Ganong's Review of Medical Physiology, 23rd Edition

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40 SECTION I Cellular & Molecular Basis of Medical Physiology gap junction channels are not simply passive, nonspecific conduits. At least 20 different genes code for connexins in humans, and mutations in these genes can lead to diseases that are highly selective in terms of the tissues involved and the type of communication between cells produced. For instance, X-linked Charcot–Marie–Tooth disease is a peripheral neuropathy associated with mutation of one particular connexin gene. Experiments in mice in which particular connexins are deleted by gene manipulation or replaced with different connexins confirm that the particular connexin subunits that make up connexons determine their permeability and selectivity. Recently it has been shown that connexons can be used as channels to release small molecules from the cytosol into the ECF. NUCLEUS & RELATED STRUCTURES A nucleus is present in all eukaryotic cells that divide. If a cell is cut in half, the anucleate portion eventually dies without dividing. The nucleus is made up in large part of the chromosomes, the structures in the nucleus that carry a complete blueprint for all the heritable species and individual characteristics of the animal. Except in germ cells, the chromosomes occur in pairs, one originally from each parent. Each chromosome is made up of a giant molecule of DNA. The DNA strand is about 2 m long, but it can fit in the nucleus because at intervals it is wrapped around a core of histone proteins to form a nucleosome. There are about 25 million nucleosomes in each nucleus. Thus, the structure of the chromosomes has been likened to a string of beads. The beads are the nucleosomes, and the linker DNA between them is the string. The whole complex of DNA and proteins is called chromatin. During cell division, the coiling around histones is loosened, probably by acetylation of the histones, and pairs of chromosomes become visible, but between cell divisions only clumps of chromatin can be discerned in the nucleus. The ultimate units of heredity are the genes on the chromosomes). As discussed in Chapter 1, each gene is a portion of the DNA molecule. The nucleus of most cells contains a nucleolus (Figure 2–1), a patchwork of granules rich in RNA. In some cells, the nucleus contains several of these structures. Nucleoli are most prominent and numerous in growing cells. They are the site of synthesis of ribosomes, the structures in the cytoplasm in which proteins are synthesized. The interior of the nucleus has a skeleton of fine filaments that are attached to the nuclear membrane, or envelope (Figure 2–1), which surrounds the nucleus. This membrane is a double membrane, and spaces between the two folds are called perinuclear cisterns. The membrane is permeable only to small molecules. However, it contains nuclear pore complexes. Each complex has eightfold symmetry and is made up of about 100 proteins organized to form a tunnel through which transport of proteins and mRNA occurs. There are many transport pathways, and proteins called importins and exportins have been isolated and characterized. Much current research is focused on transport into and out of the nucleus, and a more detailed understanding of these processes should emerge in the near future. ENDOPLASMIC RETICULUM The endoplasmic reticulum is a complex series of tubules in the cytoplasm of the cell (Figure 2–1). The inner limb of its membrane is continuous with a segment of the nuclear membrane, so in effect this part of the nuclear membrane is a cistern of the endoplasmic reticulum. The tubule walls are made up of membrane. In rough, or granular, endoplasmic reticulum, ribosomes are attached to the cytoplasmic side of the membrane, whereas in smooth, or agranular, endoplasmic reticulum, ribosomes are absent. Free ribosomes are also found in the cytoplasm. The granular endoplasmic reticulum is concerned with protein synthesis and the initial folding of polypeptide chains with the formation of disulfide bonds. The agranular endoplasmic reticulum is the site of steroid synthesis in steroid-secreting cells and the site of detoxification processes in other cells. A modified endoplasmic reticulum, the sarcoplasmic reticulum, plays an important role in skeletal and cardiac muscle. In particular, the endoplasmic or sarcoplasmic reticulum can sequester Ca 2+ ions and allow for their release as signaling molecules in the cytosol. RIBOSOMES The ribosomes in eukaryotes measure approximately 22 × 32 nm. Each is made up of a large and a small subunit called, on the basis of their rates of sedimentation in the ultracentrifuge, the 60S and 40S subunits. The ribosomes are complex structures, containing many different proteins and at least three ribosomal RNAs. They are the sites of protein synthesis. The ribosomes that become attached to the endoplasmic reticulum synthesize all transmembrane proteins, most secreted proteins, and most proteins that are stored in the Golgi apparatus, lysosomes, and endosomes. These proteins typically have a hydrophobic signal peptide at one end (Figure 2–10). The polypeptide chains that form these proteins are extruded into the endoplasmic reticulum. The free ribosomes synthesize cytoplasmic proteins such as hemoglobin and the proteins found in peroxisomes and mitochondria. GOLGI APPARATUS & VESICULAR TRAFFIC The Golgi apparatus is a collection of membrane-enclosed sacs (cisterns) that are stacked like dinner plates (Figure 2–1). There are usually about six sacs in each apparatus, but there may be more. One or more Golgi apparati are present in all eukaryotic cells, usually near the nucleus. Much of the organization of the Golgi is directed at proper glycosylation of proteins and lipids. There are more than 200 enzymes that function to add, remove, or modify sugars from proteins and lipids in the Golgi apparatus.
Free ribosome Signal sequence Growing polypeptide chain CHAPTER 2 Overview of Cellular Physiology in Medical Physiology 41 FIGURE 2–10 Rough endoplasmic reticulum and protein translation. Messenger RNA and ribosomes meet up in the cytosol for translation. Proteins that have appropriate signal peptides begin translation, then associate with the endoplasmic reticulum (ER) to complete translation. The association of ribosomes is what gives the ER its “rough” appearance. (Reproduced with permission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.) mRNA from Gene A mRNA from Gene B Vesicle Secretory vesicle Secreted protein from Gene A Exocytosis Rough endoplasmic reticulum Carbohydrate group Cleaved signal sequences Golgi apparatus Lysosome Cytoplasm Digestive protein from Gene B Plasma membrane Extracellular fluid
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168 SECTION III Central & Periphera
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302 SECTION IV Endocrine & Reproduc
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430 SECTION V Gastrointestinal Phys
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490 SECTION VI Cardiovascular Physi
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588 SECTION VII Respiratory Physiol
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688 Answers to Multiple Choice Ques
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690 INDEX Angiotensin III, 671 Angi
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692 INDEX Cannabinoids, 145, 179 Ca
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694 INDEX Climbing fibers, 255 Clin
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714 INDEX Vestibular system, 213-21
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Ganong's Review of Medical Physiology, 23rd Edition

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