Cell Differentiation and The Cytoplasm (PDF)

C H A P T E R CELL DIFFERENTIATION 2 The Cytoplasm 17 Proteasomes37...

C H A P T E R CELL DIFFERENTIATION 2 The Cytoplasm 17 Proteasomes37 Mitochondria38 THE PLASMA MEMBRANE 17 Transmembrane Proteins & Membrane Transport 19 Peroxisomes39 Transport by Vesicles: Endocytosis & Exocytosis 21 THE CYTOSKELETON 42 Signal Reception & Transduction 23 Microtubules43 CYTOPLASMIC ORGANELLES 27 Microfilaments (Actin Filaments) 44 Ribosomes27 Intermediate Filaments 45 Endoplasmic Reticulum 28 INCLUSIONS47 Golgi Apparatus 31 SUMMARY OF KEY POINTS 51 Secretory Granules 33 ASSESS YOUR KNOWLEDGE 52 Lysosomes34 C ells and extracellular material together comprise the tissues that make up animal organs. In all tissues, cells are the basic structural and functional units, the smallest living parts of the body. Animal cells are enclosed by cell membranes and are eukaryotic, each with a distinct, using these proteins to convert chemical energy into forceful contractions. Major cellular functions performed by specialized cells in the body are listed in Table 2–1. It is important to understand that the functions listed there can be performed by most cells of the body; membrane-enclosed nucleus surrounded by cytoplasm, fluid specialized cells have greatly expanded their capacity for one or containing a system of membranous organelles, nonmembra- more of these functions during differentiation. Changes in cells’ nous molecular assemblies, and a cytoskeleton. In contrast, microenvironments under normal and pathologic conditions the smaller prokaryotic cells of bacteria typically have a cell can cause the same cell type to have variable features and activi- wall and lack nuclei and membranous cytoplasmic structures. ties. Cells that appear similar structurally often have different families of receptors for signaling molecules such as hormones ››CELL DIFFERENTIATION and extracellular matrix (ECM) components, causing them to behave differently. For example, because of their diverse arrays of receptors, breast fibroblasts and uterine smooth muscle cells are The average adult human body consists of nearly 40 trillion exceptionally sensitive to female sex hormones, while most other cells, according to the best available estimate. These cells exist fibroblasts and smooth muscle cells are insensitive. as hundreds of histologically distinct cell types, all derived from the zygote, and the single cell formed by the merger of a spermatozoon with an oocyte at fertilization. The first zygotic cellular divisions produce cells called blastomeres, and as ››THE PLASMA MEMBRANE part of the early embryo’s inner cell mass blastomeres give The plasma membrane (cell membrane or plasmalemma) rise to all tissue types of the fetus. Explanted to tissue culture that envelops every eukaryotic cell consists of phospholipids, cells of the inner cell mass are called embryonic stem cells. cholesterol, and proteins, with oligosaccharide chains covalently Most cells of the fetus undergo a specialization process called linked to many of the phospholipids and proteins. This limiting differentiation in which they predominantly express sets of membrane functions as a selective barrier regulating the passage genes that mediate specific cytoplasmic activities, becoming of materials into and out of the cell and facilitating the transport efficiently organized in tissues with specialized functions and of specific molecules. One important role of the cell membrane is usually changing their shape accordingly. For example, muscle to keep constant the ion content of cytoplasm, which differs from cell precursors elongate into long, fiber-like cells containing that of the extracellular fluid. Membrane proteins also perform a large arrays of actin and myosin. All animal cells contain actin number of specific recognition and signaling functions, playing a filaments and myosins, but muscle cells are specialized for key role in the interactions of the cell with its environment. 17 02_Mescher_ch02_p017-052.indd 17 27/04/18 6:48 pm  18 CHAPTER 2 The Cytoplasm abundant in the outer half, while phosphatidylserine and         Differentiated cells typically TABLE 2–1 specialize in one activity. phosphatidylethanolamine are more concentrated in the inner layer. Some of the outer layer’s lipids, known as glycolipids, General Cellular Activity Specialized Cell(s) include oligosaccharide chains that extend outward from the cell surface and contribute to a delicate cell surface coating Movement Muscle and other contractile called the glycocalyx (Figures 2–1b and 2–2). With the trans- cells mission electron microscope (TEM) the cell membrane—as Form adhesive and tight Epithelial cells well as all cytoplasmic membranes—may exhibit a trilaminar junctions between cells appearance after fixation in osmium tetroxide; osmium binds Synthesize and secrete Fibroblasts, cells of bone and the polar heads of the phospholipids and the oligosaccharide components of the extracellular cartilage chains, producing the two dark outer lines that enclose the matrix light band of osmium-free fatty acids (Figure 2–1b). Convert physical and chemical Neurons and sensory cells Proteins are major constituents of membranes (~50% stimuli into action potentials by weight in the plasma membrane). Integral proteins are Synthesis and secretion of Cells of digestive glands incorporated directly within the lipid bilayer, whereas periph- degradative enzymes eral proteins are bound to one of the two membrane surfaces, Synthesis and secretion of Cells of mucous glands particularly on the cytoplasmic side (Figure 2–2). Peripheral glycoproteins proteins can be extracted from cell membranes with salt solu- tions, whereas integral proteins can be extracted only by using Synthesis and secretion of Certain cells of the adrenal steroids gland, testis, and ovary detergents to disrupt the lipids. The polypeptide chains of many integral proteins span the membrane, from one side to Ion transport Cells of the kidney and the other, several times and are accordingly called multipass salivary gland ducts proteins. Integration of the proteins within the lipid bilayer Intracellular digestion Macrophages and neutrophils is mainly the result of hydrophobic interactions between the Lipid storage Fat cells lipids and nonpolar amino acids of the proteins. Freeze-fracture electron-microscope studies of mem- Metabolite absorption Cells lining the intestine branes show that parts of many integral proteins protrude from both the outer or inner membrane surface (Figure 2–2b). Although the plasma membrane defines the outer limit of Like those of glycolipids, the carbohydrate moieties of glyco- the cell, a continuum exists between the interior of the cell and proteins project from the external surface of the plasma mem- extracellular macromolecules. Certain plasma membrane pro- brane and contribute to the glycocalyx (Figure 2–3). They are teins, the integrins, are linked to both the cytoskeleton and important components of proteins acting as receptors, which ECM components and allow continuous exchange of influ- participate in important interactions such as cell adhesion, cell ences, in both directions, between the cytoplasm and material recognition, and the response to protein hormones. As with in the ECM. lipids, the distribution of membrane polypeptides is different Membranes range from 7.5 to 10 nm in thickness and in the two surfaces of the cell membranes. Therefore, all mem- consequently are visible only in the electron microscope. The branes in the cell are asymmetric. line between adjacent cells sometimes seen faintly with the Studies with labeled membrane proteins of cultured cells light microscope consists of plasma membrane proteins plus reveal that many such proteins are not bound rigidly in place extracellular material, which together can reach a dimension and are able to move laterally (Figure 2–4). Such observa- visible by light microscopy. tions as well as data from biochemical, electron microscopic, Membrane phospholipids are amphipathic, consisting of and other studies showed that membrane proteins comprise two nonpolar (hydrophobic or water-repelling) long-chain a moveable mosaic within the fluid lipid bilayer, the well- fatty acids linked to a charged polar (hydrophilic or water- established fluid mosaic model for membrane structure attracting) head that bears a phosphate group (Figure 2–1a). (Figure 2–2a). Unlike the lipids, however, lateral diffusion of Phospholipids are most stable when organized into a double many membrane proteins is often restricted by their cyto- layer (bilayer) with the hydrophobic fatty acid chains located skeletal attachments. Moreover, in most epithelial cells tight in a middle region away from water and the hydrophilic polar junctions between the cells (see Chapter 4) also restrict lat- head groups contacting the water (Figure 2–1b). Molecules eral diffusion of unattached transmembrane proteins and of cholesterol, a sterol lipid, insert at varying densities among outer layer lipids, producing different domains within the the closely-packed phospholipid fatty acids, restricting their cell membranes. movements and modulating the fluidity of all membrane Membrane proteins that are components of large enzyme components. The phospholipids in each half of the bilayer are complexes are also usually less mobile, especially those different. For example, in the well-studied membranes of red involved in the transduction of signals from outside the cell. blood cells, phosphatidylcholine and sphingomyelin are more Such protein complexes are located in specialized membrane 02_Mescher_ch02_p017-052.indd 18 25/04/18 6:47 pm  The Plasma Membrane 19 FIGURE 2–1 Lipids in membrane structure. C H A P T E R Polar head group Nonpolar fatty acid chains (hydrophilic) (hydrophobic) O Saturated CH3 CH2 O C fatty acid O (straight) CH3 2 CH O C The Cytoplasm The Plasma Membrane O Unsaturated CH2 O P O X fatty acid OH O– (bent) General structure of a phospholipid Cholesterol a Sugar chains of a glycolipid Phospholipids Hydrophilic surface Hydrophobic region Extracellular fluid Hydrophilic surface Cholesterol Cytoplasm b (a) Membranes of animal cells have as their major lipid com- throughout the lipid bilayer; cholesterol affects the packing of the ponents phospholipids and cholesterol. A phospholipid is fatty acid chains, with a major effect on membrane fluidity. The amphipathic, with a phosphate group charge on the polar head outer layer of the cell membrane also contains glycolipids with and two long, nonpolar fatty acid chains, which can be straight extended carbohydrate chains. (saturated) or kinked (at an unsaturated bond). Membrane cho- Sectioned, osmium-fixed cell membrane may have a faint trilami- lesterol is present in about the same amount as phospholipid. nar appearance with the transmission electron microscope (TEM), (b) The amphipathic nature of phospholipids produces the bilayer showing two dark (electron-dense) lines enclosing a clear (electron- structure of membranes as the charged (hydrophilic) polar heads lucent) band. Reduced osmium is deposited on the hydrophilic phos- spontaneously form each membrane surface, in direct contact phate groups present on each side of the internal region of fatty acid with water, and the hydrophobic nonpolar fatty acid chains are chains where osmium is not deposited. The “fuzzy” material on the buried in the membrane’s middle, away from water. Cholesterol outer surface of the membrane represents the glycocalyx of oligo- molecules are also amphipathic and are interspersed less evenly saccharides of glycolipids and glycoproteins. (X100,000) patches termed lipid rafts with higher concentrations of cho- small molecules cross the membrane by the general mecha- lesterol and saturated fatty acids which reduce lipid fluidity. nisms shown schematically in Figure 2–5 and explained as This together with the presence of scaffold proteins that main- follows: tain spatial relationships between enzymes and signaling pro- teins allows the proteins assembled within lipid rafts to remain Diffusion transports small, nonpolar molecules directly through the lipid bilayer. Lipophilic (fat-soluble) mol- in close proximity and interact more efficiently. ecules diffuse through membranes readily, water very slowly. Transmembrane Proteins & MembraneTransport Channels are multipass proteins forming transmem- The plasma membrane is the site where materials are brane pores through which ions or small molecules exchanged between the cell and its environment. Most pass selectively. Cells open and close specific channels 02_Mescher_ch02_p017-052.indd 19 25/04/18 6:47 pm  20 CHAPTER 2 The Cytoplasm FIGURE 2–2 Proteins associated with the membrane lipid bilayer. Sugar chain Sugar chain of glycolipid of glycoprotein 2 E face Peripheral protein 1 Transmembrane protein Lipid a P face b (a) The fluid mosaic model of membrane structure emphasizes (b) When cells are frozen and fractured (cryofracture), the lipid that the phospholipid bilayer of a membrane also contains pro- bilayer of membranes is often cleaved along the hydrophobic teins inserted in it or associated with its surface (peripheral pro- center. Splitting occurs along the line of weakness formed by teins) and that many of these proteins move within the fluid lipid the fatty acid tails of phospholipids. Electron microscopy of such phase. Integral proteins are firmly embedded in the lipid layers; cryofracture preparation replicas provides a useful method for those that completely span the bilayer are called transmem- studying membrane structures. Most of the protruding mem- brane proteins. Hydrophobic amino acids of these proteins inter- brane particles seen (1) are proteins or aggregates of proteins act with the hydrophobic fatty acid portions of the membrane that remain attached to the half of the membrane adjacent to lipids. Both the proteins and lipids may have externally exposed the cytoplasm (P or protoplasmic face). Fewer particles are found oligosaccharide chains. attached to the outer half of the membrane (E or extracellular face). Each protein bulging on one surface has a corresponding depression (2) on the opposite surface. for Na+, K+, Ca2+, and other ions in response to various down a concentration gradient due to its kinetic energy. In physiological stimuli. Water molecules usually cross contrast, membrane pumps are enzymes engaged in active the plasma membrane through channel proteins called transport, utilizing energy from the hydrolysis of adenos- aquaporins. ine triphosphate (ATP) to move ions and other solutes across Carriers are transmembrane proteins that bind small membranes, against often steep concentration gradients. molecules and translocate them across the membrane via Because they consume ATP pumps, they are often referred conformational changes. to as ATPases. Diffusion, channels, and carrier proteins operate pas- These transport mechanisms are summarized with addi- sively, allowing movement of substances across membranes tional detail in Table 2–2. 02_Mescher_ch02_p017-052.indd 20 25/04/18 6:47 pm  The Plasma Membrane 21 FIGURE 2–3 Membrane proteins. C H A P T E R Interstitial fluid Phospholipid 2 Glycolipid Carbohydrate The Cytoplasm The Plasma Membrane Polar head of phospholipid molecule Phospholipid bilayer Glycoprotein Nonpolar tails of phospholipid Cholesterol Protein molecule Integral protein Peripheral protein Filaments of cytoskeleton Cytosol Functions of Plasma Membrane 1. Physical barrier: Establishes a flexible boundary, protects cellular contents, 3. Electrochemical gradients: Establishes and maintains an electrical and supports cell structure. Phospholipid bilayer separates substances charge difference across the plasma membrane. inside and outside the cell. 4. Communication: Contains receptors that recognize and respond to 2. Selective permeability: Regulates entry and exit of ions, nutrients, molecular signals. and waste molecules through the membrane. Both protein and lipid components often have covalently connections, and as selective gateways for molecules entering attached oligosaccharide chains exposed at the external mem- the cell. brane surface. These contribute to the cell’s glycocalyx, which Transmembrane proteins often have multiple hydrophobic provides important antigenic and functional properties to the regions buried within the lipid bilayer to produce a channel or cell surface. Membrane proteins serve as receptors for vari- other active site for specific transfer of substances through the ous signals coming from outside cells, as parts of intercellular membrane. Transport by Vesicles: Endocytosis & Exocytosis cytoskeletal changes. Fusion of the membranous folds encloses the bacterium in an intracellular vacuole called Macromolecules normally enter cells by being enclosed within a phagosome, which then merges with a lysosome folds of plasma membrane (often after binding specific mem- for degradation of its contents as discussed later in this brane receptors) which fuse and pinch off internally as spheri- chapter. cal cytoplasmic vesicles (or vacuoles) in a general process known as endocytosis. Three major types of endocytosis are 2. Pinocytosis (“cell drinking”) involves smaller invagina- recognized, as summarized in Table 2–2 and Figure 2–6. tions of the cell membrane which fuse and entrap extra- cellular fluid and its dissolved contents. The resulting 1. Phagocytosis (“cell eating”) is the ingestion of particles pinocytotic vesicles (~80 nm in diameter) then pinch such as bacteria or dead cell remnants. Certain blood- off inwardly from the cell surface and either fuse with derived cells, such as macrophages and neutrophils, are lysosomes or move to the opposite cell surface where specialized for this activity. When a bacterium becomes they fuse with the membrane and release their contents bound to the surface of a neutrophil, it becomes sur- outside the cell. The latter process, called transcytosis, rounded by extensions of plasmalemma and cytoplasm accomplishes bulk transfer of dissolved substances across which project from the cell in a process dependent on the cell. 02_Mescher_ch02_p017-052.indd 21 25/04/18 6:47 pm  22 CHAPTER 2 The Cytoplasm receptors causes these proteins to aggregate in special FIGURE 2–4 Experiment demonstrating the membrane regions that then invaginate and pinch off fluidity of membrane proteins. internally as vesicles. The formation and fate of vesicles in receptor-mediated endocytosis also often depend on specific peripheral proteins on the cytoplasmic side of the membrane (Figure 2–7). The occupied cell-surface receptors associate with these cytoplas- mic proteins and begin to invaginate as coated pits. The electron-dense coating on the cytoplasmic surface of such pits contains several polypeptides, the major one being clathrin. Clathrin molecules interact like the struts of a geodesic dome, a forming that region of cell membrane into a cage-like invagi- nation that soon pinches off into the cytoplasm as a coated vesicle (Figure 2–7b) with the receptor-bound ligands inside. Another type of receptor-mediated endocytosis prominent in very thin cells produces invaginations called caveolae (L. caveolae, little caves) that involve a family of integral mem- brane proteins called caveolins associated with diverse periph- eral proteins called cavins. In all these endocytotic processes, the vesicles or vacuoles produced quickly enter and fuse with the endosomal com- partment, a dynamic collection in the peripheral cytoplasm b of membranous tubules and vacuoles (Figure 2–7). The clath- rin molecules separate from the coated vesicles and recycle back to the cell membrane for the formation of new coated pits. Vesicle trafficking through the endosomal compartment is directed largely through peripheral membrane G-proteins called Rab proteins, small GTPases that bind guanine nucle- otides and associated proteins. As shown in Figure 2–7, phagosomes and pinocytotic vesicles typically fuse with lysosomes within the endosomal compartment for digestion of their contents, while molecules entering by receptor-mediated endocytosis may be directed down other pathways. The membranes of many late endo- somes have ATP-driven H+ pumps that acidify their interior, activating the hydrolytic enzymes of lysosomes, and in other endosomes causing ligands to uncouple from their receptors, c after which the two molecules are sorted into separate endo- (a) Two types of cells were grown in tissue cultures, one with fluo- somes. The receptors may be sorted into recycling endosomes rescently labeled transmembrane proteins in the plasmalemma and returned to the cell surface for reuse. Low-density lipopro- (right) and one without. tein receptors, for example, are recycled several times within (b) Cells of each type were fused together experimentally into cells. Other endosomes may release their entire contents at a hybrid cells. separate domain of the cell membrane (transcytosis), which (c) Minutes after the fusion of the cell membranes, the fluorescent occurs in many epithelial cells. proteins of the labeled cell spread to the entire surface of the Movement of large molecules from inside to outside hybrid cells. Such experiments provide important data supporting the cell usually involves vesicular transport in the process of the fluid mosaic model. However, many membrane proteins show more restricted lateral movements, being anchored in place by exocytosis. In exocytosis, a cytoplasmic vesicle containing links to the cytoskeleton. the molecules to be secreted fuses with the plasma membrane, resulting in the release of its contents into the extracellu- lar space without compromising the integrity of the plasma membrane (see “Transcytosis” in Figure 2–7a). Exocytosis 3. Receptor-mediated endocytosis: Receptors for many is triggered in many cells by a transient increase in cytosolic substances, such as low-density lipoproteins and protein Ca2+. Membrane fusion during exocytosis is highly regulated, hormones, are integral membrane proteins at the cell with selective interactions between several specific membrane surface. High-affinity binding of such ligands to their proteins. 02_Mescher_ch02_p017-052.indd 22 25/04/18 6:47 pm  The Plasma Membrane 23 FIGURE 2–5 Major mechanisms by which molecules cross membranes. 2 C H A P T E R The Cytoplasm The Plasma Membrane (a) Simple diffusion (b) Channel (c) Carrier/pump Lipophilic and some small, uncharged molecules can cross mem- conformations and release the molecule to the other side of the branes by simple diffusion (a). membrane. Most ions cross membranes in multipass proteins called chan- Diffusion, channels and most carrier proteins translocate sub- nels (b) whose structures include transmembrane ion-specific pores. stances across membranes using only kinetic energy. In contrast, Many other larger, water-soluble molecules require binding pumps are carrier proteins for active transport of ions or other to sites on selective carrier proteins (c), which then change their solutes and require energy derived from ATP. Exocytosis of macromolecules made by cells occurs via are called exosomes, which can fuse with other cells transfer- either of two pathways: ring their contents and membranes. Constitutive secretion is used for products that are released from cells continuously, as soon as synthesis is Signal Reception & Transduction complete, such as collagen subunits for the ECM. Cells in a multicellular organism communicate with one Regulated secretion occurs in response to signals com- another to regulate tissue and organ development, to control ing to the cells, such as the release of digestive enzymes their growth and division, and to coordinate their functions. from pancreatic cells in response to specific stimuli. Many adjacent cells form communicating gap junctions that Regulated exocytosis of stored products from epithelial couple the cells and allow exchange of ions and small mol- cells usually occurs specifically at the apical domains of ecules (see Chapter 4). cells, constituting a major mechanism of glandular secre- Cells also use about 25 families of receptors to detect tion (see Chapter 4). and respond to various extracellular molecules and physical stimuli. Each cell type in the body contains a distinctive set Portions of the cell membrane become part of the endo- of cell surface and cytoplasmic receptor proteins that enable cytotic vesicles or vacuoles during endocytosis; during exocy- it to respond to a complementary set of signaling molecules tosis, membrane is returned to the cell surface. This process in a specific, programmed way. Cells bearing receptors for a of membrane movement and recycling is called membrane specific ligand are referred to as target cells for that molecule. trafficking (see Figure 2–7a). Trafficking of membrane com- The routes of signal molecules from source to target provide ponents occurs continuously in most cells and is not only one way to categorize the signaling process: crucial for maintaining the cell but also for physiologically important processes such as reducing blood lipid levels. In endocrine signaling, the signal molecules (here In many cells subpopulations of vacuoles and tubules called hormones) are carried in the blood from their within the endosomal compartment accumulate small vesicles sources to target cells throughout the body. within their lumens by further invaginations of their limiting In paracrine signaling, the chemical ligand diffuses in membranes, becoming multivesicular bodies. While multi- extracellular fluid but is rapidly metabolized so that its vesicular bodies may merge with lysosomes for selective deg- effect is only local on target cells near its source. radation of their content, this organelle may also fuse with the In synaptic signaling, a special kind of paracrine inter- plasma membrane and release the intralumenal vesicles out- action, neurotransmitters act on adjacent cells through side the cell. The small (