H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003), страница 7

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.

(a) Binding of ahormone or other signaling molecule to its specific receptorscan trigger an intracellular pathway that increases or decreasesthe activity of a preexisting protein. For example, binding ofinsulin to receptors in the plasma membrane of liver and musclecells leads to activation of glycogen synthase, a key enzyme inthe synthesis of glycogen from glucose. (b) The receptors forsteroid hormones are located within cells, not on the cellsurface.

The hormone-receptor complexes activate transcriptionof specific target genes, leading to increased production of theencoded proteins. Many signals that bind to receptors on thecell surface also act, by more complex pathways, to modulategene expression.tracellular signals, signal receptors, or intracellular signaltransduction proteins, which pass along a signal through aseries of steps culminating in a particular cellular response(e.g., increased glycogen synthesis). Clearly, signaling andsignal transduction are major activities of cells.Cells Regulate Their Gene Expression to MeetChanging NeedsIn addition to modulating the activities of existing proteins,cells often respond to changing circumstances and to signalsfrom other cells by altering the amount or types of proteins theycontain.

Gene expression, the overall process of selectivelyreading and using genetic information, is commonly controlledat the level of transcription, the first step in the production ofproteins. In this way cells can produce a particular mRNA onlywhen the encoded protein is needed, thus minimizing wastedenergy. Producing a mRNA is, however, only the first in a chainof regulated events that together determine whether an activeprotein product is produced from a particular gene.1.3 • The Work of CellsThe most remarkable feature of cells and entire organisms istheir ability to reproduce. Biological reproduction, combinedNondividingcellsRestingcellsG1G0RNA andproteinsynthesisMCelldivisionDNAreplicationSRNA andproteinsynthesisG2▲ FIGURE 1-17 During growth, eukaryotic cellscontinually progress through the four stages of the cellcycle, generating new daughter cells.

In mostproliferating cells, the four phases of the cell cycle proceedsuccessively, taking from 10–20 hours depending on celltype and developmental state. During interphase, whichconsists of the G1, S, and G2 phases, the cell roughlydoubles its mass. Replication of DNA during S leaves thecell with four copies of each type of chromosome. In themitotic (M) phase, the chromosomes are evenly partitionedto two daughter cells, and the cytoplasm divides roughly inhalf in most cases. Under certain conditions such asstarvation or when a tissue has reached its final size, cellswill stop cycling and remain in a waiting state called G0.Most cells in G0 can reenter the cycle if conditions change.MEDIA CONNECTIONSCells Grow and Dividewith continuing evolutionary selection for a highly functionalbody plan, is why today’s horseshoe crabs look much as theydid 300 million years ago, a time span during which entiremountain ranges have risen or fallen.