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Chapter 4 Cell Structure and Function 53 At a Glance 4.1 What Is the Cell Theory? 4.3 What Are the Major Features 4.4 What Are the Major 4.2 What Are the Basic o...

Chapter 4 Cell Structure and Function 53 At a Glance 4.1 What Is the Cell Theory? 4.3 What Are the Major Features 4.4 What Are the Major 4.2 What Are the Basic of Prokaryotic Cells? Features of Eukaryotic Attributes of Cells? Cells? 4.1 What is the Cell Theory? C heck Your Learning Because cells are so small, no one had ever seen them until Can you … the first microscope was invented in the mid-1600s (see “How trace the historical development of the cell theory? Do We Know That? The Search for the Cell” on page 54). But list the three principles of the cell theory? seeing cells was only the first step toward understanding their importance. In 1838, the German botanist Matthias Schlei- den concluded that cells and substances produced by cells form the basic structure of plants and that plant growth oc- 4.2 What are the Basic curs by adding new cells. In 1839, German biologist Theodor Attributes of Cells? Schwann (Schleiden’s friend and collaborator) drew similar All living things, from microscopic bacteria to a giant sequoia conclusions about animal cells. The work of Schleiden and tree, are composed of cells. Cells perform an enormous vari- Schwann provided a unifying theory of cells as the funda- ety of functions, including obtaining energy and nutrients, mental units of life. In 1855, the German physician Rudolf synthesizing biological molecules, eliminating wastes, inter- Virchow ­completed the cell theory—a fundamental con- acting with other cells, and reproducing. cept of ­biology—by concluding that all cells come from pre- Most cells range in size from about 1 to 100 micrometers viously existing cells. (μm; millionths of a meter) in diameter (FIG. 4-1). Why are The cell theory consists of three principles: most cells so small? The answer lies in the need for cells to 1. Every organism is made up of one or more cells. exchange nutrients and wastes with their external environ- 2. The smallest organisms are single cells, and cells are the ment through the plasma membrane. Many nutrients and functional units of multicellular organisms. wastes move into, through, and out of cells by diffusion, 3. All cells arise from preexisting cells. the process by which molecules dissolved in fluids disperse Cells are small to facilitate efficient diffusion of nutrients and wastes, ensuring their innermost parts remain close to the external environment for metabolic processes. 100 m 10 m 1m 0.1 m 1 cm 1 mm 100 om 10 om 1 om 100 nm 10 nm 1 nm 0.1 nm longest python house fly DNA apple most flu virus C eukaryotic cells most tallest redwood human crab prokaryotic hemoglobin carbon tree louse cells atom human eye light microscope electron microscope Figure 4-1 Relative sizes Dimensions encountered in biology range from about 100 meters (the height of the tallest redwood trees) to a few nanometers (the diameter of many large molecules). M04_AUDE3001_11_SE_C04_pp052-074.indd 53 23/10/15 2:53 PM  54 UNIT 1 The Life of the Cell How Do We Know That? The Search for the Cell Although cells form the basis of life, they’re so small that it higher magnification, down to almost 1 micron (1 μm; see wasn’t until we could actually see them that we realized they Fig. 4-1). A self-taught amateur scientist, van Leeuwenhoek’s existed. In 1665, the English scientist and inventor Robert descriptions of myriad “animalcules” (mostly single-celled Hooke aimed his primitive light microscope at an “exceeding organisms) in rain, pond, and well water were greeted with thin... piece of Cork” and saw “a great many little Boxes” amazement. Over the years, he described an enormous (FIG. E4-1a). Hooke called the boxes “cells,” because he range of microscopic specimens, including blood cells, sperm thought they resembled the tiny rooms (called cells) occupied cells, and the eggs of aphids and fleas, helping overturn the by monks in a monastery. Cork comes from the dry outer bark belief that these insects emerged spontaneously from dust of the cork oak, and we now know that he was looking at the or grain. Observing white matter scraped from his teeth, van nonliving cell walls that surround all plant cells. Hooke wrote Leeuwenhoek saw swarms of cells that we now recognize as that in the living oak and other plants, “These cells [are] fill’d bacteria. Disturbed by these animalcules in his mouth, he tried with juices.” to kill them with vinegar and hot coffee—but with little suc- In the 1670s, Dutch microscopist Anton van Leeuwenhoek cess. constructed his own simple light microscopes and observed Since the pioneering efforts of early microscopists, a previously unknown living world (FIG. E4-1b). Although van biologists, physicists, and engineers have collaborated to Leeuwenhoek’s microscopes appear much more primitive than develop a variety of advanced microscopes to view the cell Hooke’s, their ­superior lenses provided clearer images and and its components. Light microscopes use lenses made of specimen focusing knob location of lens (b) van Leeuwenhoek’s light microscope (a) Robert Hooke’s light microscope and his (c) Electron microscope drawing of cork cells Figure E4-1 Microscopes yesterday and today (a) Robert Hooke saw the walls of cork cells through his elegant light microscope, and drew them with great skill. (b) Hooke and van Leeuwenhoek were contemporaries. Hooke admitted that van Leeuwenhoek’s microscopes produced better images, but described these extremely simple microscopes as “offensive to my eye.” (c) This modern machine is both a transmission electron microscope (TEM) and a scanning electron microscope (SEM). M04_AUDE3001_11_SE_C04_pp052-074.indd 54 23/10/15 2:53 PM  Chapter 4 Cell Structure and Function 55 glass or quartz to bend, focus, and transmit light rays that as DNA and even individual carbon have passed through or bounced off a specimen. The light atoms (seen here forming a six-­ microscope produces images depending on how the speci- carbon ring). men is illuminated and how it has been stained. Fluorescent Scanning electron microscopes stains targeted to specific molecules and viewed under bounce electrons off specimens that specific wavelengths of light are now revolutionizing our are dry and hard (such as shells) view of cells. The resolving power (the smallest structure or that have been covered with an distinguishable under ideal conditions) of modern light ultrathin coating of metal such as ­microscopes is about 200 nanometers (nm; see Fig. 4-1). gold. Scanning electron microscopes TEM of carbon atoms This is sufficient to see most prokaryotic cells, some struc- can be used to view the three-dimen- tures inside eukaryotic cells, and living cells such as a sional surface details of structures that range in size from swimming Paramecium (FIG. E4-2a). entire small insects down to cells and their components, Electron microscopes (FIG. E4-1c) use beams of elec- with a maximum resolution of about 1.5 nanometers trons focused by magnetic fields rather than light focused (FIGS. E4-2c, d). by lenses. Transmission electron microscopes pass electrons through a thin specimen and can reveal the details of inte- Think Critically Based on the images in Fig. E4-2, what rior cell structure (FIG. E4-2b). Some modern transmission advantages are there in visualizing microscopic structures using electron microscopes can resolve structures as small as each of these techniques? 0.05 nanometer, allowing scientists to see molecules such food vacuole mitochondria cilia nucleus contractile vacuole (a) Light micrograph (Paramecium) (b) Transmission electron micrograph mitochondria smooth ER (c) Scanning electron micrograph (Paramecia) (d) Scanning electron micrograph Figure E4-2 A comparison of microscope images (a) A living Paramecium (a single-celled freshwater protist) photographed through a light microscope. (b) A transmission electron micrograph (TEM) showing mitochondria. (c) A scanning electron micrograph (SEM) of two Paramecia. (d) An SEM showing mitochondria and smooth endoplasmic reticulum. All colors in electron micrographs (SEMs or TEMs) have been added artificially. M04_AUDE3001_11_SE_C04_pp052-074.indd 55 23/10/15 2:53 PM  1. The Plasma Membrane encloses the cell and allows interactions b/w the cell and its environment. 2. ALL cells contain cytoplasm. 56 UNIT 1 The Life of the Cell 3. All cells use DNA as a hereditary blueprint and RNA to copy the blueprint and guide construction of cell parts. from regions where their concentration is higher to regions cell to maintain essential differences in the concentrations of where their concentration is lower (see Chapter 5). Diffusion materials inside and out. In contrast, the huge variety of is a relatively slow process, so to meet the constant metabolic proteins within the bilayer facilitate communication ­between demands of cells, even their innermost parts must remain the cell and its environment. For example, channel ­proteins close to the external environment. Thus, cells maintain a very allow passage of specific molecules or ions into or out of the small diameter, whether they are round or elongated. cell (see Fig. 4-2). Glycoproteins, ­­ which have short carbohy­ drate chains ­ extending outside the cell, both facilitate ­interactions between cells and respond to external signaling All Cells Share Common Features ­molecules that promote chemical reactions within the cell All cells arose from a common ancestor that evolved about 3.5 (described in Chapter 5). billion years ago. Modern cells include the simple p ­ rokaryotic cells of bacteria and archaea and the complex eukaryotic cells of protists, fungi, plants, and animals. All cells, whether All Cells Contain Cytoplasm simple or complex, share some important features. The cytoplasm consists of all the fluid and structures that lie inside the plasma membrane but outside of the nucleus (see Figs. 4-4 and 4-5). The fluid portion of the The Plasma Membrane Encloses the Cell and Allows cytoplasm in both prokaryotic and eukaryotic cells, called Interactions Between the Cell and Its Environment the ­cytosol, contains water, salts, and an assortment of Each cell is surrounded by an extremely thin, rather fluid organic molecules, including proteins, lipids, carbohy- membrane called the plasma membrane (FIG.   4-2). The drates, sugars, amino acids, and nucleotides (described plasma membrane, like all membranes in and around cells, in Chapter 3). Most of the cell’s metabolic activities—the contains proteins embedded in a double layer, or bilayer, of biochemical reactions that support life—occur in the cell phospholipids interspersed with cholesterol molecules. cytoplasm. The phospholipid and protein components of cellular The cytoskeleton consists of a variety of protein fila- membranes play very different roles. The phospholipid bi- ments within the cytoplasm. These provide support, trans- layer helps isolate the cell from its surroundings, allowing the port structures within the cell, aid in cell division, and allow cells to move and change shape (see Figs. 4-2 and 4-7). Fluid portion- Cytosol | Protein portions- Cytoskeleton (interstitial fluid, outside) carbo- hydrate All Cells Use DNA As a Hereditary Blueprint and RNA to Copy the Blueprint and Guide glycoprotein Construction of Cell Parts A phospholipid bilayer The genetic material in all cells is deoxyribonucleic acid A gene is a segment of DNA that contains helps to isolate the (DNA), an inherited blueprint consisting of segments called the instructions for cell's contents. Proteins help the cell producing a specific genes. Genes store the instructions for making all the parts protein or functional communicate with its environment. of the cell and for producing new cells (see Chapter 12). RNA molecule. Gene carries genetic ­Ribonucleic acid (RNA), which is chemically similar to information. cholesterol DNA, copies the genes of DNA and helps construct proteins based on this genetic blueprint. The construction of pro- tein from RNA in all cells occurs on ribosomes, cellular workbenches composed of a specialized type of RNA called ­r ibosomal RNA. There Are Two Basic Types of Cells: Prokaryotic and Eukaryotic All forms of life are composed of one of two types of cells. membrane protein channel protein Prokaryotic cells (Gk. pro, before, and kary, nucleus) form the bodies of bacteria and archaea, the simplest forms of cytoskeleton life. Eukaryotic cells (Gk. eu, true) are far more complex (cytosol, fluid inside cell) and make up the bodies of animals, plants, fungi, and pro- tists. As their names suggest, one striking difference between Figure 4-2 The plasma membrane The plasma membrane prokaryotic and eukaryotic cells is that the genetic material encloses the cell in a double layer of phospholipids associated of eukaryotic cells is contained within a membrane-enclosed with a variety of proteins. The membrane is supported by the nucleus. TABLE 4-1 summarizes the principal features of cytoskeleton. prokaryotic and eukaryotic cells. In prokaryotic cells, the genetic material is not enclosed within a nucleus; instead, it is located in a region called the nucleoid, which is not membrane-bound. M04_AUDE3001_11_SE_C04_pp052-074.indd 56 23/10/15 2:53 PM  Chapter 4 Cell Structure and Function 57 cell wall- present in plants, absent in animals Table 4-1 Functions and Distribution of Cell Structures Eukaryotes: Eukaryotes: Structure Function Prokaryotes Plants Animals Cell Surface Extracellular matrix Surrounds cells, providing biochemical and structural support Absent Present Present Cilia Move the cell through fluid or move fluid past the cell surface Absent Absent (in most) Present Flagella Move the cell through fluid Present1 Absent (in most) Present Plasma membrane Isolates the cell contents from the environment; regulates movement of Present Present Present materials into and out of the cell; allows communication with other cells Organization of Genetic Material Genetic material Encodes the information needed to construct the cell and to control DNA DNA DNA ­cellular activity Chromosomes Contain and control the use of DNA Single, circular Many, linear Many, linear Nucleus2 Contains chromosomes and nucleoli Absent Present Present Nuclear envelope Encloses the nucleus; regulates movement of materials into and Absent Present Present out of the nucleus Nucleolus Synthesizes ribosomes Absent Present Present Cytoplasmic Structures Ribosomes Provide sites for protein synthesis Present Present Present 2 Mitochondria Produce energy by aerobic metabolism Absent Present Present Chloroplasts2 Perform photosynthesis Absent Present Absent Further in the Endoplasmic Synthesizes membrane components, proteins, and lipids Absent Present Present book, it is seen: reticulum2 Most prokaryotic Golgi apparatus2 Modifies, sorts, and packages proteins and lipids Absent Present Present cells have 2 Lysosomes Contain digestive enzymes; digest food and worn-out organelles Absent Absent (in most) Present plasmid outside the nucleoid. Plastids2 Store food, pigments Absent Present Absent Central vacuole2 Contains water and wastes; provides turgor pressure to support the cell Absent Present Absent Other vesicles Transport secretory products; contain food obtained through phagocytosis Absent Present Present and vacuoles2 Cytoskeleton Gives shape and support to the cell; positions and moves cell parts Present Present Present Centrioles Produce the basal bodies of cilia and flagella Absent Absent (in most) Present 1 Some prokaryotes have structures called flagella, which lack microtubules and move in a fundamentally different way than do eukaryotic flagella. Mitochondria are essential for providing 2 Indicates organelles, which are surrounded by membranes and found only in eukaryotic cells. energy, especially in non-photosynthetic tissues or when light is unavailable. C a s e S t u dy Continued Che ck Your Learning Can you … New Parts for Human Bodies describe the structure and features shared by all cells? Why was Beyene’s bioartificial trachea considered a scientific distinguish prokaryotic from eukaryotic cells? breakthrough? One reason is that the patient’s own cells were used to grow the new body part, so his immune system was unlikely to reject the cells. The plasma membranes of all cells bear surface molecules called glycoproteins that are unique to the individual and 4.3 What are the Major Features allow the person’s immune system to recognize the cells as “self.” of Prokaryotic Cells? 1. Size 2. Archaea Cells from any other person (except an identical twin), however, bear Prokaryotic cells have a relatively simple internal structure different glycoproteins. The immune system will identify the differ- ent cells as foreign and attack them, which can cause rejection of and are generally less than 5 micrometers in ­diameter (in a transplanted organ. To prevent organ rejection, physicians must comparison, eukaryotic cells range from 10 to 100 microm- find donor cells that match the patient’s as closely as possible. But eters in diameter). Prokaryotes also lack the complex internal even then, the patient must take drugs that suppress the immune membrane-enclosed structures that are the most prominent system, which increases vulnerability to cancers and infections that features of eukaryotic cells. the immune system would normally target and destroy. The cells Single prokaryotic cells make up two of life’s domains: most likely to cause problems for immune-suppressed patients are Archaea and Bacteria. Many Archaea inhabit extreme en- prokaryotic. What are the features of these simple cells? vironments, such as hot springs and cow stomachs, but ­Archaea are increasingly being discovered in more familiar M04_AUDE3001_11_SE_C04_pp052-074.indd 57 17/11/15 11:55 AM  58 UNIT 1 The Life of the Cell chromosome (within pili the nucleoid region) ribosomes plasmid (DNA) food granule (b) Spirillum capsule or prokaryotic cytoplasm slime layer flagellum plasma membrane cell wall (a) Generalized prokaryotic cell (bacillus) photosynthetic membranes chromosome cell wall plasma membrane ribosomes (e) Photosynthetic prokaryotic cell capsule (c) Cocci (d) Internal structure Figure 4-3 Prokaryotic cells Prokaryotes come in different shapes, including (a) rod-shaped bacilli, (b) spiral-shaped spirilla, and (c) spherical cocci. Internal structures are revealed in the TEMs in (d) and (e). Some photosynthetic bacteria have internal membranes where photosynthesis occurs, as shown in (e). locales, such as the soil and oceans. None are known to cause abundant attachment pili may work on their own or with cap- disease. In this chapter, we focus on the more familiar bacte- sules and slime layers to help bacteria adhere to struc­­tures. ria as representative prokaryotic cells (FIG. 4-3). For example, various types of Streptococcus bacteria (which can cause strep throat, skin infections, pneumonia, and toxic shock syndrome) use pili to help them infect their victims. Many bac- Prokaryotic Cells Have Specialized teria form sex pili, which are few in number and quite long. A Surface Features sex pilus from one bacterium binds to a nearby bacterium of Nearly all prokaryotic cells are surrounded by a cell wall, the same type and draws them together. The two bacteria form which is a relatively stiff coating that the cell secretes around a short bridge that links their cytoplasm and allows them to itself to provide protection and help maintain its shape. The transfer small rings of DNA called plasmids. cell walls of bacteria are composed of peptidoglycan (a unique Some bacteria and archaea possess flagella (singular, molecule consisting of short peptides that link chains of flagellum; “whip”), which extend from the cell surface and sugar molecules which have amino functional groups). Bac- rotate to propel these cells through a fluid environment (see propel- push teria include rod-shaped bacilli, spherical cocci, and spiral- Fig. 4-3a). Prokaryotic flagella differ from those of eukaryotic shaped spirilla (see Figs. 4-3a, b, c). cells, which are described later in this chapter. Many bacteria secrete polysaccharide coatings called ­capsules and slime layers outside their cell walls (see Fig. 4-3a). In bacteria such as those that cause tooth decay, diarrhea, pneu- Prokaryotic Cells Have Specialized monia, or urinary tract infections, capsules and slime layers Cytoplasmic Structures help them adhere to specific host tissues, such as the surface of a The cytoplasm of a typical prokaryotic cell contains tooth or the lining of the small intestine, lungs, or bladder. Cap- ­several specialized structures. A distinctive region called the sules and slime layers allow some bacteria to form surface films nucleoid (meaning “like a nucleus”; see Fig. 4-3a) contains (such as those that may coat unbrushed teeth or unwashed toilet a single circular chromosome that consists of a long, coiled bowls). They also protect the bacteria and help keep them moist. strand of DNA that carries essential genetic information. Un- Pili (singular, pilus; meaning “hairs”) are surface proteins like the nucleus of a eukaryotic cell, the nucleoid is not sepa- that project from the cell walls of many bacteria (see Fig. 4-3a). rated from the cytoplasm by a membrane. Most prokaryotic There are two types of pili: attachment pili and sex pili. Short, cells also contain plasmids outside the nucleoid. Plasmids M04_AUDE3001_11_SE_C04_pp052-074.indd 58 23/10/15 2:53 PM  Chapter 4 Cell Structure and Function 59 usually carry genes that give the cell special properties; for ex- ample, some disease-causing bacteria possess plasmids that en- Check Your Learning code proteins that inactivate antibiotics, making the bacteria Can you … much more difficult to kill. Bacterial cytoplasm also includes describe the structure and function of the major features of ribosomes, where proteins are synthesized, as well as food prokaryotic cells? describe the internal features of bacteria, including how granules that store energy-rich molecules, such as glycogen. some bacteria utilize internal membranes? Although prokaryotic cells lack internal structures sur­­­­­­ rounded by membranes, some bacteria use internal mem- branes to organize enzymes. These enzymes facilitate 4.4 WHAT ARE THE MAJOR FEATURES ­bi­­o­­­­­­chemical processes requiring several reactions, and they are situated in a specific sequence along the membrane that OF EUKARYOTIC CELLS? corresponds to the sequence in which the reactions must Eukaryotic cells make up the bodies of organisms in the do- occur. For example, photosynthetic bacteria possess exten- main Eukarya: animals, plants, protists, and fungi. As you sive internal membranes where light-capturing proteins and might imagine, these cells are extremely diverse. The cells enzymes are embedded, allowing the bacteria to harness the en- that form the bodies of unicellular protists can perform all the ergy of sunlight to synthesize high-energy molecules (Fig. 4-3e). activities necessary for independent life. Within the body of Prokaryotes also contain an extensive cytoskeleton that any multicellular organism, cells are specialized to perform a includes some proteins that resemble those of the eukaryotic variety of functions. Here, we focus on plant and animal cells. cytoskeleton (see Fig. 4-7) and others that are unique. The Unlike prokaryotic cells, eukaryotic cells (Fig. 4-4) have prokaryotic and eukaryotic cytoskeletons serve many similar organelles (“little organs”), membrane-enclosed structures functions; for example, both are essential for cell division and specialized for a specific function (see Table 4-1). Organelles contribute to regulating the shape of the cell. contribute to the complexity of eukaryotic cells. Figure 4-4 ribosomes nuclear envelope microfilaments nuclear pore (cytoskeleton) nucleus chromatin (DNA) nucleolus cytosol microtubule (cytoskeleton) flagellum (propels sperm cell) basal body rough endoplasmic reticulum vesicle intermediate cytoplasm filaments (cytoskeleton) centriole Golgi apparatus ribosomes on rough polyribosome ER lysosome smooth endoplasmic reticulum vesicles releasing substances from mitochondrion the cell plasma Figure 4-4 A generalized animal cell membrane free ribosome M04_AUDE3001_11_SE_C04_pp052-074.indd 59 23/10/15 2:53 PM  60 UNIT 1 The Life of the Cell ribosomes nuclear envelope nuclear pore nucleus chromatin microfilaments (cytoskeleton) microtubule nucleolus (cytoskeleton) cell walls of adjoining plant cells chloroplast cytoplasm rough endoplasmic reticulum intermediate filaments (cytoskeleton) vesicles smooth endoplasmic reticulum Golgi apparatus central vacuole mitochondrion vesicle cell wall plasma plasmodesmata membrane cytosol plastid free ribosome Figure 4-5 A generalized plant cell i­ llustrates a generalized animal cell, and Figure 4-5 illustrates ECM (which differs among cell types) provides both structural a generalized plant cell, each with some distinctive structures. and biochemical support, including proteins called growth Animal cells have centrioles, lysosomes, cilia, and flagella, factors, which promote cell survival and growth. The ECM which are not found in the most common plant cells, and attaches adjacent cells, transmits molecular signals between plant cells have cell walls, central vacuoles, and plastids (in- cells, and guides cells as they migrate and differentiate dur- cluding chloroplasts), which are absent in animal cells. ing development. It anchors cells within tissues and provides a supporting framework within tissues; for example, a stiff ex- tracellular matrix forms the scaffolding for bone and cartilage Extracellular Structures Surround (FIG. 4-6b). Animal and Plant Cells The extracellular matrix of plant cells is the cell wall, The plasma membrane, which is only about two molecules which protects and supports each cell. Plant cell walls, com- thick and has the consistency of viscous oil, would be torn posed mainly of overlapping cellulose fibers, are p­ orous and apart without reinforcing structures. The reinforcing structure allow oxygen, carbon dioxide, and water with its dissolved for animal cells is the complex extracellular matrix (ECM), substances to flow through them. Cell walls attach adjacent secreted by the cell. The ECM includes an array of supporting plant cells to one another and are perforated by plasma mem- and adhesive proteins embedded in a gel composed of polysac- brane-lined openings called plasmodesmata that connect the charides that are linked together by proteins (FIG. 4-6a). The cytoplasm of adjacent cells (see Fig. 4-5). M04_AUDE3001_11_SE_C04_pp052-074.indd 60 23/10/15 2:53 PM  Chapter 4 Cell Structure and Function 61 extracellular matrix (interstitial fluid, outside) support protein extracellular matrix adhesion protein cartilage cell gel-forming substance (a) The extracellular matrix (b) Extracellular matrix of a cartilage cell Figure 4-6 The extracellular matrix (a) Extracellular proteins perform a variety of functions. (b) An SEM of a cartilage cell surrounded by its extracellular matrix. The ECM signals direct or regulate this process, rather than actively helping the proteins. In this case, the ECM serves more as a signaling mechanism that guides the actions of cytoskeletal proteins. C a s e S t u dy Continued The Cytoskeleton Provides Shape, Support, and Movement New Parts for Human Bodies The cytoskeleton is a dynamic network of protein fibers within the cytoplasm (FIG. 4-7). Cytoskeletal proteins come in three Besides tracheas and noses, researchers are working on grow- major types: thin microfilaments (composed of actin pro- ing bioartificial muscles as well. In the past, major muscle injuries could mean amputation and a prosthetic limb, because tein), medium-sized intermediate filaments (composed muscles have limited ability to regenerate, and scar tissue of various proteins), and thick microtubules (composed of forms and interferes with their function. But Stephen Badylak tubulin protein). Cytoskeletal proteins provide the cell with and colleagues at the McGowen Institute for Regenerative both internal support and the ability to change shape and Medicine are investigating the use of the ECM to help muscles divide, directed by signals from the ECM. The cytoskeleton is heal and even regenerate. important in regulating the following properties of cells: After 28-year-old Marine Ron Strang’s quadriceps mus- Cell Shape Cytoskeletal proteins can alter the shapes cle was almost ripped from his leg by a roadside bomb in of cells using energy released from ATP, either by chang- Afghanistan, he volunteered for a new bioartificial muscle ing their length (by adding or removing subunits) or by treatment developed by Badylak. Badylak used ECM from pig bladders with the cells removed (which prevents tissue rejec- sliding past one another. In animal cells, a scaffolding of tion) to recreate Strang’s muscle. Badylak’s team then cut intermediate filaments supports the cell, helps determine away scar tissue from the Marine’s thigh muscle and placed its shape, and links cells to one another and to the ECM. the pig matrix in the resulting cavity. There, its unique combina- An array of microfilaments concentrated just inside the tion of natural scaffolding proteins and growth factors recruited plasma membrane provides additional support and also muscle stem cells and worked a major transformation. After connects with the surrounding ECM. 6 months, the pig matrix was broken down and replaced by Cell Movement Cell movement can occur in animal healthy human tissue, and Strang went from hobbling to hiking cells as microtubules and microfilaments extend by add- and riding a bike. ing subunits at one end and releasing subunits at the Strang’s treatment worked in part because the ECM helps other end. Microtubules and intermediate filaments support tissues and facilitates communication between cells. may be associated with motor proteins, which are special- Which structures provide support and facilitate communication ized to release energy stored in ATP and use it to generate within a cell? molecular movement. Another form of movement is generated as motor proteins cause actin microfilaments M04_AUDE3001_11_SE_C04_pp052-074.indd 61 17/11/15 11:55 AM  62 UNIT 1 The Life of the Cell subunit ribosomes rough endoplasmic reticulum 25 nm microfilaments (red) Microtubules: Composed of pairs of different polypeptides in a helical arrangement subunit 10 nm Intermediate filaments: Composed of ropelike bundles of various proteins subunits DNA in nucleus (blue) cell membrane 7 nm mitochondrion microtubules (green) Microfilaments: Composed of actin proteins that resemble twisted double strands of beads (a) Cytoskeleton (b) Light micrograph showing the cytoskeleton Figure 4-7 The eukaryotic cytoskeleton (a) Three types of protein strands form the cytoskel- eton. (b) In this light micrograph, cells treated with fluorescent stains reveal microtubules, microfila- ments, and nuclei. to slide past one another; a well-known example occurs surrounding fluid that is similar to that created by oars on a during the contraction of muscle cells. Muscle cells rowboat. A flagellum, in contrast, rotates in a corkscrew mo- must contract to increase in size, as scientists discovered tion that propels a cell through fluid, acting somewhat like when they attempted to grow muscle protein in the the propeller on a motorboat. Cells with flagella usually have ­laboratory for possible human consumption. Learn only one or two of them. more in “Earth Watch: Would You Like Fries with Your Cultured Cow Cells?” on page 65. Organelle Movement Motor proteins use microfilaments and microtubules as “railroad tracks” to transport orga- Have You Ever Over the years, scientists have wondered how many nelles within the cell. Cell Division Microtubules guide chromosome Wondered… cells are in the human body. They don’t yet agree, ­movements, and microfilaments in animal cells pinch but 10 trillion seems a reasonable estimate. There is a consensus, the dividing cell into two daughter cells. (Cell division however, that there are at least 10 times as many prokaryotic cells is covered in Chapter 9.) associated with the body, residing in a community called the microbiome. We each host a unique community consisting of about How Many Cells 3 pounds (1.4 kilograms) of prokaryotic Cilia and Flagella May Move Cells Through Form the Human life, which includes roughly 100 Fluid or Move Fluid Past Cells different types of bacteria. These cells Body? Both cilia (singular, cilium; “eyelash”) and eukaryotic colonize the nose, skin, vagina, and the ­flagella are beating hair-like structures covered by plasma digestive tract from mouth to anus. membrane that extend outward from some cell surfaces. Because the digestive tract is a tube open to the outside at both ends, our microbiome occupies a unique niche that is They are supported and moved by microtubules of the simultaneously integral to—yet outside of—our bodies. With ­cytoskeleton. Each cilium or flagellum contains a ring of recent advances allowing identification of microorganisms nine fused pairs of microtubules surrounding an unfused by their unique DNA sequences, scientists are increasingly pair (FIG. 4-8). Cilia and flagella beat almost continuously, studying our relationships with our microbial residents. Our powered by motor proteins that extend like tiny arms and gut microbiome helps digest food and synthesize vitamins, and attach neighboring pairs of microtubules (see Fig. 4-8a). ­ it allows the immune system to develop properly. Even though These sidearms use ATP energy to slide the microtubules our bacterial populations changes in response to food intake and past one another, causing the cilium or flagellum to bend. states of disease and health, one thing is clear: We would not be In general, cilia are shorter and more numerous than ourselves without them. flagella. Cilia beat in unison to produce a force on the M04_AUDE3001_11_SE_C04_pp052-074.indd 62 23/10/15 2:53 PM  Chapter 4 Cell Structure and Function 63 cilia lining trachea protein sidearms fused microtubule pair (b) Cilia central pair of microtubules TEM showing cross- section flagellum of human sperm plasma membrane basal body (extends into cytoplasm) (a) Internal structure of cilia and flagella (c) Flagellum Figure 4-8 Cilia and flagella (a) These structures are filled with microtubules produced by the basal body. (b) Cilia, shown in this SEM, line the trachea and sweep out debris. (c) A human sperm cell, shown in this SEM, uses its flagellum to swim to the egg. Think Critically What problems would arise if the trachea were lined with flagella instead of cilia? Protists use cilia or flagella to swim through water; The Nucleus, Containing DNA, Is the the Paramecium in Figures E4-2a, c (see “How Do We Know Control Center of the Eukaryotic Cell That? The Search for the Cell” on page 54) uses cilia. In ani- A cell’s DNA stores all the information needed to construct the mals, cilia usually move fluids past a surface. Ciliated cells cell and direct the countless chemical reactions necessary for line such diverse structures as the gills of oysters (where life and reproduction. A cell uses only a portion of the instruc- they circulate water rich in food and oxygen), the female tions in DNA at any given time, depending on the cell’s stage reproductive tract of vertebrates (where cilia transport the of development, its environment, and its function in a multi- egg cell to the uterus), and the respiratory tracts of most cellular body. In eukaryotic cells, DNA is housed within the nu- land vertebrates (where cilia convey mucus that carries de- cleus. The nucleus is a large organelle with three major parts: bris and microorganisms out of the air passages; see Fig. 4-8b). the ­nuclear envelope, chromatin, and the nucleolus (FIG. 4-9). Flagella propel the sperm cells of nearly all animals (see Fig. 4-8c). Each cilium or flagellum arises from a basal body The Nuclear Envelope Allows Selective just beneath the plasma membrane. Basal bodies are pro- Exchange of Materials duced by centrioles, and, like centrioles, they differ The nucleus is isolated from the rest of the cell by a double mem- from the outer portion of flagella and microtubules in brane, the nuclear envelope, which is perforated by protein- having fused triplets and no central pair of microtubules lined nuclear pores. Water, ions, and small molecules can pass (see Fig. 4-8a). A single pair of centrioles is found in ani- freely through the pores, but the passage of large molecules— mal cells (see Fig. 4-4), and these play a role in organizing particularly proteins, parts of ribosomes, and RNA—is regulated cytoskeletal proteins during cell division (described in by gatekeeper proteins called the nuclear pore complex (see Chapter 9). Fig. 4-9) that line each nuclear pore. Ribosomes stud the outer M04_AUDE3001_11_SE_C04_pp052-074.indd 63 23/10/15 2:53 PM  64 UNIT 1 The Life of the Cell nuclear envelope nuclear nucleolus pores ribosomes nucleus chromatin nuclear pores with nuclear pore complex (a) The nucleus (b) Nucleus of a yeast cell Figure 4-9 The nucleus (a) The nucleus is bounded by a double outer membrane perforated by pores. (b) SEM of the nucleus of a yeast cell. nuclear membrane, which is continuous with membranes of occurs on ribosomes (see Fig. 4-11). (Protein synthesis is de- the rough endoplasmic reticulum, described later. scribed in Chapter 13.) Chromatin Consists of Strands of DNA The Nucleolus Is the Site of Ribosome Assembly Associated with Proteins Eukaryotic nuclei contain at least one nucleolus (plural, Early observers of the nucleus noted that it was darkly colored ­nucleoli; meaning “little nuclei”) (see Fig. 4-9). The nucleo- by the stains used in light microscopy and named the nu- lus is the site of ribosome synthesis. It consists of ­ribosomal clear material chromatin (meaning “colored substance”). RNA (rRNA), parts of chromosomes that carry genes coding for Biologists have since learned that chromatin consists of rRNA, proteins, and ribosomes in various stages of ­synthesis. ­chromosomes (literally, “colored bodies”) made of DNA Ribosomes are small particles composed of ribosomal molecules and their associated proteins. When a cell is not RNA combined with proteins. A ribosome serves as a kind dividing, the chromosomes are extended into extremely long strands that are so thin that they cannot be distinguished from one another with a light microscope. During cell divi- chromatin sion, the individual chromosomes become condensed and are easily visible with a light microscope (FIG. 4-10). The genes of DNA, consisting of specific sequences of nu- cleotides, provide a molecular blueprint for the synthesis of proteins and ribosomes. Some proteins form structural com- ponents of the cell, others regulate the movement of materi- als through cell membranes, and still others are enzymes that promote chemical reactions within the cell. chromosome Proteins are synthesized in the cytoplasm, but DNA is confined to the nucleus. This means that copies of the genetic code for proteins must be ferried from the nucleus into the cy- Figure 4-10 Chromosomes Chromosomes, seen in a light toplasm. To accomplish this, the genetic information is cop- micrograph of a dividing cell (center) in an onion root tip. Chromatin ied from DNA in the nucleus into molecules of messenger RNA is visible in adjacent cells. (mRNA). The mRNA then moves through the nuclear pores Think Critically Why do the chromosomes in chromatin into the cytosol. In the cytosol, the sequence of nucleotides condense in dividing cells? in mRNA is used to direct protein synthesis, a process that M04_AUDE3001_11_SE_C04_pp052-074.indd 64 23/10/15 2:53 PM  Chapter 4 Cell Structure and Function 65 Earth Would You Like Fries with Your Cultured Cow Cells? WATCH What do you get when you combine Meat consumption changes over time 20,000 paper-thin yellowish-pink strips of cells, some lab- grown fat cells, beet coloring, egg powder, bread crumbs, 260 and a dash of salt? These unlikely ingredients make up the 240 world’s first lab-grown hamburger (FIG. E4-3). To create it, cow muscle stem cells were allowed to multiply in a nutrient 220 broth. The cells were then seeded into strips of gel and stimulated repeatedly by pulses of electricity. This caused 200 their actin-based filaments to contract and the cells to “bulk pounds per person per year up,” much as human muscle cells do when exercised. The 180 resulting artificial burger made up of 20,000 cells cost roughly $425,000 to produce, and its flavor was found to be 160 somewhat lacking by fast-food aficionados. So what was the 140 point? Demand for meat is growing, fuelled partly by an 120 expanding population, but also by increasing incomes and appetite for meat (FIG. E4-4). This is particularly true in 100 China, whose meat consumption between 1971 and 2011 increased at 10 times the rate of its population growth 80 (from 841 million to 1.3 billion people. The Food and Agri- 60 culture Organization of the UN estimates that world meat production in 2050 will be 500 million tons (compared to 40 about 350 million tons in in 2016). To accommodate our increasing demand for meat, we 20 are stripping Earth of its natural ecosystems and altering its climate. Grazing and growing food for livestock already 0 1971 1981 1991 2001 2011 require about 30% of Earth’s total land (compared to about 6% used for growing crops directly for human consumption), year and meat production accounts for roughly 18% of human- caused greenhouse gas emissions. Cattle have by far the greatest environmental impact among meat-producing India China World UK USA livestock; raising beef cattle requires about three times as much land per pound of protein as does raising chicken or Figure E4-4 Changes in meat consumption in selected pork. Increasing beef production occurs primarily at the ex- ­countries pense of rain forest, which is cleared to provide low-quality Source: FAOSTAT (Food and Agricultural Organization of the United land for cattle grazing. Nations), Food Supply As livestock compete for Earth’s limited resources, beef is likely to become an expensive luxury in the relatively near future. Clearly, our present course is unsustainable. Scien- tists growing “test-tube beef” argue that if their techniques could be refined and scaled up, meat would require almost no killing of animals and use 99% less land. It would also greatly reduce greenhouse gas emissions and energy and water use. Think Critically Using Fig. E4-4, plot the changes in each country over the 40-year period shown and use a ruler to create a trend line to predict the meat consumption per person in each country in the years 2020, 2030, 2040, and 2050 if the current trends continue. Would the ranking of the countries change over this period? Now look up the current population of each of these countries and determine which is the largest total meat consumer. Was this true in 1980? Figure E4-3 Will hamburger of the future be grown in a lab? M04_AUDE3001_11_SE_C04_pp052-074.indd 65 23/10/15 2:53 PM  66 UNIT 1 The Life of the Cell their contents outside the cell, a process called exocytosis (Gk. exo, outside). Conversely, the plasma membrane may extend and surround material just outside the cell and then fuse and pinch off to form a vesicle inside the cell, a process called endocytosis ribosome (Gk. endo, inside). As they move about the cells, vesicles not only transport their cargo but also transport their membranes, which become integrated into the membranes that they fuse with. mRNA The vesicles are transported within the cell by motor polyribosome proteins running along tracks of microtubules. How do the vesicles know where to go? Proteins embedded in vesicle mem- branes contain specific sequences of amino acids that serve as “mailing labels,” providing the address for delivery of the vesi- growing cle and its payload. Membrane proteins and proteins exported protein from the cell are synthesized in the rough endoplasmic reticu- lum, described in the next section. amino acid The Endoplasmic Reticulum Forms Membrane- Figure 4-11 A polyribosome Ribosomes strung along a Enclosed Channels Within the Cytoplasm messenger RNA molecule form a polyribosome. In the TEM (right), The endoplasmic reticulum (ER) (endoplasmic, “inside individual ribosomes are synthesizing multiple copies of a protein, the cytoplasm,” and reticulum, “network”) is a labyrinth of visible as strands projecting from some of the ribosomes. narrow channels that form interconnected sacs and tubules throughout the cytosol. The ER typically makes up at least 50% of the total cellular membrane (FIG. 4-12). This orga- of workbench for the synthesis of proteins within the cell nelle plays a major role in synthesizing, modifying, and cytoplasm. Just as a workbench can be used to construct transporting biological molecules throughout the cell. Some many different objects, a ribosome can be used to synthesize of these molecules are incorporated into the ER membranes; a multitude of different proteins (depending on the mRNA others are processed within the ER channels and tubules. The to which it attaches). In electron micrographs of cells, ribo- ER has both rough and smooth membranes, which are con- somes appear as dark granules; they may appear singly, may tinuous with one another. stud the membranes of the nuclear envelope and rough endoplasmic reticulum (see Fig. 4-4), or may be present as Rough Endoplasmic Reticulum Rough ER emerges from ­polyribosomes (GK. poly, many) strung along strands of mRNA the ribosome-covered outer nuclear membrane (see Fig. 4-4). within the cytoplasm (FIG. 4-11). Ribosomes studding the outer surface make it appear rough under the electron microscope. These ribosomes are the Eukaryotic Cytoplasm Contains Membranes most important sites of protein synthesis in the cell. As they are synthesized, some proteins on ER ribosomes are inserted That Compartmentalize the Cell into the ER membrane. Some remain there, whereas oth- All eukaryotic cells contain internal membranes that cre- ers become part of vesicle membranes budded from the ER. ate loosely connected compartments within the cytoplasm. ­Proteins destined to be secreted from the cell or used in lyso­ These membranes, collectively called the endomembrane somes are inserted into the interior of the ER, where they system, segregate molecules from the surrounding cytosol are chemically modified and folded into their proper three- and ensure that biochemical processes occur in an orderly dimensional structures (see Chapter 3). Eventually, the pro- fashion. The endomembrane system encloses regions within teins accumulate in pockets of ER membrane that pinch off which an enormous variety of molecules are synthesized, as vesicles and travel to the Golgi apparatus. Proteins pro- broken down, and transported for use inside the cell or ex- duced by the rough ER for export differ with cell type; they port outside the cell. This system of intracellular membranes include digestive system enzymes, infection-fighting anti- includes the nuclear envelope (described earlier), vesicles, the bodies, and proteins that form the extracellular matrix. Pro- endoplasmic reticulum, the Golgi apparatus, and lysosomes. teins that remain in the cell include the digestive enzymes within lysosomes (described later) and plasma membrane Vesicles Bud from the Endomembrane proteins. System and the Plasma Membrane Enzymes produced for the synthesis of membrane phos- Vesicles are temporary sacs that bud from parts of the endo- pholipids are located on the outer surfaces of ER membranes. membrane system and from the plasma membrane to ferry Phospholipids become incorporated into the ER membrane biological molecules throughout the cell. The fluid property of as they are formed, along with membrane proteins synthe- membranes permits vesicles to fuse with and release their con- sized in the rough ER. Thus, the ER produces new membrane tents into different endomembrane compartments for process- that, through vesicle fusion, becomes distributed throughout ing. Vesicles may also fuse with the plasma membrane, exporting the endomembrane system. M04_AUDE3001_11_SE_C04_pp052-074.indd 66 23/10/15 2:53 PM  Chapter 4 Cell Structure and Function 67 ribosomes smooth ER rough ER rough ER Figure 4-12 Endoplasmic reticu- lum (a) Ribosomes (black dots) stud the outside of the rough ER membrane. Rough ER is continuous with the outer nuclear smooth ER envelope. Smooth ER is less flattened and more cylindrical than rough ER and may vesicles be continuous with rough ER. (b) TEMs of (a) Endoplasmic reticulum may be rough or smooth (b) Smooth and rough ER rough and smooth ER with vesicles. Smooth Endoplasmic Reticulum Smooth ER, which lacks The Golgi apparatus performs the following functions: ribosomes, is also involved in the synthesis of cell membrane The Golgi modifies some molecules; an important phospholipids. It is scarce in most cell types, but abundant role of the Golgi is to add carbohydrates to proteins and specialized in others. For example, smooth ER packs the to make glycoproteins. Some of these carbohydrates cells of vertebrate reproductive organs that synthesize steroid sex hormones. Membranes of smooth ER of liver cells have a variety of enzymes embedded within them. Some partici- pate in converting stored glycogen into glucose to provide energy. Others promote the synthesis of the lipid portion of lipoproteins. Finally, smooth ER enzymes break down meta- bolic wastes such as ammonia, drugs such as alcohol, and poi- sons such as certain pesticides. In muscle cells, smooth ER is specialized to store calcium ions, which play a central role in muscle contraction. Protein-carrying vesicles from the ER The Golgi Apparatus merge with the Golgi Modifies, Sorts, and Packages apparatus. Important Molecules Named for the Italian physician and cell biologist Camillo Golgi, who discovered it in 1898, the Golgi apparatus (or simply Golgi) is a specialized set of membranes ­resembling a stack of flattened and interconnected sacs (FIG. 4-13). The compartments of the Golgi act like the fin- ishing rooms of a factory, where final touches are added to products to be packaged and exported. Vesicles from the rough ER fuse with the receiving side of the Golgi appa- ratus, adding their membranes to the Golgi and emptying their contents into the Golgi sacs. Within the Golgi com- Golgi Vesicles carrying apparatus modified protein leave partments, some of the proteins synthesized in the rough the Golgi apparatus. ER are modified further; many are tagged with molecules that specify their destinations in the cell. Finally, vesicles Figure 4-13 The Golgi apparatus The black arrow shows bud off from the “shipping” face of the Golgi, carrying the direction of movement of materials through the Golgi as they away finished products for use in the cell or export out of are modified and sorted. Vesicles bud from the face of the Golgi the cell. opposite the ER. M04_AUDE3001_11_SE_C04_pp052-074.indd 67 23/10/15 2:53 PM  68 UNIT 1 The Life of the Cell act as “mailing labels” that specify the proteins’ facture and export of antibodies (FIG. 4-14). Antibodies, ­destination. produced by white blood cells, are glycoproteins that bind The Golgi separates various proteins received from the ER to foreign invaders (such as disease-causing bacteria) and according to their destinations. For example, the Golgi help destroy them. Antibody proteins are synthesized on apparatus separates the digestive enzymes that are bound ribosomes of the rough ER and released into the ER chan- for lysosomes from the protein hormones that the cell will nels 1 , where they are packaged into vesicles formed secrete. from ER membrane. These vesicles travel to the Golgi 2 , The Golgi packages the finished molecules into vesicles where their membranes fuse with the Golgi membranes that are then transported to other parts of the cell or to and release the antibodies inside. Within the Golgi, car- the plasma membrane for export. bohydrates are attached to the antibodies (transforming them into glycoproteins) 3 , which are then repackaged into vesicles formed from Golgi membrane 4. The vesicle The Endomembrane System Synthesizes, Modifies, containing the completed antibodies travels to the plasma and Transports Proteins to Be Secreted