Release of transcription factors (E2F)
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FIG 3.43 The mechanism of initiation of cellular replication requires appropriate stimulation by extracellular growth factors that bind their complementary receptors on the cell surface. Activation of the receptor stimulates signaling pathways within the cell that increase cyclin proteins. The cyclins bind to cyclin-dependent kinases (Cdks) to form active enzyme complexes. The active cyclin-Cdk enzymes phosphorylate Rb protein (pRb), inducing it to release E2F transcription factors that initiate replication. In the absence of appropriate growth factor signals, the Rb protein functions to inhibit unwanted cell proliferation.
Detailed knowledge of cell physiology is essential to understanding disease processes. Cells are complex, membrane-bound units that perform a variety of functions necessary to the maintenance of life. The major cell components and their functions are summarized in Table 3.2. The cell membrane is an important cellular structure that protects the cell interior and mediates information transfer to and from the extracellular environment. Proteins embedded in the membrane lipid bilayer perform most of the membrane functions, including transduction of extracellular messages, membrane transport, electrical excitation, and cell-to-cell communication.
Human cells have several important intracellular organelles. These include the cytoskeleton, which organizes the intracellular compartment;
S U M M A R Y the nucleus, which holds the cell’s genetic material and directs the daily activities of the cell; the ER and the Golgi apparatus, which produce, package, and transport proteins and lipids to the plasma membrane and lysosomes; the lysosomes and peroxisomes, which perform the task of intracellular digestion of organic waste; and the mitochondria, which produce cellular energy in the form of ATP. The energy released by ATP hydrolysis is used by the cell to drive the many energetically unfavorable reactions needed to maintain cellular functions. Multicellular organisms have developed complex communication systems to control cell behavior, such as growth and differentiation into specialized cell types. Disruption of these cellular processes is at the root of pathophysiologic processes and disease.