Cell-structure-and-function.pdf

mad21975_ch03pg45_66 8/4/04 1:05 Page 45 EQA chapter 3 Cell Structure and Function A CHICKEN’s egg is large enough to hold in your C h a p t e r C o n c e p t s hand while most cells take a microscope to see them. A chicken’s egg is big because much of 3.1 the cellular level of Organization it is stored food for growth of an embryo. What does the cell theory state? 46 Cells are incredibly variable. Bacterial cell What instruments would a scientist use to study and view looks simple compared to those within our small cells? 46–48 nervous system. However, the bacterial cell is 3.2 Eukaryotic Cells the entire organism; that senses the What boundary is found in all cells? What additional boundary is environment, obtains food, gets rid of waste, found in plant cells? 49 and reproduces. In contrast, a human being is What do you call the small structures in eukaryotic cells that carry out specific functions? 49 composed of approximately 10 trillion cells. Each type of human cell is specialized to What is the function of the nucleus? 52 perform specific functions, but it still has to do What membranous system within eukaryotic cells is involved in the production, modification, transportation, storage, secretion, and/or many of the same things a bacterial cell is digestion of macromolecules? 53–55 required to do. What energy transformation structures are present in plant and animal Cells have still other differences. Some cells? What does each structure produce? 56–57 bacteria, such as thermophilic (heat-loving) What is the composition of the cytoskeleton, and what is its ones, live in boiling sulfur springs, while others function? 58–59 exist as parasites within the human body. Most What structures are responsible for movement of the cell? 60–61 bacterial and fungal cells, acquire energy by 3.3 Prokaryotic Cells decomposing the dead remains of organisms, What is the major difference between prokaryotic and but plant cells are able to get their energy from eukaryotic cells? 62 the sun. Regardless of their differences, there are 3.4 Evolution of the Eukaryotic Cell certain structures that every cell must have and What hypothesis suggests how eukaryotic cells arose? 63 certain functions it must perform. This chapter discusses the contents of a generalized cell. mad21975_ch03pg45_66 30/07/2004 9:37 Page 46 FPG-04 FPG-04:Desktop Folder:GQ229: EQA t science focus 46 Part I cell biology 3.1The Cellular Level Microscopy Today of Organization T hree types of microscopes are most commonly used to- The cell marks the boundary between the nonliving and the day: the compound light microscope, transmission elec- living. The molecules that serve as food for a cell and the tron microscope, and scanning electron microscope. macromolecules that make up a cell are not alive, and yet Figure 3A depicts these microscopes, along with a micrograph of the cell is alive. Thus, the answer to what life is must lie red blood cells viewed with each one. within the cell, because the smallest living organisms are In a compound light microscope, light rays passing through a unicellular, while larger organisms are multicellular—that specimen are brought to a focus by a set of glass lenses, and the is, composed of many cells. The diversity of cells is exem- resulting image is then viewed by the human eye. In the transmis- plified by the many types in the human body, such as sion electron microscope, electrons passing through a specimen are muscle cells and nerve cells. But despite variety of form and brought to a focus by a set of magnetic lenses, and the resulting function, cells contain the same components. The basic components that are common to all cells regardless of their specializations are the subject of this chapter. The Science Focus on these two pages introduces you to the micro- scopes most used today to study cells. Electron microscopy and biochemical analysis have revealed that a cell actually contains tiny specialized structures called organelles that perform speciﬁc cellular functions. Today, we are accustomed to thinking of living things as being constructed of cells. But the word cell didn’t enter biol- ogy until the seventeenth century. Antonie van Leeuwenhoek of Holland is now famous for making his own microscopes and observing all sorts of tiny things that no one had seen before. Robert Hooke, an Englishman, conﬁrmed Leeuwenhoek’s observations and was the ﬁrst to use the term cell. The tiny chambers he observed in the honeycomb structure of cork Tissue was stained. 25 µm reminded him of the rooms, or cells, in a monastery. A hundred years later—in the 1830s—the German blood vessel microscopist Matthias Schleiden said that plants are com- eye posed of cells; his counterpart, Theodor Schwann, said red blood cells that animals are also made up of living units called cells. ocular lens This was quite a feat, because aside from their own exhaust- ing work, both had to take into consideration the studies of many other microscopists. Rudolf Virchow, another German microscopist, later came to the conclusion that cells don’t suddenly appear; rather, they come from pre- existing cells. Today, the cell theory, which states that all organ- isms are made up of basic living units called cells and that cells come only from preexisting cells, is a basic theory of objective lens specimen biology. condenser The cell theory states the following: All organisms are composed of one or more cells. light source Cells are the basic living unit of structure and function in organisms. All cells come only from other cells. Compound light microscope FIGURE 3A Blood vessels and red blood cells viewed with three different types of microscopes. mad21975_ch03pg45_66 8/4/04 2:50 AM Page 47 EQA Chapter 3 cell structure and function 47 image is projected onto a fluorescent screen or resolving power of the electron microscope been coated with a thin layer of metal. The photographic film. is due to the fact that electrons travel at a metal gives off secondary electrons, which The magnification produced by an elec- much shorter wavelength than do light rays. are collected to produce a television-type pic- tron microscope is much higher than that of However, because electrons travel only in a ture of the specimen’s surface on a screen. a light microscope (50,000X compared to vacuum, the object is always dried out be- A picture obtained using a light micro- 1,000X). Also, the ability of the electron micro- fore viewing, whereas even living objects scope is sometimes called a photomicro- scope to make out detail is much greater. can be observed with a light microscope. graph, and a picture resulting from the use The distance needed to distinguish two A scanning electron microscope provides of an electron microscope is called a trans- points as separate is much less for an elec- a three-dimensional view of the surface of an mission electron micrograph (TEM) or a tron microscope than for a light microscope object. A narrow beam of electrons is scanned scanning electron micrograph (SEM), depend- (10 nm compared to 200 nm1). The greater over the surface of the specimen, which has ing on the type of microscope used. 1 nm nanometer. See Appendix C, Metric System. Micrograph was colored. Micrograph was colored. 10 µm 14 µm blood vessel blood vessel red blood red blood cells electron beam cells electron beam condenser condensers specimen objective lens scanning coil objective lens electron projector lens detector secondary electrons observation TV specimen or viewing photograph screen Transmission electron microscope Scanning electron microscope mad21975_ch03pg45_66 30/07/2004 9:37 Page 48 FPG-04 FPG-04:Desktop Folder:GQ229: EQA 48 Part I cell biology 0.1 nm 1 nm 10 nm 100 nm 1 µm 10 µm 100 µm 1 mm 1 cm 0.1 m 1m 10 m 100 m 1 km protein chloroplast mouse plant and animal cells frog amino acid egg virus blue whale atom human most bacteria egg ant electron microscope light microscope human human eye FIGURE 3.1 The sizes of living things and their components. It takes a microscope to see most cells and lower levels of biological organization. Cells are visible with the light microscope, but not in much detail. An electron microscope is needed to see organelles in detail and to make out viruses and molecules. Notice that in this illustration each measurement is 10 times greater than the lower one. (In the metric system, 1 meter ⫽ 102 cm ⫽ 103 mm ⫽ 106 ␮m ⫽ 109 nm—see Appendix C.) Cell Size A small cube, 1 mm tall, has a volume of 1 mm3 (height ⫻ width ⫻ depth is 1 mm3). The surface area is 6 mm2. (Each Figure 3.1 outlines the visual ranges of the eye, light micro- side has a surface area of 1 mm2, and 6 ⫻ 1 mm2 is 6 mm2). scope, and electron microscope. Cells are usually quite small. Therefore, the ratio of surface area to volume is 6:1 because A frog’s egg, at about one millimeter (mm) in diameter, is the surface area is 6 mm2 and the volume is 1 mm3. large enough to be seen by the human eye. But most cells are Contrast this with a larger cube that is 2 mm tall. The far smaller than one millimeter; some are even as small as surface area is 24 mm2. (Each side has a surface area of 4 mm2, one micrometer (␮m)—one-thousandth of a millimeter. Cell and 6 ⫻ 4 is 24 mm2). The volume of this larger cube is inclusions and macromolecules are even smaller than a 8 mm3; therefore, the ratio of surface area to volume of the micrometer and are measured in terms of nanometers (nm). larger cube is 3:1. We can conclude then that a small cell has To understand why cells are so small and why we are a greater surface area to volume ratio than does a larger cell. multicellular, consider the surface/volume ratio of cells. Therefore, small cells, not large cells, are likely to have Nutrients enter a cell and wastes exit a cell at its surface; an adequate surface area for exchanging wastes for nutrients. therefore, the amount of surface affects the ability to get ma- We would expect, then, a size limitation for an actively terial in and out of the cell. A large cell requires more metabolizing cell. You can hold a chicken’s egg in your hand, nutrients and produces more wastes than a small cell. In but the egg is not actively metabolizing. Once the egg is incu- other words, the volume represents the needs of the cell. Yet, bated and metabolic activity begins, the egg divides as cells get larger in volume, the proportionate amount of repeatedly without growth. Cell division restores the amount surface area actually decreases, as you can see by comparing of surface area needed for adequate exchange of materials. these two cells: Further, cells that specialize in absorption have modiﬁcations that greatly increase the surface area per volume of the cell. For example, the columnar cells along the surface of the intestinal wall have surface foldings called microvilli (sing., microvillus), which increase their surface area. A cell needs a surface area that can adequately ex- change materials with the environment. Surface-area- small cell— large cell— to-volume considerations require that cells stay small. more surface area less surface area per volume per volume mad21975_ch03pg45_66 30/07/2004 9:37 Page 49 FPG-04 FPG-04:Desktop Folder:GQ229: EQA Chapter 3 cell structure and function 49 3.2 Eukaryotic Cells TABLE 3.1 EUKARYOTIC STRUCTURES IN ANIMAL CELLS AND PLANT CELLS Eukaryotic cells, one of the two major types of cells, have a nucleus. A nucleus is a large structure that controls the Name Composition Function workings of the cell because it contains the genes. Both ani- Cell wall* Contains cellulose Support and protection mals and plants have eukaryotic cells. ﬁbrils Plasma Phospholipid bilayer Deﬁnes cell boundary; membrane with embedded regulation of molecule Outer Boundaries of Animal and Plant Cells proteins passage into and out of cells Animal and plant cells are surrounded by a plasma membrane, that consists of a phospholipid bilayer in which Nucleus Nuclear envelope, Storage of genetic nucleoplasm, information; synthesis of protein molecules are embedded: chromatin, and DNA and RNA nucleoli protein Nucleolus Concentrated area Ribosomal subunit molecules of chromatin, RNA, formation and proteins Ribosome Protein and RNA in Protein synthesis two subunits Endoplasmic Membranous Synthesis and/or phospholipid reticulum ﬂattened channels modiﬁcation of proteins bilayer (ER) and tubular canals and other substances, and distribution by vesicle formation Rough ER Studded with Protein synthesis The plasma membrane is a living boundary that separates ribosomes the living contents of the cell from the nonliving surround- Smooth ER Having no ribosomes Various; lipid synthesis ing environment. Inside the cell, the nucleus is surrounded in some cells by the cytoplasm, a semiﬂuid medium that contains Golgi Stack of membranous Processing, packaging, organelles. The plasma membrane regulates the entrance apparatus saccules and distribution of and exit of molecules into and out of the cytoplasm. proteins and lipids Plant cells (but not animal cells) have a permeable but Lysosome Membranous vesicle Intracellular digestion protective cell wall, in addition to a plasma membrane. containing digestive Many plant cells have both a primary and secondary cell enzymes wall. A main constituent of a primary cell wall is cellulose Vacuole and Membranous sacs Storage of substances molecules. Cellulose molecules form ﬁbrils that lie at right vesicle angles to one another for added strength. The secondary cell Peroxisome Membranous vesicle Various metabolic tasks wall, if present, forms inside the primary cell wall. Such sec- containing speciﬁc ondary cell walls contain lignin, a substance that makes enzymes them even stronger than primary cell walls. Mitochondrion Inner membrane Cellular respiration (cristae) bounded by an outer membrane Organelles of Animal and Plant Cells Chloroplast* Membranous grana Photosynthesis Originally the term organelle referred to only membranous bounded by two structures, but we will use it to include any well-deﬁned membranes subcellular structure (Table 3.1). Just as all the assembly Cytoskeleton Microtubules, Shape of cell and lines of a factory are in operation at the same time, so all the intermediate movement of its organelles of a cell function simultaneously. Raw materials ﬁlaments, actin parts enter a factory and then are turned into various products by ﬁlaments different departments. In the same way, chemicals are taken Cilia and 9 + 2 pattern of Movement of cell up by the cell and then processed by the organelles. The cell ﬂagella microtubules is a beehive of activity the entire 24 hours of every day. Centriole** 9 + 0 pattern of Formation of basal Both animal cells (Fig. 3.2) and plant cells (Fig. 3.3) con- microtubules bodies tain mitochondria, while only plant cells have chloroplasts. Only animal cells have centrioles. All the organelles have an *Plant cells only **Animal cells only assigned color that is used to represent them in the illustra- tions throughout the text. mad21975_ch03pg45_66 30/07/2004 10:04 Page 50 FPG-04 FPG-04:Desktop Folder:GQ229: EQA 50 Part I cell biology nuclear pore chromatin nucleus nucleolus nuclear envelope polyribosome smooth ER actin filament (within cytoskeleton) peroxisome rough ER vacuole ribosome cytoplasm (attached to rough ER) ribosomes (in cytoplasm) centriole Golgi apparatus mitochondrion vesicle lysosome plasma membrane microtubule (within cytoskeleton) a. plasma membrane nuclear envelope chromatin nucleolus endoplasmic reticulum b. 50 nm FIGURE 3.2 Animal cell anatomy. a. Generalized drawing. b. Transmission electron micrograph. See Table 3.1 for a description of these structures, along with a listing of their functions. mad21975_ch03pg45_66 30/07/2004 9:37 Page 51 FPG-04 FPG-04:Desktop Folder:GQ229: EQA Chapter 3 cell structure and function 51 microtubule (within cytoskeleton) nuclear pore central vacuole chromatin nucleus nucleolus chloroplast nuclear envelope ribosomes (in cytoplasm) rough ER ribosome (attached to rough ER) smooth ER cell wall plasma membrane Golgi apparatus cytoplasm mitochondrion actin filament (within cytoskeleton) cell wall of adjacent cell a. peroxisome mitochondrion nucleus ribosomes plasma membrane central vacuole chloroplast cell wall 1 µm b. FIGURE 3.3 Plant cell anatomy. a. Generalized drawing. b. Transmission electron micrograph of a young leaf cell. See Table 3.1 for a description of these structures, along with a listing of their functions. mad21975_ch03pg45_66 30/07/2004 9:37 Page 52 FPG-04 FPG-04:Desktop Folder:GQ229: EQA 52 Part I cell biology The Nucleus Most likely, too, when you look at an electron micro- graph of a nucleus, you will see one or more regions that look The nucleus, which has a diameter of about 5 ␮m, is a darker than the rest of the chromatin. These are nucleoli (sing., prominent structure in the eukaryotic cell. The nucleus is of nucleolus), where another type of RNA, called ribosomal primary importance because it stores the genetic material RNA (rRNA), is produced and where rRNA joins with pro- DNA which governs the characteristics of the cell and its teins to form the subunits of ribosomes. (Ribosomes are small metabolic functioning. Every cell in the same individual bodies in the cytoplasm that contain rRNA and proteins.) contains the same DNA, but, in each cell type, certain genes The nucleus is separated from the cytoplasm by a dou- are turned on and certain others are turned off. Activated ble membrane known as the nuclear envelope, which is DNA, with RNA acting as an intermediary, speciﬁes the se- continuous with the endoplasmic reticulum discussed on quence of amino acids when a protein is synthesized. The the next page. The nuclear envelope has nuclear pores of proteins of a cell determine its structure and the functions it sufﬁcient size (100 nm) to permit the passage of proteins into can perform. the nucleus and ribosomal subunits out of the nucleus. When you look at the nucleus, even in an electron micro- graph, you cannot see a DNA molecule. You can see chro- matin, which consists of DNA and associated proteins (Fig. The structural features of the nucleus include the 3.4). Chromatin looks grainy, but actually it is a threadlike ma- following. terial that undergoes coiling to form rodlike structures, called Chromatin: DNA and proteins chromosomes, just before the cell divides. Chromatin is im- Nucleolus: Chromatin and ribosomal mersed in a semiﬂuid medium called the nucleoplasm. A dif- subunits ference in pH between the nucleoplasm and the cytoplasm Nuclear envelope: Double membrane with pores suggests that the nucleoplasm has a different composition. nuclear envelope chromatin nucleolus nuclear pores inner membrane outer membrane Electron micrographs of nuclear envelope showing pores. FIGURE 3.4 The nucleus and the nuclear envelope. The nucleoplasm contains chromatin. Chromatin has a special region called the nucleolus, where rRNA is produced and ribosomal subunits are assembled. The nuclear envelope, consisting of two membranes separated by a narrow space, contains pores. The electron micrographs show that the pores cover the surface of the envelope. mad21975_ch03pg45_66 30/07/2004 9:37 Page 53 FPG-04 FPG-04:Desktop Folder:GQ229: EQA Chapter 3 cell structure and function 53 Ribosomes Ribosomes are composed of two subunits, one large and one small. Each subunit has its own mix of proteins and rRNA. Protein synthesis occurs at the ribosomes. Ribosomes can be found within the cytoplasm, either singly or in groups called polyribosomes. Ribosomes can also be found attached to the endoplasmic reticulum, a membranous sys- tem of saccules and channels discussed in the next section. Proteins synthesized at ribosomes attached to the endoplas- mic reticulum have a different fate. They are eventually se- ribosome creted from the cell or become a part of its external surface. smooth ER Ribosomes are small organelles where protein synthesis occurs. Ribosomes occur in the cytoplasm, both singly and in groups (i.e., polyribosomes). a. Numerous ribosomes are also attached to the rough ER endoplasmic reticulum. The Endomembrane System The endomembrane system consists of the nuclear enve- lope, the endoplasmic reticulum, the Golgi apparatus, and several vesicles (tiny membranous sacs). This system com- partmentalizes the cell so that particular enzymatic reac- b. tions are restricted to speciﬁc regions. Organelles that make 400 nm ribosome up the endomembrane system are connected either directly or by transport vesicles. The Endoplasmic Reticulum The endoplasmic reticulum (ER), a complicated system of polypeptide carbohydrate membranous channels and saccules (ﬂattened vesicles), is chain physically continuous with the outer membrane of the glycoprotein nuclear envelope. Rough ER is studded with ribosomes on the side of the membrane that faces the cytoplasm (Fig. 3.5). Here, proteins are synthesized and enter the ER interior, where processing and modiﬁcation begin. Most proteins are vesicle modiﬁed by the addition of a sugar chain, which makes formation them a glycoprotein. Smooth ER, which is continuous with rough ER, does not have attached ribosomes. Smooth ER synthesizes the phospholipids that occur in membranes and has various other functions depending on the particular cell. In the transport testes, it produces testosterone, and in the liver, it helps vesicle detoxify drugs. Regardless of any specialized function, smooth ER also forms vesicles in which proteins are trans- ported to the Golgi apparatus. c. FIGURE 3.5 The endoplasmic reticulum (ER). a. Rough ER has attached ribosomes, but smooth ER does not. ER is involved in protein synthesis (rough ER) and b. Rough ER appears to be ﬂattened saccules, while smooth ER is a various other processes, such as lipid synthesis network of interconnected tubules. c. A protein made at a ribosome (smooth ER). Vesicles transport proteins from the moves into the lumen of the system, is modiﬁed, and is eventually ER to the Golgi apparatus. packaged in a transport vesicle for distribution to the Golgi apparatus. mad21975_ch03pg45_66 30/07/2004 9:38 Page 54 FPG-04 FPG-04:Desktop Folder:GQ229: EQA 54 Part I cell biology smooth ER rough ER synthesizes lipids and synthesizes proteins and performs other functions. packages them in vesicles. transport vesicles from smooth ER. transport vesicles from rough ER. lysosome digest molecules or old cell parts. Golgi apparatus modifies lipids and proteins; sorts and packages them in vesicles. secretory vesicles incoming vesicle fuse with the plasma brings substances into the membrane as secretion cell. occurs. FIGURE 3.6 Endomembrane system. The organelles in the endomembrane system work together to carry out the functions noted. The Golgi Apparatus inner face to the outer face by shuttle vesicles. It is likely that The Golgi apparatus is named for Camillo Golgi, who dis- both models apply, depending on the organism and the type covered its presence in cells in 1898. The Golgi apparatus of cell. consists of a stack of three to twenty slightly curved saccules During their passage through the Golgi apparatus, whose appearance can be compared to a stack of pan- glycoproteins have their sugar chains modiﬁed before they cakes (Fig. 3.6). In animal cells, one side of the stack (the are repackaged in secretory vesicles. Secretory vesicles pro- inner face) is directed toward the ER, and the other side of ceed to the plasma membrane, where they discharge their the stack (the outer face) is directed toward the plasma contents. Because this is secretion, the Golgi apparatus is membrane. Vesicles can frequently be seen at the edges of said to be involved in processing, packaging, and secretion. the saccules. The Golgi apparatus is also involved in the formation The Golgi apparatus receives protein and also lipid- of lysosomes, vesicles that contain proteins and remain ﬁlled vesicles that bud from the smooth ER. These molecules within the cell. How does the Golgi apparatus direct trafﬁc— then move through the Golgi from the inner face to the outer in other words, what makes it direct the ﬂow of proteins to face. How this occurs is still being debated. According to the different destinations? It now seems that proteins made at maturation saccule model, the vesicles fuse to form an inner the rough ER have speciﬁc molecular tags that serve as “zip face saccule, which matures as it gradually becomes a sac- codes” to tell the Golgi apparatus whether they belong in a cule at the outer face. According to the stationary saccule lysosome or in a secretary vesicle. The ﬁnal sugar chain model, the molecules move through stable saccules from the serves as a tag that directs proteins to their ﬁnal destination. mad21975_ch03pg45_66 30/07/2004 9:38 Page 55 FPG-04 FPG-04:Desktop Folder:GQ229: EQA Chapter 3 cell structure and function 55 Lysosomes Lysosomes are membrane-bounded vesicles produced by the Golgi apparatus. Lysosomes contain hydrolytic diges- tive enzymes. Sometimes macromolecules are brought into a cell by vesicle formation at the plasma membrane (Fig. 3.6). When a lysosome fuses with such a vesicle, its contents are digested by lysosomal enzymes into simpler subunits that then enter the cytoplasm. Some white blood cells defend the body by engulﬁng pathogens by vesicle formation. When lysosomes fuse with these vesicles, the bacteria are digested. Even parts of a cell are digested by its own lysosomes (called autodigestion). Normal cell rejuvenation takes place in this manner. Lysosomes contain many enzymes for digesting all sorts of molecules. The absence or malfunction of one of 0.5 µm these results in a so-called lysosomal storage disease. Occa- sionally, a child inherits the inability to make a lysosomal en- FIGURE 3.7 Peroxisomes. zyme, and therefore has a lysosomal storage disease. Instead Peroxisomes are vesicles that oxidize organic substances with a of being degraded, the molecule accumulates inside lyso- resulting buildup of hydrogen peroxide. Peroxisomes contain the enzyme catalase, which breaks down hydrogen peroxide (H2O2) to somes, and illness develops when they swell and crowd the water and oxygen. other organelles. In Tay Sachs disease, the cells that surround nerve cells cannot break down a particular lipid, and the nervous system is affected. At about six months, the infant Peroxisomes can no longer see and, then, gradually loses hearing and Peroxisomes, similar to lysosomes, are membrane-bounded even the ability to move. Death follows at about three years vesicles that enclose enzymes (Fig. 3.7). However, the of age. enzymes in peroxisomes are synthesized by cytoplasmic ri- bosomes and transported into a peroxisome by carrier pro- The endomembrane system consists of the teins. Typically, peroxisomes contain enzymes whose action endoplasmic reticulum, Golgi apparatus, results in hydrogen peroxide (H2O2): lysosomes, and transport vesicles. RH2 ⫹ O2 → R ⫹ H2O2 R ⫽ remainder of molecule Hydrogen peroxide, a toxic molecule, is immediately broken Vacuoles down to water and oxygen by another peroxisomal enzyme A vacuole is a large membranous sac. A vesicle is smaller called catalase. than a vacuole. Animal cells have vacuoles, but they are The enzymes in a peroxisome depend on the function much more prominent in plant cells. Typically, plant cells of the cell. Peroxisomes are especially prevalent in cells that have a large central vacuole so ﬁlled with a watery ﬂuid that are synthesizing and breaking down fats. In the liver, some it gives added support to the cell (see Fig. 3.3). peroxisomes break down fats and others produce bile salts Vacuoles store substances. Plant vacuoles contain not from cholesterol. In the movie Lorenzo’s Oil, the cells lacked only water, sugars, and salts but also pigments and toxic a carrier protein to transport an enzyme into peroxisomes. As molecules. The pigments are responsible for many of the a result, long chain fatty acids accumulate in his brain and red, blue, or purple colors of ﬂowers and some leaves. The Lorenzo suffers from neurological damage. toxic substances help protect a plant from herbivorous ani- Plant cells also have peroxisomes. In germinating seeds, mals. The vacuoles present in unicellular protozoans are they oxidize fatty acids into molecules that can be converted to quite specialized, and they include contractile vacuoles for sugars needed by the growing plant. In leaves, peroxisomes ridding the cell of excess water and digestive vacuoles for can carry out a reaction that is opposite to photosynthesis— breaking down nutrients. the reaction uses up oxygen and releases carbon dioxide. Vacuoles are larger than vesicles. Plants are well Typically, the enzymes in peroxisomes break down known for having a large central vacuole area for molecules and as a result produce hydrogen storage of various molecules. peroxide molecules. mad21975_ch03pg45_66 30/07/2004 10:04 Page 56 FPG-04 FPG-04:Desktop Folder:GQ229: EQA 56 Part I cell biology Energy-Related Organelles Only plants, algae, and cyanobacteria are capable of carrying on photosynthesis in this manner: Life is possible only because of a constant input of energy used for maintenance and growth. Chloroplasts and mito- chondria are the two eukaryotic membranous organelles solar energy + carbon dioxide + water carbohydrate + oxygen that specialize in converting energy to a form that can be used by the cell. Chloroplasts use solar energy to synthesize carbohydrates, and carbohydrate-derived products are bro- Plants and algae have chloroplasts while cyanobacteria ken down in mitochondria (sing., mitochondrion) to pro- carry on photosynthesis within independent thylakoids. duce ATP molecules, as shown in the following diagram: Solar energy is the ultimate source of energy for cells because nearly all organisms, either directly or indirectly, use the carbohydrates produced by photosynthesizers as an energy source. carbohydrate All organisms carry on cellular respiration, the process (high chemical energy) by which the chemical energy of carbohydrates is converted to that of ATP (adenosine triphosphate). ATP is the common solar carrier of chemical energy in cells. All organisms, except bac- energy teria, complete the process of cellular respiration in mito- chondria. Cellular respiration can be represented by this equation: carbohydrate + oxygen carbon dioxide + water + energy chloroplast mitochondrion usable Here energy is in the form of ATP molecules. When a cell ATP needs energy, ATP supplies it. The energy of ATP is used for energy CO2 + H2O for cells synthetic reactions, active transport, and all energy-requiring (low chemical energy) processes in cells. outer membrane double inner membrane membrane granum independent thylakoids stroma overlapping thylakoids 500 nm a. b. FIGURE 3.8 Chloroplast structure. a. Electron micrograph. b. Generalized drawing in which the outer and inner membranes have been cut away to reveal the grana. mad21975_ch03pg45_66 30/07/2004 9:38 Page 57 FPG-04 FPG-04:Desktop Folder:GQ229: EQA Chapter 3 cell structure and function 57 Chloroplasts Mitochondria Plant and algal cells contain chloroplasts, the organelles that All eukaryotic cells, including those of plants and algae, allow them to produce their own organic food. Chloroplasts contain mitochondria. This means that plant cells contain are about 4–6 ␮m in diameter and 1–5 ␮m in length; they both chloroplasts and mitochondria. Most mitochondria are belong to a group of organelles known as plastids. Among usually 0.5–1.0 ␮m in diameter and 2–5 ␮m in length. the plastids are also the amyloplasts, common in roots, which Mitochondria, like chloroplasts, are bounded by a dou- store starch, and the chromoplasts, common in leaves, which ble membrane (Fig. 3.9). In mitochondria, the inner ﬂuid- contain red and orange pigments. A chloroplast is green, of ﬁlled space is called the matrix. The matrix contains DNA, course, because it contains the green pigment chlorophyll. ribosomes, and enzymes that break down carbohydrate A chloroplast is bounded by two membranes that products, releasing energy to be used for ATP production. enclose a ﬂuid-ﬁlled space called the stroma (Fig. 3.8). A The inner membrane of a mitochondrion invaginates membrane system within the stroma is organized into in- to form cristae. Cristae provide a much greater surface area terconnected ﬂattened sacs called thylakoids. In certain re- to accommodate the protein complexes and other partici- gions, the thylakoids are stacked up in structures called pants that produce ATP. grana (sing., granum). There can be hundreds of grana Mitochondria and chloroplasts are able to make some within a single chloroplast (Fig. 3.8). Chlorophyll, which is proteins, but others are imported from the cytoplasm. located within the thylakoid membranes of grana, captures the solar energy needed to enable chloroplasts to produce Chloroplasts and mitochondria are membranous carbohydrates. The stroma also contains DNA, ribosomes, organelles whose structures lend themselves to the and enzymes that synthesize carbohydrates from carbon energy transfers that occur within them. dioxide and water. a. 200 nm outer membrane double cristae matrix membrane inner membrane b. FIGURE 3.9 Mitochondrion structure. a. Electron micrograph. b. Generalized drawing in which the outer membrane and portions of the inner membrane have been cut away to reveal the cristae. mad21975_ch03pg45_66 30/07/2004 9:38 Page 58 FPG-04 FPG-04:Desktop Folder:GQ229: EQA 58 Part I cell biology The Cytoskeleton The cytoskeleton is a network of interconnected ﬁlaments and tubules that extends from the nucleus to the plasma membrane in eukaryotic cells. Prior to the 1970s, it was believed that the cytoplasm was an unorganized mixture of organic molecules. Then, high-voltage electron microscopes, which can penetrate thicker specimens, showed that the cytoplasm is instead highly organized. It contains actin ﬁla- a. microtubules in cell ments, microtubules, and intermediate filaments. The technique of immunofluorescence microscopy identified the makeup of these protein ﬁbers within the cytoskeletal vesicle network (Fig. 3.10). kinesin The name cytoskeleton is convenient in that it compares receptor ATP the cytoskeleton to the bones and muscles of an animal. kinesin Bones and muscles give an animal structure and produce movement. Similarly, the ﬁbers of the cytoskeleton maintain cell shape and cause the cell and its organelles to move. The cytoskeleton is dynamic; assembly occurs when monomers join a ﬁber and disassembly occurs when monomers leave a fiber. Assembly and disassembly occur at rates that are meas- ured in seconds and minutes. The entire cytoskeletal net- b. vesicles move along microtubules work can even disappear and reappear at various times in FIGURE 3.11 Microtubules. the life of a cell. a. Microtubules are visible in this cell due to a technique called immunoﬂuorescence. b. Microtubules act as tracks along which organelles move. The motor molecule kinesin, bound to a vesicle, breaks down ATP and uses the energy to move along the microtubule. plasma Microtubules microtubule membrane Microtubules are small, hollow cylinders about 25 nm in diameter and from 0.2 to 25 ␮m in length. centrosome Microtubules are made of a globular protein called tubulin. When microtubules assemble, tubulin molecules come together as dimers, and the dimers arrange them- selves in rows. Microtubules have 13 rows of tubulin dimers surrounding what appears in electron micrographs to be an empty central core. In many cells, microtubule assembly is under the control of a microtubule organizing center, MTCO, called the centrosome. The centrosome lies near the nucleus. Before a cell divides, the microtubules assemble into a structure called a spindle that distributes chromosomes in an orderly manner. At the end of cell di- vision, the spindle disassembles, and the microtubules re- assemble once again into their former array. intermediate actin When the cell is not dividing, microtubules help filament filament maintain the shape of the cell and act as tracks along FIGURE 3.10 The cytoskeleton. which organelles can move. Motor molecules are proteins Diagram comparing the size relationship of microtubules, intermediate that derive energy from ATP to propell themselves along a ﬁlaments, and intermediate ﬁlaments. Microtubule construction is protein ﬁlament or microtubule. Whereas, the motor mole- controlled by the centrosome. cule myosin is associated with actin ﬁlaments, the motor mad21975_ch03pg45_66 8/4/04 1:05 Page 59 EQA Chapter 3 cell structure and function 59 a. Actin filaments in cell actin filament a. Intermediate filaments (IF) fibrous in cell subunits ATP ADP + P myosin molecules anchor head membrane b. Actin and myosin interact b. IF anchor organelles FIGURE 3.12 Actin filaments. a. Actin filaments are visible in this cell due to a technique called FIGURE 3.13 Intermediate filaments. immunofluorescence. b. In the presence of ATP, myosin, a motor a. Intermediates are visible in this cell due to a technique called molecule, attaches to an actin filament, pulls it, and then reattaches at a immunofluorescence. b. Fibrous subunits account for the ropelike different location. This is the mechanism that allows muscle to contract. structure of intermediate filaments. molecules kinesin and dynein move along microtubules. How are actin filaments involved in the movement of One type of kinesin is responsible for moving vesicles along the cell and its organelles? They interact with motor mole- microtubules, including microtubules, including the transport cule called myosin. Myosin has both a head and a tail. In the vesicles of the endomembrane system. The vesicle is bonded presence of ATP, the myosin head attaches, and then reat- to the kinesin, and then kinensin “walks” along the micro- taches to an actin filament at a more distant location (Fig. 3.12). tubule by attaching and reattaching itself further along the In muscle cells, the tails of several muscle myosin molecules microtubule. There are different types of kinesin proteins, each are joined to form a thick filament. In nonmuscle cells, cyto- specialized to move one kind of vesicle or cellular organelle. plasmic myosin tails are bound to membranes, but the heads One type of dynein molecule, called cytoplasmic dynein, is still interact with actin. During animal cell division, the two closely related to the dynein found in flagella (Fig. 3.11). new cells form when actin, in conjunction with myosin, pinches off the cells from one another. Actin Filaments Actin filaments (formerly called microfilaments) are long, Intermediate Filaments extremely thin fibers (about 7 nm in diameter) that occur in Intermediate filaments (8–11 nm in diameter) are intermedi- bundles or meshlike networks. The actin filament contains ate in size between actin filaments and microtubules. They two chains of globular actin monomers twisted about one are ropelike assemblies of fibrous polypeptides (Fig. 3.13) another in a helical manner. that support the nuclear envelope and the plasma mem- Actin filaments play a structural role by forming a brane. In the skin, intermediate filaments made of the protein dense complex web just under the plasma membrane, to keratin give great mechanical strength to skin cells. Recent which they are anchored by special proteins. Also, the as- work has shown intermediate filaments to be highly dy- sembly and disassembly of a network of actin filaments ly- namic. They also are able to assemble and disassemble in the ing beneath the plasma membrane accounts for the forma- same manner as actin filaments and microtubules. tion of pseudopods, extensions that allow certain cells to move in an amoeboid fashion. Actin filaments are seen in the microvilli that project The cytoskeleton contains microtubules, actin from intestinal cells, and their presence most likely accounts filaments, and intermediate filaments. These for the ability of microvilli to alternately shorten and extend maintain cell shape and allow organelles to move into the intestine. In plant cells, actin filaments apparently within the cytoplasm. Sometimes they are also form the tracks along which chloroplasts circulate or stream involved in movement of the cell itself. in a particular direction. mad21975_ch03pg45_66 30/07/2004 9:38 Page 60 FPG-04 FPG-04:Desktop Folder:GQ229: EQA 60 Part I cell biology one microtubule one pair of centrioles triplet two pairs of centrioles 200 nm FIGURE 3.14 Centrioles. Left and top right. A nondividing cell contains a pair of centrioles in a centrosome outside the nucleus. Bottom, right. Just before a cell divides, the centrosome divides so that there are two pairs of centrioles. During cell division, the centrosomes separate so that each new cell has one pair of centrioles. Centrioles Cilia and Flagella Centrioles are short cylinders with a 9 ⫹ 0 pattern of Cilia and ﬂagella are hairlike projections that can move either microtubule triplets—that is, a ring having nine sets of in an undulating fashion, like a whip, or stifﬂy, like an oar. triplets with none in the middle (Fig. 3.14). In animal cells, a Cells that have these organelles are capable of movement. For centrosome contains two centrioles lying at right angles to example, unicellular paramecia move by means of cilia, each other. The centrosome is the major microtubule organ- whereas sperm cells move by means of ﬂagella. The cells that izing center for the cell, and centrioles may be involved in line our upper respiratory tract have cilia that sweep debris the process of microtubule assembly and disassembly. trapped within mucus back up into the throat, where it can be Before an animal cell divides, the centrioles replicate, swallowed. This action helps keep the lungs clean. and the members of each pair are again at right angles to one In eukaryotic cells, cilia are much shorter than ﬂagella, another (Fig. 3.14). Then, each pair becomes part of a sepa- but they have a similar construction. Both are membrane- rate centrosome. During cell division, the centrosomes move bounded cylinders enclosing a matrix area. In the matrix are apart and may function to organize the mitotic spindle. nine microtubule doublets arranged in a circle around two Plant cells have the equivalent of a centrosome, but it does central microtubules. Therefore, they have a 9 ⫹ 2 pattern of not contain centrioles, suggesting that centrioles are not microtubules. Cilia and ﬂagella move when the microtubule necessary to the assembly of cytoplasmic microtubules. doublets slide past one another (Fig. 3.15). Centrioles are believed to give rise to basal bodies that As mentioned, each cilium and ﬂagellum has a basal direct the organization of microtubules within cilia and body lying in the cytoplasm at its base. Basal bodies have the ﬂagella. In other words, a basal body does for a cilium (or same circular arrangement of microtubule triplets as centri- ﬂagellum) what the centrosome does for the cell. oles and are believed to be derived from them. The basal body initiates polymerization of the nine outer doublets of a cilium or ﬂagellum Centrioles, which are short cylinders with a 9 ⫹ 0 pattern of microtubule triplets, may be involved in microtubule formation and in the organization of Cilia and ﬂagella, which have a 9 ⫹ 2 pattern of cilia and ﬂagella. microtubules, enable some cells to move. mad21975_ch03pg45_66 30/07/2004 9:38 Page 61 FPG-04 FPG-04:Desktop Folder:GQ229: EQA - visual focus outer The shaft of microtubule flagellum has a ring doublet of nine microtubule doublets anchored to a central pair of dynein microtubules. side arms central microtubules radial spokes Flagellum flagellum cross section 25 nm dynein side arm The side arms of each doublet are composed of dynein, a motor molecule. plasma membrane Sperm Flagellum shaft ATP triplets In the presence of ATP, the dynein side arms reach out to their neighbors, and bending occurs. Basal body The basal body of a flagellum has a ring of nine microtubule triplets with no central microtubules. Basal body cross section 100 nm FIGURE 3.15 Structure of a ﬂagellum or cilium. A basal body, derived from a centriole, is at the base of a ﬂagellum or cilium. The shaft of a ﬂagellum (or cilium) contains microtubule doublets whose side arms are motor molecules that cause the ﬂagellum (such as those of sperm) to move. Without the ability of sperm to move to the egg, human reproduction would not be possible. 61 mad21975_ch03pg45_66 30/07/2004 9:38 Page 62 FPG-04 FPG-04:Desktop Folder:GQ229: EQA 62 Part I cell biology 3.3 Prokaryotic Cells cyanobacteria have light-sensitive pigments, usually within the membranes of ﬂattened disks called thylakoids. Prokaryotic cells, the other major type of cell, does not Although prokaryotes are structurally simple, they are have a nucleus as eukaryotic cells do. Archaea and bacteria actually metabolically complex and contain many different are both prokaryotes, cells so small they are just visible kinds of enzymes. Prokaryotes are adapted to living in with the light microscope. almost any kind of environment and are diversiﬁed to the Figure 3.16 illustrates the main features of bacterial extent that almost any kind of organic matter can be used as anatomy. The cell wall contains peptidoglycan, a complex a nutrient for some particular type. The cytoplasm is the site molecule with chains of a unique amino disaccharide joined of thousands of chemical reactions, and prokaryotes are by peptide chains. In some bacteria, the cell wall is further more metabolically competent than are human beings. surrounded by a capsule and/or gelatinous sheath called a Given adequate nutrients, most prokaryotes are able to syn- slime layer. Motile bacteria usually have long, very thin thesize any kind of molecule they may need. Indeed, the appendages called ﬂagella (sing., ﬂagellum) that are com- metabolic capability of bacteria is exploited by humans, who posed of subunits of the protein called ﬂagellin. The ﬂagella, use them to produce a wide variety of chemicals and prod- which rotate like propellers, rapidly move the bacterium in ucts for human use. a ﬂuid medium. Some bacteria also have ﬁmbriae, which are short appendages that help them attach to an appropriate Bacteria are prokaryotic cells with these constant surface. features. The cytoplasm of prokaryotic cells like that of eukary- otic cells is bounded by a plasma membrane. Prokaryotes Outer boundaries: Cell wall have a single chromosome (loop of DNA) located within a re- Plasma membrane gion called the nucleoid but it is not bounded by membrane. Cytoplasm: Ribosomes Many prokaryotes also have small accessory rings of DNA Thylakoids (cyanobacteria) called plasmids. The cytoplasm has thousands of ribosomes Innumerable enzymes for the synthesis of proteins. In addition, the photosynthetic Nucleoid: Chromosome (DNA only) flagellum ribosome plasma membrane cytoplasm cell wall nucleoid ribosome slime layer nucleoid capsule thylakoid cell wall cytoplasm plasma membrane 250 nm 25 µm a. b. FIGURE 3.16 Bacterial cells. a. Nonphotosynthetic bacterium. b. Cyanobacterium, a photosynthetic bacterium, formerly called a blue-green alga. mad21975_ch03pg45_66 30/07/2004 9:38 Page 63 FPG-04 FPG-04:Desktop Folder:GQ229: EQA Chapter 3 cell structure and function 63 3.4Evolution of the vesicle, and the inner one may be derived from the plasma membrane of the original prokaryote. Eukaryotic Cell 3. Mitochondria and chloroplasts contain a limited amount of genetic material and divide by splitting. How did the eukaryotic cell arise? Invagination of the Their DNA (deoxyribonucleic acid) is a circular loop plasma membrane might explain the origin of the nuclear like that of prokaryotes. envelope and organelles, such as the endoplasmic reticu- 4. Although most of the proteins within mitochondria lum and the Golgi apparatus. Some believe that the other and chloroplasts are now produced by the eukaryotic organelles could also have arisen in this manner. host, they do have their own ribosomes and they do Another, more interesting, hypothesis has been put produce some proteins. Their ribosomes resemble forth. It has been observed that, in the laboratory, an amoeba those of prokaryotes. infected with bacteria can become dependent upon them. 5. The RNA (ribonucleic acid) base sequence of the Some investigators believe that mitochondria and chloro- ribosomes in chloroplasts and mitochondria also plasts are derived from prokaryotes that were taken up by a suggests a prokaryotic origin of these organelles. much larger cell (Fig. 3.17). Perhaps mitochondria were orig- inally aerobic heterotrophic bacteria, and chloroplasts were It is also just possible that the ﬂagella of eukaryotes are originally cyanobacteria. The host eukaryotic cell would derived from an elongated bacterium that became attached have beneﬁted from an ability to utilize oxygen or synthesize to a host cell (Fig. 3.17). However, it is important to remem- organic food when, by chance, the prokaryote was taken up ber that the ﬂagella of eukaryotes are constructed differ- and not destroyed. In other words, after these prokaryotes ently. In any case, the acquisition of basal bodies, which entered by endocytosis, a symbiotic relationship would have could have become centrioles, may have led to the ability to been established. Some of the evidence for this endosymbiotic form a spindle during cell division. hypothesis is as follows: 1. Mitochondria and chloroplasts are similar to bacteria According to the endosymbiotic hypothesis, in size and in structure. heterotrophic bacteria became mitochondria, and 2. Both organelles are bounded by a double membrane— cyanobacteria became chloroplasts after being taken the outer membrane may be derived from the engulﬁng up by precursors to modern-day eukaryotic cells. aerobic bacterium plasma membrane bacterium Animal cell has a flagellum and endoplasmic mitochondria. nucleus reticulum nuclear envelope cyanobacterium Cell has a nucleus Cell has mitochondria. Plant cell has chloroplasts and and other organelles. mitochondria. FIGURE 3.17 Evolution of the eukaryotic cell. Invagination of the plasma membrane could account for the formation of the nucleus and certain other organelles. The endosymbiotic hypothesis suggests that mitochondria, chloroplasts, and ﬂagella are derived from prokaryotes that were taken up by a much larger eukaryotic cell. mad21975_ch03pg45_66 8/4/04 1:05 Page 64 EQA ubioethical focus Use of Stem Cells S tem cells are immature cells that death camp inmates—“after all, they are go- blood for future use. Once researchers have develop into mature, differentiated ing to be killed anyway.” the know-how, it may eventually be possible cells that make up the adult body. For Parkinson disease and Alzheimer disease to use any type of stem cell to cure many of the example, the red bone marrow contains stem are debilitating neurological disorders that disorders afflicting human beings. cells for all the many different types of blood people fear. It is possible that one day these cells in the bloodstream. Embryonic cells are disorders could be cured by supplying the pa- an even more suitable source of stem cells. tient with new nerve cells in a critical area of Decide Your Opinion The early embryo is simply a ball of cells, and the brain. Suppose you had one of these disor- 1. Should researchers have access to embry- each of the

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