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The Fundamental Units of Life In this chapter, we begin by looking at the great variety of forms that cells can show, and we take a preliminary glimpse at the chemical machinery that all cells have in common. We then consider how cells are made vis- ible under the microscope and what we see when we peer inside them.
Finally, we discuss how we can exploit the similarities of living things to achieve a coherent understanding of all forms of life on Earth—from the tiniest bacterium to the mightiest oak. But cells are not all alike; in fact, they can be wildly different.
Biologists estimate that there may be up to million distinct species of living things on our planet. Before delving deeper into cell biology, we must take stock: What does a bacterium have in common with a butter- fly?
What do the cells of a rose have in common with those of a dolphin? And in what ways do the plethora of cell types within an individual mul- ticellular organism differ? A frog egg—which is also a single cell—has a diameter of about 1 millimeter.
If we scaled them up to make the Lactobacillus the size of a person, the frog egg would be half a mile high. Cells vary just as widely in their shape Figure 1—1. A typical nerve cell in your brain, for example, is enormously extended; it sends out its electrical signals along a fine protrusion that is 10, times longer than it is thick, and it receives signals from other nerve cells through a mass of shorter processes that sprout from its body like the branches of a tree see Figure 1—1A.
A Paramecium in a drop of pond water is shaped like a submarine and is covered with thousands of cilia—hairlike extensions whose sinu- ous beating sweeps the cell forward, rotating as it goes Figure 1—1B. Note the very different scales of these micrographs. A Drawing of a single nerve cell from a mammalian brain. This cell has a huge branching tree of processes, through which it receives signals from as many as , other nerve cells.
B Paramecium. This protozoan—a single giant cell—swims by means of the beating cilia that cover its surface.
C Chlamydomonas. This type of single-celled green algae is found all over the world—in soil, fresh water, oceans, and even in the snow at the top of mountains. The cell makes its food like plants do—via photosynthesis—and it pulls itself through the water using its paired flagella to do the breaststroke.
D Saccharomyces cerevisiae. This yeast cell, used in baking bread, reproduces itself by a process called budding. E Helicobacter pylori.
This bacterium—a causative agent of stomach ulcers—uses a handful of whiplike flagella to propel itself through the stomach lining. Unity and Diversity of Cells 3 by a rigid box of cellulose with an outer waterproof coating of wax. A neutrophil or a macrophage in the body of an animal, by contrast, crawls QUESTION 1—1 through tissues, constantly pouring itself into new shapes, as it searches for and engulfs debris, foreign microorganisms, and dead or dying cells.
According to one popular biology text, living things: Cells are also enormously diverse in their chemical requirements. Some 1. Are highly organized compared require oxygen to live; for others this gas is deadly. Some cells consume to natural inanimate objects.
Display homeostasis, maintaining need a complex mixture of molecules produced by other cells. These differences in size, shape, and chemical requirements often reflect 3. Reproduce themselves. Some cells are specialized factories for the 4. Grow and develop from simple production of particular substances, such as hormones, starch, fat, latex, beginnings.
Others are engines, like muscle cells that burn fuel to do 5. Take energy and matter from the mechanical work. Still others are electricity generators, like the modified environment and transform it. Respond to stimuli. Show adaptation to their Some modifications specialize a cell so much that they spoil its chances environment.
Such specialization would be senseless Score a person, a vacuum cleaner, for a cell that lived a solitary life. In a multicellular organism, however, and a potato with respect to these there is a division of labor among cells, allowing some cells to become characteristics. Even the most basic need of all, that of passing on the genetic instructions of the organism to the next generation, is delegated to specialists—the egg and the sperm.
Living Cells All Have a Similar Basic Chemistry Despite the extraordinary diversity of plants and animals, people have recognized from time immemorial that these organisms have something in common, something that entitles them all to be called living things. But while it seemed easy enough to recognize life, it was remarkably dif- ficult to say in what sense all living things were alike. DNA The discoveries of biochemists and molecular biologists have provided an elegant solution to this awkward situation.
Although the cells of all living things are infinitely varied when viewed from the outside, they RNA synthesis are fundamentally similar inside. They are RNA composed of the same sorts of molecules, which participate in the same types of chemical reactions discussed in Chapter 2. In all organisms, genetic information—in the form of genes—is carried in DNA molecules. Thus, in every cell, the long DNA polymer chains are made amino acids from the same set of four monomers, called nucleotides, strung together in different sequences like the letters of an alphabet to convey informa- Figure 1—2 In all living cells, genetic tion.
A sub- transcription and from RNA to protein translation —a sequence known as set of these RNA molecules is in turn translated into yet another type of the central dogma. The sequence of polymer called a protein. Only a small part of the gene, RNA, protein molecules, which serve as structural supports, chemical catalysts, and protein are shown. Proteins are built from amino acids, and all constructed from cells.
A colony organisms use the same set of 20 amino acids to make their proteins. In this A, courtesy of Janice Carr; C, courtesy of way, the same basic biochemical machinery has served to generate the the John Innes Foundation; D, courtesy of whole gamut of life on Earth Figure 1—3. If cells are the fundamental unit of living matter, then nothing less than a cell can truly be called living. Viruses, for example, are compact pack- ages of genetic information—in the form of DNA or RNA—encased in protein but they have no ability to reproduce themselves by their own efforts.
Instead, they get themselves copied by parasitizing the reproduc- tive machinery of the cells that they invade. Thus, viruses are chemical zombies: That is why daughter cells resemble the parent cell.
However, the copying is not always perfect, and the instructions are occasionally corrupted by mutations that change the DNA. For this reason, daughter cells do not always match the parent cell exactly. Mutations can create offspring that are changed for the worse in that they are less able to survive and reproduce , changed for the better in that they are better able to survive and reproduce , or changed in a neutral way in that they are genetically different but equally viable. The struggle for survival eliminates the first, favors the second, and tolerates the third.
The genes of the next generation will be the genes of the survivors. On occasion, the pattern of descent may be complicated by sexual repro- duction, in which two cells of the same species fuse, pooling their DNA. The genetic cards are then shuffled, re-dealt, and distributed in new com- QUESTION 1—2 binations to the next generation, to be tested again for their ability to promote survival and reproduction. Mutations are mistakes in the DNA These simple principles of genetic change and selection, applied repeat- that change the genetic plan from the previous generation.
Imagine edly over billions of cell generations, are the basis of evolution—the a shoe factory. Would you expect process by which living species become gradually modified and adapted mistakes i. Evolution in copying the shoe design to lead offers a startling but compelling explanation of why present-day cells to improvements in the shoes are so similar in their fundamentals: Explain your answer.
It is estimated that this ancestral cell existed between 3. Through a very long process of mutation and natural selection, the descendants of this ancestral cell have gradually diverged to fill every habitat on Earth with organisms that exploit the potential of the machinery in an endless variety of ways. For the cells of plant and animal embryos, the genome directs the growth and development of an adult organism with hundreds of dif- ferent cell types.
Within an individual plant or animal, these cells can be extraordinarily varied, as we discuss in Chapter Fat cells, skin cells, bone cells, and nerve cells seem as dissimilar as any cells could be. Yet all these differentiated cell types are generated during embryonic develop- ment from a single fertilized egg cell, and all contain identical copies of the DNA of the species.
Their varied characters stem from the way that individual cells use their genetic instructions. Different cells express dif- ferent genes: The DNA, therefore, is not just a shopping list specifying the molecules that every cell must make, and a cell is not just an assembly of all the items on the list.
Each cell is capable of carrying out a variety of biologi- cal tasks, depending on its environment and its history, and it selectively uses the information encoded in its DNA to guide its activities. Later in this book, we will see in detail how DNA defines both the parts list of the cell and the rules that decide when and where these parts are to be made. But cell biology started without these tools. The earliest cell biologists began by simply looking at tissues and cells, and later breaking them open or slicing them up, attempting to view their contents.
What they saw was to them profoundly baffling—a collection of tiny and scarcely visible objects whose relation- ship to the properties of living matter seemed an impenetrable mystery. Nevertheless, this type of visual investigation was the first step toward understanding cells, and it remains essential in the study of cell biology. Cells were not made visible until the seventeenth century, when the microscope was invented.
For hundreds of years afterward, all that was known about cells was discovered using this instrument. Light microscopes use visible light to illuminate specimens, and they allowed biologists to see for the first time the intricate structure that underpins all living things. Although these instruments now incorporate many sophisticated improvements, the properties of light itself set a limit to the fineness of detail they reveal.
Electron microscopes, invented in the s, go beyond this limit by using beams of electrons instead of beams of light as the source of illumination, greatly extending our ability to see the fine details of cells and even making some of the larger molecules visible individ- ually. These and other forms of microscopy remain vital tools in the modern cell biology laboratory, where they continue to reveal new and sometimes surprising details about the way cells are built and how they operate. By the seventeenth century, lenses were pow- erful enough to make out details invisible to the naked eye.
Using an instrument equipped with such a lens, Robert Hooke examined a piece of cork and in reported to the Royal Society of London that the cork was composed of a mass of minute chambers. The name stuck, even though the structures Hooke described were actually the cell walls that remained after the living plant cells inside them had died. Later, Hooke and his Dutch contempo- rary Antoni van Leeuwenhoek were able to observe living cells, seeing for the first time a world teeming with motile microscopic organisms.
For almost years, such instruments—the first light microscopes— remained exotic devices, available only to a few wealthy individuals. It was not until the nineteenth century that microscopes began to be widely used to look at cells.
The emergence of cell biology as a distinct science was a gradual process to which many individuals contributed, but its offi- cial birth is generally said to have been signaled by two publications: In these papers, Schleiden and Schwann documented the results of a systematic investigation of plant and animal tissues with the light microscope, showing that cells were the universal building blocks of all living tissues.
Their work, and that of other nine- teenth-century microscopists, slowly led to the realization that all living cells are formed by the growth and division of existing cells—a principle sometimes referred to as the cell theory Figure 1—4.
A In , Eduard Strasburger drew a living plant cell a hair cell from a Tradescantia flower , which he observed dividing into two daughter cells over a period of 2. B A comparable living plant cell photographed recently through a modern light microscope. B, courtesy of Peter Hepler. Cells Under the Microscope 7 living organisms do not arise spontaneously but can be generated only from existing organisms was hotly contested, but it was finally confirmed QUESTION 1—3 in the s by an elegant set of experiments performed by Louis Pasteur.
You have embarked on an ambitious The principle that cells are generated only from preexisting cells and research project: You boil up a rich mixture the subject a unique flavor: To understand why in a flask along with a sprinkling present-day cells and organisms behave as they do, we need to under- of the inorganic salts known to be stand their history, all the way back to the misty origins of the first cells essential for life.
You seal the flask 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.
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