cell precursors elongate into spindle-shaped cells that synthesize and accumulate myofibrillar proteins (actin, myosin). The resulting cell efficiently converts chemical energy into contractile force.

The main cellular functions performed by specialized cells in the body are listed in Table 2–1.

Table 2–1. Cellular Functions in Some Specialized Cells.

Function Specialized Cell(s) Movement Muscle cell Synthesis and secretion of enzymes Pancreatic acinar cells Synthesis and secretion of mucous substances Mucous-gland cells Synthesis and secretion of steroids Some adrenal gland, testis, and ovary cells Ion transport Cells of the kidney and salivary gland ducts Intracellular digestion Macrophages and some white blood cells Transformation of physical and chemical stimuli into nervous impulses Sensory cells Metabolite absorption Cells of the intestine

Cell Ecology Because the body experiences considerable environmental diversity (eg, normal and pathological conditions), the same cell type can exhibit different characteristics and behaviors in different regions and circumstances. Thus, macrophages and neutrophils (both of which are phagocytic defense cells) will shift from oxidative metabolism to glycolysis in an anoxic, inflammatory environment. Cells that appear to be structurally similar may react in different ways because they have different families of receptors for signaling molecules (such as hormones and extracellular matrix macromolecules). For example, because of their diverse library of receptors, breast fibroblasts and uterine smooth muscle cells are exceptionally sensitive to female sex hormones. References Afzelius BA, Eliasson R: Flagellar mutants in man: on the heterogeneity of the immotile-cilia syndrome. J Ultrastruct Res 1979;69:43. [PMID: 501788] Aridor M, Balch WE: Integration of endoplasmic reticulum signaling in health and disease. Nat Med 1999;5:745. [PMID: 10395318] Barrit GJ: Communication Within Animal Cells. Oxford University Press, 1992. Becker WM et al: The World of the Cell, 4th ed. Benjamin/Cummings, 2000. Bretscher MS: The molecules of the cell membrane. Sci Am 1985;253:100. [PMID: 2416050] Brinkley BR: Microtubule organizing centers. Annu Rev Cell Biol 1985;1:145. [PMID: 3916316] Brown MS et al: Recycling receptors: the round-trip itinerary of migrant membrane proteins. Cell 1983;32:663. [PMID: 6299572] Cooper GM: The Cell: A Molecular Approach. ASM Press/Sinauer Associates, Inc., 1997. DeDuve C: A Guided Tour of the Living Cell. Freeman, 1984. DeDuve C: Microbodies in the living cell. Sci Am 1983;248:74. Dustin P: Microtubules, 2nd ed. Springer-Verlag, 1984. Farquhar MG: Progress in unraveling pathways of Golgi traffic. Annu Rev Cell Biol 1985;1:447. [PMID: 3916320] Fawcett D: The Cell, 2nd ed. Saunders, 1981. Krstíc RV: Ultrastructure of the Mammalian Cell. Springer-Verlag, 1979. Mitchison TJ, Cramer LP: Actin-based cell motility and cell locomotion. Cell 1996;84:371. [PMID: 8608590] Osborn M, Weber K: Intermediate filaments: cell-type-specific markers in differentiation and pathology. Cell 1982;31:303. [PMID: 6891619] Pfeffer SR, Rothman JE: Biosynthetic protein transport and sorting in the endoplasmic reticulum. Annu Rev Biochem 1987;56:829. [PMID: 3304148] Rothman J: The compartmental organization of the Golgi apparatus. Sci Am 1985;253:74. [PMID: 3929377] Simons K, Ikonen E: How cells handle cholesterol. Science 2000;290:1721. [PMID: 11099405] Tzagoloff A: Mitochondria. Plenum, 1982. Weber K, Osborn M: The molecules of the cell matrix. Sci Am 1985;253:110. [PMID: 4071030]