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𝗣𝗗𝗙 | It is said that a book is known by its cover. With the title of this book Essential cell biology 4th edition, it is clear that this book is meant to. Essential Cell Biology, Fourth Edition- Alberts, Bray, Hopkin. Alexandra Duarte. CHAPTER ONE 1 Cells: The Fundamental Units of Life What does it mean to be. Art of Essential Cell Biology, Fourth Edition References The images from the book are The “References” PDF document is available on both the instructor and.
For almost years, such instruments—the first light microscopes— remained exotic devices, available only to a few wealthy individuals. Other dyes can be coupled slices, or sections, that can be mounted on to antibody molecules, which then serve as highly a glass microscope slide and subsequently specific and versatile staining reagents that bind stained to reveal different components of selectively to particular large molecules, allowing us the cells. The compact genomes of typical bacteria are dwarfed by the genomes of typical eukaryotes. Their varied characters stem from the way that individual cells use their genetic instructions. In the process, they release chloroplasts chlorophyll- containing membranes Figure 1—19 Chloroplasts in plant cells inner capture the energy of sunlight. Because this 0. How is each of us similar to—yet different from—everyone else on Earth?
Charles Darwin provided the key insight that makes this his- and allow it to cool. After several tory comprehensible. His theory of evolution, published in , explains months, the liquid is as clear as how random variation and natural selection gave rise to diversity among ever, and there are no signs of life.
A friend suggests that excluding organisms that share a common ancestry. When combined with the cell the air was a mistake, since most theory, the theory of evolution leads us to view all life, from its beginnings life as we know it requires oxygen. Although You repeat the experiment, but this this book is primarily about how cells work today, we will encounter the time you leave the flask open to the theme of evolution again and again.
To your great delight, the liquid becomes cloudy after a Light Microscopes Allow Examination of Cells and Some of few days and under the microscope you see beautiful small cells that Their Components are clearly growing and dividing.
If you cut a very thin slice from a suitable plant or animal tissue and view Does this experiment prove that it using a light microscope, you will see that the tissue is divided into you managed to generate a novel thousands of small cells. These may be either closely packed or separated life-form? How might you redesign from one another by an extracellular matrix, a dense material often made your experiment to allow air into the flask, yet eliminate the of protein fibers embedded in a polysaccharide gel Figure 1—5.
If you have taken care of your the explanation for the results? And if you watch patiently, you the classic experiments of Louis may even see a cell slowly change shape and divide into two see Figure Pasteur. To see the internal structure of a cell is difficult, not only because the parts are small, but also because they are transparent and mostly color- less.
One way around the problem is to stain cells with dyes that color particular components differently see Figure 1—5. Alternatively, one can exploit the fact that cell components differ slightly from one another in Figure 1—5 Cells form tissues in plants and animals.
A Cells in the root tip of a fern. The nuclei are stained red, and each cell is surrounded by a thin cell wall light blue. B Cells in the urine-collecting ducts of the kidney. Each duct appears in this cross section as a ring of closely packed cells with nuclei stained red. The ring is surrounded by extracellular matrix, stained purple. A, courtesy of James Mauseth; B, from P.
Wheater et al. Churchill A B Livingstone, A A cell taken other. The small differences in refractive index can be made visible by from human skin and grown in culture was photographed through a light microscope specialized optical techniques, and the resulting images can be enhanced using interference-contrast optics see Panel further by electronic processing. The nucleus is especially prominent. B A pigment cell from a frog, The cell thus revealed has a distinct anatomy Figure 1—6A.
It has stained with fluorescent dyes and viewed a sharply defined boundary, indicating the presence of an enclosing with a confocal fluorescence microscope membrane. A large, round structure, the nucleus, is prominent in the see Panel 1—1. The nucleus is shown in middle of the cell. With a good light microscope, one green.
A, courtesy of Casey Cunningham; can begin to distinguish and classify some of the specific components in B, courtesy of Stephen Rogers and the the cytoplasm, but structures smaller than about 0. In recent years, however, new types of fluorescence microscopes have been developed that use sophisticated methods of illumination and elec- tronic image processing to see fluorescently labeled cell components in much finer detail Figure 1—6B.
The most recent super-resolution flu- orescence microscopes, for example, can push the limits of resolution down even further, to about 20 nanometers nm. That is the size of a single ribosome, a large macromolecular complex composed of 80—90 individual proteins and RNA molecules. The Fine Structure of a Cell Is Revealed by Electron Microscopy For the highest magnification and best resolution, one must turn to an electron microscope, which can reveal details down to a few nano- meters.
Cell samples for the electron microscope require painstaking preparation. Even for light microscopy, a tissue often has to be fixed that is, preserved by pickling in a reactive chemical solution , supported by embedding in a solid wax or resin, cut or sectioned into thin slices, and stained before it is viewed.
For electron microscopy, similar procedures are required, but the sections have to be much thinner and there is no possibility of looking at living, wet cells. A Thin section of a liver cell showing the enormous amount of detail that ized functions that are often only hazily defined with a light microscope. Some of the components to be A delicate membrane, only about 5 nm thick, is visible enclosing the cell, discussed later in the chapter are labeled; and similar membranes form the boundary of many of the organelles they are identifiable by their size and shape.
The membrane that separates the interior of the B A small region of the cytoplasm at higher cell from its external environment is called the plasma membrane, while magnification. The smallest structures that are clearly visible are the ribosomes, each the membranes surrounding organelles are called internal membranes.
C Portion of a long, Chapter With an electron microscope, even individual large mole- threadlike DNA molecule isolated from a cules can be seen Figure 1—7C. A and B, courtesy of Daniel S. Friend; The type of electron microscope used to look at thin sections of tissue is C, courtesy of Mei Lie Wong. This is, in principle, simi- lar to a light microscope, except that it transmits a beam of electrons rather than a beam of light through the sample.
Another type of electron microscope—the scanning electron microscope—scatters electrons off the surface of the sample and so is used to look at the surface detail of cells and other structures. A survey of the principal types of microscopy used to examine cells is given in Panel 1—1 pp. Three things are required condenser Fluorescent dyes used for staining cells are detected with the for viewing cells in a light microscope.
This is similar to an First, a bright light must be focused ordinary light microscope except that the illuminating light onto the specimen by lenses in the light is passed through two sets of filters. The first 1 filters the condenser. Second, the specimen must source light before it reaches the specimen, passing only those be carefully prepared to allow light to wavelengths that excite the particular fluorescent dye. The pass through it. Third, an appropriate second 2 blocks out this light and passes only those set of lenses objective and eyepiece the light path in a wavelengths emitted when the dye fluoresces.
Dyed objects must be arranged to focus an image of light microscope show up in bright color on a dark background. The two latter systems exploit differences in the way light travels through regions of the cell B with differing refractive indexes. All three images can be obtained on the same microscope simply by interchanging optical components. Some such Most tissues are neither small enough nor dyes bind specifically to particular molecules in cells transparent enough to examine directly in and can reveal their location when examined with a the microscope.
Typically, therefore, they fluorescence microscope. An example is the stain for are chemically fixed and cut into very thin DNA shown here green.
Other dyes can be coupled slices, or sections, that can be mounted on to antibody molecules, which then serve as highly a glass microscope slide and subsequently specific and versatile staining reagents that bind stained to reveal different components of selectively to particular large molecules, allowing us the cells. A stained section of a plant root to see their distribution in the cell.
In the example tip is shown here D. Courtesy of shown, a microtubule protein in the mitotic spindle Catherine Kidner. D is stained red with a fluorescent antibody. The beam is focused onto a single point at a specific depth in the specimen, and a pinhole aperture in the detector allows only fluorescence emitted from this same point to be included in the image.
Scanning the beam across the specimen generates a sharp image of the plane of focus—an optical section. A series of optical sections at different depths allows a three-dimensional image to be constructed. An intact insect embryo is shown here stained with a fluorescent probe for actin filaments.
A Conventional fluorescence microscopy gives a blurry image due to the presence of fluorescent structures above and below the plane of focus.
B Confocal microscopy provides an optical section showing the individual cells clearly. Courtesy of Richard Warn and Peter Shaw. The tissue has projector lens lens been chemically fixed, embedded in plastic, and cut into very thin sections that have video electrons from viewing then been stained with salts of screen specimen screen or detector uranium and lead.
Courtesy of photographic specimen film Daniel S. In the scanning electron microscope SEM , the specimen, which has been coated with a very thin film of a heavy metal, is scanned by a beam of electrons brought to a focus on the specimen by magnetic coils that act as lenses. The quantity of electrons scattered or emitted as the beam bombards each successive point on the surface of the specimen is measured by the detector, and is used to control the intensity of successive points in an image built up on a video screen.
The microscope creates striking images of three-dimensional objects with great depth of focus and can resolve details down to somewhere between 3 nm and 20 nm, depending on the instrument. The specimen, which is placed in a vacuum, must be hair cell in the inner ear left. Contrast is usually introduced by staining the specimen For comparison, the same with electron-dense heavy metals that locally absorb or scatter structure is shown by light electrons, removing them from the beam as it passes through microscopy, at the limit of its resolution above.
Courtesy of the specimen. The TEM has a useful magnification of up to a Richard Jacobs and James million-fold and can resolve details as small as about 1 nm in Hudspeth. The Fundamental Units of Life 0. A The sizes of cells and of their component parts, plus the units in which they are measured.
B Drawings to convey a sense of scale between living cells and atoms. Each panel shows an image that is magnified by a factor of 10 compared to its predecessor—producing an imaginary progression from a thumb, to skin, to skin cells, to a mitochondrion, to a ribosome, and ultimately to a cluster of atoms forming part of one of the many protein molecules in our bodies.
Note that ribosomes are present inside mitochondria as shown here , as well as in the cytoplasm.
Details of molecular structure, as shown in the last two panels, are beyond the power of the electron microscope. Even the most powerful electron microscopes, however, cannot visualize the individual atoms that make up biological molecules Figure 1—8. A technique called X-ray crystallography, for example, is used to determine the precise three-dimensional structure of protein molecules discussed in Chapter 4. Indeed, a bacterium contains essentially no organelles—not even a nucleus to hold its DNA.
This property—the presence or absence of a nucleus—is used as the basis for a simple but fundamental classifica- tion of all living things. Typical spherical, rodlike, and spiral-shaped bacteria are drawn to scale. The spiral cells shown are the organisms that cause syphilis. They are also small—generally just a few micrometers long, A bacterium weighs about 10—12 g although there are some giant species as much as times longer than and can divide every 20 minutes.
If a single bacterial cell carried on this. Prokaryotes often have a tough protective coat, or cell wall, sur- dividing at this rate, how long would rounding the plasma membrane, which encloses a single compartment it take before the mass of bacteria containing the cytoplasm and the DNA.
Contrast your result obvious organized internal structure Figure 1— The cells reproduce with the fact that bacteria originated quickly by dividing in two.
Under optimum conditions, when food is plen- at least 3. In 11 hours, by repeated divisions, a single prokaryote can give Explain the apparent paradox. The rise to more than 8 billion progeny which exceeds the total number of number of cells N in a culture at time t is described by the equation humans presently on Earth. Prokaryotes Are the Most Diverse and Numerous Cells on Earth Most prokaryotes live as single-celled organisms, although some join together to form chains, clusters, or other organized multicellular struc- tures.
In shape and structure, prokaryotes may seem simple and limited, but in terms of chemistry, they are the most diverse and inventive class of cells. Members of this class exploit an enormous range of habitats, from hot puddles of volcanic mud to the interiors of other living cells, and they vastly outnumber all eukaryotic organisms on Earth. Some are aerobic, using oxygen to oxidize food molecules; some are strictly anaerobic and are killed by the slightest exposure to oxygen.
As we discuss later in this cytoplasm Figure 1—10 The bacterium Escherichia coli E.
A Anabaena cylindrica forms long, multicellular filaments. This light micrograph shows specialized cells that either fix nitrogen that is, capture N2 from the atmosphere and incorporate it into organic compounds; labeled H , fix CO2 through photosynthesis labeled V , or become resistant spores labeled S.
B An electron micrograph of a related species, Phormidium laminosum, shows the intracellular membranes where photosynthesis occurs. These micrographs illustrate that even some prokaryotes can form simple multicellular organisms.
A, courtesy of David Adams; B, courtesy of D. Hill and C. Thus our own oxygen-based metabolism can be regarded as a product of the activities of bacterial cells. Virtually any organic, carbon-containing material—from wood to petro- leum—can be used as food by one sort of bacterium or another.
Even more remarkably, some prokaryotes can live entirely on inorganic sub- stances: Some of these prokaryotic cells, like plant cells, perform photosynthesis, using energy from sunlight to produce organic molecules from CO2 Figure 1—11 ; oth- ers derive energy from the chemical reactivity of inorganic substances in the environment Figure 1— In either case, such prokaryotes play a unique and fundamental part in the economy of life on Earth: Plants, too, can capture energy from sunlight and carbon from atmos- pheric CO2.
But plants unaided by bacteria cannot capture N2 from the atmosphere, and in a sense even plants depend on bacteria for photo- synthesis. It is almost certain that the organelles in the plant cell that Figure 1—12 A sulfur bacterium gets its energy from H2S. Beggiatoa, a prokaryote that lives in sulfurous environments, oxidizes H2S to produce sulfur and can fix carbon even in the dark. Courtesy of Ralph W. The Eukaryotic Cell 15 perform photosynthesis—the chloroplasts—have evolved from photosyn- thetic bacteria that long ago found a home inside the cytoplasm of a plant cell ancestor.
Bacteria and Archaea Traditionally, all prokaryotes have been classified together in one large group. But molecular studies reveal that there is a gulf within the class of prokaryotes, dividing it into two distinct domains called the bacteria and the archaea.
Remarkably, at a molecular level, the members of these two domains differ as much from one another as either does from the eukaryotes. Most of the prokaryotes familiar from everyday life—the spe- cies that live in the soil or make us ill—are bacteria.
Archaea are found not only in these habitats, but also in environments that are too hostile for most other cells: Many of these extreme environments resemble the harsh conditions that must have existed on the primitive Earth, where living things first evolved before the atmosphere became rich in oxygen.
Some live independent lives as single-celled organisms, such as amoebae and yeasts Figure 1—13 ; others live in multicellular assemblies. All of the more complex multicellular organisms—including plants, animals, and fungi—are formed from eukaryotic cells. By definition, all eukaryotic cells have a nucleus. But possession of a nucleus goes hand-in-hand with possession of a variety of other organelles, most of which are membrane-enclosed and common to all eukaryotic organisms.
In this section, we take a look at the main organelles found in eukaryotic cells from the point of view of their func- tions, and we consider how they came to serve the roles they have in the life of the eukaryotic cell.
It is enclosed within two concentric membranes that form the nuclear envelope, and it contains molecules of DNA—extremely long polymers that encode the genetic information of the organism. In the light microscope, these giant DNA molecules become visible as individual chromosomes when they become more compact before a cell divides into two daughter cells Figure 1— DNA also carries the genetic infor- mation in prokaryotic cells; these cells lack a distinct nucleus not because they lack DNA, but because they do not keep their DNA inside a nuclear envelope, segregated from the rest of the cell contents.
Figure 1—13 Yeasts are simple free-living eukaryotes. The cells shown in this micrograph belong to the species of yeast, Saccharomyces cerevisiae, used to make dough rise and turn malted barley juice into beer.
As can be seen in this image, the cells reproduce by growing a bud and then dividing asymmetrically into a large mother cell and a small daughter cell; for this reason, they are called budding yeast. A This drawing of a typical animal cell shows its extensive system of membrane-enclosed organelles. The nucleus is colored brown, the nuclear envelope is green, and the cytoplasm the interior of the cell outside the nucleus is white.
B An electron micrograph of the nucleus in a mammalian cell. B, courtesy of Daniel S. In a fluorescence microscope, they appear as worm-shaped struc- tures that often form branching networks Figure 1— When seen with an electron microscope, individual mitochondria are found to be enclosed in two separate membranes, with the inner membrane formed into folds that project into the interior of the organelle Figure 1— Microscopic examination by itself, however, gives little indication of what mitochondria do.
Their function was discovered by breaking open cells and then spinning the soup of cell fragments in a centrifuge; this nucleus nuclear envelope condensed chromosomes Figure 1—15 Chromosomes become visible when a cell is about to divide.
As a eukaryotic cell prepares to divide, its DNA molecules become progressively more compacted condensed , forming wormlike chromosomes that can be distinguished in the light microscope. Courtesy of Conly L. This budding yeast cell, which contains a green fluorescent protein in its mitochondria, was viewed in a super-resolution confocal fluorescence microscope.
In this three-dimensional image, the mitochondria are seen to form complex branched networks. From A. Egner et al. Natl Acad. USA With permission from the National Academy of Sciences.
Purified mitochondria were then tested to see what chemical processes they could perform. This revealed that mitochondria are generators of chemi- cal energy for the cell. Without mitochondria, animals, fungi, and plants would be unable to use oxygen to extract the energy they need from the food molecules that nourish them. The process of cel- lular respiration is considered in detail in Chapter A An electron micrograph of a cross section of a mitochondrion reveals the extensive infolding of the inner membrane.
B This three-dimensional representation of the arrangement of the mitochondrial membranes shows the smooth outer membrane gray and the highly convoluted inner membrane red.
C In this schematic cell, the interior space of the mitochondrion is colored orange. A, courtesy of Daniel S. The Fundamental Units of Life Figure 1—18 Mitochondria most likely anaerobic early aerobic evolved from engulfed bacteria. It is pre-eukaryotic cell eukaryotic cell virtually certain that mitochondria originate internal from bacteria that were engulfed by an membranes nucleus ancestral pre-eukaryotic cell and survived inside it, living in symbiosis with their host.
Note that the double membrane of present- day mitochondria is thought to have been derived from the plasma membrane and outer membrane of the engulfed bacterium. Because they resemble bacteria in so many ways, they are thought to have been derived from bacteria that were engulfed by some ancestor of present-day eukaryotic cells Figure 1— This evidently created a symbiotic relationship in which the host eukaryote and the engulfed bac- terium helped one another to survive and reproduce.
Chloroplasts Capture Energy from Sunlight Chloroplasts are large, green organelles that are found only in the cells of plants and algae, not in the cells of animals or fungi. These organelles have an even more complex structure than mitochondria: Chloroplasts carry out photosynthesis—trapping the energy of sun- light in their chlorophyll molecules and using this energy to drive the manufacture of energy-rich sugar molecules.
In the process, they release chloroplasts chlorophyll- containing membranes Figure 1—19 Chloroplasts in plant cells inner capture the energy of sunlight.
B A drawing of one of the chloroplasts, showing the inner and outer membranes, as well as the highly folded system of internal membranes containing the green chlorophyll molecules that absorb A B light energy. A, courtesy of Preeti Dahiya. The bacteria are thought to have been taken up by early eukaryotic cells that already contained mitochondria.
Plant cells can then extract this stored chemical energy when they need it, by oxidizing these sugars in their mitochondria, just as animal cells do. Chloroplasts thus enable plants to get their energy directly from sunlight.
And they allow plants to produce the food molecules—and the oxygen—that mitochondria use to generate chemical energy in the form of ATP. How these organelles work together is discussed in Chapter Like mitochondria, chloroplasts contain their own DNA, reproduce by dividing in two, and are thought to have evolved from bacteria—in this case, from photosynthetic bacteria that were engulfed by an early eukaryotic cell Figure 1— Internal Membranes Create Intracellular Compartments with Different Functions Nuclei, mitochondria, and chloroplasts are not the only membrane- enclosed organelles inside eukaryotic cells.
The cytoplasm contains a profusion of other organelles that are surrounded by single membranes see Figure 1—7A. The endoplasmic reticulum ER is an irregular maze of interconnected spaces enclosed by a membrane Figure 1— It is the site where most cell-membrane components, as well as materials destined for export from the cell, are made.
This organelle is enormously enlarged in cells that are specialized for the secretion of proteins. Stacks of flattened, membrane-enclosed sacs constitute the Golgi apparatus Figure 1—22 , which modifies and packages molecules made in the ER that are destined to be either secreted from the cell or transported to another cell com- partment. Lysosomes are small, irregularly shaped organelles in which intracellular digestion occurs, releasing nutrients from ingested food par- ticles and breaking down unwanted molecules for either recycling within the cell or excretion from the cell.
Indeed, many of the large and small molecules within the cell are constantly being broken down and remade. Peroxisomes are small, membrane-enclosed vesicles that provide a safe environment for a variety of reactions in which hydrogen peroxide is used to inactivate toxic molecules. Membranes also form many different types of small transport vesicles that ferry materials between one mem- brane-enclosed organelle and another. All of these membrane-enclosed organelles are sketched in Figure 1—23A.
The Fundamental Units of Life Figure 1—21 The endoplasmic reticulum nucleus nuclear envelope endoplasmic reticulum produces many of the components of a eukaryotic cell. A Schematic diagram of an animal cell shows the endoplasmic reticulum ER in green. B Electron micrograph of a thin section of a mammalian pancreatic cell shows a small part of the ER, of which there are vast amounts in this cell type, which is specialized for protein secretion.
Note that the ER is continuous with the membranes of the nuclear envelope. The black particles studding the particular region of the ER shown here are ribosomes, structures that translate RNAs into proteins. B, courtesy of Lelio Orci. The exchange is mediated by transport vesicles that pinch off from the membrane of one organelle and fuse with another, like tiny soap bubbles budding from and rejoining larger bubbles.
At the surface of the cell, for example, portions of the plasma membrane tuck inward and pinch off to form vesicles that carry material captured from the external medium into the cell—a process called endocytosis Figure 1— Animal cells can nuclear envelope A B membrane- Figure 1—22 The Golgi apparatus is enclosed vesicles composed of a stack of flattened discs. A Schematic diagram of an animal cell with the Golgi apparatus colored red. B More Golgi apparatus realistic drawing of the Golgi apparatus.
Some of the vesicles seen nearby have endoplasmic reticulum pinched off from the Golgi stack; others are destined to fuse with it. Only one stack is shown here, but several can be present in a cell. C Electron micrograph that shows the C Golgi apparatus from a typical animal cell. C, courtesy of Brij J. A The peroxisome membrane-enclosed organelles, shown cytosol in different colors, are each specialized to perform a different function.
B The cytoplasm that fills the space outside nuclear Golgi of these organelles is called the cytosol apparatus colored blue. In the reverse process, called exocytosis, vesicles from inside the cell fuse with the plasma membrane and release their contents into the external medium see Figure 1—24 ; most of the hormones and signal molecules that allow cells to communicate with one another are secreted from cells by exocytosis.
How membrane-enclosed organelles move proteins and other molecules from place to place inside the cell is discussed in detail in Chapter The Cytosol Is a Concentrated Aqueous Gel of Large and Small Molecules If we were to strip the plasma membrane from a eukaryotic cell and then remove all of its membrane-enclosed organelles, including the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, and so on, we would be left with the cytosol see Figure 1—23B.
In other words, the cytosol is the part of the cytoplasm that is not contained within intracellular membranes. In most cells, the cytosol is the largest single compartment. It contains a host of large and small molecules, crowded together so closely that it behaves more like a water-based gel than a liquid solution Figure 1— The Cytoskeleton Is Responsible for Directed Cell Movements plasma membrane The cytoplasm is not just a structureless soup of chemicals and organelles.
Using an electron microscope, one can see that in eukaryotic cells the cytosol is criss-crossed by long, fine filaments. Frequently, the filaments are seen to be anchored at one end to the plasma membrane or to radi- ate out from a central site adjacent to the nucleus. This system of protein filaments, called the cytoskeleton, is composed of three major filament types Figure 1— The thickest fila- Figure 1—24 Eukaryotic cells engage in continual endocytosis and exocytosis.
In dividing cells, they become reorganized into a endocytosis and secrete intracellular spectacular array that helps pull the duplicated chromosomes in opposite materials by exocytosis. The Fundamental Units of Life Figure 1—25 The cytoplasm is stuffed with directions and distribute them equally to the two daughter cells Figure organelles and a host of large and small 1— Intermediate in thickness between actin filaments and microtu- molecules.
This schematic drawing, which extends across two pages and is based bules are the intermediate filaments, which serve to strengthen the cell. Proteins are blue, mechanical strength, controls its shape, and drives and guides its move- membrane lipids are yellow, and ribosomes ments Movie 1. The panorama begins on the far left at the plasma membrane, moves Because the cytoskeleton governs the internal organization of the cell through the endoplasmic reticulum, Golgi as well as its external features, it is as necessary to a plant cell—boxed apparatus, and a mitochondrion, and ends in by a tough wall of extracellular matrix—as it is to an animal cell that on the far right in the nucleus.
Courtesy of D. In a plant cell, for example, organelles such as mitochondria are driven in a constant stream around the cell interior along cytoskeletal tracks Movie 1. And animal cells and plant cells alike depend on the cytoskeleton to separate their internal components into two daughter cells during cell division see Figure 1— Even bacteria contain proteins that are distantly related to those of eukaryotic actin filaments and microtubules, forming filaments that play a part in prokaryotic cell division.
We examine the cytoskeleton in detail in Chapter 17, discuss its role in cell division in Chapter 18, and review how it responds to signals from outside the cell in Chapter The cytoskeleton is a dynamic evolve elaborate internal membrane jungle of protein ropes that are continually being strung together and systems that allow them to import taken apart; its filaments can assemble and then disappear in a matter substances from the outside, as of minutes.
Motor proteins use the energy stored in molecules of ATP to shown in Figure 1— Figure 1—26 The cytoskeleton is a network of protein filaments that criss- crosses the cytoplasm of eukaryotic cells. The three major types of filaments can be detected using different fluorescent stains. Shown here are A actin filaments, B microtubules, and C intermediate filaments.
A few of the key discoveries are listed in Table 1—1. In addition, Panel 1—2 sum- marizes the differences between animal, plant, and bacterial cells. Eukaryotic Cells May Have Originated as Predators Eukaryotic cells are typically 10 times the length and times the vol- ume of prokaryotic cells, although there is huge size variation within each category.
They also possess a whole collection of features—a cytoskeleton, mitochondria, and other organelles—that set them apart from bacteria and archaea. When and how eukaryotes evolved these systems remains something of a mystery. Although eukaryotes, bacteria, and archaea must have diverged from one another very early in the history of life on Earth discussed in Chapter 14 , the eukaryotes did not acquire all of their distinctive features at the same time Figure 1— According to one theory, the ancestral eukaryotic cell was a predator that fed by capturing other cells.
The nuclear compartment may have evolved to keep the DNA segregated from this physical and chemical Discuss the relative advantages hurly-burly, so as to allow more delicate and complex control of the way and disadvantages of light and the cell reads out its genetic information. How could Such a primitive cell, witha nucleus and cytoskeleton, was most likely you best visualize a a living skin cell, b a yeast mitochondrion, c a the sort of cell that engulfed the free-living, oxygen-consuming bacte- bacterium, and d a microtubule?
This partnership is thought to have been established 1. A subset of duplicated chromosomes Figure 1—27 Microtubules help distribute the chromosomes in a dividing cell. When a cell divides, its nuclear envelope breaks down and its DNA condenses into visible chromosomes, each of which has duplicated to form a pair of conjoined chromosomes that will ultimately be pulled apart into separate cells by microtubules.
In the transmission electron micrograph left , microtubules the microtubules are seen to radiate from foci at opposite ends of the dividing cell. Photomicrograph courtesy of Conly L. Nine years later, he sees bacteria for the first time. In one of the first applications of these techniques, Huxley shows that muscle contains arrays of protein filaments—the first evidence of a cytoskeleton. Perutz proposes a lower-resolution structure for hemoglobin. The likely history of these endosymbiotic events is illustrated in Figure 1— That single-celled eukaryotes can prey upon and swallow other cells is borne out by the behavior of many of the free-living, actively motile nonphotosynthetic photosynthetic fungi plants animals archaea bacteria bacteria chloroplasts Figure 1—28 Where did eukaryotes mitochondria come from?
The eukaryotic, bacterial, and archaean lineages diverged from one another very early in the evolution of life TIME on Earth. Some time later, eukaryotes are bacteria anaerobic ancestral eukaryote archaea thought to have acquired mitochondria; later still, a subset of eukaryotes acquired chloroplasts. Mitochondria are essentially the same in plants, animals, and fungi, and therefore were presumably acquired before these lines diverged.
The same colors are used, however, to distinguish the organelles chromatin DNA flagellum of the cell. The animal cell drawing is based on a nuclear pore fibroblast, a cell that inhabits connective tissue cell wall and deposits extracellular matrix. A micrograph of a living fibroblast is shown in microtubule Figure 1—6A.
The plant cell drawing is typical of a young leaf cell. The bacterium shown vacuole ribosomes in is rod-shaped and has a single fluid-filled cytosol flagellum for motility; note its much smaller size compare scale bars. B Didinium is seen ingesting another ciliated protozoan, a Paramecium.
It has a globular body encircled by two fringes of cilia, and its front end is flattened except for a single pro- trusion rather like a snout Figure 1—29A. Didinium swims at high speed by means of its beating cilia.
When it encounters a suitable prey, usually another type of protozoan, it releases numerous small, paralyzing darts from its snout region. Didinium then attaches to and devours the other cell, inverting like a hollow ball to engulf its victim, which can be almost as large as itself Figure 1—29B. Not all protozoans are predators. They can be photosynthetic or carnivo- rous, motile or sedentary.
Their anatomy is often elaborate and includes such structures as sensory bristles, photoreceptors, beating cilia, stalk- like appendages, mouthparts, stinging darts, and musclelike contractile bundles Figure 1— Although they are single cells, protozoans can be as intricate and versatile as many multicellular organisms. Much remains to be learned about fundamental cell biology from studies of these fasci- nating life-forms. Thus knowledge gained from the study of one organism contributes to our understanding of others, including ourselves.
But certain organisms are easier than others to study in the laboratory. Some reproduce rapidly and are convenient for genetic manipulations; others are multicellular but transparent, so that one can directly watch the development of all their internal tissues and organs. For reasons such as these, large communi- ties of biologists have become dedicated to studying different aspects of the biology of a few chosen species, pooling their knowledge to gain a deeper understanding than could be achieved if their efforts were spread over many different species.
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