Plasma Membrane PDF

The Plasma Membrane The plasma membrane serves many functions: interior The boundary between the _________ extracellular of the cell and the ________________ environment Controls what enters and exits the cell Houses enzymes...

The Plasma Membrane The plasma membrane serves many functions: interior The boundary between the _________ extracellular of the cell and the ________________ environment Controls what enters and exits the cell Houses enzymes involved in many chemical reactions _____________________ of the cell  & carbohydrates The Plasma Membrane The plasma membrane is a fluid mosaic of lipids and proteins flexible layer Lipid molecules form a _______________ Proteins are embedded in the plasma membrane carbohydrates on the surface of the _______________ plasma membrane act as cell identification tags The Lipid Bilayer Phospholipids are amphipathic hydrophilic Amphipathic = molecules that have _______________ and hydrophobic properties __________________ In water, phospholipids form a stable bilayer heads Hydrophilic __________ face outward (towards the water) tails face inward (away from water) Hydrophobic _________  Membrane Fluidity Membranes must be fluid to function properly, but not TOO fluid Lipids have rapid lateral movement, but they rarely flip-flop Why would you expect flip-flops to be rare events? Lipid composition affects membrane fluidity Saturated fatty acids have no double ce bond resulting in ________________, straight chains that allow for maximum interaction between the fatty acid tails This makes membranes more viscous (thicker) Solid at room temperature, e.g., animal fats some double c bond resulting Unsaturated fatty acids have _________________, in bent chains that space tails apart This makes membranes more fluid Liquid at room temperature, e.g., vegetable fats Lipid composition affects membrane fluidity Cholesterol act as a fluidity buffer diff temperatures Has different effects on membrane fluidity at ___________________ At warm temperatures (e.g., 37°C), cholesterol restrains the movement __________________ of phospholipids preventing At cool temperatures, it maintains fluidity by ________________ tight packing of phospholipids Let’s practice! Which part of a phospholipid can interact with water? 1. The saturated fatty acid 2. The unsaturated fatty acid 3. The phosphate 4. None. Phospholipids do not interact with water  Membrane Proteins Peripheral membrane proteins Touch the membrane or another protein superficially ionic-hydrogen-bond interactions with hydrophilic lipid and _________________________ protein groups Integral membrane proteins transmembrane proteins  Membrane proteins do cellular work! different proteins embedded in The plasma membrane is a mosaic of ___________ the lipid bilayer of types proteinsdetermine most of the membrane’s specific functions The _______________ Membrane Carbohydrates Carbohydrates are found on the exterior surface of the cell membrane _____________ cell-cell They provide specificity for _________ or _____________ cell-protein interactions Glycolipids: carbohydrate attached to a lipid (e.g., blood antigens that determine blood type) Glycoproteins: carbohydrate attached to a protein (e.g., receptors) ABO blood group antigens are glycolipids Let’s review! In which of the following locations could you find an integral membrane protein? 1. Completely on the outside surface of the cell membrane 2. Completely on the inside surface of the cell membrane 3. Floating freely in the cytoplasm 4. Spanning the cell membrane, with some parts visible to the outside and inside of the cell Membranes are selectively permeable The cell membrane controls traffic into and out of the cell by being selectively permeable → It allows some substances to cross but not others  What makes the membrane selectively permeable? 1. Permeability of the lipid bilayer ______________________ hydrophobic molec pass through the membrane rapidly (e.g., CO2, O2, steroid hormones) polar molecules and __________ ______________________ ions do not cross the membrane easily 2. Transport proteins (transmembrane) Each transport protein is _______________ specific to the solutes it can transport Molecules pass through the membrane via passive transport or active transport  Passive Transport diffuse Substances ___________ through membranes without work by the cell No energy (ATP) is required Diffusion = tendency for particles to spread out spontaneously from where they are ____________ more concentrated to where they are less concentrated __________________ Dynamic equilibrium is present when molecules move in opposite equal rates directions at __________________ Passive Transport Each solute in solution follows their own independent concentration gradient  Passive Transport: Facilitated Diffusion Molecules that cannot cross the lipid bilayer can still be passively transported across the plasma membrane via facilitated diffusion → Does not require energy Facilitated diffusion = substances moving ______ down a concentration gradient through transport proteins E.g., _______________ hydrophilic substances: water, ions Let’s practice! Which way is the net flow of salt in each beaker? outel in a content Let’s practice! The cell membrane is permeable to water and glucose, but not sucrose. Water is the 5% glucose solvent in the beaker and in the cell. 20% sucrose In which direction will glucose move? 20% glucose 1. Into the cell 5% sucrose 2. Out of the cell 3. Stays where it is  Osmosis Solutions are solute(s) dissolved in a solvent Water is the solvent for most of life’s solutions Osmosis is the diffusion of ________ water across selectively permeable a _________________________ membrane Water diffuses across a membrane from the region of lower solute concentration (i.e., high water concentration) to the region of higher solute concentration (i.e., low water concentration)  Let’s consider… Recall that water is a polar molecule. Can water cross the plasma membrane easily?  Water and Facilitated Transport Water is a polar molecule that CAN diffuse across the lipid bilayer because it Aquaporin is small, but, it can only do so _________ slowly Some cell types (e.g., kidney cells) faster transport require __________________ of water either in or out of the cell transmembrane protein Aquaporins are_____________________ ___________ channels that facilitate the transport of water across the cell membrane  Tonicity Tonicity = the ability of a solution to cause a cell to gain or lose water Hypertonic = environment has ________ higher [solute] than inside of the cell Hypotonic = environment has _________ lower [solute] than inside of the cell Isotonic = equal [solutes] inside and outside the cell Osmoregulation = the control of water balance Many marine organisms have cells that are isotonic to seawater, but organisms that live in ______________ or _____________ environments need a mechanism to prevent excessive uptake or loss of water Osmosis in Organisms Without Cell Walls Shrink in a hypertonic Osmosis causes cells to ______ solution, and ______ swell in a hypotonic solution Osmosis in Organisms with Cell Walls hypotonic solutions: rigid cell Plant cells do best in ___________ wall prevents membranes from rupturing, making them turgid In _________ isotonic environments, plant cells become flaccid Cells of the aquatic plant Elodea in a hypotonic solution In _________ hypertenic environments they shrivel Cells of the aquatic plant Elodea in a hypertonic solution Active Transport: Transport Proteins Some transport proteins can move solutes across the membrane against a concentration gradient ___________________________ Active transport requires energy in the form of ATP E.g., Na+/K+ pump  + Na /K +pump actively transports ions against their concentration gradient Always more Na+ outside of the cell ATP ATP Always more K+ inside of the cell Let’s practice! Cells actively transport calcium out of the cell. Is calcium more concentrated inside or outside the cell? Explain!  Active Transport: Bulk Transport Large molecules/large quantities of smaller molecules exocytosis can be transported across the membrane by __________ endocytosis or _____________ inside Exocytosis: membrane-bound vesicle from ___________ the cell fuses with the cell membrane and expels its Exocytosis to the outside contents ____________________ Endocytosis: membrane folds inward, trapping material outside from _______________ inside for use ______________ the cell Alp-energy In exo- and endocytosis, ________ is required for vesicle fusion/creation and transport Endocytosis  Learning Outcomes 1. Explain how surface area to volume ratio influences the minimum or maximum size of a cell. 2. Compare and contrast prokaryotic and eukaryotic cells with respect to internal organization, size, genetic content, and Domain classification. 3. Summarize the benefits of cell compartmentalization. 4. Compare and contrast the organelles present in plant and animal cells. 5. Explain the endosymbiont theory and summarize the evidence that supports it. 6. Summarize the general functions of the cytoskeleton, and the specific functions of microfilaments, intermediate filaments, and microtubules. 7. Contrast the structure and function of cilia and flagella. 8. Compare and contrast extracellular matrices in animal cells and cell walls in plant cells, with respect to location, function, and macromolecular composition. 9. Explain the role of tight junctions, anchoring junctions, gap junctions, and plasmodesmata.  Visualizing Cell Structure The cell is the fundamental unit of life All organisms are made of cells, all cells come from pre-existing cells Cells discovered by Robert Hooke in 1665 following the invention of the microscope light microscopes Today, we use ________________, scanning electron microscope ___________________________, and transmission electron microscope ___________________________ to view external and internal structures of cells Cell size and shape vary with function bird eggs The largest cells are __________________ muscle of nerve cell The longest cells are _________________ The smallest cells are mycoplasmas (type of bacteria, ~0.1-1 μm in diameter) M. gallisepticum, Catroxo & Martins Common Units of Length 1/1000 mm  Why are cells so small? At minimum, cells must be large enough to contain the necessary parts for reproduction and survival The maximum size is limited by the surface area to volume ratio needed to efficiently exchange of materials with the environment A cell must have a sufficiently large surface area to volume ratio  Small cells have a greater SA/V than large cells of the same shape Surface area of a cube: SA = 6L2 T Volume of a cube: 3) -. 5 V = L3 Surface Area to Volume Ratio: SA/V = 6L2/L3 = 6/L As L ___________, increases decreases SA/V __________! Me surface area 6 : 2Sx6 = 150 volume : LxWxH SX5x5 125 = ( x(x) = 1 # 7 6= = = 2  Cell shape affects surface area: volume ratio Neurons can reach up to a meter in length, but they long and thin maintain a high SA:V by being _____________ Red blood cells are already small (6-8 μm in flattened disc-like shape further diameter), but their _______ increases SA:V  Let’s practice! If a cell increases in size (while maintaining the same shape), which of the following will occur? 1. Surface area, volume, and surface area to volume ratio will ↑ 2. Surface area, volume, and surface area to volume ratio will ↓ 3. Surface area and volume will ↓, surface area to volume ratio will ↑ 4. Surface area and volume will ↑, surface area to volume ratio will ↓  Cells are diverse in shape and function, but every cell has… 1. A plasma membrane Separates internal and external environment Selectively permeable (permits only certain substances to enter/exit) 2. Chromosomes Strands of DNA that carry _________ gene 3. Ribosomes Make ___________ proteins according to instructions from genes ? 4. Cytoplasm eukaryotic cell cytosol Fluid component is called the ____________ plasma membrane nucleus and the _________________ In eukaryotes, it is the area between the _______ Plasma eutoplasm Key ne genera / & - cytosol N... S ·.. enromosome osomes Outline 1. Cell Structure and Function 2. General Cell Types 3. Organelles The Nucleus and Ribosomes The Endomembrane System Mitochondria and Chloroplasts The Cytoskeleton Cellular Appendages and Connections Two General Types of Cells 1. Prokaryotic cells 1/10th the size of average eukaryotic cell membrane bound organelles No nucleus or complex __________________________ Domains ____________ Bacteria Archaea and ________________ 2. Eukaryotic cells Have nucleus and membrane-bound organelles with distinct functions in the cell - proteins Have a __________________ cytoskeleton Domain ______________ eukary a (animals, plants, fungi, protists)  Prokaryotic cells are small and structurally simple Prokaryotic cells are enclosed by a plasma membrane, and usually cell wall encased in a rigid _______________ Inside the cell, the DNA is located in a nucleoid region appendages On the exterior surface are __________ called fimbriae and flagella um ~ helps cells sticktg t &membranese in there  Prokaryotes Vs. Eukaryotes Commonalities: cell membrane _______________ cytoplasm/cytosol _______________ Chromosomes/genes/DNA _______________ ribosomes Differences: - > Size eukaryotic , is bigger than prokaryotic 1-20 μm vs 10-100 μm (and beyond, e.g., neurons, bird eggs) nucleoid region _______________ vs. nucleus _______________ no complex organelle vs. complex membrane-bound organelles Let’s practice! Which of the following is present in prokaryotic cells but not eukaryotic cells? 1. Nucleus 2. Nucleoid region 3. Cell wall 4. DNA  Eukaryotic cells are partitioned into functional compartments efficiency Compartmentalization increases __________! Allows incompatible chemical reactions to separated be ________________ from each other Keeps enzymes that cooperate close together Allows higher local concentrations of molecules Plant Cell  Animal vs. Plant Cells Animals and plants have most of the same organelles, with exceptions: g Animal cells only Plant cells only (absent in plants): (absent in animals): ___________ lysosome chloroplasts ______________ Centrosomes cell wall ______________ central vacuole ______________ ↓ cell wall made of : carbohydrates (cellulose)  RNA is made here The Nucleus genetic information in Houses _________________ ↑ the form of DNA - I The nucleolus is the region inside the nucleus where ribosomal RNA ______________ is synthesized The nucleus is separated from the cytoplasm by the nuclear envelope, 2 lipid bilayers which is made of ______________________ Nuclear pores regulate entry and exit of materials · proteins go in and. out · nucleotides also The Nucleus DNA and proteins form In the nucleus, __________________ genetic material called chromatin Chromatin condenses to form discrete chromosomes Why does DNA condense into chromatin? Why does chromatin condense into chromosomes?  Ribosomes Ribosomes are particles made of ribosomal una and ________________ ____________ protein They carry out protein synthesis In eukaryotes, they may be _____________ free in the cytosol ___________ or bound to the endoplasmic reticulum DNA to Protein in a Eukaryotic Cell S - · DNA-> RNA-- PROTEIN E = = Organelles of the Endomembrane System The endomembrane system is a collection of membranous organelles manufacture These organelles _____________, distribute ____________, break down and ____________ cell products The endomembrane system includes: Nuclear envelope Endoplasmic reticulum Golgi apparatus Lysosomes Vesicles and vacuoles Plasma membrane  Endoplasmic Reticulum (ER) ER is a _________________________ network of membranes next to the nucleus It is ______________ continuous with the nuclear envelope The interior space is distinct from the __________ cytoplasm Two types, distinguished by the ______________ absence or ____________ presence of ribosomes: Smooth ER Rough ER  Smooth Endoplasmic Reticulum Structure No ribosomes = “smooth” Functions synthesize lipids _____________ _____________ store calcium & Breaks down toxins and drugs↓ Istorage) can break down toxic stuff from your body  Rough Endoplasmic Reticulum Structure Dotted with ribosomes = “rough” appearance Functions O-> pinched Off Manufacture ______________ more membrane Ribosomes on its surface produce ________________________ proteins for selection makes proteins O I amino acids  The Golgi Apparatus The Golgi apparatus consists of flattened stacks of ______________ membranous sacs _________ receive modify products and _________ from the ER, then send the products to other organelles or to the cell membrane each sac has diff enzymes J = - sacs - Golgi -  autophagy phagocytosis - Lysosomes - eat cell self eat digestive enzymes Lysosomes are sacs of _________________ They fuse with food vacuoles containing particles/bacteria brought in from outside the cell ____________ (phagocytosis), or vesicles that have engulfed damaged organelles __________________ (autophagy) Enzymes from the lysosome hydrolyze proteins, fats, polysaccharides, and nucleic acids Proteins in the lysosomal membrane pump H+ pH of 5 from cytosol to lumen to maintain a _________  Vacuoles Vacuoles are large membranous sacs and have diverse roles within the cell Food vacuoles store materials to be ________ digested excess water Contractile vacuoles expel _______________ Plant cells contain a large central vacuole that absorbs water, stores vital chemicals and toxic waste products quarantines _____________________  The Plasma Membrane cell moves easily Y + X - A flexible, sturdy barrier that surrounds and contains the cytoplasm of a cell phospholipid bilayer Made up of a ___________________ with embedded proteins Responsible for directional transport of molecules, e.g., oxygen, nutrients, waste  Overview of the endomembrane system ⑳ Nucleus Ab. BI. > - protein synthesis Rough ER C# - > endoplasm C. D2. Smooth ER E3. Y Nuclear envelope cis Golgi makes lipids F5. Transport vesicle trans Golgi & Plasma membrane outside cell  Peroxisomes Structure Membranous sacs containing detoxification enzymes Not part of the endomembrane system Function Break down fatty acids in a metabolic reaction that produces hydrogen peroxide (toxic!) senergy Enzymes in peroxisomes convert hydrogen peroxide to water → Peroxisomes ________________ protect the cell by quarantining these potentially harmful reactions Let’s recap! smooth M True or false? ER makes lipids break down stuff ? > - 1. The main function of lysosomes is to synthesize lipids. false 2. One function of the rough endoplasmic reticulum is to synthesize proteins. true  Mitochondria eukaryotic organisms Involved in energy processing in ____________________ The sites of cellular respiration; chemical energy is harvested from food and stored as ATP C6H12O6 + O2 → CO2 + H2O + ATP 2 membranes and 2 internal compartments: the intermembrane space, and an inner compartment called the mitochondrial matrix The inner membrane is highly folded into cristae and ATP synthesis embedded with the proteins involved in _____________  Chloroplasts In plants and algae solar The sites of photosynthesis (process that converts _____ chemical ______________ to ________________): energy energy Light + CO2 + H2O → C6H12O6 + O2 intermembrane 2 membranes, an _________________, inner and an ________ comportment _________________ Inner compartment contains stroma (fluid) and thylakoids stacked in grana (singular = granum) Thylakoids contain chlorophyll and are where the photosynthesis reaction occurs  Endosymbiosis Mitochondria and chloroplasts originated via endosymbiosis Both share characteristics with bacteria: 1. Inner membranes of mitochondria and plastids have enzymes homologous to those in membranes of living prokaryotes ribosomes 2. Contain own ______________, more similar to those in prokaryotes than eukaryotes circular DNA molecule 3. Contain own ______________________ Lynn Margulis (1938-2011) Let’s practice! Which of the following is found in eukaryotic but not prokaryotic cells? 1. Cytosol 2. Plasma membrane 3. Ribosome 4. DNA 5. Mitochondria Let’s practice! True or false? 1. Rough endoplasmic reticulum is part of the endomembrane system. 2. Mitochondria are part of the endomembrane system.  The Cytoskeleton The cytoskeleton (“cell skeleton”) is a network of protein fibres that runs throughout the cell _______________ Functions: maintains cell shape 1. Establishes and ____________ mechanical strength 2. Provides _________________ 3. Facilitates locomotion 4. Segregates chromosomes during cell division organelles within the cell 5. Moves ____________  Three filaments make up the cytoskeleton 1. Microfilaments: enable cells to change shape or move. Made of thin, twisted filaments made of a protein called actin 2. Intermediate filaments: reinforce the cell and anchor organelles. Made of rope-like filaments made of many different types of protein 3. Microtubules: give the cell rigidity, provide anchors for organelles, and act as tracks for organelle movement. Hollow tubular filaments made of a protein called tubulin  Cilia and flagella help cells move Cilia are made of microtubules that bend Cilia movement (Paramecium) Short and hair-like Beat like oars through water to move cells Can also move substances over cells (e.g., mucus)  Cilia and flagella help cells move Flagella are also microtubules that bend Flagellum movement (Euglena) Longer than cilia Beat like a whip to move cells OR spin like a propeller (bacteria, mostly) E.g., Sperm  Cell surfaces protect, support, and join cells interact with their environments and Cells __________ surfaces each other via their _________ Animal cells secrete an extracellular matrix Sticky layer on the outside of animal cells Made of large polysaccharides and protein filaments Functions binds cells together in tissues __________ __________ protects cells from invasion  Cell-Cell Junctions in Animal Cells Tight junctions bind cells together into leak-proof sheets so nothing can get ____________ thru Anchoring junctions ________ bind animal cells to each other Gap junctions allow substances to _______ flow from cell to cell communication messengers ,  Plant Cell Walls and Junctions Plant cells are supported by rigid cell walls made largely of cellulose Plant cells are connected by plasmodesmata that permeate their cell (what walls, channels that allow them to share food ______________, ______________, and water mical messages _____________________________ they use to communicate Let’s practice! True or false? in animals ~ 1. - Gap junctions are found in plant cells to allow cytosol and solutes to travel between cells. X 2. Tight junctions form a water-tight seal between adjacent cells. ~  1. Mitochondria, which break down 4.0 Microscopes reveal a startling new view of life glucose to produce cellular energy, are found in cells, Imagine living 350 years ago and being told "Your body is composed whi le chloroplasts, which use of invisibly tiny liquid-filled rooms." Egads! What utter nonsense! sunlight to produce sugars, are Now imagine the shock and surprise when in 1665 Robert found in cells. Hooke used a crude microscope to examine bark from an a. eukaryotic... plant oak tree. Hooke cal led the structures he saw cellulae ("little b. animal... plant c. prokaryotic... eukaryotic rooms " in Latin) and the term cell stuck. A few decades later, d. eukaryotic... prokaryot ic Dutch scientist Antoni van Leeuwenhoek used a more refi ned e. plant... animal microscope to view numerous subjects, including blood, sperm, 2. What kinds of cel ls can you see and pond water. He produced drawings and enthusiastic with your unaided eye? descriptions of his discoveries, such as the tiny "animalcules , a. only really large cell s, such very prettily a-moving" he found in the scrapings from his teeth. as eggs A previously unknown and invisible world had been revealed. b. none In the ensuing centuries , improvements in technology have c. most anima l cel ls vastly expanded our view of the microscop ic world. For example, d. bacteria e. most plant and animal cells an immunofluorescent light microscope revealed the specialized epithe lial cells that line the inner surface of blood ce lls (shown 3. How does the structure of a at left). Throughout this book, you will see many micrographs phospholipid correspond to (microscope photographs), often paired with drawings that its function? emphasize details. a. Its chemical makeup ensures that it will organize as a semi- In this chapter, we will explore the cellular basis of life. As you permeable membrane. study the images in this chapter, keep in mind that the parts of a b. The hydrophi lic tails wi ll always cell are actually moving and interacting. Indeed , the phenomenon of orient toward water. life emerges from the interactions of the many components of a cell. c. The hydrophobic head will always point toward the cytoplasm. d. Its protein allows only certa in substances to pass. e. The genes it carries control most ce ll functions. Introduction to the Cell 4.1 Microscopes reveal the world of the cell Before microscop es were first used in the 1600s, no on e as small as about 2 n anometers (nm), a 100-fold improvement knew that living organisms were composed of t h e tiny units over the light microscope. This high resolution has enabled we call cells. The first microscopes were light microscopes, biologists to explore cell ultrastructure, the complex internal like the on es you may use in a biology laboratory. In a light anatomy of a cell. Figures 4.18 and 4.1C show images microscope (LM), visible light is passed through a spec- produced by two kinds of electron m icroscopes. imen, such as a microorganism or a thin slice of animal or plant tissue, and then through glass lenses. Th e lenses bend the light in such a way that the image of the specimen is magnified as it is projected into your eye or a camera. Magnification is the increase in an object's image size compared with its actual size. Figure 4.lA sh ows a micrograph of a single-celled organism called Paramecium. The notation "LM 230X" printed along the right edge tells you that this photograph was taken t h rough a light m icroscope and that the image is 230 times the actual size of the organism. This Paramecium is about 0.33 millimeter (mm) in len gth. Table 4.1 shows the most comm on units of len gth that biologists use. An important factor in microscopy is resolution, a m ea- sure of the clarity of an image. Resolution is the ability to.6. Figure 4.lA Light micrograph of the unicellular organism Paramecium distinguish two n earby objects as separate. For example, what you see as a single star in t he sky may be resolved as twin stars with a telescope. Each optical instrument-be it an eye, a telescope, or a microscope-has a limit to its resolution. The human eye can distinguish p oints as close togeth er as 0.1 mm, about the size of a very fine grain of sand. A typical light m icroscope cann ot resolve detail finer t h an about 0.2 m icrometer (µm), about th e size of the smallest bacterium. No matter how many times the image of such a sm all cell is magnified , the light microscope cannot resolve the details of its structure. Indeed, light microscopes can effectively m agnify objects only about 1,000 times. From the time that Hooke discovered cells in 1665 until the m iddle of the 1900s, biologists had only light micro- scopes for viewing cells. With t hese microscopes and various.6. Figure 4.18 Scanning electron micrograph of Paramecium staining techniques to increase contrast between parts of cells, these early biologists discovered microorganisms, ani- mal and plan t cells, and even some structures within cells. By t he mid-1800s, this accumulation of evidence led to the cell theory, which states t hat all living things are composed of cells an d that all cells come from other cells. Our knowledge of cell structure took a giant leap forward as biologists began using t h e electron microscope in the 1950s. In stead of using light, an electron microscope (EM) focuses a beam of electrons through a specimen or onto its surface. Electron microscopes can distinguish biological structures TABLE 4.1 Metric Measurement Equivalents.6. Figure 4.1C Transmission electron micrograph of Toxop/asma 1 meter (m) = 100 cm= 1 ,000 mm= 39.4 inches (This parasite of cats can be transmitted to humans, causing the 1 centimet er (cm) = 10- 2 m (0.01 or 1/ 100 m) = 0.4 inch disease toxoplasmosis.) 1 millimeter (mm) = 10- 3 m (0.001or1/ 1,000 m) TRY THIS Describe a major difference between the Paramecium 1 micrometer (µm) = 10-6 m (0.000001 m) = 10- 3 mm in Figure 4.1B and the Tox oplasma in this figure. (Hint: Compare 1 nanometer (nm) = 10-9 m = 10- 3 µm the notations along the right sides of the micrographs.) 56 CHAPTER 4 A Tour of the Cell  Biologists use the scanning electron microscope (SEM) sh own beside each micrograph , it is often hard to im agine to study the detailed architecture of cell surfaces. Th e SEM uses just how sm all cells are. Figure 4.1E sh ows t he size range of an electron beam to scan the surface of a cell or other sample, cells compared with objects both larger and sm aller and t he which is usually coated with a thin film of gold. The beam excites optical instrument that allows us to view them. Notice t hat electrons on the surface, and these electrons are then detected t h e scale along the left side of the figure is logarithmic to by a device that translates their pattern into an image projected accom modate the range of sizes shown. Starting at the top onto a video screen. The scanning electron micrograph in with 10 meters (m) , each referen ce measurem ent m arks a ten- Figure 4.lB highlights the numerous cilia on Paramecium, pro- fold decrease in length. Most cells are between 1 and 100 µm jections it uses for movement. Notice the indentation, called in diam eter (yellow region of the figure) and are therefore the oral groove, through which food enters the cell. As you can visible only with a microscope. Certain bacteria are as small see, the SEM produces images that look three-dimension al. as 0.2 µm and can barely be seen with a light microscope, The transmission electron microscope (TEM) is used whereas chicken eggs are large enough to be seen with the to study the details of internal cell structure. Th e TEM aims an unaided eye. A single nerve cell running from the base of your electron beam through a very thin section of a specimen , just spinal cord to your big toe may be 1 m in length , although it as a light microscope aims a beam of light through a specimen. is so t hin you would still n eed a m icroscope to see it. In the Th e section is stained with atom s of h eavy metals, which attach n ext m odule, we explore why cells are so sm all. to certain cellular structures more than others. Electrons are f t Which type of microscope would you use to study (a) t he scattered by these more den se parts, and the image is created U changes in shape of a living human white blood cell; (b) the by the pattern of tran smitted electron s. Instead of using glass finest details of surface texture of a human hair; (c) the detailed len ses, both th e SEM and TEM use electrom agnets as len ses to structure of an organelle in a liver cell? bend the paths of the electrons, m agnifying and focusing the ado:>SOJ:>!W UOJl:>a1a image onto a monitor. The transm ission electron micrograph UO!SS!WSUBJl (:>) !ado:>SOJ:>!W UOJl:>a1a (q) (e). in Figure 4.l C shows internal details of a single-celled organism called Toxoplasma. SEMs and TEMs are initially black and wh ite 10m,,..----------- but are often artificially colorized, as they are h ere, to highlight or clarify structural features. Electron microscopes have truly revolutionized t h e study of Human height 1m cells and their structures. Non etheless, th ey h ave not replaced (1,000 mm) Length of some the light microscope: Electron microscopes cannot be u sed to nerve and muscle cells study living specimens because the m ethods used to prepare 0.1 m the specimen kill the cells. For a biologist studying a living pro- (100 mm) Chicken egg cess, such as the movement of Paramecium, a light microscope equipped with a video cam era is more suitable than either 0.01 m an SEM or a TEM. (10 mm) There are different types of light microscopy, and major tech- @ Frog egg nical advances in the past several decades h ave greatly expand- 1 mm ed our ability to visualize cells. Figure 4.10 shows Paramecium (1 ,000 µm) as seen using differential interference contrast microscopy. Paramecium This optical technique amplifies differen ces in den sity so that 100 µm Human egg the structures in living cells appear almost three-dimension al. Other techniques use fluorescent stains that selectively bind Most plant and to various cellular molecules (see t he chapter introduction). animal cells 10 µm You will see m any beautiful and illuminating examples of Nucleus microscopy in this textbook. But even with the m agnification Most bacteria Mitochondrion 1 µm (1,000 nm) 100 nm Smallest bacteria 10 nm 1 nm } Small molecules 57 - 0.1 nm.__A_t_o_m_s_@ _@:_ - - - - - - - - - - - - - - - - ' Figure 4.10 Differential interference contrast micrograph of Paramecium Figure 4.1E The size range of cells and related objects Introduction to the Cell 4.2 The small size of cells relates to the need to exchange materials across the plasma membrane As you saw in Figure 4.lE, most cells are microscopic. Are there advantages to being so small? The logistics of carrying out a cell's functions appear to set both lower and upper limits on Outside cell Hydrophilic cell size. At minimum, a cell must be large enough to house heads enough DNA, protein molecules, and structures to survive and reproduce. But why aren't most cells as large as chicken eggs? The maximum size of a cell is influenced by geometry-the need to have a surface area large enough to service the volume of a cell. Active cells have a huge amount of traffic across t heir outer surface. A chicken egg cell isn't very active, but once a Phospholipid chick embryo starts to develop, the egg is divided into many microscopic cells, each bounded by a membrane that allows the essential flow of oxygen, nutrients, and wastes across its surface. Channel Hydrophilic Hydrophobic protein reg ions of regions of Surface-to-Volume Ratio Large cells have more surface a protein a protein area than sm all cells, but t hey have a much smaller surface area relative to their volume than small cells. Figure 4.2A.A. Figure 4.28 The structure of a plasma membrane illustrates t his by comparing l large cube to 27 small ones. Using arbitrary units of measurement, the total volume Phospholipid molecules are well suited to their role as a is t he same in bot h cases: 27 units 3 (height x width x major constituent of biological membranes. Each phospho- length). The total surface areas, however, are quite different. lipid is composed of two distinct regions-a head with a neg- A cube has six sides; thus, its surface area is six times the area atively charged phosphate group and two nonpolar fatty acid of each side (h eight x widt h). The surface area of the large tails (see Module 3.10). Phospholipids group togeth er to form cube is 54 units 2, wh ile the total surface area of all 27 cubes a two-layer sheet called a phospholipid bilayer. As you can see is 162 units2 (27 x 6 x 1 x 1), three t imes greater than the in Figure 4.28, the phospholipids' hydrophilic (water-loving) surface area of the large cube. Thus, the combined smaller heads face outward, exposed to the aqueous solutions on both cubes h ave a much greater surface-to-volume ratio than the sides of a membrane. Their hydrophobic (water-fearin g) tails large cube. How about t h ose neurons that extend from t he point inward, mingling together and shielded from water. base of your spine to your toes? Very thin, elongated shapes Embedded in this lipid bilayer are diverse proteins, floating also provide a large surface area relative to a cell's volume. like icebergs in a phospholipid sea. The regions of t h e proteins The Plasma Membrane So what is a cell's surface like? within the center of the membrane are hydrophobic; the exte- And how does it control t he traffic of molecules across it? Th e rior sections exposed to water are hydrophilic. plasma membrane, also referred to as the cell membrane, Illustrating our theme of , the forms a flexible boundary between the living cell and its sur- properties of the phospholipid bilayer and the proteins sus- roundings. For a structure that separates life from nonlife, this pended in it relate to the plasma membrane's job as a traffic membrane is amazingly thin. It would take a stack of more than cop, regulating the flow of material into and out of the cell. 8,000 plasma membranes to equal the thickness of this page. Nonpolar molecules, such as 0 2 and C0 2 , can easily move And, as you have come to expect with all things biological, the across the membrane's hydrophobic interior. Some of the structure of the plasma membrane correlates with its function. membrane's proteins form channels (tunnels) that shield ions and polar molecules as they pass through the hydrophobic cen- ter of the membrane. Still other proteins serve as pumps, using energy to actively transport molecules into or out of the cell. We will return to the structure and function of biological membranes later (see Chapter 5). In t he next module, we consider other features common to all cells and take a closer look at the prokaryotic cells found in two of the three major groups of organisms. Total volume 27 units3 27 units3 Total su rface f t To convince yourself that a small cell has a greater surface 54 units2 162 units2 U area relative to volume than a large cell, compare the area surface-to-volume ratios of the large cube and one of the small Surface-to- 2 6 cubes in Figure 4.2A. volume ratio (e:l!Un l x l x l S! awn10A 9 = sap!S g.A. Figure 4.2A Effect of cell size on surface area and volume x i: x i: S! eaJe aoeµ ns) g = i;/ g :aqno news :1: = L1:/ 1'S :aqno al!Jel 58 CHAPTER 4 A Tour of the Cell 4.3 Prokaryotic cells are structurally simpler than eukaryotic cells Cells are of two distinct types: prokaryotic and eukaryotic. differences are the basis for the action of some antibiotics, Prokaryo tic cells were the first to evolve and were Earth's which specifically target prokaryotic ribosomes. Thus, pro- sole inhabitants for more than 1.5 billion years. Eviden ce tein synthesis can be blocked for the bacterium that's invaded indicates that eukaryot ic cells evolved from some of these you, but not for you, the eukaryote who is taking the drug. ancestral cells about 1.8 billion years ago. Biologists recognize Outside the p lasma membrane of most prokaryotes is a three domains or major groups of organisms. The m icroor- fairly rigid, chemically complex cell wall. The wall protects ganisms placed in domains Bacteria and Archaea consist of the cell and helps maintain its shape. Some antibiotics, such prokaryotic cells. These organisms are known as prokaryotes. as penicillin, prevent the formation of these protective walls. All other forms of life are placed in domain Eukarya. They are Again, because your cells don't have such walls, these antibi- composed of eukaryotic cells and are referred to as eukaryotes. otics can kill invading bacteria without harming your cells. Eukaryotic cells are distinguish ed by having a membrane- Certain prokaryotes have a sticky outer coat called a capsule enclosed nucleus, which houses most of their DNA, and many around the cell wall, helping to glue the cells to surfaces or membrane-enclosed organelles that perform specific functions. to other cells in a colony. In addition to capsules, some pro- Prokaryotic cells are smaller and simpler in structure. karyotes have surface projections. Short projections help Both types of cells, however, sh are certain basic features. attach prokaryotes to each other or their substrate. Longer In addition to being bounded by a plasma membrane, the projections called flagella (singular, flagellum) propel a cell interior of all cells is filled with a thick, jellylike fluid called through its liquid environment. cyt osol , in which cellular components are suspended. All It takes an electron microscope to see the internal details cells have one or more chromosomes, which carry genes of any cell, and this is especially true of prokaryotic cells. made of DNA. They also contain ribosomes, tiny structures Notice that the TEM of the bacterium in Figure 4.3 has a mag- that make proteins according to instructions from the genes. nification of 20,940X. Most prokaryotic cells are about one- The inside of both types of cells is called the cytoplasm. tenth t he size of a typical eukaryotic cell. (Prokaryotes will be However, in eukaryotic cells, this term refers only to the described in more detail in Chapter 16.) Eukaryotic cells are region between the nucleus and the plasma membrane. the main focus of this chapter, so we turn to these next. Figure 4.3 explores the structure of a generalized prokary- otic cell. Notice that the DNA is coiled into a region called n List three features that are common to prokaryotic U and eukaryotic cells. List three features that differ. the n udeoid ("nucleus-like") , but no membrane surrounds ·sawosoq!J lUaJaJJ.!P le4Mawos a11.e4 pue paso1::>ua-aueJqwaw the DNA. The ribosomes of prokaryotes are smaller and dif- JaLno Jo sna1onu e aAe4 lOU op 'Ja11ews aJe s11aa :mo.\Je>jOJd ·sawosoqp pue fer somewhat from t h ose of eukaryotes. These molecular '\fNO sawosowonp 'saueJqwaw ewse1d a11.e4 s11a:> !O sadAl 1.noa Fimbriae: attachment structures on Helicobactor pylori, the surface of some prokaryotes a bacterium that causes stomach ulcers rigid structure outside the Bacterial plasma membrane Ca psule: jellylike outer coating of many prokaryotes A typical rod-shaped bacterium Flagella: locomotion 59 -.t. Figure 4.3 A diagram (left) and electron micrograph (right) of a typical prokaryotic cell Introduction to the Cell 4.4 Eukaryotic cells are partitioned into functional compartments All eukaryotic cells- whether from protists (a diverse group of which perform specific tasks. Just as the cell itself is wrapped in mostly un icellular organisms), fungi, animals, or plan ts- are a membrane made of phospholipids an d proteins that perform fundamentally similar to on e another an d profoun d ly differ- various functions, each organelle is bounded by a membrane en t from prokaryotic cells. Let's look at an animal cell an d a with a lipid and protein composition that suits its function. plan t cell as representatives of the eukaryotes. The organelles and other structures of eukaryotic cells Figure 4.4A is a diagram of a generalized animal cell, and can be organized into four basic fun ctional groups: (1) The Figure 4.48 sh ows a generalized plan t cell. We color-code th e n ucleus an d ribosomes carry out th e genetic control of th e various structures in the diagrams for easier identification, cell. (2) Organelles involved in t he manufacture, distribu- and you will see miniature versions of these cells to orient you tion, and breakdown of molecules include the endoplasmic during our in-depth tour in the rest of the chapter. But no cells reticulum, Golgi apparatus, lysosomes, vacuoles, and peroxi- would look exactly like these. For one thing, cells have multi- somes. (3) Mitochondria in all cells and chloroplasts in plant ple copies of all of these structures (except for the n ucleus). cells function in energy processing. (4) Structural support, Your cells h ave hundreds of mitochondria and millions of movemen t , and communication between cells are the func- ribosomes. A plant cell may have 30 ch loroplasts packed tions of the cytoskeleton, plasma membrane, and plant cell in side. Cells also h ave differen t shapes and relat ive propor- wall. Th e cellular components iden tified in these two figures tions of cell parts, depending on th eir specialized function s. will be examined in detail in the modules t hat follow. Th e most obvious h allmark of a eukaryotic cell is its n ucleus. In essence, t he internal membranes of a eukaryotic cell But it also con tains various other organelles ("little organs"), partition it into functional compartments in which many of its chemical activities- collectively called cellular thick cell wall. Chemically different from prokaryotic cell metabolism-take place. In fact, various enzymes essen- walls, plant cell walls contain the polysaccharide cellulose. tial for metabolic processes are built into the membranes Plasmodesmata (singular, plasmodesma) are cytoplasmic of organelles. The fluid-filled spaces within such compart- channels through cell walls that connect adjacent cells. An ments are locations where specific chemical conditions are important organelle found in plant cells is the chloroplast, maintained. These conditions vary among organelles and where photosynthesis occurs. Unique to plant cells is a large favor the metabolic processes occurring in each. For exam- central vacuole, a compartment that stores water and a ple, while a part of the endoplasmic reticulum is engaged variety of chemicals. in making hormones, neighboring peroxisomes may be Eukaryotic cells contain nonmembranous structures as detoxifying harmful compounds and making hydrogen per- well. The cytoskeleton, which you were introduced to in oxide (H 2 0 2) as a poisonous by-product of their activities. the chapter introduction, is composed of different types But because the H2 0 2 is confined within the peroxisomes, of protein fibers that extend throughout the cell. And ribo- where it is converted to H2 0 by resident enzymes, the rest somes are found in the cytosol as well as attached to certain of the cell is protected. membranes. Except for lysosomes and centrosomes, the organelles and After you preview these cell diagrams, let's move to the other structures of animal cells are found in plant cells. Also, first stop on our detailed tour of the eukaryotic cell-the although some animal cells have flagella or cilia (not shown nucleus. in Figure 4.4A), among plants, only the sperm cells of a few species have flagella. f t Identify the structures i n the plant cell that are not present in A plant cell (Figure 4.4B) also has some structures that an U the animal cell. animal cell lacks. For example, a plant cell has a rigid, rather The Nucleus and Ribosomes 4.5 The nucleus contains the cell's genetic instructions You just saw a preview of the many intricate structures t hat Enclosing the nucleus is a double membrane called the can be found in a eukaryotic cell. A cell must build and nuclear envelope. Each of the two membranes is a separate maintain these structures and also process energy to support phospholipid bilayer with associated proteins. Similar in func- its work of transport, movement, and communication. But tion to the p lasma membrane, the nuclear envelope controls who is in charge of this bustling factory? Who stores the the flow of materials into and out of the nucleus. As you can see master plans, gives the orders, changes course in response to in the diagram of a nucleus in Figure 4.5, the nuclear envelope environmental input, and, when called on, makes another is perforated with protein-lined pores. These pores regulate the factory just like itself? The cell's nucleus functions as this entry and exit of large molecules and also connect with the command center. cell's network of membranes called the endoplasmic reticulum. The nucleus contains the cell's genetic instructions The nucleolus, a prominent structure in t h e nucleus, encoded in DNA. These master plans control the cell's activ- is the site where a special type of RNA called ribosomal RNA ities by directing protein synthesis. The DNA is associated (rRNA) is synthesized according to instructions in the DNA. with many proteins and organized into structures called chro- Proteins brought in from the cytoplasm are assembled with mosomes. The proteins help coil these long DNA molecules. this rRNA to form the subunits of ribosomes. These subunits Indeed, the DNA of the 46 chromosomes in one of your cells then exit to the cytoplasm, where they will join to form laid end to end would stretch to a length of more than 2 m, functional ribosomes. but it must coil up to fit into a nucleus only 5 µm in diameter. Another type of RNA, messenger RNA (mRNA), directs When a cell is not dividing, this complex of proteins and protein synthesis. Essentially, mRNA is a transcription of DNA, called chromatin, appears as a diffuse mass within protein-synthesizing instructions written in a gene's DNA the nucleus, as shown in the TEM (right half) and diagram (see Figure 10.7). The mRNA moves into t h e cytoplasm, (left half) of a nucleus in Figure 4.5. where ribosomes translate it into the amino acid sequences As a cell prepares to divide, the DNA is copied so that of proteins. Let's look at ribosomes next. each daughter cell can later receive an identical set of genetic instructions. Just prior to cell division, the thin chromatin fibers coil up further, becoming thick enough to be visible 0 Describe the processes that occur in the nucleus. "VN!JW OIU! paq11osue11 am VNO U! with a light microscope as the familiar separate structures you suo11:mJ1su1 Sl!Unqns 1ewosoq1J pue apew S! would probably recognize as chromosomes. 11a:> LI! s11a:> 01 uo passed pue pa!doo S! 'VNG.A. Figure 4.5 A cross section of the nucleus with a superimposed TEM 62 CHAPTER 4 A Tour of the Cell 4.6 Ribosomes make proteins for use in the cell and for export If the nucleus is the cell's command center, then ribosomes are the machines that carry out those commands. Ribosomes are the cellular components that use instructions from the nucleus, written in mRNA, to build proteins. Cells that make a lot of proteins have a large number of ribosomes. For example, a cell in your pancreas that produces digestive enzymes may contain a few million ribosomes. What other structure is prominent in cells that are active in protein synthesis? Remember that the nucleolus in the nucleus is the site wh ere the subunits of ribosomes are assembled. As shown in Figure 4.6, ribosomes are found in two locations in the cell. Free ribosomes are suspended in the cytosol, while bound ribosom es are attached to the outside of the en doplasmic reticulum or nuclear envelope. Free and bound ribosomes are structurally identical, and they can function in either location, depen ding on the protein they are making. Most of the proteins made on free ribosomes function within the cytosol; examples are enzymes th at catalyze the first steps of sugar breakdown for cellular respiration. In Module 4.8, you will see how boun d ribosomes make proteins that will be exported from the cell. At the bottom right in Figure 4.6, you see how ribosomes interact with messenger RNA (carrying the instructions from a gene) to build a protein. The nucleotide sequen ce.A. Figure 4.6 The locations and structure of ribosomes of an mRNA molecule is translated into the amino acid sequen ce of a polypeptide. The pathway from DNA to RNA f t What role do ribosomes play in carrying out the genetic to protein is a prime example of our theme of the flow of U instructions of a cell? [email protected];j.l.f-ii!@. (Protein synthesis is explored in more detail ·sna1onu a41 LI! \INO WOJj paq!JOSUBJl SBM 40!4M '\IN!l in Chapter 10.) !O suo11onJ1su 1a1..n 01 su1a10Jd az1sa1..nuAs The Endomembrane System 4. 7 Many organelles are connected in the endomembrane system Ribosomes may be a cell's protein-making machines, but The largest component of the endomembrane system is running a factory as complex as a cell requires infrastructure the endoplasmic reticulum (ER), an extensive n etwork and many different departments that perform separate but of flattened sacs and tubules. (The word endoplasmic means related functions. Internal membranes, a distinguishing "within the cytoplasm," and reticulum is Latin for "little net.") feature of eu karyotic cells, are involved in most of a cell's The ER is a prime example of the direct and indirect interre- function s. Many of the membranes of the eukaryotic cell are latedness of parts of the endomembrane system. As sh own in part of an endomembrane system. Some of these mem- Figure 4.5 on the facing page, membranes of the ER are con - branes are physically connected and others are linked when t inuous with the nuclear envelope. And when vesicles bud tiny vesicles (sacs made of membrane) transfer membrane from the ER, they travel to many other components o f the segments between t h em. endomembrane system. Th e endomembrane system includes the nuclear enve- The membranes of the ER enclose a space separate from the lope, endoplasmic reticulum, Golgi app aratus, lysosom es, cytosol. Indeed, an important aspect of the components of the various types of vesicles and vacuo les, and the plasma endomembrane system is dividing the cell into functional com- membrane. (The plasma membrane is not exactly an endo partments, each of which may require different conditions. (inner) membran e in physical location, but it is related to 63 - the oth er membranes by the transfer of vesicles.) Many of f t Which structure includes all ot hers in the list: ER, vesicle, these organelles interact in the synthesis, distribution, U endomembrane system, nuclear envelope? storage, and export of molecules. aueJqwawopu3 The Endomembrane System 4.8 The endoplasmic reticulum is a biosynthetic workshop One of t he major manufacturing sites in a cell is the endoplas- mic reticulum. Th e diagram in Figure 4.SA shows a cutaway view of t h e interconnecting membranes of the smooth and rough ER, which can be distinguished in the superimposed electron micrograph. Smooth endoplasmic reticu lum is called smooth because its outer surface lacks attached ribo- somes. Rough endoplasmic ret iculum has bound ribo- somes that stud t he outer surface of t h e membrane; thus, it appears rough in the electron micrograph. Smooth ER Th e smooth ER of various cell types functions in a variety of metabolic processes. Enzymes of the smoot h ER are important in the synthesis of lipids, including oils, phos- pholipids, and steroids. In vertebrates, for example, cells of the ovaries and testes synthesize the steroid sex hormones. These cells are rich in smooth ER, a structural feature t h at fits their function by providing ample machinery for steroid synthesis. Our liver cells also have large amoun ts o f smooth ER, with enzymes that help process drugs, alcohol, and ot her potentially h armful substances. Th e sedative phenobarbital and other barbiturates are examples of drugs detoxified by t hese en zymes. As liver cells are exposed to such chemicals, th e amou nt of smooth ER and its detoxifying enzymes increases, thereby increasing t he rate of detoxification and th us the body's tolerance to the drugs. Th e result is a need for higher doses of a drug to achieve a particular effect, su ch Figure 4.SA Sm ooth and rough en doplasmic reticulum as sedation. Also, because detoxifying enzymes often cannot distinguish among related chemicals, the growth of smooth ER in response to one drug can increase t h e need for higher doses of oth er drugs. Barbiturate abuse, for example, can decrease t h e effectiveness of certain antibiotics and Transport vesic le buds off other useful drugs. Smooth ER h as yet another function , the storage Secretary of calcium ions. In muscle cells, for example, a specialized protein inside trans- smooth ER membrane pumps calcium ions into the interior port vesicle of the ER. When a nerve signal stimulates a muscle cell, calci- um ions rush from the smooth ER into the cytosol and trigger contraction of the cell. Rough ER Many types of cells secrete proteins produced by Sugar ribosomes attached to rough ER. An example of a secretary chain protein is insulin , a hormone produced and secreted by cer- tain cells of the pancreas and transported in the bloodstream. Type 1 diabetes results when these cells are destroyed and a Rough ER lack of insulin disrupts glucose metabolism in the body. Figure 4.SB follows the synthesis, modification, and packaging of a secretory protein. As the polypeptide is syn- Figure 4.SB Synthesis and packaging of a secretory protein by the rough ER th esized by a bound ribosome following the instructions of an mRNA, 0 it is t hreaded into the cavity of the rough ER. TRY THIS Explain where the protein-making Instructions carried As it enters, the new protein folds into its t h ree-dimension al by the mRNA came from. shape. f) Sh ort chains of sugars are often linked to the poly- peptide, making th e molecule a g l ycoprot ein (glyco means "sugar"). Q When the molecule is ready for export from The vesicle now carries the protein to the Golgi apparatus for t h e ER, it is packaged in a t ra n sp o rt vesicle , a vesicle t hat further processing. From there, a transport vesicle containing moves from one part of the cell to another. 0 This vesicle the finished molecule makes its way to the plasma membrane buds off from the ER membrane. and releases its contents from the cell. 64 CHAPTER 4 A Tour of the Cell  In addition to making secretory proteins, rough ER is a mem- Now let's follow a transport vesicle carrying products of brane-making machine for the cell. It grows in place by adding the rough ER to the Golgi apparatus. membrane proteins and phospholipids to its own membrane. As polypeptides destined to be membrane proteins grow from bound ribosomes, they are inserted into the ER membrane. Phospholipids are made by enzymes of the rough ER and also f t Explain why we say that the endoplasmic reticulum U is a biosynthetic workshop. inserted into the membrane. Thus, the ER membrane grows, ·uao a1,n ,(q UO!laJ:>aS pue JaLUO 'saueJqWaW JOJ and portions of it are transferred to other components of the (sawosoqp punoq Aq paz!sa1.nuAs) SU!alOJd pue 'sauowJ04 P!OJalS 'saueJqwaw endomembrane system in the form of transport vesicles. 11ao JOI SP!d1104dso4d llU!P"IOU! 'sa1noa1ow 10 A1a!JeA ajjn4 e saonpoJd !l3 a41 4.9 The Golgi apparatus modifies, sorts, and ships cell products After leaving the ER, many transport vesicles travel to the depot, dispatching its products in vesicles that bud off and Golgi apparatus. Using a light microscope and a staining travel to other sites. technique he developed, Italian scientist Camillo Golgi dis- How might ER products be processed during their transit covered this membranous organelle in 1898. The electron through the Golgi? Various Golgi enzymes modify the carbo- microscope confirmed his discovery more than 50 years later, hydrate portions of the glycoproteins made in the ER, removing revealing a stack of flattened sacs, looking much like a pile of some sugars and substituting others. Molecular identification pita bread. A cell may contain many, even hundreds, of these tags, such as phosphate groups, may be added that help the Golgi stacks. The number of Golgi stacks correlates with how active sort molecules into different batches for different destinations. the cell is in secreting proteins-a multistep process that, Finished secretory products, packaged in transport vesi- as you have just seen, is initiated in the rough ER. cles, move to the plasma membrane for export from the The Golgi apparatus serves as a molecular warehouse and cell. Alternatively, finished products may become part of the processing station for products manufactured by the ER. You plasma membrane itself or part of another organelle, such can follow these activities in Figure 4.9. Note that, unlike the as a lysosome, which we discuss next. ER sacs, the flattened Golgi sacs are not connected. O One side of a Golgi stack serves as a receiving dock for transport f t What is the relationship of the Golgi apparatus to the ER vesicles produced by the ER. f) A vesicle fuses with a Golgi U in a protein-secreting cell? sac, adding its membrane and contents to the "receiving" side. ·palaJ:>as am su1at0Jd a4l 'aueJqwaw ewse1d alll Ol sa101saA lJOdsueJl sa4:>leds1p pue su1al0Jd a1..n $ Products of the ER are modified as they progress through sa4s!U!:J a1.u ·sawosoq!J punoq ,(q paz1sa1.nui1soii wJads pue 'l::>eJl i\JoteJ!dSaJ aLn asueap "feet" that "walk" along an adjacent doublet. The walking touue::> e111::> smu ·puaq touue::> sa1nqn10Jo1 w 'su1a10Jd JOlOW lOOLn!M 4.19 The extracellular matrix of animal cells functions in support and regulation The plasma membrane is usually regarded as the boundary of the cell, but most cells synthesize and secrete materials that are external to the plasma membrane. Animal cells produce an extracellular matrix (ECM) (Figure 4.19). This elabo- rate layer helps hold cells together in t issues and protects and supports the plasma membrane. The main components of the ECM are glycoproteins, proteins bonded with carbohydrates. The most abundant gly- lntegrin coprotein is collagen, which forms strong fibers outside the cell. In fact, collagen accounts for about 40% of the protein in your body. The collagen fibers are embedded in a network Plasma --{ woven from large complexes that include hundreds of small membrane glycoproteins connected to a long polysaccharide molecule (shown as green in the figure). The ECM may attach to the cell through other glycoproteins that then bind to membrane Microfilaments proteins called integrins. Integrins span t he membrane, of cytoskeleton attaching on the other side to proteins con nected to microfil- ,. Figure 4.19 The extracellular matrix (ECM) of an animal cell aments of the cytoskeleton. As their name implies, integrins have the function of cell's ECM can even influence the activity of genes through integration : They transmit signals between t he ECM and the signals it relays. the cytoskeleton and can communicate changes occurring outside and inside the cell. Curren t resea rch is revealing new f t Referring to Figure 4.19, describe the structures that provide 71 - U support to the plasma membrane. and influential functions of the ECM. For example, it can 'WJ3 04110 regulate a cell's behavior by directing the path along which SJ9Q!I ualienoo pue liu11oauuoo 01 pue 04110 embryonic cells move. Researchers h ave also learned t hat a s1uawe1110J01w 01 su1a10Jd aueJqwaw 4linoJ4I pa4oene S! aueJqwaw a41 The Cytoskeleton and Cell Surfaces 4.20 Three types of cell junctions are found in animal tissues Neighboring cells in animal tissues often adhere, interact, and -....... , junctions communicate through specialized junctions between them. '' prevent fluid from Figure 4.20 uses cells lining the digestive tract to illustrate ' moving across a layer of cells three types of cell junctions. (The projections at the top of the cells increase the surface area for absorption of nutrients.) At tight junctions, the p lasma membranes of neigh boring cells are knit tightly together by proteins. Tight junctions pre- vent leakage of fluid across a layer of cells. The dotted green Tight junction arrows show how tight junctions prevent the contents of the digestive tract from leaking into surrounding tissues. Anchoring junctions function like rivets, fastening cells Anchoring together into strong sheets. Intermediate filaments made junction of sturdy proteins anchor these junctions in the cytoplasm. Anchoring junctions are common in tissues subject to stretching or mechanical stress, such as skin and muscle. Gap junction Gap junctions, also called communicating junctions, are channels that allow small molecules to flow through protein-lined pores between cells. The flow of ions through Plasma membranes gap junctions in the cells of heart muscle coordinates their of adjacent cells contraction. Gap junctions are common in embryos, where communication between cells is essential for development. f t A muscle tear injury would probably involve t he rupture U of which type of cell junction? Ions or small molecules Extracellular matrix.6. Figure 4.20 Three types of cell j unctions in an imal t issues 4.21 Cell walls enclose and support plant cells The cell wall is one of t h e features that distinguishes plant cells from animal cells. This rigid extracellular structure not - -- _.::"'--Plant cell only protects the cells but also provides the skeletal support walls th at keeps plants uprigh t on land. Plant cell walls consist of fibers of cellulose (see Figure 3.7) embedded in a matrix of other polysaccharides and proteins. This fibers-in-a-matrix construction resembles that of steel-reinforced concrete, which is also noted for its strength. Figure 4.21 shows t h e layered structure of p lant cell walls. Cells initially lay down a relatively thin and flexible p rimary wall, which allows the growing cell to continue to Primary cell wall - - - - + - - -- - -.. enlarge. Between adjacent cells is a layer of sticky polysac- charides called pectins (shown here in dark brown), which Secondary cell glue the cells together. (Pectin is used to thicken jams and jellies.) When a cell stops growing, it strengthens its wall. Plasma membrane ----+-- -- - - -. Some cells add a secondary wall deposited in laminated _ _ _ _ _ __ __,...___ _-""L...J layers next to the plasma membrane. Wood consists mainly of secondary walls, which are strengthened with rigid mol-.6. Figure 4.21 Plant cell walls and plasmodesmata ecules called lignin. Despite their thickness, plant cell walls do not totally plasmodesmata, the cells of a plant tissue share water, nour- isolate the cells from each other. Figure 4.21 shows the ishment, and chemical messages. numerous channels that connect adjacent plant cells, called plasmodesmata (singular, plasmodesma). Cytosol pass- ing through t h e plasmodesmata allows water and other 0 Which animal cell junction is analogous to a plasmodesma? small molecules to freely move from cell to cell. Through uo1iounf dell 'tl 72 CHAPTER 4 A Tour of the Cell 4.22 Review: Eukaryotic cell structures can be grouped on the basis of four main functions il·iMf%jH@iifillfi@!liiifj,j·ll,@iiii.!4!1.!,f Con gratulations: You h ave comp leted t h e grand tour of the cell. In t h e process, you have been introduced 11. Genetic Control to many important cell structures. To provide a fram e- Nucleus DNA replicat ion, RNA synthesis; assembly of ribosomal subunits (in nucleolus) work for this information an d rein force the theme that structure is correlated with function, we have grouped the eukaryotic cell structures into four cate- gories by gen eral function , as reviewed in Table 4.22. Th e first category is genetic control. Here we include the nucleus that h ouses a cell's genetic instruction s and the ribosom es that produce the proteins coded for in those instruction s. Th e second category includes Ribosomes Polypeptide (protein) synt hesis organelles of the endomembran e system that are 2. Manufacturing, Distribution, and Breakdown involved in the m anufacture, distribution, and break- Rough ER Synt hesis of membrane lipids and proteins , secretory down of materials. The third category includes the proteins, and hydrolytic enzymes; formation of two en ergy-processing organelles, mitoch ondria and t ransport vesicles chloroplasts. And the fourth category- structural sup- Smooth ER Lipid synthesis; detoxification in liver cells; ca lcium port, movement, and intercellular communication- ion storage in muscle cells includes the cytoskeleton , extracellular structures, and Modification and sorting of ER products ; formation of connection s between cells. lysosomes and transport vesicles Within most o f these categories, a structural similarity underlies the gen eral functio n of each compon ent. Manufacturing dep en ds h eavily on a n etwork of structurally and function ally connected mem branes. All the organ elles involved in the breakdown or recycling of m aterials are m embra- Lysosomes (in animal cells Digestion of ingested food or bacteria and recycling n ous sacs, inside o f which en zym atic digestion can and some protists) of a ce ll's damaged organelles and macromolecules safely occur. In the energy-processing category, Vacuoles Digest ion (food vacuole); water balance (contractile vacuole); storage of chem icals and cell enlargement expanses of m etabolically active m embran es and (central vacuole in plant cells ) intermem bran e comp artments within the organ- Peroxisomes (not part of Diverse metabolic processes, with breakdown of toxic elles en able chloroplasts and m itoch ondria to per- endomembrane system) hydrogen peroxide by-product form the com plex en ergy conversions that power the cell. Even in t h e diverse fourth category, there is 3. Energy Processing a com mon structural t h em e in th e various protein Mit ochondria Cellular respiration: conversion of chemical energy in fibers of m ost of these cellular system s. food to chemical energy of ATP We can summarize furth er by n oting that th e over- all structure of a cell is closely related to its specific function. Thus, cells that produce proteins for export contain a large quantity of ribosom es and rough ER, Chloroplasts (in plants Photosynthesis: conversion of light energy to chemical while muscle cells are packed with microfilam ents, and algae) energy of sugars myosin m otor proteins, and mitoch ondria. And, 4. Structural Support, Movement, and Communication Between Cells finally, let us em phasize t h at these cellular structures Cytoskeleton. - - - --.........,.. Maintenance of cell s hape; anchorage for form an integrated team-wit h the p roperty of life (microfilaments, organelles; movement of organelles within cells; emerging at the level of the cell from the coordinated intermediate cell movement (crawling, musc le cont raction, functions of the team members. A cell beautifully fi laments, and bending of cilia and flagella) illustrates our theme of iili13;f.\tii[o!M: it is a living microtubules) unit that is greater than the sum of its parts. Plasma membrane Regulate traffic in and out of cel l Extracellular matrix (in animals ) Support; regulat ion of cell ular activities ft How do mitochondria, smooth ER, and the Cell j unct ions Communication between cells; binding of cells U cytoskeleton all contribute to the contraction in tissues 73 - of a muscle cell? Cell wall s Support and protection; binding of cells in t issues ·sn1eJedde amoeJ1uoo 1enpe a41 U! uo1punJ s1uawe11JOJO!W ·suo1 (in plant s) wnp1eo JO asea1aJ pue a>1e1dn a41 Aq uo1peJ1uoo a1e1nl!aJ sd1a4 1:13 41oows a41 al\I JO WJOJ a41 u1Al!Jaua 1dd ns e1Jpuo400111111 The Cytoskeleton and Cell Surfaces 0:: 4 UJ I- For practice quizzes, BioFlix animations, MP3 tutorials, video tutors, and REVIEW C.. J4M LI! JaleM puod a1u 5.6 Transport proteins can facilitate diffusion across membranes Recall t hat n onpolar m olecules, such as 0 2 and C02 , can d is- Sl! terolemia, LDL receptor proteins are defective or m issing. 'aueiqwaw ewserd a41 41!M sasn1aro1saA11odsue11 e ua4M :s1so1.loox3 Membrane Structure and Function