Exam 2 PDF - Biology

6 OVERVIEW The Fundamental Units of Life Given the scope of biology, you may wonder sometimes how...

6 OVERVIEW The Fundamental Units of Life Given the scope of biology, you may wonder sometimes how you will ever learn all the material in this course! The answer involves cells, which are as fundamental to the living systems of biology as the atom is to chemistry. The contraction of mus- cle cells moves your eyes as you read this sentence. The words A Tour of the Cell on the page are translated into signals that nerve cells carry to your brain. Figure 6.1 shows extensions from one nerve cell (purple) making contact with another nerve cell (orange) in the brain. As you study, your goal is to make connections like these that solidify memories and permit learning to occur. All organisms are made of cells. In the hierarchy of biolog- ical organization, the cell is the simplest collection of matter that can be alive. Indeed, many forms of life exist as single- celled organisms. More complex organisms, including plants and animals, are multicellular; their bodies are cooperatives of many kinds of specialized cells that could not survive for long on their own. Even when cells are arranged into higher levels of organization, such as tissues and organs, the cell re- mains the organism’s basic unit of structure and function. All cells are related by their descent from earlier cells. How- ever, they have been modified in many different ways during the long evolutionary history of life on Earth. But although cells can differ substantially from one another, they share common features. In this chapter, we’ll first examine the tools and techniques that allow us to understand cells, then tour the cell and become acquainted with its components. 䉱 Figure 6.1 How do your brain cells help CONCEPT 6.1 you learn about biology? Biologists use microscopes and the tools of biochemistry to study cells KEY CONCEPTS How can cell biologists investigate the inner workings of a cell, 6.1 Biologists use microscopes and the tools of usually too small to be seen by the unaided eye? Before we biochemistry to study cells tour the cell, it will be helpful to learn how cells are studied. 6.2 Eukaryotic cells have internal membranes that Microscopy compartmentalize their functions 6.3 The eukaryotic cell’s genetic instructions are The development of instruments that extend the human housed in the nucleus and carried out by the senses has gone hand in hand with the advance of science. ribosomes The discovery and early study of cells progressed with the in- 6.4 The endomembrane system regulates protein vention of microscopes in 1590 and their refinement during traffic and performs metabolic functions in the the 1600s. Cell walls were first seen by Robert Hooke in 1665 cell as he looked through a microscope at dead cells from the 6.5 Mitochondria and chloroplasts change energy bark of an oak tree. But it took the wonderfully crafted lenses from one form to another of Antoni van Leeuwenhoek to visualize living cells. Imagine 6.6 The cytoskeleton is a network of fibers that Hooke’s awe when he visited van Leeuwenhoek in 1674 and organizes structures and activities in the cell the world of microorganisms—what his host called “very lit- 6.7 Extracellular components and connections tle animalcules”—was revealed to him. between cells help coordinate cellular activities The microscopes first used by Renaissance scientists, as well as the microscopes you are likely to use in the laboratory, are 94 UNIT TWO The Cell all light microscopes. In a light microscope (LM), visible 10 m light is passed through the specimen and then through glass Human height lenses. The lenses refract (bend) the light in such a way that 1m the image of the specimen is magnified as it is projected into Length of some nerve and the eye or into a camera (see Appendix D). Unaided eye muscle cells Three important parameters in microscopy are magnifica- 0.1 m tion, resolution, and contrast. Magnification is the ratio of an Chicken egg object’s image size to its real size. Light microscopes can mag- nify effectively to about 1,000 times the actual size of the 1 cm specimen; at greater magnifications, additional details can- not be seen clearly. Resolution is a measure of the clarity of the Frog egg 1 mm image; it is the minimum distance two points can be sepa- rated and still be distinguished as two points. For example, Light microscopy what appears to the unaided eye as one star in the sky may be Human egg 100 μm resolved as twin stars with a telescope, which has a higher re- Most plant and solving ability than the eye. Similarly, using standard tech- animal cells niques, the light microscope cannot resolve detail finer than 10 μm Nucleus about 0.2 micrometer (μm), or 200 nanometers (nm), regard- Most bacteria less of the magnification (Figure 6.2). The third parameter, Electron microscopy Mitochondrion contrast, accentuates differences in parts of the sample. Im- 1 μm provements in light microscopy have included new methods for enhancing contrast, such as staining or labeling cell com- Smallest bacteria Super- 100 nm ponents to stand out visually. Figure 6.3, on the next page, Viruses resolution shows different types of microscopy; study this figure as you microscopy Ribosomes read the rest of this section. 10 nm Until recently, the resolution barrier prevented cell biolo- Proteins gists from using standard light microscopy to study Lipids organelles, the membrane-enclosed structures within eu- 1 nm karyotic cells. To see these structures in any detail required the Small molecules development of a new instrument. In the 1950s, the electron 0.1 nm Atoms microscope was introduced to biology. Rather than light, the electron microscope (EM) focuses a beam of electrons 1 centimeter (cm) = 10 –2 meter (m) = 0.4 inch 1 millimeter (mm) = 10 –3 m through the specimen or onto its surface (see Appendix D). 1 micrometer (μm) = 10 –3 mm = 10 –6 m Resolution is inversely related to the wavelength of the radia- 1 nanometer (nm) = 10 –3 μm = 10 –9 m tion a microscope uses for imaging, and electron beams have 䉱 Figure 6.2 The size range of cells. Most cells are between 1 much shorter wavelengths than visible light. Modern electron and 100 μm in diameter (yellow region of chart) and are therefore microscopes can theoretically achieve a resolution of about visible only under a microscope. Notice that the scale along the left side is logarithmic to accommodate the range of sizes shown. Starting 0.002 nm, though in practice they usually cannot resolve at the top of the scale with 10 m and going down, each reference structures smaller than about 2 nm across. Still, this is a hun- measurement marks a tenfold decrease in diameter or length. For a dredfold improvement over the standard light microscope. complete table of the metric system, see Appendix C. The scanning electron microscope (SEM) is especially useful for detailed study of the topography of a specimen (see atoms of heavy metals, which attach to certain cellular struc- Figure 6.3). The electron beam scans the surface of the sam- tures, thus enhancing the electron density of some parts of ple, usually coated with a thin film of gold. The beam excites the cell more than others. The electrons passing through the electrons on the surface, and these secondary electrons are de- specimen are scattered more in the denser regions, so fewer tected by a device that translates the pattern of electrons into are transmitted. The image displays the pattern of transmitted an electronic signal to a video screen. The result is an image of electrons. Instead of using glass lenses, the TEM uses electro- the specimen’s surface that appears three-dimensional. magnets as lenses to bend the paths of the electrons, ulti- The transmission electron microscope (TEM) is used mately focusing the image onto a monitor for viewing. to study the internal structure of cells (see Figure 6.3). The Electron microscopes have revealed many organelles and TEM aims an electron beam through a very thin section of the other subcellular structures that were impossible to resolve specimen, similar to the way a light microscope transmits with the light microscope. But the light microscope offers ad- light through a slide. The specimen has been stained with vantages, especially in studying living cells. A disadvantage of CHAPTER 6 A Tour of the Cell 95 䉲 Figure 6.3 Exploring Microscopy Light Microscopy (LM) Brightfield (unstained specimen). Confocal. The top image is a standard Light passes directly through the fluorescence micrograph of fluores- specimen. Unless the cell is naturally cently labeled nervous tissue (nerve pigmented or artificially stained, the cells are green, support cells are image has little contrast. (The first four orange, and regions of overlap are 50 μm light micrographs show human cheek yellow); below it is a confocal image epithelial cells; the scale bar pertains to of the same tissue. Using a laser, this all four micrographs.) “optical sectioning” technique elimi- nates out-of-focus light from a thick Brightfield (stained specimen). sample, creating a single plane of Staining with various dyes enhances fluorescence in the image. By capturing contrast. Most staining procedures require sharp images at many different planes, 50 μm that cells be fixed (preserved). a 3-D reconstruction can be created. The standard image is blurry because out-of-focus light is not excluded. Deconvolution. The top of this split image is a compilation of standard Phase-contrast. Variations in density fluorescence micrographs through the within the specimen are amplified to depth of a white blood cell. Below is an enhance contrast in unstained cells, which image of the same cell reconstructed is especially useful for examining living, from many blurry images at different unpigmented cells. planes, each of which was processed using deconvolution software. This 10 μm process digitally removes out-of-focus light and reassigns it to its source, creat- ing a much sharper 3-D image. Differential-interference-contrast (Nomarski). As in phase-contrast micro- Super-resolution. On the top is a scopy, optical modifications are used to confocal image of part of a nerve cell, exaggerate differences in density, making using a fluorescent label that binds to a the image appear almost 3-D. molecule clustered in small sacs in the cell (vesicles) that are 40 nm in diameter. The greenish-yellow spots are blurry because 40 nm is below the 200-nm limit of resolution for standard light Fluorescence. The locations of specific mol- microscopy. Below is an image of the ecules in the cell can be revealed by labeling the same part of the cell, seen using a new molecules with fluorescent dyes or antibodies; “super-resolution” technique. Sophisti- some cells have molecules that fluoresce on cated equipment is used to light up indi- their own. Fluorescent substances absorb vidual fluorescent molecules and record ultraviolet radiation and emit visible light. In their position. Combining information this fluorescently labeled uterine cell, nuclear from many molecules in different places material is blue, organelles called mitochondria “breaks” the limit of resolution, result- 1 μm are orange, and the cell’s “skeleton” is green. ing in the sharp greenish-yellow dots 10 μm seen here. (Each dot is a 40-nm vesicle.) Electron Microscopy (EM) Scanning electron microscopy (SEM). Micrographs taken with a scan- Longitudinal section Cross section Transmission electron ning electron microscope show a 3-D image of the surface of a specimen. of cilium of cilium microscopy (TEM). This SEM shows the surface of a cell from a trachea (wind- A transmission electron Cilia pipe) covered with cilia. Beating of the cilia helps move microscope profiles a thin inhaled debris upward toward the throat. The SEM and section of a specimen. Here TEM shown here have been artificially colorized. (Electron we see a section through micrographs are black and white, but are often artificially a tracheal cell, revealing colorized to highlight particular structures.) its internal structure. In preparing the TEM, some cilia were cut along their Abbreviations used in this book: lengths, creating longitudi- LM = Light Micrograph nal sections, while other cilia SEM = Scanning Electron Micrograph were cut straight across, TEM = Transmission Electron Micrograph 2 μm creating cross sections. 2 μm 96 UNIT TWO The Cell electron microscopy is that the methods used to prepare the 䉲 Figure 6.4 RESEARCH METHOD specimen kill the cells. For all microscopy techniques, in fact, specimen preparation can introduce artifacts, structural fea- Cell Fractionation tures seen in micrographs that do not exist in the living cell. APPLICATION Cell fractionation is used to isolate (fractionate) cell In the past several decades, light microscopy has been revi- components based on size and density. talized by major technical advances (see Figure 6.3). Labeling TECHNIQUE Cells are homogenized in a blender to break them up. The individual cellular molecules or structures with fluorescent resulting mixture (homogenate) is centrifuged. The supernatant (liquid) is poured into another tube and centrifuged at a higher speed for a markers has made it possible to see such structures with in- longer time. This process is repeated several times. This “differential cen- creasing detail. In addition, both confocal and deconvolution trifugation” results in a series of pellets, each containing different cell microscopy have sharpened images of three-dimensional tis- components. sues and cells. Finally, over the past ten years, a group of new techniques and labeling molecules have allowed researchers to “break” the resolution barrier and distinguish subcellular structures as small as 10–20 nm across. As this “super- resolution microscopy” becomes more widespread, the im- ages we’ll see of living cells may well be as awe-inspiring to us as van Leeuwenhoek’s were to Robert Hooke 350 years ago. Homogenization Microscopes are the most important tools of cytology, the Tissue study of cell structure. To understand the function of each cells structure, however, required the integration of cytology and Homogenate biochemistry, the study of the chemical processes (metabo- Centrifuged at lism) of cells. 1,000 g (1,000 times the Centrifugation Cell Fractionation force of gravity) for 10 min A useful technique for studying cell structure and function is Supernatant poured into next cell fractionation, which takes cells apart and separates tube major organelles and other subcellular structures from one an- Differential other (Figure 6.4). The instrument used is the centrifuge, 20,000 g centrifugation which spins test tubes holding mixtures of disrupted cells at a 20 min series of increasing speeds. At each speed, the resulting force causes a fraction of the cell components to settle to the bottom of the tube, forming a pellet. At lower speeds, the pellet con- 80,000 g sists of larger components, and higher speeds yield a pellet 60 min with smaller components. Pellet rich in nuclei and Cell fractionation enables researchers to prepare specific cellular debris cell components in bulk and identify their functions, a task 150,000 g not usually possible with intact cells. For example, on one of 3 hr the cell fractions, biochemical tests showed the presence of Pellet rich in enzymes involved in cellular respiration, while electron mitochondria microscopy revealed large numbers of the organelles called (and chloro- plasts if cells mitochondria. Together, these data helped biologists deter- are from a plant) mine that mitochondria are the sites of cellular respiration. Biochemistry and cytology thus complement each other in Pellet rich in “microsomes” correlating cell function with structure. (pieces of plasma membranes and CONCEPT CHECK 6.1 cells’ internal membranes) Pellet rich in 1. How do stains used for light microscopy compare ribosomes with those used for electron microscopy? 2. WHAT IF? Which type of microscope would you use RESULTS In early experiments, researchers used microscopy to identify to study (a) the changes in shape of a living white blood the organelles in each pellet and biochemical methods to determine their metabolic functions. These identifications established a baseline for cell and (b) the details of surface texture of a hair? this method, enabling today’s researchers to know which cell fraction For suggested answers, see Appendix A. they should collect in order to isolate and study particular organelles. CHAPTER 6 A Tour of the Cell 97 CONCEPT 6.2 the word prokaryotic means “before nucleus” (from the Greek pro, before), reflecting the fact that prokaryotic cells evolved before eukaryotic cells. Eukaryotic cells have internal The interior of either type of cell is called the cytoplasm; in membranes that compartmentalize eukaryotic cells, this term refers only to the region between the their functions nucleus and the plasma membrane. Within the cytoplasm of a eukaryotic cell, suspended in cytosol, are a variety of organelles Cells—the basic structural and functional units of every of specialized form and function. These membrane-bounded organism—are of two distinct types: prokaryotic and eukary- structures are absent in prokaryotic cells. Thus, the presence or otic. Organisms of the domains Bacteria and Archaea consist absence of a true nucleus is just one aspect of the disparity in of prokaryotic cells. Protists, fungi, animals, and plants all structural complexity between the two types of cells. consist of eukaryotic cells. Eukaryotic cells are generally much larger than prokary- otic cells (see Figure 6.2). Size is a general feature of cell struc- Comparing Prokaryotic and Eukaryotic Cells ture that relates to function. The logistics of carrying out All cells share certain basic features: They are all bounded by cellular metabolism sets limits on cell size. At the lower limit, a selective barrier, called the plasma membrane. Inside all cells the smallest cells known are bacteria called mycoplasmas, is a semifluid, jellylike substance called cytosol, in which which have diameters between 0.1 and 1.0 μm. These are subcellular components are suspended. All cells contain perhaps the smallest packages with enough DNA to program chromosomes, which carry genes in the form of DNA. And all metabolism and enough enzymes and other cellular equip- cells have ribosomes, tiny complexes that make proteins ac- ment to carry out the activities necessary for a cell to sustain cording to instructions from the genes. itself and reproduce. Typical bacteria are 1–5 μm in diameter, A major difference between prokaryotic and eukaryotic cells about ten times the size of mycoplasmas. Eukaryotic cells are is the location of their DNA. In a eukaryotic cell, most of the typically 10–100 μm in diameter. DNA is in an organelle called the nucleus, which is bounded by Metabolic requirements also impose theoretical upper limits a double membrane (see Figure 6.8, on pp. 100–101). In a on the size that is practical for a single cell. At the boundary of prokaryotic cell, the DNA is concentrated in a region that every cell, the plasma membrane functions as a selective is not membrane-enclosed, called the nucleoid (Figure 6.5). barrier that allows passage of enough oxygen, nutrients, and The word eukaryotic means “true nucleus” (from the Greek eu, wastes to service the entire cell (Figure 6.6). For each square mi- true, and karyon, kernel, here referring to the nucleus), and crometer of membrane, only a limited amount of a particular Fimbriae: attachment structures on the surface of some prokaryotes Nucleoid: region where the cell’s DNA is located (not enclosed by a membrane) Ribosomes: complexes that synthesize proteins Plasma membrane: membrane enclosing the cytoplasm Bacterial Cell wall: rigid structure outside chromosome the plasma membrane Capsule: jellylike outer coating of many prokaryotes 0.5 μm Flagella: locomotion (a) A typical organelles of (b) A thin section through the rod-shaped bacterium some bacteria bacterium Bacillus coagulans (TEM) 䉱 Figure 6.5 A prokaryotic cell. Lacking a true nucleus and the other membrane-enclosed organelles of the eukaryotic cell, the prokaryotic cell is much simpler in structure. Prokaryotes include bacteria and archaea; the general cell structure of the two domains is essentially the same. 98 UNIT TWO The Cell  (a) TEM of a plasma membrane. The Surface area increases while Outside of cell plasma membrane, here in a red blood total volume remains constant cell, appears as a pair of dark bands separated by a light band. 5 Inside of cell 1 0.1 μm 1 Carbohydrate side chains Total surface area [sum of the surface areas 6 150 750 (height × width) of all box Hydrophilic sides × number of boxes] region Total volume [height × width × length 1 125 125 × number of boxes] Hydrophobic region Surface-to-volume Hydrophilic (S-to-V) ratio 6 1.2 6 region Phospholipid Proteins [surface area ÷ volume] (b) Structure of the plasma membrane 䉱 Figure 6.7 Geometric relationships between surface area 䉱 Figure 6.6 The plasma membrane. The plasma membrane and and volume. In this diagram, cells are represented as boxes. Using the membranes of organelles consist of a double layer (bilayer) of arbitrary units of length, we can calculate the cell’s surface area (in phospholipids with various proteins attached to or embedded in it. The square units, or units2), volume (in cubic units, or units3), and ratio of hydrophobic parts, including phospholipid tails and interior portions of surface area to volume. A high surface-to-volume ratio facilitates the membrane proteins, are found in the interior of the membrane. The exchange of materials between a cell and its environment. hydrophilic parts, including phospholipid heads, exterior portions of proteins, and channels of proteins, are in contact with the aqueous solution. Carbohydrate side chains may be attached to proteins or lipids on the outer surface of the plasma membrane. A Panoramic View of the Eukaryotic Cell MAKE CONNECTIONS Review Figure 5.12 (p. 76) and describe the characteristics of a phospholipid that allow it to function as the major In addition to the plasma membrane at its outer surface, a eu- component in the plasma membrane. karyotic cell has extensive and elaborately arranged internal membranes that divide the cell into compartments—the or- substance can cross per second, so the ratio of surface area to ganelles mentioned earlier. The cell’s compartments provide volume is critical. As a cell (or any other object) increases in different local environments that facilitate specific metabolic size, its volume grows proportionately more than its surface functions, so incompatible processes can go on simultane- area. (Area is proportional to a linear dimension squared, ously inside a single cell. The plasma membrane and organelle whereas volume is proportional to the linear dimension membranes also participate directly in the cell’s metabolism, cubed.) Thus, a smaller object has a greater ratio of surface area because many enzymes are built right into the membranes. to volume (Figure 6.7). Because membranes are so fundamental to the organiza- The need for a surface area sufficiently large to accommo- tion of the cell, Chapter 7 will discuss them in detail. The date the volume helps explain the microscopic size of most basic fabric of most biological membranes is a double layer of cells and the narrow, elongated shapes of others, such as phospholipids and other lipids. Embedded in this lipid bi- nerve cells. Larger organisms do not generally have larger layer or attached to its surfaces are diverse proteins (see cells than smaller organisms—they simply have more cells Figure 6.6). However, each type of membrane has a unique (see Figure 6.7). A sufficiently high ratio of surface area to vol- composition of lipids and proteins suited to that membrane’s ume is especially important in cells that exchange a lot of specific functions. For example, enzymes embedded in the material with their surroundings, such as intestinal cells. membranes of the organelles called mitochondria function Such cells may have many long, thin projections from their in cellular respiration. surface called microvilli, which increase surface area without Before continuing with this chapter, examine the eukary- an appreciable increase in volume. otic cells in Figure 6.8, on the next two pages. The general- The evolutionary relationships between prokaryotic and ized diagrams of an animal cell and a plant cell introduce the eukaryotic cells will be discussed later in this chapter, and various organelles and highlight the key differences between prokaryotic cells will be described in detail in Chapter 27. animal and plant cells. The micrographs at the bottom of the Most of the discussion of cell structure that follows in this figure give you a glimpse of cells from different types of eu- chapter applies to eukaryotic cells. karyotic organisms. CHAPTER 6 A Tour of the Cell 99  䉲 Figure 6.8 Exploring Eukaryotic Cells Animal Cell (cutaway view of generalized cell) Nuclear envelope: double ENDOPLASMIC RETICULUM (ER): network membrane enclosing the of membranous sacs and tubes; active in nucleus; perforated by membrane synthesis and other synthetic pores; continuous with ER Flagellum: motility and metabolic processes; has rough structure present in (ribosome-studded) and smooth regions Nucleolus: nonmembranous some animal cells, structure involved in production composed of a cluster of of ribosomes; a nucleus has NUCLEUS Rough ER Smooth ER microtubules within an one or more nucleoli extension of the plasma membrane Chromatin: material consisting of DNA and proteins; visible in a dividing cell as individual Centrosome: region condensed chromosomes where the cell’s microtubules are initiated; contains a pair of centrioles Plasma membrane: membrane enclosing the cell CYTOSKELETON: reinforces cell’s shape; functions in cell movement; components are made of protein. Includes: Microfilaments Intermediate filaments Ribosomes (small brown Microtubules dots): complexes that make proteins; free in cytosol or bound to rough ER or nuclear envelope Microvilli: projections that increase the cell’s surface area Golgi apparatus: organelle active in synthesis, modification, sorting, and secretion of cell products Peroxisome: organelle with various specialized In animal cells but not plant cells: metabolic functions; Lysosome: digestive Lysosomes produces hydrogen Mitochondrion: organelle where organelle where Centrosomes, with centrioles peroxide as a by-product, cellular respiration occurs and macromolecules are Flagella (but present in some plant sperm) then converts it to water most ATP is generated hydrolyzed Parent 1 μm 10 μm cell Cell wall Animal Cells Fungal Cells Buds Vacuole Cell 5 μm Nucleus Nucleolus Yeast cells: reproducing by budding Nucleus (above, colorized SEM) and a single cell Human cells from lining of uterus (colorized TEM) (right, colorized TEM) Mitochondrion 100 UNIT TWO The Cell Plant Cell (cutaway view of generalized cell) ANIMATION Visit the Study Area at Nuclear envelope Rough www.masteringbiology.com for the NUCLEUS endoplasmic Smooth BioFlix® 3-D Animations Tour of an Nucleolus reticulum endoplasmic Animal Cell and Tour of a Plant Cell. Chromatin reticulum Ribosomes (small brown dots) Central vacuole: prominent organelle in older plant cells; functions include storage, breakdown of waste products, hydrolysis of Golgi apparatus macromolecules; enlargement of vacuole is a major mechanism of plant growth Microfilaments Intermediate filaments CYTOSKELETON Microtubules Mitochondrion Peroxisome Chloroplast: photosynthetic Plasma membrane organelle; converts energy of sunlight to chemical energy stored in sugar molecules Cell wall: outer layer that maintains cell’s shape and protects cell from mechanical damage; made of cellulose, other polysaccharides, and protein In plant cells but not animal cells: Plasmodesmata: cytoplasmic channels through cell walls that Chloroplasts connect the cytoplasms of Central vacuole Wall of adjacent cell adjacent cells Cell wall Plasmodesmata Cell Flagella 1 μm 5 μm 8 μm Cell wall Protistan Cells Plant Cells Chloroplast Nucleus Mitochondrion Nucleolus Nucleus Vacuole Nucleolus Unicellular green alga Chlamy- Chloroplast Cells from duckweed (Spirodela oligorrhiza), domonas (above, colorized SEM; a floating plant (colorized TEM) right, colorized TEM) Cell wall CHAPTER 6 A Tour of the Cell 101 CONCEPT CHECK 6.2 the DNA molecule of each chromosome, reducing its length and allowing it to fit into the nucleus. The complex of DNA 1. After carefully reviewing Figure 6.8, briefly describe and proteins making up chromosomes is called chromatin. the structure and function of the nucleus, the mito- When a cell is not dividing, stained chromatin appears as a dif- chondrion, the chloroplast, and the endoplasmic fuse mass in micrographs, and the chromosomes cannot be reticulum. distinguished from one another, even though discrete chromo- 2. WHAT IF? Imagine an elongated cell (such as a somes are present. As a cell prepares to divide, however, the nerve cell) that measures 125 ⫻ 1 ⫻ 1 arbitrary units. chromosomes coil (condense) further, becoming thick enough Predict how its surface-to-volume ratio would com- to be distinguished as separate structures. Each eukaryotic pare with those in Figure 6.7. Then calculate the ratio species has a characteristic number of chromosomes. For ex- and check your prediction. ample, a typical human cell has 46 chromosomes in its nu- For suggested answers, see Appendix A. cleus; the exceptions are the sex cells (eggs and sperm), which have only 23 chromosomes in humans. A fruit fly cell has 8 chromosomes in most cells and 4 in the sex cells. CONCEPT 6.3 A prominent structure within the nondividing nucleus is the nucleolus (plural, nucleoli), which appears through the The eukaryotic cell’s genetic electron microscope as a mass of densely stained granules and instructions are housed fibers adjoining part of the chromatin. Here a type of RNA in the nucleus and carried called ribosomal RNA (rRNA) is synthesized from instructions in the DNA. Also in the nucleolus, proteins imported from out by the ribosomes the cytoplasm are assembled with rRNA into large and small On the first stop of our detailed tour of the cell, let’s look at two subunits of ribosomes. These subunits then exit the nucleus cellular components involved in the genetic control of the cell: through the nuclear pores to the cytoplasm, where a large and the nucleus, which houses most of the cell’s DNA, and the ribo- a small subunit can assemble into a ribosome. Sometimes somes, which use information from the DNA to make proteins. there are two or more nucleoli; the number depends on the species and the stage in the cell’s reproductive cycle. The Nucleus: Information Central As we saw in Figure 5.25, the nucleus directs protein syn- The nucleus contains most of the genes in the eukaryotic thesis by synthesizing messenger RNA (mRNA) according to cell. (Some genes are located in mitochondria and chloro- instructions provided by the DNA. The mRNA is then trans- plasts.) It is generally the most conspicuous organelle in a eu- ported to the cytoplasm via the nuclear pores. Once an mRNA karyotic cell, averaging about 5 μm in diameter. The nuclear molecule reaches the cytoplasm, ribosomes translate the envelope encloses the nucleus (Figure 6.9), separating its mRNA’s genetic message into the primary structure of a spe- contents from the cytoplasm. cific polypeptide. This process of transcribing and translating The nuclear envelope is a double membrane. The two mem- genetic information is described in detail in Chapter 17. branes, each a lipid bilayer with associated proteins, are sepa- rated by a space of 20–40 nm. The envelope is perforated by Ribosomes: Protein Factories pore structures that are about 100 nm in diameter. At the lip of Ribosomes, which are complexes made of ribosomal RNA each pore, the inner and outer membranes of the nuclear en- and protein, are the cellular components that carry out pro- velope are continuous. An intricate protein structure called a tein synthesis (Figure 6.10). Cells that have high rates of pro- pore complex lines each pore and plays an important role in the tein synthesis have particularly large numbers of ribosomes. cell by regulating the entry and exit of proteins and RNAs, as For example, a human pancreas cell has a few million ribo- well as large complexes of macromolecules. Except at the somes. Not surprisingly, cells active in protein synthesis also pores, the nuclear side of the envelope is lined by the nuclear have prominent nucleoli. lamina, a netlike array of protein filaments that maintains Ribosomes build proteins in two cytoplasmic locales. At the shape of the nucleus by mechanically supporting the nu- any given time, free ribosomes are suspended in the cytosol, clear envelope. There is also much evidence for a nuclear while bound ribosomes are attached to the outside of the en- matrix, a framework of protein fibers extending throughout doplasmic reticulum or nuclear envelope (see Figure 6.10). the nuclear interior. The nuclear lamina and matrix may help Bound and free ribosomes are structurally identical, and ribo- organize the genetic material so it functions efficiently. somes can alternate between the two roles. Most of the pro- Within the nucleus, the DNA is organized into discrete units teins made on free ribosomes function within the cytosol; called chromosomes, structures that carry the genetic infor- examples are enzymes that catalyze the first steps of sugar mation. Each chromosome contains one long DNA molecule breakdown. Bound ribosomes generally make proteins that associated with many proteins. Some of the proteins help coil are destined for insertion into membranes, for packaging 102 UNIT TWO The Cell  Nucleus 1 μm Nucleus Nucleolus Chromatin Nuclear envelope: Inner membrane Outer membrane Nuclear pore Rough ER Pore complex 䉱 Surface of nuclear envelope. TEM of a specimen prepared by Ribosome a technique known as freeze-fracture. 䉳 Close-up 0.25 μm of nuclear 䉱 Chromatin. Part of a envelope chromosome from a non- dividing cell shows two states of coiling of the DNA (blue) and protein (purple) complex. The thicker 1 μm form is sometimes also organized into long loops. 䉱 Pore complexes (TEM). Each pore is ringed by protein particles. 䉳 Nuclear lamina (TEM). The netlike lamina lines the inner surface of the nuclear envelope. 䉱 Figure 6.9 The nucleus and its which function in ribosome synthesis. The MAKE CONNECTIONS Since the chromosomes envelope. Within the nucleus are the nuclear envelope, which consists of two contain the genetic material and reside in the chromosomes, which appear as a mass of membranes separated by a narrow space, is nucleus, how does the rest of the cell get access chromatin (DNA and associated proteins), and perforated with pores and lined by the nuclear to the information they carry? See Figure 5.25, one or more nucleoli (singular, nucleolus), lamina. page 86. 0.25 μm Ribosomes ER Free ribosomes in cytosol 䉴 Figure 6.10 Ribosomes. This electron Endoplasmic reticulum (ER) micrograph of part of a pancreas cell shows many ribosomes, both free (in the cytosol) and Ribosomes bound to ER bound (to the endoplasmic reticulum). The simplified diagram of a ribosome shows its Large two subunits. subunit DRAW IT After you have read the section on ribosomes, circle a ribosome in the micro- Small graph that might be making a protein that will subunit be secreted. TEM showing ER and ribosomes Diagram of a ribosome CHAPTER 6 A Tour of the Cell 103 within certain organelles such as lysosomes (see Figure 6.8), Latin for “little net.”) The ER consists of a network of mem- or for export from the cell (secretion). Cells that specialize in branous tubules and sacs called cisternae (from the Latin protein secretion—for instance, the cells of the pancreas that cisterna, a reservoir for a liquid). The ER membrane separates secrete digestive enzymes—frequently have a high propor- the internal compartment of the ER, called the ER lumen tion of bound ribosomes. You will learn more about ribo- (cavity) or cisternal space, from the cytosol. And because the some structure and function in Chapter 17. ER membrane is continuous with the nuclear envelope, the space between the two membranes of the envelope is contin- CONCEPT CHECK 6.3 uous with the lumen of the ER (Figure 6.11). 1. What role do ribosomes play in carrying out genetic instructions? 2. Describe the molecular composition of nucleoli and explain their function. 3. WHAT IF? As a cell begins the process of dividing, its chromatin becomes more and more condensed. Does the number of chromosomes change during this process? Explain. For suggested answers, see Appendix A. Smooth ER CONCEPT 6.4 Rough ER Nuclear envelope The endomembrane system regulates protein traffic and performs metabolic functions in the cell Many of the different membranes of the eukaryotic cell are part of the endomembrane system, which includes the ER lumen nuclear envelope, the endoplasmic reticulum, the Golgi appa- Cisternae Transitional ER ratus, lysosomes, various kinds of vesicles and vacuoles, and Ribosomes the plasma membrane. This system carries out a variety of Transport vesicle tasks in the cell, including synthesis of proteins, transport of 200 nm Smooth ER Rough ER proteins into membranes and organelles or out of the cell, metabolism and movement of lipids, and detoxification of poisons. The membranes of this system are related either through direct physical continuity or by the transfer of mem- brane segments as tiny vesicles (sacs made of membrane). Despite these relationships, the various membranes are not identical in structure and function. Moreover, the thickness, molecular composition, and types of chemical reactions car- ried out in a given membrane are not fixed, but may be modi- fied several times during the membrane’s life. Having already discussed the nuclear envelope, we will now focus on the en- doplasmic reticulum and the other endomembranes to which the endoplasmic reticulum gives rise. 䉱 Figure 6.11 Endoplasmic reticulum (ER). A membranous The Endoplasmic Reticulum: system of interconnected tubules and flattened sacs called cisternae, Biosynthetic Factory the ER is also continuous with the nuclear envelope. (The drawing is a cutaway view.) The membrane of the ER encloses a continuous The endoplasmic reticulum (ER) is such an extensive compartment called the ER lumen (or cisternal space). Rough ER, which is studded on its outer surface with ribosomes, can be distinguished network of membranes that it accounts for more than half from smooth ER in the electron micrograph (TEM). Transport vesicles bud the total membrane in many eukaryotic cells. (The word off from a region of the rough ER called transitional ER and travel to the endoplasmic means “within the cytoplasm,” and reticulum is Golgi apparatus and other destinations. 104 UNIT TWO The Cell  There are two distinct, though connected, regions of the lumen through a pore formed by a protein complex in the ER ER that differ in structure and function: smooth ER and rough membrane. As the new polypeptide enters the ER lumen, it ER. Smooth ER is so named because its outer surface lacks ri- folds into its native shape. Most secretory proteins are bosomes. Rough ER is studded with ribosomes on the outer glycoproteins, proteins that have carbohydrates covalently surface of the membrane and thus appears rough through the bonded to them. The carbohydrates are attached to the pro- electron microscope. As already mentioned, ribosomes are teins in the ER by enzymes built into the ER membrane. also attached to the cytoplasmic side of the nuclear envelope’s After secretory proteins are formed, the ER membrane outer membrane, which is continuous with rough ER. keeps them separate from proteins that are produced by free ribosomes and that will remain in the cytosol. Secretory pro- Functions of Smooth ER teins depart from the ER wrapped in the membranes of vesi- The smooth ER functions in diverse metabolic processes, cles that bud like bubbles from a specialized region called which vary with cell type. These processes include synthesis transitional ER (see Figure 6.11). Vesicles in transit from one of lipids, metabolism of carbohydrates, detoxification of part of the cell to another are called transport vesicles; we drugs and poisons, and storage of calcium ions. will discuss their fate shortly. Enzymes of the smooth ER are important in the synthesis In addition to making secretory proteins, rough ER is a of lipids, including oils, phospholipids, and steroids. Among membrane factory for the cell; it grows in place by adding the steroids produced by the smooth ER in animal cells are membrane proteins and phospholipids to its own mem- the sex hormones of vertebrates and the various steroid hor- brane. As polypeptides destined to be membrane proteins mones secreted by the adrenal glands. The cells that synthe- grow from the ribosomes, they are inserted into the ER size and secrete these hormones—in the testes and ovaries, membrane itself and anchored there by their hydrophobic for example—are rich in smooth ER, a structural feature that portions. Like the smooth ER, the rough ER also makes fits the function of these cells. membrane phospholipids; enzymes built into the ER mem- Other enzymes of the smooth ER help detoxify drugs and brane assemble phospholipids from precursors in the cy- poisons, especially in liver cells. Detoxification usually in- tosol. The ER membrane expands and portions of it are volves adding hydroxyl groups to drug molecules, making transferred in the form of transport vesicles to other compo- them more soluble and easier to flush from the body. The nents of the endomembrane system. sedative phenobarbital and other barbiturates are examples of drugs metabolized in this manner by smooth ER in liver cells. The Golgi Apparatus: In fact, barbiturates, alcohol, and many other drugs induce Shipping and Receiving Center the proliferation of smooth ER and its associated detoxifica- After leaving the ER, many transport vesicles travel to the Golgi tion enzymes, thus increasing the rate of detoxification. This, apparatus. We can think of the Golgi as a warehouse for re- in turn, increases tolerance to the drugs, meaning that higher ceiving, sorting, shipping, and even some manufacturing. Here, doses are required to achieve a particular effect, such as seda- products of the ER, such as proteins, are modified and stored tion. Also, because some of the detoxification enzymes have and then sent to other destinations. Not surprisingly, the Golgi relatively broad action, the proliferation of smooth ER in re- apparatus is especially extensive in cells specialized for secretion. sponse to one drug can increase tolerance to other drugs as The Golgi apparatus consists of flattened membranous well. Barbiturate abuse, for example, can decrease the effec- sacs—cisternae—looking like a stack of pita bread (Figure 6.12, tiveness of certain antibiotics and other useful drugs. on the next page). A cell may have many, even hundreds, of The smooth ER also stores calcium ions. In muscle cells, these stacks. The membrane of each cisterna in a stack sepa- for example, the smooth ER membrane pumps calcium ions rates its internal space from the cytosol. Vesicles concentrated from the cytosol into the ER lumen. When a muscle cell is in the vicinity of the Golgi apparatus are engaged in the trans- stimulated by a nerve impulse, calcium ions rush back fer of material between parts of the Golgi and other structures. across the ER membrane into the cytosol and trigger contrac- A Golgi stack has a distinct structural directionality, with tion of the muscle cell. In other cell types, calcium ion release the membranes of cisternae on opposite sides of the stack dif- from the smooth ER triggers different responses, such as se- fering in thickness and molecular composition. The two sides cretion of vesicles carrying newly synthesized proteins. of a Golgi stack are referred to as the cis face and the trans face; these act, respectively, as the receiving and shipping depart- Functions of Rough ER ments of the Golgi apparatus. The cis face is usually located Many types of cells secrete proteins produced by ribosomes near the ER. Transport vesicles move material from the ER to attached to rough ER. For example, certain pancreatic cells the Golgi apparatus. A vesicle that buds from the ER can add synthesize the protein insulin in the ER and secrete this hor- its membrane and the contents of its lumen to the cis face by mone into the bloodstream. As a polypeptide chain grows fusing with a Golgi membrane. The trans face gives rise to from a bound ribosome, the chain is threaded into the ER vesicles that pinch off and travel to other sites. CHAPTER 6 A Tour of the Cell 105  䉲 Figure 6.12 The Golgi apparatus. The Golgi apparatus consists of stacks of flattened sacs, or cisternae, which, unlike ER cisternae, are not physically connected. (The drawing is a cutaway view.) A Golgi stack receives and dispatches transport vesicles and Golgi the products they contain. A Golgi stack has a structural and functional directionality, with apparatus a cis face that receives vesicles containing ER products and a trans face that dispatches vesicles. The cisternal maturation model proposes that the Golgi cisternae themselves “mature,” moving from the cis to the trans face while carrying some proteins along. In cis face addition, some vesicles recycle enzymes that had been carried forward in moving cisternae, (“receiving” side of transporting them “backward” to a less mature region where their functions are needed. Golgi apparatus) 1 Vesicles move 2 Vesicles coalesce to 6 Vesicles also from ER to Golgi. form new cis Golgi cisternae. 0.1 μm transport certain proteins back to ER, their site of function. Cisternae 3 Cisternal maturation: Golgi cisternae move in a cis- to-trans direction. 4 Vesicles form and leave Golgi, carrying proteins to specific products to other locations or to the plasma mem- brane for secretion. 5 Vesicles transport some proteins trans face backward to less mature Golgi (“shipping” side of cisternae, where they function. Golgi apparatus) TEM of Golgi apparatus Products of the endoplasmic reticulum are usually modified Before a Golgi stack dispatches its products by budding during their transit from the cis region to the trans region of vesicles from the trans face, it sorts these products and targets the Golgi apparatus. For example, glycoproteins formed in the them for various parts of the cell. Molecular identification ER have their carbohydrates modified, first in the ER itself, tags, such as phosphate groups added to the Golgi products, then as they pass through the Golgi. The Golgi removes some aid in sorting by acting like ZIP codes on mailing labels. Fi- sugar monomers and substitutes others, producing a large vari- nally, transport vesicles budded from the Golgi may have ex- ety of carbohydrates. Membrane phospholipids may also be al- ternal molecules on their membranes that recognize “docking tered in the Golgi. sites” on the surface of specific organelles or on the plasma In addition to its finishing work, the Golgi apparatus also membrane, thus targeting the vesicles appropriately. manufactures some macromolecules. Many polysaccharides secreted by cells are Golgi products. For example, pectins and Lysosomes: Digestive Compartments certain other noncellulose polysaccharides are made in the A lysosome is a membranous sac of hydrolytic enzymes that Golgi of plant cells and then incorporated along with cellu- an animal cell uses to digest (hydrolyze) macromolecules. lose into their cell walls. Like secretory proteins, nonprotein Lysosomal enzymes work best in the acidic environment Golgi products that will be secreted depart from the trans face found in lysosomes. If a lysosome breaks open or leaks its con- of the Golgi inside transport vesicles that eventually fuse tents, the released enzymes are not very active because the cy- with the plasma membrane. tosol has a neutral pH. However, excessive leakage from a large The Golgi manufactures and refines its products in stages, number of lysosomes can destroy a cell by self-digestion. with different cisternae containing unique teams of enzymes. Hydrolytic enzymes and lysosomal membrane are made Until recently, biologists viewed the Golgi as a static structure, by rough ER and then transferred to the Golgi apparatus for with products in various stages of processing transferred from further processing. At least some lysosomes probably arise by one cisterna to the next by vesicles. While this may occur, re- budding from the trans face of the Golgi apparatus (see cent research has given rise to a new model of the Golgi as a Figure 6.12). How are the proteins of the inner surface of the more dynamic structure. According to the cisternal maturation lysosomal membrane and the digestive enzymes themselves model, the cisternae of the Golgi actually progress forward from spared from destruction? Apparently, the three-dimensional the cis to the trans face, carrying and modifying their cargo as shapes of these proteins protect vulnerable bonds from enzy- they move. Figure 6.12 shows the details of this model. matic attack. 106 UNIT TWO The Cell  Lysosomes carry out intracellular digestion in a variety of are returned to the cytosol for reuse. With the help of lyso- circumstances. Amoebas and many other protists eat by en- somes, the cell continually renews itself. A human liver cell, gulfing smaller organisms or food particles, a process called for example, recycles half of its macromolecules each week. phagocytosis (from the Greek phagein, to eat, and kytos, The cells of people with inherited lysosomal storage dis- vessel, referring here to the cell). The food vacuole formed in eases lack a functioning hydrolytic enzyme normally present this way then fuses with a lysosome, whose enzymes digest in lysosomes. The lysosomes become engorged with indi- the food (Figure 6.13a, bottom). Digestion products, includ- gestible substrates, which begin to interfere with other cellular ing simple sugars, amino acids, and other monomers, pass activities. In Tay-Sachs disease, for example, a lipid-digesting into the cytosol and become nutrients for the cell. Some enzyme is missing or inactive, and the brain becomes im- human cells also carry out phagocytosis. Among them are paired by an accumulation of lipids in the cells. Fortunately, macrophages, a type of white blood cell that helps defend the lysosomal storage diseases are rare in the general population. body by engulfing and destroying bacteria and other in- vaders (see Figure 6.13a, top, and Figure 6.33). Vacuoles: Diverse Maintenance Lysosomes also use their hydrolytic enzymes to recycle the Compartments cell’s own organic material, a process called autophagy. Dur- Vacuoles are large vesicles derived from the endoplasmic ing autophagy, a damaged organelle or small amount of cy- reticulum and Golgi apparatus. Thus, vacuoles are an integral tosol becomes surrounded by a double membrane (of part of a cell’s endomembrane system. Like all cellular mem- unknown origin), and a lysosome fuses with the outer mem- branes, the vacuolar membrane is selective in transporting brane of this vesicle (Figure 6.13b). The lysosomal enzymes solutes; as a result, the solution inside a vacuole differs in dismantle the enclosed material, and the organic monomers composition from the cytosol. 1 μm Nucleus Vesicle containing 1 μm two damaged organelles Mitochondrion fragment Peroxisome fragment Lysosome 1 Lysosome contains 2 Food vacuole fuses 3 Hydrolytic 1 Lysosome fuses with 2 Hydrolytic enzymes active hydrolytic with lysosome. enzymes digest vesicle containing digest organelle enzymes. food particles. damaged organelles. components. Digestive enzymes Lysosome Lysosome Peroxisome Plasma membrane Digestion Food vacuole Mitochondrion Digestion Vesicle (a) Phagocytosis: lysosome digesting food (b) Autophagy: lysosome breaking down damaged organelles 䉱 Figure 6.13 Lysosomes. Lysosomes Macrophages ingest bacteria and viruses and will fuse with a lysosome in the process of digest (hydrolyze) materials taken into the cell destroy them using lysosomes. Bottom: This autophagy (TEM). Bottom: This diagram shows and recycle intracellular materials. (a) Top: In diagram shows one lysosome fusing with a fusion of such a vesicle with a lysosome. This this macrophage (a type of white blood cell) food vacuole during the process of type of vesicle has a double membrane of from a rat, the lysosomes are very dark because phagocytosis by a protist. (b) Top: In the unknown origin. The outer membrane fuses of a stain that reacts with one of the products cytoplasm of this rat liver cell is a vesicle with the lysosome, and the inner membrane is of digestion within the lysosome (TEM). containing two disabled organelles; the vesicle degraded along with the damaged organelles. CHAPTER 6 A Tour of the Cell 107  Vacuoles perform a variety of functions in different kinds of cells. Food vacuoles, formed by phagocytosis, have al- Central vacuole ready been mentioned (see Figure 6.13a). Many freshwater protists have contractile vacuoles that pump excess water Cytosol out of the cell, thereby maintaining a suitable concentration of ions and molecules inside the cell (see Figure 7.16). In plants and fungi, certain vacuoles carry out enzymatic hydrol- ysis, a function shared by lysosomes in animal cells. (In fact, some biologists consider these hydrolytic vacuoles to be a Central type of lysosome.) In plants, smaller vacuoles can hold re- Nucleus vacuole serves of important organic compounds, such as the proteins stockpiled in the storage cells in seeds. Vacuoles may also help Cell wall protect the plant against herbivores by storing compounds that are poisonous or unpalatable to animals. Some plant vac- Chloroplast uoles contain pigments, such as the red and blue pigments of 5 μm petals that help attract pollinating insects to flowers. 䉱 Figure 6.14 The plant cell vacuole. The central vacuole is Mature plant cells generally contain a large central usually the largest compartment in a plant cell; the rest of the vacuole (Figure 6.14), which develops by the coalescence of cytoplasm is often confined to a narrow zone between the vacuolar smaller vacuoles. The solution inside the central vacuole, called membrane and the plasma membrane (TEM). cell sap, is the plant cell’s main repository of inorganic ions, in- cluding potassium and chloride. The central vacuole plays a The Endomembrane System: A Review major role in the growth of plant cells, which enlarge as the vacuole absorbs water, enabling the cell to become larger with a Figure 6.15 reviews the endomembrane system, showing the minimal investment in new cytoplasm. The cytosol often occu- flow of membrane lipids and proteins through the various or- pies only a thin layer between the central vacuole and the ganelles. As the membrane moves from the ER to the Golgi plasma membrane, so the ratio of plasma membrane surface to and then elsewhere, its molecular composition and metabolic cytosolic volume is sufficient, even for a large plant cell. functions are modified, along with those of its contents. The Nucleus 1 Nuclear envelope is connected to rough ER, which is also continuous with smooth ER. Rough ER Smooth ER 2 Membranes and proteins produced by the ER flow in the cis Golgi form of transport vesicles to the Golgi. 3 Golgi pinches off transport vesicles and other vesicles that give rise to lysosomes, other types of specialized vesicles, Plasma and vacuoles. membrane trans Golgi 4 Lysosome is available 5 Transport vesicle carries 6 Plasma membrane expands for fusion with another proteins to plasma membrane by fusion of vesicles; proteins vesicle for digestion. for secretion. are secreted from cell. 䉱 Figure 6.15 Review: relationships among organelles of the endomembrane system. The red arrows show some of the migration pathways for membranes and the materials they enclose. 108 UNIT TWO The Cell endomembrane system is a complex and dynamic player in Endoplasmic Nucleus the cell’s compartmental organization. reticulum We’ll continue our tour of the cell with some organelles that Nuclear are not closely related to the endomembrane system but play Engulfing of oxygen- envelope crucial roles in the energy transformations carried out by cells. using nonphotosynthetic prokaryote, which becomes a mitochondrion CONCEPT CHECK 6.4 1. Describe the structural and functional distinctions be- Ancestor of Mitochondrion tween rough and smooth ER. eukaryotic cells (host cell) 2. Describe how transport vesicles integrate the en- domembrane system. 3. WHAT IF? Imagine a protein that functions in the ER but requires modification in the Golgi apparatus Engulfing of photosynthetic before it can achieve that function. Describe the pro- prokaryote tein’s path through the cell, starting with the mRNA At least one cell Chloroplast molecule that specifies the protein. Nonphotosynthetic eukaryote For suggested answers, see Appendix A. CONCEPT 6.5 Mitochondrion Mitochondria and chloroplasts Photosynthetic eukaryote change energy from one form 䉱 Figure 6.16 The endosymbiont theory of the origin of to another mitochondria and chloroplasts in eukaryotic cells. According to this theory, the proposed ancestors of mitochondria were oxygen- Organisms transform the energy they acquire from their sur- using nonphotosynthetic prokaryotes, while the proposed ancestors of roundings. In eukaryotic cells, mitochondria and chloroplasts chloroplasts were photosynthetic prokaryotes. The large arrows represent change over evolutionary time; the small arrows inside the are the organelles that convert energy to forms that cells can cells show the process of the endosymbiont becoming an organelle. use for work. Mitochondria (singular, mitochondrion) are the sites of cellular respiration, the metabolic process that uses other cell). Indeed, over the course of evolution, the host cell oxygen to generate ATP by extracting energy from sugars, fats,