A Tou of 4. Focus on the Concepts

Transcription

1 A Tou of 4 Focus on the Concepts Cells are the building blocks of life. The characteristics of life emerge from the arrangement and interactions of the parts within ceils. This chapter explores the structure and function of ceils. As you study the chapter, focus on these major concepts: Most ceils are microscopic. Cell size is limited by the amount of surface area required for exchange with the environment and the distance materials must diffuse within the ceil. All ceils are bounded by a plasma membrane, have d~r0mosomes carrying genetic information, and possess ribosomes that manufacture protein. Eukaryotic cells--those of plants and animals--possess a membrane-enclosed nucleus and are compartmentalized by internal membranes into other orgm~elles that carry out various functions. Prokaryotic cells are smaller and simpler, having no nucleus or other membrane-enclosed organelles. The plasma membrane and internal membranes are composed of two layers of phospholipid molecules and associated proteins. Membranes carry out a variety of functions, most notably controlling movement of substances in and out of the cell and membrane-enclosed compartments. The permeability of a membrane depends on the properties of the phospholipid bilayer and associated proteins. A number of cell structures function in chemical synthesis and breakdown. The nucleus contains most of the cell s genetic information and controls the cell by direcnng protein synthesis by the ribosomes. Some ribosomes float free in the cytoplasm, while some are bound to the ER, pa~ t of the endomembrane system. Smooth and rough ER make and break down a variety of substances. ER products are packaged and modified by the Golgi apparatus. Some products leaving the Golgi are exported, while others are retained by structures such as lysosomes, which may digest food in food vacuoles. Other vacuoles function in storage. Mitochondria and chloroplasts are energy-converting organelles that evolved from free-living prokaryotes. Mitochondria, found in nearly all eukaryotic cells, carry out cellular respiration, converting food energy into the chemical energy of ATP. Photosynthetic eukaryotes have chloroplasts, which conduct photosynthesis, converting the light energy into the chemical energy of food molecules. Microfilaments, intermediate filaments, and microtubules are rods, fibers, rand tubes that form the cytoskeleton, suppor~g and orgm~izing the cell s internal structure and facilita~g functions such as movement. For example, microtubules m~d associated motor proteins move cilia and flagella, locomotor appendages that project from cells. There are important structures that lie outside the plasma membrane. Animal cells produce a flexible glycoprotein extracellular matrix that helps support

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3 A Tour of the Cell cells and hold them together. Plant cells are characterized by rigid cell walls, made largely of cellulose, that support the plant. Various kinds of junctions enable cells to adhere, interact, and communicate. Review the Concepts Work through the following exercises to review the concepts in this chapter. For additional review, check out the activities at The website offers a pre-test that will help you plan your studies. Microscopes allow us to explore the world of the cell. Cells are very small--but just how small are they? Figure 4.1B compares the size range of objects we can observe with our unaided eyes and different kinds of microscopes. It is not that important to remember exact dimensions of these sma~ objects, but it is important to understand size relationships. Study the chart, and see if you can estimate the following size relationships. (Note that each of the reference measurements is ten times larger than the one shown below it.) A frog egg is about times larger than a human egg. The smallest objects we can see under a light microscope are about we can see with our unaided eyes. 3. An average virus is about times larger in diameter than a protein molecule. times smaller than objects 4. An electron microscope enables us to see things about times smaller than we can see with our eyes alone. 5. A large animal cell is about times larger than its nucleus and about times larger than a mitochondrion. 6. An average plant or animal cell is about times larger in diameter than a protein molecule. 7. The smallest bacteria are about times smaller than the average plant or animal cell. 8. A mitochondrion is about times larger than a ribosome Different kinds of microscopes enable us to study cells in different ways. Use the information module and Figure 4.1B to complete the following table comparing microscopes and the unaided human eye. Electron Microscope Unaided Eye Light Microscope (SEM or TEM) Kind of radiation o (beam) used Parts that 1 focus beam Maximum o o magnification Smallest objects visible Ability to separate close objects (resolution) Limitations

4 Chapter 4 We need to use a microscope to see cells because cells are so small. Why can t a cell be as big as a house, or at least as big as a baseball? Compare the two cells diagrammed in the following. For each cell, calculate the surface area, volume, and ratio of surface area to volume. Then answer the questions. Cell I Cell 2 Surface area: s = 6 x (I x t) = Surface area: s = 6 x (1 x 1) = Volume: v = I x 1 x I = Volume: v = I x I x 1 = Surface/volume: s / v = Surface/volume: s / v = 1. Which cell has the greater surface area? 2. Which cell has the greater volume? 3. Which cell has the greater ratio of surface area to volume? 4. In which cell would the surface area of the plasma membrane most efficiently service the cytoplasm? 5. What modification of the less-efficient cell would make it as efficient as the more efficient one?

5 Bacteria and archaea consist of small, simple prokaryotic cells. Label the following on this diagram of a prokaryotic cell: capsule, cell wall, plasma membrane, nucleoid, ribosome, prokaryotic flagella, fimbriae, bacterial chromosome. Briefly state the function of each structure next to its label. A Tour of the Cell

6 44 Chapter 4 Eukaryotic cells are larger than prokaryotic cells. More importantly, eukaryotic cells are divided into compartments called organelles that carry out various functions. Examine the diagrams and text, and then compare the structures of the cells of prokaryotes, plants, and animals by checking off their characteristics below. You may want to revise or refer to this checklist as you complete the chapter. Characteristic Prokaryotic Ceil Plant Cell Animal Cell Prokaryotic structure Eukaryotic structure Relatively large size Relatively small size Membranous organelles Plasma membrane Cell wall Cytoplasm Ribosomes Bacterial flagellum Nucleus Nuclear envelope Rough endoplasmic reticulum Smooth endoplasmic reticulum Golgi apparatus Lysosome Peroxisome Central vacuole Mitochondrion Chloroplast Cytoskeleton Flagellum Centriole Extracellular matrix Plasmodesmata Cell junctions,,

7 A Tour of the Cell Review the structures and functions of the nucleus and the various components that make up the endomembrane system by matching each phrase on the left with a structure from the list on the right. Answers can be used more than once. 1. Lipids manufactured here 2. Small structure that makes protein 3. Contains chromatin 4. Sac of digestive enzymes 5. Carries secretions for export from cell 6. Breaks down drugs and toxins in liver 7. Makes cell membranes 8. Cell control center 9. Numerous ribosomes give it its name 10. "Ships" products to plasma membrane, outside, or other organelles 11. May store water, needed chemicals, wastes, in plant cell 12. Buds off from Golgi apparatus 13. Defective in Tay-Sachs disease 14. Proteins made and modified here for secretion from cell 15. Pumps out excess water from some cells 16. Nonmembranous organelle 17. Takes in transport vesicles from ER and modifies their contents 18. Boundary between the cell and its surroundings 19. Carries out mrna instructions from the nucleus 20. How proteins and other substances get from ER to Golgi apparatus 21. Stores calcium in muscle cells 22. "Labels" and sorts molecules to be sent to different destinations 23. A digestive sac not related to other endomembrane structures 24. Digests food, wastes, foreign substances 25. Phosopholipid bilayer with proteins controls flow in and out of cell 26. Drug tolerance results from its enlargement 27. Fuses with food vacuole and digests contents 28. Structure in the nucleus where ribosomal RNA is made 29. Maybe "free" or "bound" 30. Surrounded by an double-membrane envelope with protein-lined pores 31. Looks like a stack of flattened sacs 32. Sends transport vesicles loaded with proteins to the Golgi apparatus 33. Can store plant pigments and/or poisons 34. Buds from here can become lysosomes A. Nucleus B. Transport vesicle C. Central vacuole D. Smooth ER E. Lysosome F. Golgi apparatus G. Rough ER H. Contractile vacuole I. Ribosome J. Plasma membrane K. Nucleolus L. Peroxisome

8 Chapter 4 Review the functions of the endomembrane system by completing this diagram and labelling the parts. Sketch and label rough ER, smooth ER, vacuole, ribosomes, Golgi apparatus, lysosome, nuclear envelope, transport vesicles, and plasma membrane. (1) Trace the path of a protein from its site of manufacture to the outside of the cell with a red arrow. (2) Trace the path of a protein incorporated into a lysosome in blue. (3) Trace the path of a protein incorporated into the plasma membrane in green. (4) Trace the path of a lipid secreted from the cell in yellow. Both mitochondria and chloroplasts are energy converters, but their functions are quite different. Compare them by filling in the chart below. Found in the following organisms... Chloroplast Mitochondrion Carries out process of... Converts energy of... Into chemical energy in... Evolved by endosymbiosis from...

9 A Tour of the Cell The cytoskeleton is important in cell shape, organization, and movement. Compare the components of the cytoskeleton by checking which of the following are characteristics of microfilaments, intermediate filaments, or microtubules. Hollow tubes Solid rods. Ropelike structure Made of tubulin Made of actin Made of supercoiled fibrous proteins Form a cage that anchors nucleus in place Work with myosin to change cell shape Reinforce cell shape, anchor organelles Act in muscle cell contraction Act as tracks for organelle movement Shape and support cell In cilia Inflagella In centrioles pattem Dynein feet cause bending movement Extend from basal body Guide chromosomes when cells divide Don t function in the disease PCD Intermediate Micro.filaments Filaments Microtubules The surfaces of cells have some important structural features. Match each of the cell surface characteristics or structures on the left with a phrase on the right. Answers may be used more than once. 1. Channel between animal cells 2. Rigid covering of a plant cell 3. Link animal cells in leakproof layer 4. Allows water and solutes to move between plant cells 5. Connects animal cells into a strong sheet 6. Sticky layer holds animal cells together 7. Integrins in plasma membrane link this to cytoskeleton 8. Helps support plant against gravity 9. Connects cytoplasm of adjacent plant cells 10. Acts like a "rivet" between animal cells 11. Made mostly of cellulose 12. Made mostly of collagen A. Tight junction B. Plasmodesma C. Anchoring junction D. Cell wall E. Gap junction F. Extracellular matrix

10 Chapter 4 Label the organelles listed in Module 4.22 on these diagrams of animal and plant cells. (If you get stuck, refer to Module 4.4.) Try to group your labels according to the four functional categories in Module 4.22 so that you can also circle and label each category. Complete your diagrams by underlining in red the names of structures found in animal cells but not in most plant cells. Underline in green the names of structures found in plant cells but not in animal cells.

11 A Tour of the Cell Test Your Knowledge Multiple Choice 1. To enter or leave a cell, substances must pass through a. a microtubule. b. the Golgi apparatus. c. a ribosome. d. the nucleus. e. the plasma membrane. 2. Which of the following would not be considered part of a cell s cytoplasm? a. a ribosome b. the nucleus c. a mitochondrion d. a microtubule e. fluid between the organelles 3. Which of the following consist of prokaryotic cells? a. plants and animals b. bacteria and archaea c. plants, fungi, bacteria, and archaea d. animals e. plants, bacteria, and archaea o 4. Organelles involved in energy conversion are the a, rough ERand Golgi apparatus. b. nucleus and smooth ER. c. nucleus and chloroplast. d. lysosome and ribosome. e. mitochondrion and chloroplast. The maximum size of a cell is limited by a, its need for enough surface area for exchange with its environment. b. the number of organelles that.can be packed inside. c. the materials needed to build it. d. the amount of flexibility it needs to be able to move. e. the amount of food it needs to survive. You would expect a cell with an extensive Golgi apparatus to a. make a lot of ATP. b. secrete a lot of material. c. move actively. d. perform photosynthesis. e. store large quantities of food. Which of the following correctly matches a structure with its function? a. mitochondrion =-- photosynthesis b. nucleus--cellular respiration c. central vacuole--storage d. lysosome--movement e. ribosome--manufacture of lipids Cellular metabolism is a. a type of cell division.. b. the process by which certain parts cause a cell to "self-destruct." c. the chemical activity of a cell. d. movement of a cell. e. control of the cell by the nucleus..9. Which of the following stores calcium, important in muscle contraction? a. mitochondria b. smooth ER c. the Golgi apparatus d. contractile vacuoles e. rough ER 10. Which group below is involved in manufacturing and distributing substances needed by the cell? a. lysosome, vacuole, ribosome b. Golgi, rough ER, smooth ER c. vacuole, rough ER, smooth ER d. smooth ER, ribosome, microtubule e. rough ER, lysosome, peroxisome 11. The internal skeleton of a cell is composed of a. microtubules, intermediate filaments, and microfilaments. b. cellulose and intermediate filaments. c. cellulose, microtubules, and centrioles. d. microfilaments and collagen. e. microfilaments and cellulose. 12. Mitochondria and chloroplasts are thought to have once been free-living organisms. What is the best evidence for this? a. They can make their own ATP. b. They can move around the cell. c. There is substantial fossil evidence. d. The have their own DNA and ribosomes. e. They originated by endosymbiosis. Essay 1. What are the advantages of an electron microscope over a light microscope? For what tasks would it be preferable to use a light microscope?

12 Chapter 4 2. Briefly describe the major differences between prokaryotic and eukaryotic cells. 3. Name the structures present in plant cells but lacking in animal cells, and describe their functions. 5o Explain the advantages eukaryotic cells derive from being compartmentalized by many internal membranes. Compare the functions of chloroplasts and mitochondria in a plant cell. App y the Concepts Multiple Choice 1. A cell has mitochondria, ribosomes, smooth ER, and other parts. Based on this information, it could not be a. a cell from a pine tree. b. a grasshopper cell. c. a yeast (fungus) cell. d. a bacterium. e. Actually, it could be any of the above. Dye injected into a cell might be able to enter an adjacent cell through a a. tight junction. b. microtubule. c. vacuole. d. plasmodesma. e. lysosome. If a cell s chromatin were damaged, the cell would a. swell up and burst. b. run out of energy needed for its activities. c. go out of control. d. not be able to absorb light. e. divide immediately. 4. A researcher made an interesting observation about a protein made by the rough ER and eventually used to build a cell s plasma membrane. The protein in the membrane was actually slightly different from the protein made in the ER. The protein was probably changed in the a. Golgi apparatus. b. smooth ER. c. mitochondrion. d. nucleus. e. chloroplast. o If the nucleus is a cell s "control center," and chloroplasts its "solar collectors," which of the following might be called the cell s combination "food processor" and "garbage disposer"? a. lysosome b. Golgi apparatus c. flagellum. d. ribosome eo nucleolus When elongated, tube-shaped cells from the lining of the intestine are treated with a certain chemical, the cells sag and become round blobs. The internal structures disrupted by this chemical are probably a. cell junctions. b. microtubules. c. smooth and rough ER. d. mitochondria. e. microfilaments. The electron microscope has been particularly useful in studying prokaryotes, because a. electrons can penetrate tough prokaryotic cell walls. b. prokaryotes are so small. prokaryotes move so quickly they are hard to photograph. do they aren t really alive, so it doesn t hurt to "kill" them for viewing. eo their organelles are small and tightly packed together. 8. A cell possesses ribosomes, a plasma membrane, a cell wall, and other parts. It could not be a. a bacterium. b. a cell from a fungus. c. a cell from a mouse. d. an oak tree cell. ~ e. a bacterium or a plant cell. 9. A mutant plant cell unable to manufacture cellulose would be unable to a. build a cell wall. b. divide. c. capture sunjight. d. move. e. store food.

13 A Tour of the Cell 10. A plant cell was grown in a test tube containing radioactive nucleotides, the parts from which DNA is built. Later examination of the cell showed the radioactivity to be concentrated in the a. rough ER. b. Golgi apparatus. c. smooth ER. d. central vacuole. e. nucleus. Essay Which of the following is correctly paired with a description of its structure? a. chloroplast: double membrane, stacks of thylakoid sacs in liquid stroma b. plasma membrane: phospholipid bllayer with embedded proteins c. extracellular matrix: collagen fibers in a network of other glycoproteins d. rough endopasmic reticulum: folded membranes with attached ribosomes e. All of the above are correct. Explain whether you would use a light microscope, a transmission electron microscope (TEM), or a scanning electron microscope (SEM) to perform each of the following tasks, and explain why: examining fine structural details within cell organelles, observing how a cell changes shape as it moves, studying tiny bumps on the cell surface, filming changes in the shape of the nucleus as a cell prepares to divide. Imagine a cell shaped like a cube, 5/~m on each side. (Cells are not perfect cubes, but this assumption simplifies the question.) What is the surface area of the cell, in/~m27 What is its volume, in/~m3? What is the ratio of surface area to volume for this cell? (Sketches may help.) Now imagine a second cell, this one 10/~m on each side. What are its surface area, volume, and surface-to-volume ratio? Compare the surface-to-volume ratios of the two cells. How is this comparison significant to the functioning of cells? How could the surface-tovolume ratio of the larger cell be increased? An enzyme (a type of protein) called salivary amylase is manufactured in the cells of your salivary glands and secreted in saliva. Explain how these parts of the cell cooperate to produce and secrete salivary amylase: transport vesicles, rough ER, plasma membrane, nucleus, Golgi apparatus, ribosomes. A poison that acts specifically on mitochondria was found to interfere with the movement of cilia, slow down protein synthesis, reduce the frequency of cell division, and slow down the manufacture of lipids. Explain how one chemical Could affect so many different cell activities. When you work harder, your muscle cells work harder and increase in size. How might various organelles in a muscle cell increase in size, number, or activity to respond to the challenge of an increased workload? Put Words to Work Correctly use as many of the following words as possible when reading, talking, and writing about biology: ATP, actin, anchoring junction, basal body, cell cell theory, cell wail celhdar metabolism, central vacuole, centriole, chloroplast, chromatin, chromosome, cilium, collagen, cytoskeleton, cytoplasm, electron microscope (EM), endomembrane system, endoplasmic reticulum (ER), endosymbiont theony, eukaryotic cell extracelhdar matrix, yqagellum, gap junction, glycoprotein, Golgi apparatus, integ~ ins, h~termediatejilament, light microscope (LM), lysosome, microjilament, micrograph, microtubule, mitochondrion, nuclear envelope, nucleoid, nucleus, nucleolus, organelle, peroxisome, plasma membrane, plasmodesma, prol~aryotic ceil, ribosome, rough endoplasmic reticulum, scanning electron microscope (SEM), surface-to-volume ratio, tight junction, transmission electron microscope (TEM), transport vesicle, tubulin, vacuole, vesicle Use the Web To best understand and remember how.cells are built and how they function, see the activities, questions, and animations at

14 The Working Cell Focus on the Concepts This chapter discusses the roles of membranes, energy.transformations, and enzymes in the functioning of cells. As you study the chapter, focus on the following concepts: Membranes are fluid mosaics of phospholipids and proteins. The phospholipids form a hydrophobic bilayer that is selectively permeable. Membrane proteins carry out many functions, such as transport and cell-cell recognition. Molecules may diffuse across a membrane in a process called passive transport. Diffusion of water through a selectively permeable membrane is called osmosis; osmosis and water balance are important to living things. Transport proteins may facilitate diffusion of a solute across a membrane or expend energy to actively transport a solute against its concentration gradient. Endocytosis and exocytosis can transport particles and bulk materials into and out of cells. Energy is the ability to do work. Kinetic energy is energy of movement. Potential energy is energy stored in position or chemical structure. Chemical reactions may release or store energy. ATP shuttles energy from energy-producing to energy-consuming processes and drives cellular work. An enzyme is a protein (or sometimes an RNA molecule) that speeds up a specific chemical reaction by lowering activation energy needed to start the reaction. The enzyme s active site fits a specific substrate. Environmental conditions affect the enzyme. Inhibitors may block it and cofactors may assist it. Many drugs, pesticides, and poisons are enzyme inhibitors. Review the Concepts work through the following exercises to review the concepts in this chapter. For additional review, check out the activities at The website offers a pre-test that will help you plan your studies.