Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 7
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Theenergy released by the splitting (hydrolysis) of Pi from ATPdrives many cellular processes.1.3 • The Work of Cellsmerous proteins composing the various working compartments must be transported from where they are made totheir proper locations (Chapters 16 and 17). Some proteinsare made on ribosomes that are free in the cytosol.
Proteinssecreted from the cell and most membrane proteins, however,are made on ribosomes associated with the endoplasmicreticulum (ER). This organelle produces, processes, and shipsout both proteins and lipids. Protein chains produced on theER move to the Golgi apparatus, where they are furthermodified before being forwarded to their final destinations.Proteins that travel in this way contain short sequences ofamino acids or attached sugar chains (oligosaccharides) thatserve as addresses for directing them to their correct destinations.
These addresses work because they are recognizedand bound by other proteins that do the sorting and shippingin various cell compartments.Animal Cells Produce Their Own ExternalEnvironment and GluesThe simplest multicellular animals are single cells embeddedin a jelly of proteins and polysaccharides called the extracellular matrix. Cells themselves produce and secrete these materials, thus creating their own immediate environment(Chapter 6). Collagen, the single most abundant protein inthe animal kingdom, is a major component of the extracellular matrix in most tissues. In animals, the extracellular matrix cushions and lubricates cells.
A specialized, especiallytough matrix, the basal lamina, forms a supporting layer underlying sheetlike cell layers and helps prevent the cells fromripping apart.The cells in animal tissues are “glued” together by celladhesion molecules (CAMs) embedded in their surfacemembranes. Some CAMs bind cells to one another; othertypes bind cells to the extracellular matrix, forming a cohesive unit. The cells of higher plants contain relatively fewsuch molecules; instead, plants cells are rigidly tied togetherby extensive interlocking of the cell walls of neighboringIntermediate filamentscells.
The cytosols of adjacent animal or plant cells often areconnected by functionally similar but structurally different“bridges” called gap junctions in animals and plasmodesmata in plants. These structures allow cells to exchange smallmolecules including nutrients and signals, facilitating coordinated functioning of the cells in a tissue.Cells Change Shape and MoveAlthough cells sometimes are spherical, they more commonlyhave more elaborate shapes due to their internal skeletonsand external attachments. Three types of protein filaments,organized into networks and bundles, form the cytoskeletonwithin animal cells (Figure 1-15).
The cytoskeleton preventsthe plasma membrane of animal cells from relaxing into asphere (Chapter 5); it also functions in cell locomotion andthe intracellular transport of vesicles, chromosomes, andmacromolecules (Chapters 19 and 20). The cytoskeleton canbe linked through the cell surface to the extracellular matrixor to the cytoskeleton of other cells, thus helping to form tissues (Chapter 6).All cytoskeletal filaments are long polymers of proteinsubunits. Elaborate systems regulate the assembly and disassembly of the cytoskeleton, thereby controlling cell shape. Insome cells the cytoskeleton is relatively stable, but in othersit changes shape continuously. Shrinkage of the cytoskeletonin some parts of the cell and its growth in other parts can produce coordinated changes in shape that result in cell locomotion.
For instance, a cell can send out an extension thatattaches to a surface or to other cells and then retract the cellbody from the other end. As this process continues due to coordinated changes in the cytoskeleton, the cell moves forward. Cells can move at rates on the order of 20 m/second.Cell locomotion is used during embryonic development ofmulticellular animals to shape tissues and during adulthoodto defend against infection, to transport nutrients, and to healwounds.
This process does not play a role in the growth anddevelopment of multicellular plants because new plant cellsMicrotubules▲ FIGURE 1-15 The three types of cytoskeletal filamentshave characteristic distributions within cells. Three views ofthe same cell. A cultured fibroblast was treated with threedifferent antibody preparations. Each antibody binds specificallyto the protein monomers forming one type of filament and ischemically linked to a differently colored fluorescent dye (green,15Microfilamentsblue, or red).
Visualization of the stained cell in a fluorescencemicroscope reveals the location of filaments bound to a particulardye-antibody preparation. In this case, intermediate filaments arestained green; microtubules, blue; and microfilaments, red. Allthree fiber systems contribute to the shape and movements ofcells. [Courtesy of V. Small.]16CHAPTER 1 • Life Begins with Cells(a)Surface receptorsare generated by the division of existing cells that share cellwalls. As a result, plant development involves cell enlargement but not movement of cells from one position to another.Bound signalCells Sense and Send InformationA living cell continuously monitors its surroundings and adjusts its own activities and composition accordingly.
Cellsalso communicate by deliberately sending signals that canbe received and interpreted by other cells. Such signals arecommon not only within an individual organism, but alsobetween organisms. For instance, the odor of a pear detectedby us and other animals signals a food source; consumptionof the pear by an animal aids in distributing the pear’s seeds.Everyone benefits! The signals employed by cells include simple small chemicals, gases, proteins, light, and mechanicalmovements. Cells possess numerous receptor proteins for detecting signals and elaborate pathways for transmitting themwithin the cell to evoke a response. At any time, a cell may beable to sense only some of the signals around it, and how acell responds to a signal may change with time.
In somecases, receiving one signal primes a cell to respond to a subsequent different signal in a particular way.Both changes in the environment (e.g., an increase or decrease in a particular nutrient or the light level) and signalsreceived from other cells represent external information thatcells must process. The most rapid responses to such signalsgenerally involve changes in the location or activity of preexisting proteins.
For instance, soon after you eat a carbohydrate-rich meal, glucose pours into your bloodstream. Therise in blood glucose is sensed by cells in the pancreas,which respond by releasing their stored supply of the proteinhormone insulin. The circulating insulin signal causes glucose transporters in the cytoplasm of fat and muscle cells tomove to the cell surface, where they begin importing glucose.Meanwhile, liver cells also are furiously taking in glucose viaa different glucose transporter.
In both liver and muscle cells,an intracellular signaling pathway triggered by binding of insulin to cell-surface receptors activates a key enzyme neededto make glycogen, a large glucose polymer (Figure 1-16a).The net result of these cell responses is that your blood glucose level falls and extra glucose is stored as glycogen, whichyour cells can use as a glucose source when you skip a mealto cram for a test.The ability of cells to send and respond to signals is crucial to development. Many developmentally important signals are secreted proteins produced by specific cells atspecific times and places in a developing organism.
Often areceiving cell integrates multiple signals in deciding how tobehave, for example, to differentiate into a particular tissuetype, to extend a process, to die, to send back a confirmingsignal (yes, I’m here!), or to migrate.The functions of about half the proteins in humans,roundworms, yeast, and several other eukaryotic organismshave been predicted based on analyses of genomic sequences(Chapter 9).
Such analyses have revealed that at least 10–15percent of the proteins in eukaryotes function as secreted ex-Inactive enzymeActive enzyme(b)CytosolicreceptorReceptor-hormonecomplexmRNAProteinmRNANucleusIncreased transcriptionof specific genes▲ FIGURE 1-16 External signals commonly cause a changein the activity of preexisting proteins or in the amountsand types of proteins that cells produce.
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