3 36 ✦ CHAPTER THREE ◗ The Role of Cells The cell (sel) is the basic unit of all life. It is the simplest structure that shows all the characteristics of life, including organization, metabolism, responsiveness, homeostasis, growth, and reproduction. In fact, it is possible for a single cell to live independently of other cells. Examples of some free-living cells are microscopic organisms such as protozoa and bacteria, some of which produce disease. In a multicellular organism, cells make up all tissues. All the activities of the human body, which is composed of trillions of cells, result from the activities of individual cells. Cells produce all the materials manufactured within the body. The study of cells is cytology (si-TOL-o-je). Cilia Cilia A B Cilia C Figure 3-1 Cilia photographed under three different microscopes. (A) Cilia (hairlike projections) in cells lining the trachea under the highest magnification of a compound light microscope (1000⫻) (B) Cilia in the bronchial lining viewed with a transmission electron microscope (TEM). Internal components are visible at this much higher magnification. (C) Cilia on cells lining an oviduct as seen with a scanning electron microscope (SEM) (7000⫻). A three dimensional view can be seen. (A, Reprinted with permission from Cormack DH. Essential histology. 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2001. B, Reprinted with permission from Quinton P, Martinez R, eds. Fluid and electrolyte transport in exocrine glands in cystic fibrosis. San Francisco: San Francisco Press, 1982. C, Reprinted with permission from Hafez ESE, ed. Scanning electron microscopic atlas of mammalian reproduction. Tokyo: Igaku Shoin, 1975.) ZOOMING IN ✦ Which microscope shows the most internal structure of the cilia? Which shows the cilia in three dimensions? CELLS AND THEIR FUNCTIONS ✦ 37 ◗ Microscopes Plasma Membrane The outer limit of the cell is the plasma membrane, forThe outlines of cells were first seen in dried plant tissue merly called the cell membrane (Fig. 3-3). The plasma almost 350 years ago. Study of their internal structure, membrane not only encloses the cell contents but also however, depended on improvements in the design of participates in many cellular activities, such as growth, the microscope, a magnifying instrument needed to exreproduction, and interactions between cells, and is espeamine structures not visible with the naked eye. The sincially important in regulating what can enter and leave th gle-lens microscope used in the late 17 century was the cell. The main substance of this membrane is a doulater replaced by the compound light microscope most ble layer of lipid molecules, described as a bilayer. Becommonly used in laboratories today. This instrument, cause these lipids contain the element phosphorus, they which can magnify an object up to 1000 times, has two are called phospholipids. Some molecules of cholesterol, lenses and uses visible light for illumination. A much another type of lipid, are located between the phosphomore powerful microscope, the transmission electron lipids. Cholesterol strengthens the membrane. microscope (TEM), uses an electron beam in place of A variety of different proteins float within the lipid bivisible light and can magnify an image up to 1 million layer. Some of these proteins extend all the way through times. Another type of microscope, the scanning electhe membrane, and some are located near the inner or tron microscope (SEM), does not magnify as much outer surfaces of the membrane. The importance of these (100,000⫻) and shows only surface features, but gives a proteins will be revealed in later chapters, but they are three-dimensional view of an object. Figure 3-1 shows listed here along with their functions (Table 3-2): some cell structures viewed with each of these types of ◗ Channels—pores in the membrane that allow specific microscopes. The structures are cilia, short, hairlike prosubstances to enter or leave. Certain ions travel through jections from the cell that move nearby fluids. The channels in the membrane. metric unit used for microscopic measurements is the micrometer (MI-kro-me-ter), formerly called a micron. This unit is Smooth endoplasmic Plasma 1/1000 of a millimeter and is symbolreticulum (ER) membrane ized with the Greek letter mu (␮), as Centriole Nucleus ␮m. Before a scientist can examine cells Cytosol and tissues under a microscope, he or Nucleolus she must usually color them with special dyes called stains to aid in viewLysosome ing. These stains produce the variety of colors seen in pictures of cells and tisRough endoplasmic sues taken under a microscope. reticulum (ER) Checkpoint 3-1 The cell is the basic unit of life. What characteristics of life does it show? Vesicle Ribosomes Checkpoint 3-2 Name three types of microscopes. ◗ Cell Structure Just as people may look different but still have certain features in common—two eyes, a nose, and a mouth, for example—all cells share certain characteristics. Refer to Figure 3-2 as we describe some of the parts that are common to most animal cells. Table 31 summarizes information about the main cell parts. Nuclear membrane Peroxisome Golgi apparatus Mitochondrion Microvilli Cilia Figure 3-2 A generalized animal cell, sectional view. ZOOMING IN ✦ What is attached to the ER to make it look rough? What is the liquid part of the cytoplasm called? 3 38 ✦ CHAPTER THREE Table 3•1 Cell Parts NAME DESCRIPTION FUNCTION Plasma membrane Outer layer of the cell; composed mainly of lipids and proteins Microvilli Nucleus Mitochondria Golgi apparatus Short extensions of the cell membrane Large, dark-staining organelle near the center of the cell, composed of DNA and proteins Small body in the nucleus; composed of RNA, DNA, and protein Colloidal suspension that fills the cell from the nuclear membrane to the plasma membrane The fluid portion of the cytoplasm Network of membranes within the cytoplasm. Rough ER has ribosomes attached to it; smooth ER does not. Small bodies free in the cytoplasm or attached to the ER; composed of RNA and protein Large organelles with folded membranes inside Layers of membranes Encloses the cell contents; regulates what enters and leaves the cell; participates in many activities, such as growth, reproduction, and interactions between cells Absorb materials into the cell Contains the chromosomes, the hereditary units that direct all cellular activities Makes ribosomes Saclike bodies Small, membrane-enclosed bodies Nucleolus Cytoplasm Cytosol Endoplasmic reticulum (ER) Ribosomes Lysosomes Peroxisomes Vesicles Centrioles Surface projections Cilia Flagellum ◗ ◗ ◗ ◗ ◗ Small sacs of digestive enzymes Membrane-enclosed organelles containing enzymes Small membrane-bound bubbles in the cytoplasm Rod-shaped bodies (usually two) near the nucleus Structures that extend from the cell Short, hairlike projections from the cell Long, whiplike extension from the cell Transporters—shuttle substances from one side of the membrane to the other. Glucose, for example, is carried into cells using transporters. Receptors—points of attachment for materials coming to the cell in the blood or tissue fluid. Some hormones, for example, must attach to receptors on the cell surface before they can act upon the cell, as described in Chapter 12 on the endocrine system. Enzymes—participate in reactions occurring at the plasma membrane. Linkers—give structure to the membrane and help attach cells to other cells. Cell identity markers—proteins unique to an individual’s cells. These are important in the immune system and are also a factor in transplantation of tissue from one person to another. Carbohydrates are present in small amounts in the plasma membrane, combined either with proteins (glycoproteins) or with lipids (glycolipids). These carbohydrates help cells to recognize each other and to stick together. Site of many cellular activities, consists of cytosol and organelles Surrounds the organelles Rough ER sorts proteins and forms them into more complex compounds; smooth ER is involved with lipid synthesis Manufacture proteins Convert energy from nutrients into ATP Makes compounds containing proteins; sorts and prepares these compounds for transport to other parts of the cell or out of the cell Store materials, transport materials through the plasma membrane, or destroy waste material Digest substances within the cell Break down harmful substances Store materials and move materials into or out of the cell in bulk Help separate the chromosomes during cell division Move the cell or the fluids around the cell Move the fluids around the cell Moves the cell In some cells, the plasma membrane is folded out into multiple small projections called microvilli (mi-kroVIL-li). Microvilli increase the surface area of the membrane, allowing for greater absorption of materials from the cell’s environment, just as a sponge absorbs water. Microvilli are found on cells that line the small intestine, where they promote absorption of digested foods into the circulation. They are also found on kidney cells, where they reabsorb materials that have been filtered out of the blood. Checkpoint 3-3 The outer limit of the cell is a complex membrane. What is the main substance of this membrane and what are three types of materials found within the membrane? The Nucleus Just as the body has different organs to carry out special functions, the cell contains specialized structures that perform different tasks. These structures are called or- CELLS AND THEIR FUNCTIONS ✦ 39 The Cytoplasm Carbohydrate The remaining organelles are part of the cytoplasm (SI-to-plazm), the material that fills the cell from the nuclear membrane to the plasma membrane. The liquid part of the cytoplasm is the cytosol, a suspension of nutrients, minerals, enzymes, and other specialized materials in water. The main organelles are described here (see Table 3-1). The endoplasmic reticulum (endo-PLAS-mik re-TIK-u-lum) is a network of membranes located between the nuclear membrane and the plasma membrane. Its name literally means Lipid bilayer “network” (reticulum) “within the cyProteins Cholesterol toplasm” (endoplasmic), but for ease, Cytoplasm Protein it is almost always called simply the Phospholipids channel ER. In some areas, the ER appears to have an even surface, and is described as smooth ER. This type of ER is involved with the synthesis of lipids. In other areas, the ER has a gritty, uneven surface, causing it to be described as rough ER. The texture of rough ER comes from small bodies, called ribosomes (RI-bo-somz), attached to its surface. Ribosomes are necessary for the manufacture of proFigure 3-3 The plasma membrane. This drawing shows the current concept of its structure. teins, as described later. They may be ZOOMING IN ✦ How many layers make up the main substance of the plasma membrane? attached to the ER or be free in the cytoplasm. The mitochondria (mi-to-KON-dre-ah) are large organelles, which means “little organs.” The largest of the ganelles that are round or bean-shaped with folded memorganelles is the nucleus (NU-kle-us). branes on the inside. Within the mitochondria, the enThe nucleus is often called the control center of the ergy from nutrients is converted to energy for the cell in cell because it contains the chromosomes, the threadlike the form of ATP. Mitochondria are the “power plants” of units of heredity that are passed on from parents to their the cell. Active cells, such as muscle cells or sperm cells, offspring. It is information contained in the chromoneed lots of energy and thus have large numbers of mitosomes (KRO-mo-somes) that governs all cellular activichondria. ties, as described later in this chapter. Most of the time, Another organelle in a typical cell is the Golgi (GOLthe chromosomes are loosely distributed throughout the je) apparatus (also called Golgi complex), a stack of nucleus, giving that organelle a uniform, dark appearance membranous sacs involved in sorting and modifying prowhen stained and examined under a microscope (see Fig. teins and then packaging them for export from the cell. 3-2). When the cell is dividing, however, the chromoSeveral types of organelles appear as small sacs in the somes tighten into their visible threadlike forms. cytoplasm. These include lysosomes (LI-so-somz), which Within the nucleus is a smaller globule called the nucontain digestive enzymes. Lysosomes remove waste and cleolus (nu-KLE-o-lus), which means “little nucleus.” foreign materials from the cell. They are also involved in The job of the nucleolus is to assemble ribosomes, small destroying old and damaged cells as needed for repair and bodies outside the nucleus that are involved in the manremodeling of tissue. Peroxisomes (per-OK-sih-somz) ufacture of proteins. have enzymes that destroy harmful substances produced in metabolism (see Box 3-1, Lysosomes and Peroxisomes: Checkpoint 3-4 What are cell organelles? Cellular Recycling). Vesicles (VES-ih-klz) are small, membrane-bound bubbles used for storage. They can be Checkpoint 3-5 Why is the nucleus called the control center of used to move materials into or out of the cell, as described the cell? later. Extracellular fluid 3 40 ✦ CHAPTER THREE Table 3•2 Proteins in the Plasma Membrane and Their Functions TYPE OF PROTEIN FUNCTION Channels Pores in the membrane that allow passage of specific substances, such as ions Transporters Shuttle substances, such as glucose, across the membrane Receptors Allow for attachment of substances, such as hormones, to the membrane Enzymes Participate in reactions at the surface of the membrane Linkers Give structure to the membrane and attach cells to other cells Cell identity markers Proteins unique to a person’s cells; important in the immune system and in transplantation of tissue from one person to another Box 3-1 ILLUSTRATION Clinical Perspectives Lysosomes and Peroxisomes: Cellular Recycling T wo organelles that play a vital role in cellular disposal and recycling are lysosomes and peroxisomes. Lysosomes contain enzymes that break down carbohydrates, lipids, proteins, and nucleic acids. These powerful enzymes must be kept within the lysosome because they would digest the cell if they escaped. In a process called autophagy (aw-TOF-ah-je), the cell uses lysosomes to safely recycle cellular structures, fusing with and digesting worn out organelles. The digested components then return to the cytoplasm for reuse. Lysosomes also break down foreign material, as when cells known as phagocytes (FAG-o-sites) engulf bacteria and then use lysosomes to destroy them. The cell may also use lysosomes to digest itself during autolysis (aw-TOL-ih-sis), a normal part of development. Cells that are no longer needed “self-destruct” by releasing lysosomal enzymes into their own cytoplasm. Peroxisomes are small membranous sacs that resemble lysosomes but contain different kinds of enzymes. They break down toxic substances that may enter the cell, such as drugs and alcohol, but their most important function is to break down free radicals. These substances are byproducts of normal metabolic reactions but can kill the cell if not neutralized by peroxisomes. Disease may result if either lysosomes or peroxisomes are unable to function. In Tay-Sachs disease, nerve cells’ lysosomes lack an enzyme that breaks down certain kinds of lipids. These lipids build up inside the cells, causing malfunction that leads to brain injury, blindness, and death. Disease may also result if lysosomes or peroxisomes function when they should not. Some investigators believe this is the case in autoimmune diseases, in which the body develops an immune response to its own cells. Phagocytes engulf the cells and lysosomes destroy them. In addition, body cells themselves may self-destruct through autolysis. The joint disease rheumatoid arthritis is one such example. CELLS AND THEIR FUNCTIONS ✦ 41 Centrioles (SEN-tre-olz) are rod-shaped bodies near the nucleus that function in cell division. They help to organize the cell and divide the cell contents during this process. Surface Organelles Some cells have structures projecting from their surface that are used for motion. Cilia (SIL-e-ah) are small, hairlike projections that wave, creating movement of the fluids around the cell. For example, cells that line the passageways of the respiratory tract have cilia that move impurities out of the system. Ciliated cells in the female reproductive tract move the egg cell along the oviduct toward the uterus. A long, whiplike extension from the cell is a flagellum (flah-JEL-lum). The only type of cell in the human body that has a flagellum is the sperm cell of the male. Each human sperm cell has a flagellum that is used to propel the sperm cell toward the egg in the female reproductive tract. Cellular Diversity Although all body cells have some fundamental similarities, individual cells may vary widely in size, shape, and composition according to the function of each. The average cell size is 10 to 15 ␮m, but cells may range in size from the 7 ␮m of a red blood cell to the 200 ␮m or more in the length of a muscle cell. Cell shape is related to cell function (Fig. 3-4). A neuron (nerve cell) has long fibers that transmit electrical energy from place to place in the nervous system. Cells in surface layers have a modified shape that covers and protects the tissue beneath. Red blood cells are small and round, which lets them slide through tiny blood vessels. They also have a thin outer membrane to allow for pas- sage of gases into and out of the cell. As red blood cells mature, they lose the nucleus and most of the other organelles, making the greatest possible amount of space available to carry oxygen. Aside from cilia and flagella, most human cells have all the organelles described above. These may vary in number, however. For example, cells producing lipids have lots of smooth ER. Cells that secrete proteins have lots of ribosomes and a prominent Golgi apparatus. All active cells have lots of mitochondria to manufacture the ATP needed for energy. Checkpoint 3-6 What are the two types of organelles used for movement, and what do they look like? ◗ Protein Synthesis Because protein molecules play an indispensable part in the structure and function of the body, we need to identify the cellular substances that direct the production of proteins. As noted, the hereditary units that govern the cell are the chromosomes in the nucleus. Each chromosome in turn is divided into multiple subunits, called genes (Fig. 3-5). It is the genes that carry the messages for the development of particular inherited characteristics, such as brown eyes, curly hair, or blood type, and they do so by directing the manufacture of proteins in the cell. Nucleic Acids: DNA and RNA The genes are distinct segments of the complex organic chemical that makes up the chromosomes, a substance called deoxyribonucleic (de-ok-se-RI-bo-nu-kle-ik) acid, or DNA. DNA is composed of subunits called nucleotides (NU-kle-o-tides) (see Fig. 3-5). A related compound, ribonucleic (RI-bo-nu-kle-ik) acid, or RNA, which also B A D C E Figure 3-4 Cellular diversity. Cells vary in structure according to their functions. (A) A neuron has long extensions that pick up and transmit electrical impulses. (B) Epithelial cells cover and protect underlying tissue. (C) Muscle cells have fibers that produce contraction. (D) Red blood cells lose most organelles, which maximizes their oxygen-carrying capacity, and have a small, round shape that lets them slide through blood vessels. (E) A sperm cell is small and light and swims with a flagellum. ZOOMING IN ✦ Which of the cells shown would best cover a large surface area? 3 42 ✦ CHAPTER THREE Nucleotide Gene DNA segment Chromosome Figure 3-5 Subdivisions of a chromosome. A gene is a distinct region of a chromosome. The entire chromosome is made of DNA. Nucleotides are the building blocks of DNA. participates in protein synthesis but is not part of the chromosomes, is also composed of nucleotides. There are four different nucleotides in DNA and four in RNA, but only three of these are common to both. Both DNA and RNA have the nucleotides adenine (A), guanine (G), and cytosine (C), but DNA has thymine (T), whereas RNA has uracil (U). Table 3-3 compares the structure and function of DNA and RNA. Moving one step deeper into the structure of the nucleic acids, each nucleotide is composed of four units: ◗ ◗ ◗ A sugar, which in RNA is ribose and in DNA is a ribose that is missing one oxygen atom (that is, deoxyribose) A phosphorus-containing portion, or phosphate A nitrogen-containing portion known as a nitrogen base The sugar and phosphate alternate to form a long chain to which the nitrogen bases are attached. It is variTable 3•3 Location Composition Structure Function ation in the nitrogen bases that accounts for the differences in the five different nucleotides. DNA Most of the DNA in the cell is organized into chromosomes within the nucleus (a small amount of DNA is in the mitochondria located in the cytoplasm). Looking at Figure 3-6, which shows a section of a chromosome, you can see that the DNA exists as a double strand. Visualizing the complete molecule as a ladder, the sugar and phosphate units of the nucleotides make up the “side rails” of the ladder, and the nitrogen bases project from the side rails to make up the “steps” of the ladder. The two DNA strands are paired very specifically according to the identity of the nitrogen bases in the nucleotides. The adenine (A) nucleotide always pairs with the thymine (T) nucleotide; the guanine (G) nucleotide always pairs with the cytosine (C) nucleotide. The two strands of DNA are held together by weak bonds (hydrogen binds; see Box 2- Comparison of DNA and RNA DNA RNA Almost entirely in the nucleus Nucleotides: adenine (A), guanine (G), cytosine (C), thymine (T) Sugar: deoxyribose Double-stranded helix formed by nucleotide pairing A-T; G-C Makes up the chromosomes, hereditary units that control all cell activities; divided into genes that carry the nucleotide codes for the manufacture of proteins Almost entirely in the cytoplasm Nucleotides: adenine (A), guanine (G), cytosine (C), uracil (U) Sugar: ribose Single strand Manufacture proteins according to the nucleotide codes carried in the DNA; three types: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA) CELLS AND THEIR FUNCTIONS ✦ 43 Nitrogen bases: Adenine (A) Thymine (T) Guanine (G) Cytosine (C) Symbols show how the nitrogen bases of the nucleotides pair in DNA. Adenine bonds with thymine; guanine bonds with cytosine. pearance and function if they all have the same amount and same kind of DNA. The answer to this question is that only portions of the DNA in a given cell are active at any one time. In some cells, regions of the DNA can be switched on and off, under the influence of hormones, for example. However, as cells differentiate during development and become more specialized, regions of the DNA are permanently shut down, leading to the variations in the different cell types. Scientists now realize that the control of DNA action throughout the life of the cell is a very complex matter involving not only the DNA itself but proteins as well. Checkpoint 3-7 What are the building blocks of nucleic acids? Phosphate unit Checkpoint 3-8 What category of compounds does DNA code for in the cell? Sugar unit The Role of RNA A blueprint is only a guide. The in- Nitrogen base B Nucleotide A DNA Figure 3-6 Structure of DNA. (A) This schematic representation of a chromosome segment shows the paired nucleic acid strands twisted into a double helix. (B) Each structural unit, or nucleotide, consists of a phosphate unit and a sugar unit attached to a nitrogen base. The sugar unit in DNA is deoxyribose. There are four different nucleotides in DNA. Their arrangement “spells out” the genetic instructions that control all activities of the cell. ZOOMING IN ✦ Two of the DNA nucleotides (A and G) are larger in size than the other two (T and C). How do the nucleotides pair up with regard to size? 1). The doubled strands then coil into a spiral, giving DNA the descriptive name double helix. The message of the DNA that makes up the individual genes is actually contained in the varying pattern of the four nucleotides along the strand. The nucleotides are like four letters in an alphabet that can be combined in different ways to make a variety of words. The words represent the amino acids used to make proteins, and a long string of words makes up a gene. Each gene thus codes for the building of amino acids into a specific cellular protein. Remember that all enzymes are proteins, and enzymes are essential for all cellular reactions. DNA is thus the master blueprint for the cell. In light of observations on cellular diversity, you may wonder how different cells in the body can vary in ap- formation it contains must be interpreted by appropriate actions, and RNA is the substance needed for these steps. RNA is much like DNA except that it exists as a single strand of nucleotides and has the nucleotide uracil (U) instead of thymine (T). Thus, when RNA pairs up with another molecule of nucleic acid to manufacture proteins, as explained below, adenine (A) bonds with uracil (U) in the RNA instead of thymine (T). A detailed account of protein synthesis is beyond the scope of this book, but a highly simplified description and illustrations of the process are presented. The process begins with the transfer of information from DNA to RNA in the nucleus, a process known as transcription (Fig. 37). Before transcription begins, the DNA breaks its weak bonds and uncoils into single strands. Then a matching strand of RNA forms along one of the DNA strands by the process of nucleotide pairing. (For example, if the DNA strand reads CGAT, the corresponding mRNA will read GCUA. Recall that RNA uses the nucleotide U instead of A.) When complete, this messenger RNA (mRNA) leaves the nucleus and travels to a ribosome in the cytoplasm (Fig. 3-8). Recall that ribosomes are the site of protein synthesis in the cell. Ribosomes are composed of a type of RNA called ribosomal RNA (rRNA) and also protein. At the ribosomes, the genetic message now contained within mRNA is decoded to build amino acids into the long chains that form proteins, a process termed translation. This final step requires a third type of RNA, transfer RNA (tRNA), small molecules present in the cytoplasm (see Fig. 3-8). Each transfer RNA carries a specific amino acid that can be added to a protein chain. A nucleotide code on each tRNA determines whether or not its amino acid will be added. After the amino acid chain is formed, it must be coiled and folded into the proper shape for that protein, as noted in Chapter 2. Table 3-4 summarizes information on the 3 44 ✦ CHAPTER THREE Cytoplasm Nucleus DNA mRNA Figure 3-7 Transcription. In the first step of protein synthesis the DNA code is transcribed into messenger RNA (mRNA) by nucleotide base pairing. An enlarged view of the nucleic acids during transcription shows how mRNA forms according to the nucleotide pattern of the DNA. Note that adenine (A, red) in DNA bonds with uracil (U, brown) in RNA. Ribosome Protein chain Amino acid Messenger RNA (mRNA) Cytoplasm Transfer RNA (tRNA) with amino acid Ribosomal RNA (rRNA) Figure 3-8 Translation. In protein synthesis, messenger RNA (mRNA) travels to the ribosomes in the cytoplasm. The information in the mRNA codes for the building of proteins from amino acids. Transfer RNA (tRNA) molecules bring amino acids to the ribosomes to build each protein. CELLS AND THEIR FUNCTIONS ✦ 45 Before mitosis can occur, the genetic information (DNA) in the parTYPES FUNCTION ent cell must be doubled, so that each of the two new daughter cells will reMessenger RNA (mRNA) Is built on a strand of DNA in the nucleus ceive a complete set of chromosomes. and transcribes the nucleotide code; moves For example, a cell that divides by to cytoplasm and attaches to a ribosome Ribosomal RNA (rRNA) With protein makes up the ribosomes, the mitosis in the human body must prosites of protein synthesis in the cytoplasm; duce two cells with 46 chromosomes involved in the process of translating the each, the same number of chromogenetic message into a protein somes that was present in the original Transfer RNA (tRNA) Works with other forms of RNA to translate parent cell. DNA duplicates during the genetic code into protein; each molecule of tRNA carries an amino acid that interphase, the stage in the life of a can be used to build a protein at the ribocell between one mitosis and the some next. During this phase, DNA uncoils from its double-stranded form, and each strand takes on a matching strand of nucleotides according to the different types of RNA. Also see Box 3-2, Proteomics: So pattern of A-T, G-C pairing. There are now two strands, Many Proteins, So Few Genes. each identical to the original double helix. The strands are held together at a region called the centromere (SENCheckpoint 3-9 What three types of RNA are active in protein tro-mere) until they separate during mitosis. A typical synthesis? cell lives in interphase for most of its cycle and spends only a relatively short period in mitosis. For example, a cell reproducing every 20 hours spends only about 1 ◗ Cell Division hour in mitosis and the rest of the time in interphase. Table 3•4 RNA For growth, repair, and reproduction, cells must multiply to increase their numbers. The cells that form the sex cells (egg and sperm) divide by the process of meiosis (mi-Osis), which cuts the chromosome number in half to prepare for union of the egg and sperm in fertilization. If not for this preliminary reduction, the number of chromosomes in the offspring would constantly double. The process of meiosis is discussed in Chapter 23. All other body cells, known as somatic cells, divide by the process of mitosis (mi-TO-sis). In this process, described below, each original parent cell becomes two identical daughter cells. Box 3-2 Checkpoint 3-10 What must happen to the DNA in a cell before mitosis can occur? During what stage in the life of a cell does this occur? Stages of Mitosis Although mitosis is a continuous process, distinct changes can be seen in the dividing cell at four stages (Fig. 3-9). ◗ In prophase (PRO-faze), the doubled strands of DNA return to their tightly wound spiral organization and Hot Topics Proteomics: Proteomics: So So Many Many Proteins, Proteins, So So Few Few Genes Genes T o build the many different proteins that make up the body, cells rely on instructions encoded in genes on chromosomes. Collectively, all the different genes on all the chromosomes make up the genome. Genes contain the instructions for making proteins, while the proteins themselves perform the body’s functions. Scientists are now studying the human proteome—all the proteins that can be expressed in a cell—to help them understand the proteins’ structure and function. Unlike the genome, the proteome changes as the cell’s activities and needs change. In 2003, after a decade of intense scientific activity, investigators mapped the entire human genome and realized that it contained only 35,000 genes, far fewer than initially expected. How could this relatively small number of genes code for several million proteins? They concluded that genes were not the whole story. Gene transcription is only the beginning of protein synthesis. In response to cell conditions, enzymes can snip newly transcribed mRNA into several pieces, each of which a ribosome can use to build a different protein. After each protein is built, enzymes can further modify the amino acid strands to produce several more different proteins. Other molecules help the newly formed proteins to fold into precise shapes and interact with each other, resulting in even more variations. Thus, while a gene may code for a specific protein, modifications after gene transcription can produce many more unique proteins. There is much left to discover about the proteome, but scientists hope that future research will lead to new techniques for detecting and treating disease. 3 46 ✦ CHAPTER THREE Interphase cell Centrioles Nucleus Nucleolus DNA Prophase MITOSIS ◗ ◗ Centrioles ◗ Chromosomes Metaphase Anaphase Telophase Spindle fibers Separating chromosomes Plasma membrane divides the cell Two new cells in interphase become visible under the microscope as dark, threadlike chromosomes. The nucleolus and the nuclear membrane begin to disappear. In the cytoplasm, the two centrioles move toward opposite ends of the cell and a spindle-shaped structure made of thin fibers begins to form between them. In metaphase (MET-ah-faze), the chromosomes line up across the center (equator) of the cell attached to the spindle fibers. In anaphase (AN-ah-faze), the centromere splits and the duplicated chromosomes separate and begin to move toward opposite ends of the cell. As mitosis continues into telophase (TEL-o-faze), a membrane appears around each group of separated chromosomes, forming two new nuclei. Also during telophase, the plasma membrane pinches off to divide the cell. The midsection between the two areas becomes progressively smaller until, finally, the cell splits in two. There are now two new cells, or daughter cells, each with exactly the same kind and amount of DNA as was present in the parent cell. In just a few types of cells, skeletal muscle cells for example, the cell itself does not divide following nuclear division. The result, after multiple mitoses, is a giant single cell with multiple nuclei. This pattern is extremely rare in the human body. During mitosis, all the organelles, except those needed for the division process, temporarily disappear. After the cell splits, these organelles reappear in each daughter cell. Also at this time, the centrioles usually duplicate in preparation for the next cell division. Body cells differ in the rate at which they reproduce. Some, such as nerve cells and muscle cells, stop dividing at some point in development and are not replaced if they die. They remain in interphase. Others, such as blood cells, sperm cells, and skin cells, multiply rapidly to replace cells destroyed by injury, disease, or natural wearand-tear. Cells that multiply slowly may be triggered to divide when tissue is injured, as in repair of a bone fracture. Immature cells that retain the ability to divide and mature when necessary are known as stem cells (see Box 4-1 in Chapter 4). All blood cells, for example, are produced from stem cells in the red bone marrow. Research has been done on stimulating stem cells to divide into various cell types in the laboratory, but these studies have been controversial. Although it may be possible some day to use such cells to replace cells injured by disease, some people consider these studies to be unethical. Checkpoint 3-11 What are the four stages of mitosis? Figure 3-9 The stages of mitosis. When it is not dividing, the cell is in interphase. The cell shown is for illustration only. It is not a human cell, which has 46 chromosomes. (Photomicrographs reprinted with permission from Cormack DH. Essential Histology. 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2001.) ZOOMING IN ✦ If the original cell shown has 46 chromosomes, how many chromosomes will each new daughter cell have? ◗ Movement of Substances Across the Plasma Membrane The plasma membrane serves as a barrier between the cell and its environment. Nevertheless, nutrients, oxygen, and many other substances needed by the cell must be CELLS AND THEIR FUNCTIONS ✦ 47 Movement That Does Not Require Cellular Energy Figure 3-10 Diffusion of a solid in a liquid. The molecules of the solid tend to spread evenly throughout the liquid as they dissolve. taken in and waste products must be eliminated. Clearly, some substances can be exchanged between the cell and its environment through the plasma membrane. For this reason, the plasma membrane is described at a simple level as semipermeable (sem-e-PER-me-ah-bl). It is permeable, or passable, to some molecules but impassable to others. Some particles, proteins for example, are too large to travel through the membrane unaided. The ability of a substance to travel through the membrane is based on several factors. Molecular size is the main factor that determines passage through the membrane, but solubility and electrical charge are also considerations. Water, a tiny molecule, is usually able to penetrate the membrane with ease. Nutrients, however, must be split into small molecules by the process of digestion so that they can travel through the plasma membrane. Sucrose (table sugar), for example, is converted to glucose and fructose, smaller molecules that enter the cell and serve as sources of energy. Various physical processes are involved in exchanges through the plasma membrane. One way of grouping these processes is according to whether they do or do not require cellular energy. The adjective passive describes movement through the plasma membrane that does not require energy output by the cell. Passive mechanisms depend on the internal energy of the moving particles or the application of some outside source of energy. The methods include: ◗ ◗ Diffusion is the constant movement of particles from a region of relatively higher concentration to one of lower concentration. Just as couples on a crowded dance floor spread out into all the available space to avoid hitting other dancers, diffusing substances spread throughout their available space until their concentration everywhere is the same—that is, they reach equilibrium (Fig. 3-10). This movement from higher to lower concentrations uses the internal energy of the particles and does not require cellular energy, just as a sled will move from the top to the bottom of a snowy hill. The particles are said to follow their concentration gradient from higher concentration to lower concentration. When substances diffuse through a membrane, such as the intact plasma membrane, passage is limited to those particles small enough to pass through spaces between molecules in the membrane, as shown with a large-scale example in Figure 3-11. In the body, soluble materials, such as nutrients, electrolytes, gases, and waste materials, are constantly moving into or out of the cells by diffusion. Osmosis (os-MO-sis) is a special type of diffusion. The term applies specifically to the diffusion of water through a semipermeable membrane. The water molecules move, as expected, from an area where there are more of them to an area where there are fewer of them. Basketball Tennis ball Ping-Pong ball Marble BB Figure 3-11 Diffusion through a semipermeable membrane. In this example, large objects (basketballs, tennis balls) cannot pass through the net, whereas the smaller ones (ping-pong balls, marbles, BBs) can. In the human body, large particles in the blood, such as proteins and blood cells, cannot pass through the walls of the capillaries, whereas small particles, such as nutrients, electrolytes, and gases can. ZOOMING IN ✦ If this picture represented diffusion in the body, what would the net be? 3 48 ✦ CHAPTER THREE is the osmotic pressure. In practice, the term osmotic pressure is used to describe the tendency of a solution to draw water into it. This force is Net flow Solute directly related to concentration: the of water molecules higher the concentration of a solution, the greater is its tendency to draw water in. ◗ Filtration is the passage of water containing dissolved materials through a membrane as a result of a mechanical (“pushing”) force on Semipermeable one side (Fig. 3-14). One example of membrane filtration in the body is the movement of materials out of the capillarMembrane A B ies and into the tissues under the force of blood pressure (see Chapter Figure 3-12 A simple demonstration of osmosis. Solute molecules are shown in 15). Another example occurs in the yellow. All of the solvent (blue) is composed of water molecules. (A) Two solutions kidneys as materials are filtered out with different concentrations of solute are separated by a semipermeable membrane. Water can flow through the membrane, but the solute cannot. (B) Water flows into the of the blood in the first step of urine more concentrated solution, raising the level of the liquid in that side. ZOOMING IN formation (see Chapter 22). ✦ What would happen in this system if the solute could pass through the membrane? ◗ Facilitated diffusion is the movement of materials across the plasma That is, the solvent (the water molecules) moves from membrane in the direction of the concentration gradian area of lower solute concentration to an area of ent (from higher to lower concentration) but using higher solute concentration (Fig. 3-12). transporters to move the material at a faster rate (Fig. For a physiologist studying the flow of water across 3-15). Glucose, the sugar that is the main energy membranes, as in exchange of fluids through capillaries source for cells, moves through the plasma membrane in the circulation, it is helpful to know the direction in by means of facilitated diffusion. which water will flow and at what rate it will move. A measure of the force driving osmosis is called the osMovement That Requires Cellular motic pressure. This force can be measured, as illusEnergy trated in Figure 3-13, by applying enough pressure to the surface of a liquid to stop the inward flow of water Movement across the membrane that requires energy by osmosis. The pressure needed to counteract osmosis is describes as active. These methods include: Force ◗ Active transport. The plasma membrane has the ability to move small solute particles into or out of the cell opposite to the direction in which they would normally flow by diffusion. That is, the membrane moves them against the concentration gradient from an area where they are in relatively lower concentration to an area where they are in higher concentration. Because this Force A Large solute particles B Membrane Figure 3-13 Osmotic pressure. Osmotic pressure is the force needed to stop the flow of water by osmosis. Pressure on the surface of the fluid in side B counteracts the osmotic flow of water from side A to side B. ZOOMING IN ✦ What would happen to osmotic pressure if the concentration of solute were increased on side B of this system? Small solute particles Membrane Filtrate Figure 3-14 Filtration. A mechanical force pushes a substance through a membrane, although the membrane limits which particles can pass through based on size. The small particles go through the membrane and appear in the filtered solution (filtrate). CELLS AND THEIR FUNCTIONS ✦ 49 Extracellular fluid Higher solute concentration Solute particle Concentration gradient movement goes against the natural flow of particles, it requires energy, just as getting a sled to the top of a hill requires energy. It also requires proteins in the cell membrane that act as transporters for the particles. Transporter This process of active transport is Plasma one important function of the living membran cell membrane. The nervous system e and muscular system, for example, depend on the active transport of sodium, Cytoplasm A potassium, and calcium ions for proper C function. The kidneys also carry out acB tive transport in regulating the compoLower solute sition of urine. By means of active concentration transport, the cell can take in what it needs from the surrounding fluids and remove materials from the cell. Because the cell membrane can carry on active transport, the membrane is most accurately described, not as simply semiper- Figure 3-15 Facilitated diffusion. Transporters (proteins in the plasma membrane) move solute particles through a membrane from an area of higher concentrameable, but as selectively permeable. It tion to an area of lower concentration. (A) A solute particle enters the transporter. (B) regulates what can enter and leave The transporter changes shape. (C) The transporter releases the solute particle on the based on the needs of the cell. other side of the membrane. ZOOMING IN ✦ How would a change in the number of There are several active methods transporters affect the movement of a solute by facilitated diffusion? for moving large quantities of material into or out of the cell. These methods are grouped to◗ In phagocytosis (fag-o-si-TO-sis), relatively large gether as bulk transport, because of the amounts of maparticles are engulfed by the plasma membrane terial moved, or vesicular transport, because small buband moved into the cell (Fig. 3-16). Certain white bles, or vesicles, are needed for the processes. blood cells carry out phagocytosis to rid the body of foreign material and dead cells. Material taken ◗ Endocytosis (en-do-si-TO-sis) is a term that describes into a cell by phagocytosis is first enclosed in a the bulk movement of materials into the cell. There are vesicle made from the plasma membrane and is two examples: later destroyed by lysosomes. ◗ In pinocytosis (pi-no-si-TO-sis), the cell membrane engulfs droplets of fluid. This is a way for large protein molecules in suspension to travel Particle into the cell. The word pinocytosis means “cell Extracellular drinking.” fluid ne ◗ In exocytosis, the cell moves materials out in vesicles bra m e m (Fig. 3-17). One example of exocytosis is the export of Plasma neurotransmitters from neurons (neurotransmitters are chemicals that control the activity of the nervous system). Cytoplasm The transport methods described above are summarized in Table 3-5. Phagocytic vesicle Figure 3-16 Phagocytosis. The plasma membrane encloses a particle from the extracellular fluid. The membrane then pinches off, forming a vesicle that carries the particle into the cytoplasm. ZOOMING IN ✦ What organelle would likely help to destroy a particle taken in by phagocytosis? Checkpoint 3-12 Substances are constantly moving into and out of cells through the plasma membrane. What types of movement do not require cellular energy and what types of movement do require cellular energy? How Osmosis Affects Cells As stated earlier, water usually moves easily through the cell membrane. Therefore, for a normal fluid balance to be maintained, the fluid outside all cells must have the 3 50 ✦ CHAPTER THREE A solution that is less concentrated than the intracellular fluid is described as hypotonic. Based on the principles of osmosis already explained, a cell placed in a hypotonic solution draws Extracellular fluid water in, swells, and may burst. When a red blood cell draws in water and m e b rane Pl a s m a m bursts in this way, the cell is said to undergo hemolysis (he-MOL-ih-sis). Cytoplasm If a cell is placed in a hypertonic solution, which is more concentrated than Vesicle the cellular fluid, it loses water to the surrounding fluids and shrinks, a process termed crenation (kre-NAFusion with shun) (see Fig. 3-18). plasma membrane Fluid balance is an important facet of homeostasis and must be properly Stored material regulated for health. You can figure out in which direction water will Figure 3-17 Exocytosis. A vesicle fuses with the plasma membrane then ruptures move through the plasma membrane and releases its contents. if you remember the saying “water follows salt,” salt meaning any dissolved material (solute). The total amount and distribution of same concentration of dissolved substances (solutes) as the fluids inside the cells (Fig. 3-18). If not, water will body fluids is discussed in Chapter 21. Table 3-6 summove rapidly into or out of the cell by osmosis. Solutions marizes the effects of different solution concentrations with concentrations equal to the concentration of the cyon cells. toplasm are described as isotonic (i-so-TON-ik). Tissue Checkpoint 3-13 The concentration of fluids in and around the fluids and blood plasma are isotonic for body cells. Mancell is important in homeostasis. What term describes a fluid that ufactured solutions that are isotonic for the cells and can is the same concentration as the fluid within the cell (intracellular thus be used to replace body fluids include 0.9% salt, or fluid)? What type of fluid is less concentrated? More concentrated? normal saline, and 5% dextrose (glucose). Release of stored material Table 3•5 Membrane Transport PROCESS Do not require cellular energy (passive) Diffusion Osmosis Filtration Facilitated diffusion Require cellular energy Active transport DEFINITION EXAMPLE Random movement of particles with the concentration gradient (from higher concentration to lower concentration) until they reach equilibrium Diffusion of water through a semipermeable membrane Movement of nutrients, electrolytes, gases, wastes, and other soluble materials into and out of the cell Movement of water across the plasma membrane Movement of materials out of the blood under the force of blood pressure Movement of glucose into the cells Movement of materials through a membrane under mechanical force Movement of materials across the plasma membrane along the concentration gradient using transporters to speed the process Movement of materials through the plasma membrane against the concentration gradient using transporters Endocytosis Transport of bulk amounts of materials into the cell using vesicles Exocytosis Transport of bulk amounts of materials out of the cell using vesicles Transport of ions (e.g., Na⫹, K⫹, Ca2⫹) in the nervous system and muscular system Phagocytosis, intake of large particles, as when white blood cells take in waste materials; also pinocytosis—intake of fluid Release of neurotransmitters from neurons CELLS AND THEIR FUNCTIONS ✦ 51 Isotonic solution (normal) Hypotonic solution (dilute) Hypertonic solution (concentrated) Water Solute A Normal red B Swollen red blood cell blood cell C Shrunken (crenated) red blood cell Direction of osmotic water movement Figure 3-18 The effect of osmosis on cells. Water moves through a red blood cell membrane in solutions with three different concentrations of solute. (A) The isotonic (normal) solution has the same concentration as the cell fluid, and water moves into and out of the cell at the same rate. (B) A cell placed in a hypotonic (more dilute) solution draws water in, causing the cell to swell and perhaps undergo hemolysis (bursting). (C) The hypertonic (more concentrated) solution draws water out of the cell, causing it to shrink, an effect known as crenation. ZOOMING IN ✦ What would happen to red blood cells if blood lost through injury were replaced with pure water? tions, are a natural occurrence in the process of cell division and are increased by exposure to harmful substances and radiation in the environment. Mutations usually harm cells and may lead to cancer. As a person ages, the overall activity of the body cells slows. One example of this change is the slowing down of repair processes. A bone fracture, for example, takes considerably longer to heal in an old person than in a young person. One theory on aging holds that cells are preprogrammed to divide only a certain number of times before they die. Support for this idea comes from the fact that cells taken from a young person divide more times when grown in the laboratory than similar cells taken from an older individual. This programmed cell death, known as apoptosis (ah-pop-TO-sis), is a natural part of growth and remodeling before birth in the developing embryo and in repair and remodeling of tissue throughout life (see Box 3-3, Necrosis and Apoptosis: Cellular Homicide and Suicide). ◗ Cell Aging ◗ Cells and Cancer As cells multiply throughout life, changes occur that may lead to their damage and death. Harmful substances known as free radicals, produced in the course of normal metabolism, can injure cells unless these materials are destroyed. Chapter 20 covers free radicals in more detail. Lysosomes may deteriorate as they age, releasing enzymes that can harm the cell. Alteration of the genes, or muta- Certain mutations (changes) in the genetic material of a cell may cause that cell to reproduce without control. Cells that normally multiply at a fast rate, such as epithelial cells, are more likely than slower-growing cells to undergo such transformations. If these altered cells do not die naturally or get destroyed by the immune system, they will continue to multiply and may spread (metastasize) to Table 3•6 Solutions and Their Effects on Cells TYPE OF SOLUTION Isotonic Hypotonic Hypertonic DESCRIPTION EXAMPLES EFFECT ON CELLS Has the same concentration of dissolved substances as the fluid in the cell Has a lower concentration of dissolved substances than the fluid in the cell Has a higher concentration of dissolved substances than the fluid in the cell 0.9% salt (normal saline); 5% dextrose (glucose) None; cell in equilibrium with its environment Less than 0.9% salt or 5% dextrose Cell takes in water, swells, and may burst; red blood cell undergoes hemolysis Cell will lose water and shrink; cell undergoes crenation Higher than 0.9% salt or 5% dextrose 3 52 ✦ CHAPTER THREE Box 3-3 A Closer Look Necrosis and Apoptosis: Cellular Homicide and Suicide C ell death happens in two ways: by necrosis, because the cell is injured; or by apoptosis, because the cell is programmed to die. One way of remembering the difference is to think of necrosis as “cellular homicide” and apoptosis as “cellular suicide.” Necrosis disrupts the cell’s normal water-balancing mechanisms and stimulates autolysis (see Box 3-1). As a result, the cell swells and its organelles break down. Finally, the cell ruptures, releasing its contents into the surrounding tissue. These contents contain digestive enzymes that damage adjacent cells, producing more injury and necrosis. Apoptosis is an orderly, genetically programmed cell death triggered by the cell’s own genes. Under the right circumstances, these “suicide genes” produce enzymes called capsases that destroy the cell swiftly and neatly. The cell shrinks, and phagocytes quickly digest it. In contrast to necrosis, apoptotic cells do not cause further chaos when they die. Apoptosis is a normal bodily process. It is especially important during embryonic development because it removes unneeded cells, such as those from limb buds to form fingers and toes. Apoptosis also occurs after birth, as when cells subject to extreme wear and tear regularly undergo apoptosis and are replaced. For example, the cells lining the digestive tract are removed and replaced every 2 to 3 days. other tissues, producing cancer. Cancer cells form tumors, which interfere with normal functions, crowding out normal cells and robbing them of nutrients. There is more information on the various types of tumors in Chapter 4. ◗ Cancer Risk Factors The causes of cancer are complex, involving interactions between cellular factors and the environment. Because cancer may take a long time to develop, it is often difficult to identify its cause or causes. Certain forces increase the chances of developing the disease and are considered risk factors. These include the following: ◗ ◗ Heredity. Certain types of cancer occur more frequently in some families than in others, indicating that there is some inherited predisposition to the development of cancer. Chemicals. Certain industrial and environmental chemicals are known to increase the risk of cancer. Any chemical that causes cancer is called a carcinogen ◗ ◗ ◗ (kar-SIN-o-jen). The most common carcinogens in our society are those present in cigarette smoke. Carcinogens are also present, both naturally and as additives, in foods. Certain drugs also may be carcinogenic. Ionizing radiation. Certain types of radiation can produce damage to cellular DNA that may lead to cancer. These include x-rays, rays from radioactive substances, and ultraviolet rays. For example, the ultraviolet rays received from exposure to the sun are very harmful to the skin. Physical irritation. Continued irritation, such as the contact of a hot pipe stem on the lip, increases cell division and thus increases the chance of mutation. Diet. It has been shown that diets high in fats and total calories are associated with an increased occurrence of certain forms of cancer. A general lack of fiber and insufficient amounts of certain fruits and vegetables in the diet can leave one susceptible to cancers of the digestive tract. Viruses have been implicated in cancers of the liver, the blood (leukemias), and lymphatic tissues (lymphomas). Word Anatomy Medical terms are built from standardized word parts (prefixes, roots, and suffixes). Learning the meanings of these parts can help you remember words and interpret unfamiliar terms. WORD PART MEANING EXAMPLE The Role of Cells cyt/o cell Cytology is the study of cells. Microscopes micr/o small Microscopes are used to view structures too small to see with the naked eye. Cell Structure bi-some chrom/o- two body color The lipid bilayer is a double layer of lipid molecules. Ribosomes are small bodies outside the cell’s nucleus that help make proteins. Chromosomes are small, threadlike bodies that stain darkly with basic dyes. CELLS AND THEIR FUNCTIONS ✦ 53 WORD PART MEANING EXAMPLE in, within loosening, dissolving, separating The endoplasmic reticulum is a network of membranes within the cytoplasm. Lysosomes are small bodies (organelles) with enzymes that dissolve materials (see also hemolysis). Cell Functions interprometa- between before, in front of change anatel/osemiphag/o upward, back, again end partial, half to eat, ingest pino ex/o- to drink outside, out of, away from same, equal deficient, below, beneath blood above, over, excessive Interphase is the stage between one cell division (mitosis) and the next. Prophase is the first stage of mitosis. Metaphase is the second stage of mitosis when the chromosomes change position and line up across the equator. In the anaphase stage of mitosis, chromosomes move to opposite sides of the cell. Telophase is the last stage of mitosis. A semipermeable membrane lets some molecules pass through but not others. In phagocytosis the cell membrane engulfs large particles and moves them into the cell. In pinocytosis the cell membrane “drinks” (engulfs) droplets of fluid. In exocytosis the cell moves material out in vesicles. Cell Structure end/olys/o isohypohem/o hyperCells and Cancer carcin/o -gen cancer, carcinoma agent that produces or originates An isotonic solution has the same concentration as that of the cytoplasm. A hypotonic solution’s concentration is lower than that of the cytoplasm. Hemolysis is the destruction of red blood cells. A hypertonic solution’s concentration is higher than that of the cytoplasm. A carcinogen is a chemical that causes cancer. See preceding example. Summary I. The Role of Cells 1. Basic unit of life 2. Show all characteristics of life—organization, metabolism, responsiveness, homeostasis, growth, reproduction II. Microscopes A. Types 1. Compound light microscope 2. Transmission electron microscope—magnifies up to 1 million times 3. Scanning electron microscope—gives three-dimensional image B. Micrometer—metric unit commonly used for microscopic measurements C. Stains—dyes used to aid in viewing cells under the microscope III. Cell Structure A. Plasma membrane—regulates what enters and leaves cell 1. Phospholipid bilayer with proteins, carbohydrates, cholesterol a. Proteins—channels, transporters, receptors, enzymes, linkers, cell identity markers B. Nucleus 1. Control center of the cell 2. Contains the chromosomes (units of heredity) 3. Contains the nucleolus, which manufactures ribosomes C. Cytoplasm—colloidal suspension that holds organelles 1. Cytosol—liquid portion 2. Organelles—structures that carry out special functions a. ER (endoplasmic reticulum), ribosomes, mitochondria, Golgi apparatus, lysosomes, peroxisomes, vesicles, centrioles b. Cilia, flagellum—surface organelles used for movement IV. Protein synthesis A. Nucleic acids—DNA and RNA 1. Composed of nucleotides a. Each nucleotide has sugar, phosphate, nitrogen base b. Nitrogen bases vary, giving five nucleotides 2. DNA a. Carries the genetic message b. Located almost entirely in the nucleus c. Composed of nucleotides adenine (A), guanine (G), cytosine (C), thymine (T) d. Double stranded by pairing of A-T, G-C, and wound into helix 3. The role of RNA a. Single strand of nucleotides—A, G, C, and uracil (U) b. Located in the cytoplasm c. Translates DNA message into proteins d. Three types (1) Messenger RNA (mRNA)—transcribes the message of the DNA (2) Ribosomal RNA (rRNA)—makes up the ribosomes, the site of protein synthesis (3) Transfer RNA (tRNA)—brings amino acids to be made into proteins 3 54 ✦ CHAPTER THREE V. Cell division 1. Meiosis—forms the sex cells (egg and sperm) a. Divides the chromosome number in half 2. Mitosis—division of somatic (body) cells a. Chromosomes first duplicate during interphase b. Division of cell into two identical daughter cells A. Stages of mitosis—prophase, metaphase, anaphase, telophase VI. Movement of substances across plasma membrane A. Movement that does not require cellular energy (passive) 1. Diffusion—molecules move from area of higher concentration to area of lower concentration 2. Osmosis—diffusion of water through semipermeable membrane a. Osmotic pressure—measure of tendency of a solution to draw in water 3. Filtration—movement of materials through plasma membrane under mechanical force 4. Facilitated diffusion—movement of materials with aid of transporters in plasma membrane B. Movement that requires cellular energy (active) 1. Active transport a. Movement of solute particles from area of lower concentration to area of higher concentration b. Requires transporters 2. Endocytosis—movement of bulk amounts of material into the cell in vesicles a. Phagocytosis—engulfing of large particles b. Pinocytosis—intake of droplets of fluid 3. Exocytosis—movement of bulk amounts of materials out of the cell in vesicles C. How osmosis affects cells 1. Isotonic solution—same concentration as cell fluids; cell remains the same 2. Hypotonic solution—lower concentration than cell fluids; cell swells and may undergo hemolysis (bursting) 3. Hypertonic solution—higher concentration than cell fluids; cell undergoes crenation (shrinking) VII. Cell aging 1. Mutations (changes) occur in genes 2. Slowing of cellular activity 3. Apoptosis—programmed cell death VIII. Cells and cancer 1. Cancer a. Uncontrolled growth of cells b. Spread (metastasize) to other tissues A. Cancer risk factors 1. Heredity 2. Chemicals—carcinogens 3. Ionizing radiation 4. Physical irritation 5. Diet 6. Viruses Questions for Study and Review Building Understanding Fill in the blanks 1. The part of the cell that regulates what can enter or leave is the _____. 2. Distinct segments of DNA that code for specific proteins are called _____. 3. The cytosol and organelles make up the _____. 4. If Solution A has more solute and less water than Solution B, then Solution A is _____ to Solution B. 5. Mechanisms that require energy to move substances across the plasma membrane are called _____ transport mechanisms. Matching Match each numbered item with the most closely related lettered item. ___6. DNA duplication takes place ___7. DNA is tightly wound into chromosomes ___8. Chromosomes line up along the cell’s equator ___9. Chromosomes separate and move toward opposite ends of the cell ___10. Cell membrane pinches off, dividing the cell into two new daughter cells Multiple choice ___ 11. The main component of the plasma membrane is a. phospholipid b. cholesterol c. carbohydrate d. protein a. b. c. d. e. metaphase anaphase telophase interphase prophase CELLS AND THEIR FUNCTIONS ✦ 55 ___ 12. ATP is synthesized in the a. nucleus b. Golgi apparatus c. endoplasmic reticulum d. mitochondria ___ 13. Transcription of the DNA strand TGAAC would produce an mRNA strand with the sequence a. CAGGU b. ACTTG c. CAGGT d. ACUUG ___ 14. Somatic cells divide by the process called a. mitosis b. meiosis c. crenation d. hemolysis ___ 15. Movement of solute from a region of high concentration to one of lower concentration is called a. exocytosis b. diffusion c. endocytosis d. osmosis Understanding Concepts 16. List the components of the plasma membrane and state a function for each. 17. Compare and contrast the following cellular components: a. microvilli and cilia b. nucleus and nucleolus c. rough ER and smooth ER d. lysosome and peroxisome e. DNA and RNA f. chromosome and gene 18. Describe the role of each of the following in protein synthesis: DNA, nucleotide, RNA, ribosomes, rough ER, and Golgi apparatus. 19. List and define six methods by which materials cross the cell membrane. Which of these requires cellular energy? 20. Why is the cell membrane described as selectively permeable? 21. What will happen to a red blood cell placed in a 5.0% salt solution? In distilled water? 22. Discuss the link between genetic mutation and cancer. List six risk factors associated with cancer. Conceptual Thinking 23. Cigarette smoke paralyzes the cilia of cells lining the respiratory tract. Explain the effects of this on respiratory system function. 24. A particular type of cell manufactures a protein needed elsewhere in the body. Beginning with events in the nucleus, describe the process of making that protein and exporting it out of the cell. 25. Kidney failure causes a buildup of waste and water in the blood. A procedure called hemodialysis removes these substances from the blood. During this procedure, the patient’s blood passes by a semipermeable membrane within the dialysis machine. Waste and water from the blood diffuses across the membrane into dialysis fluid on the other side. Based on this information, compare the osmotic concentration of the blood with that of the dialysis fluid. 3