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Chapter Menu Chapter Introduction The Basic Unit of Life 6.1 Cell Study and Technology 6.2 Two Basic Types of Cells Cell Structure 6.3 Prokaryotic Cell Structure 6.4 Eukaryotic Cell Structure Multicellular Organization 6.5 Cooperation Among Cells 6.6 Division of Labor 6.7 Systems Chapter Highlights Chapter Animations Chapter Menu Contents

By the end of this chapter you will be able to: Learning Outcomes By the end of this chapter you will be able to: A Explain the basic tenets of the cell theory. B Predict the possible effects of improved technology on the study of cells. C Distinguish between prokaryotic and eukaryotic cells. D Identify prokaryotic cell structures and explain functions of eukaryotic organelles. E Describe ways in which cells cooperate with each other. F Summarize the advantages of multicellular organization. Learning Outcomes

Cell Structures and Their Functions A micrograph of Paramecium aurelia, x100 How many different structures can you see in these cells? What functions could they have? Chapter Introduction 1

Cell Structures and Their Functions A micrograph of Paramecium aurelia, x100 Cells are so complex that we are continually learning more about how they are made and how they work. Biologists are making rapid progress toward unlocking the mysteries of the cell. In this chapter, you will learn what research has revealed about the cell so far. Chapter Introduction 2

End of the Introduction

6.1 Cell Study and Technology 1 The Basic Unit of Life 6.1 Cell Study and Technology Whatever their size, all living things are composed of cells—the basic unit of life. Many biologists contributed data and ideas that led to the cell theory, which can be stated in two parts: 1. Cells, or products made by cells, are the unit of structure and function in organisms. 2. All cells come from preexisting cells. 6.1 Cell Study and Technology 1

6.1 Cell Study and Technology 2 Examples of the variety of cells that make up all organisms (color added). Unicellular (single-celled) bacteria, x20,000 Unicellular algae, x150 Photosynthetic cells in a leaf, x200 Cells from the liver of a salamander, x400 6.1 Cell Study and Technology 2

6.1 Cell Study and Technology 3 The Basic Unit of Life 6.1 Cell Study and Technology (cont.) Once the cell theory was established, scientists began to study cell structure and function in detail. Some cell structures are too small to see without the electron microscope, which was developed in the 1930s. Electron microscopes reveal very tiny cell parts and even some large molecules down to 0.5 nm—a magnification of more than a million. 6.1 Cell Study and Technology 3

6.1 Cell Study and Technology 4 Transmission electron microscope (a) and scanning electron microscope (b), with typical images of similar white blood cells produced by each. Note the differences in the images produced of the same subject (color added). b 6.1 Cell Study and Technology 4

6.1 Cell Study and Technology 5 The Basic Unit of Life 6.1 Cell Study and Technology (cont.) The major drawback of the electron microscope is that the steps needed to prepare samples for examination kill any living cells before they can be observed. Scanning tunneling microscopes can be more powerful than electron microscopes and do not require such harsh treatment of samples. Both the electron and scanning tunneling microscopes can reveal only surface features. 6.1 Cell Study and Technology 5

6.1 Cell Study and Technology 6 Cells differ in size but average 10 to 20 µm in diameter. Note that most cells are too small to be seen with the unaided eye. 6.1 Cell Study and Technology 6

6.2 Two Basic Types of Cells 1 The Basic Unit of Life 6.2 Two Basic Types of Cells Many prokaryotes are visible in this scanning electron micrograph of the point of a pin. x290 Living cells can be separated into two groups, prokaryotes and eukaryotes, that differ in structure. Prokaryotes—the bacteria— are the simplest living cells and are found almost anywhere on Earth. Prokaryotic organisms are nearly always unicellular. Prokaryotes range in size from about 0.3 µm to 5 µm in diameter. 6.2 Two Basic Types of Cells 1

6.2 Two Basic Types of Cells 2 The Basic Unit of Life 6.2 Two Basic Types of Cells (cont.) The cells of eukaryotes are larger (10–50 µm) and more complex than prokaryotes. Eukaryotic cells can form multicellular organisms such as plants, animals, and fungi. Eukaryotic cells have many parts, each with a specific function, that gives them the flexibility to develop into hundreds of specialized cell types. The membrane-enclosed nucleus is the most obvious difference between prokaryotes and eukaryotes. 6.2 Two Basic Types of Cells 2

6.2 Two Basic Types of Cells 3 Note the greater structural complexity of the eukaryotic cell (an amoeba) and its many membrane enclosed parts, or organelles. Eukaryotic cell Prokaryotic cell 6.2 Two Basic Types of Cells 3

End of Section 1

6.3 Prokaryotic Cell Structure 1 Nearly all prokaryotic cells have: a rigid cell wall made of lipids, carbohydrates other than cellulose, and protein. a plasma membrane that encloses the cell. one chromosome that is attached to the plasma membrane in an area of the cell known as the nuclear region, or nucleoid. 6.3 Prokaryotic Cell Structure 1

6.3 Prokaryotic Cell Structure 2 6.3 Prokaryotic Cell Structure (cont.) Most prokaryotes—bacteria—are unicellular but can associate in clusters, chains, and films. In addition, bacteria usually contain one or more smaller circular DNA molecules called plasmids. Some have flagella (singular: flagellum), long, whiplike extensions made of protein that rotate like propellers, enabling cells to swim through water or the body fluids of larger organisms. 6.3 Prokaryotic Cell Structure 2

6.3 Prokaryotic Cell Structure 3 The structure of a prokaryotic cell 6.3 Prokaryotic Cell Structure 3

6.3 Prokaryotic Cell Structure 4 6.3 Prokaryotic Cell Structure (cont.) Most bacteria have one of three shapes—rod, sphere, or corkscrew. cocci (spheres), x45,000 bacilli (rods), x31,000 (note the flagella) spirochetes (corkscrews), x700 6.3 Prokaryotic Cell Structure 4

6.3 Prokaryotic Cell Structure 5 6.3 Prokaryotic Cell Structure (cont.) Many of the prokaryotic metabolic processes, such as glycolysis, are similar to those of eukaryotes. However, others are unique. All ecosystems include many types of bacterial decomposers that help recycle nutrients such as carbon, nitrogen, and sulfur compounds. 6.3 Prokaryotic Cell Structure 5

6.3 Prokaryotic Cell Structure 6 6.3 Prokaryotic Cell Structure (cont.) Many prokaryotes are autotrophs and are important primary producers in lakes and oceans. Although some bacteria can cause human diseases, such as skin infections and strep throat, most are beneficial. Bacteria in your intestines help you digest food. 6.3 Prokaryotic Cell Structure 6

6.4 Eukaryotic Cell Structure 1 Eukaryotic cells are divided into small functional parts called organelles. Any part of a eukaryotic cell that has its own structure and function can be considered an organelle. Compartmentation makes eukaryotic cells more efficient by separating specific processes and enabling a division of labor within the cell. 6.4 Eukaryotic Cell Structure 1

6.4 Eukaryotic Cell Structure 2 6.4 Eukaryotic Cell Structure (cont.) Animal cell A plasma membrane encloses the contents of both eukaryotic cells and prokaryotic cells. A cell wall surrounds the plasma membrane of plant and fungal cells, as well as some unicellular eukaryotes. Plasma membrane Plant cell Animal cells lack a rigid cell wall. Cell wall 6.4 Eukaryotic Cell Structure 2

6.4 Eukaryotic Cell Structure 3 6.4 Eukaryotic Cell Structure (cont.) Animal cell The nucleus contains the chromosomes and is a cell’s genetic control center. Nucleus A double layer of membranes forms the nuclear envelope, or nuclear membrane, that surrounds the chromosomes. Plant cell 6.4 Eukaryotic Cell Structure 3

6.4 Eukaryotic Cell Structure 4 6.4 Eukaryotic Cell Structure (cont.) Animal cell One or more drops of concentrated RNA are usually visible in the nucleus, in bodies called nucleoli (singular: nucleolus). The nucleoli are the sites where types of RNA are synthesized. Nucleus Plant cell Nucleolus 6.4 Eukaryotic Cell Structure 4

6.4 Eukaryotic Cell Structure 5 6.4 Eukaryotic Cell Structure (cont.) Animal cell Within the plasma membrane, but outside the nucleus, is the cellular material, or cytoplasm. The cytosol is the protein-rich, semifluid material in the cell that surrounds and bathes the organelles. Cytosol Plant cell The cytoplasm includes the cytosol and the organelles. 6.4 Eukaryotic Cell Structure 5

6.4 Eukaryotic Cell Structure 6 6.4 Eukaryotic Cell Structure (cont.) Animal cell A network of several types of very fine protein fibers, known as the cystoskeleton, helps to shape the cell and organize the cytoplasm. Cytoskeleton The cytoskeleton includes hollow microtubules and connecting intermediate filaments. Plant cell 6.4 Eukaryotic Cell Structure 6

6.4 Eukaryotic Cell Structure 6B The cytoskeleton network of proteins in the cytoplasm of eukaryotic cells facilitates movement and helps the cell maintain its shape. 6.4 Eukaryotic Cell Structure 6B

6.4 Eukaryotic Cell Structure 7 6.4 Eukaryotic Cell Structure (cont.) Animal cell Many small bodies composed of RNA and protein, called ribosomes, are scattered throughout the cytoplasm. ER Ribosomes catalyze the synthesis of a cell’s proteins. In eukaryotes, some ribosomes are attached to a system of membranes called the endoplasmic reticulum (ER). Plant cell 6.4 Eukaryotic Cell Structure 7

6.4 Eukaryotic Cell Structure 8 6.4 Eukaryotic Cell Structure (cont.) Animal cell The ER membranes form tubes and channels throughout the cytoplasm connecting many of the organelles in the cell. ER Proteins are synthesized at the ribosomes attached to the ER. Proteins and other substances are transported through the ER to their final destinations in the cell. Plant cell 6.4 Eukaryotic Cell Structure 8

6.4 Eukaryotic Cell Structure 9 6.4 Eukaryotic Cell Structure (cont.) Animal cell Many substances that are exported from the cell pass through the ER to the Golgi apparatus. Material passing through the Golgi apparatus is packaged in vesicles that appear to pinch off the Golgi membranes. Golgi apparatus Plant cell 6.4 Eukaryotic Cell Structure 9

6.4 Eukaryotic Cell Structure 10 6.4 Eukaryotic Cell Structure (cont.) Animal cell Together the ER, Golgi apparatus, and vesicles form a connected internal membrane system. The structure of the system enables it to direct proteins to target points inside the cell and to the plasma membrane for passage out of the cell. Golgi apparatus Plant cell 6.4 Eukaryotic Cell Structure 10

6.4 Eukaryotic Cell Structure 11 Internal membrane system of a eukaryotic cell 6.4 Eukaryotic Cell Structure 11

6.4 Eukaryotic Cell Structure 12 6.4 Eukaryotic Cell Structure (cont.) Animal cell Lysosomes are special vesicles in animal cells and some other eukaryotes that contain enzymes that break down the cell’s old macromolecules for recycling. Lysosome Lysosomes can also fuse with vesicles formed by endocytosis, digesting the food particles within. Some animal cells have lysosomes that fuse with the plasma membrane, releasing digestive enzymes outside the cell. Plant cell 6.4 Eukaryotic Cell Structure 12

6.4 Eukaryotic Cell Structure 13 6.4 Eukaryotic Cell Structure (cont.) Animal cell The vacuoles present in most plant cells are vesicles that enlarge as the cells mature. Vacuoles contain water, organic acids, digestive enzymes, salts, and pigments. Up to 90% of the volume of a mature plant cell may consist of its vacuole. Plant cell Vacuole 6.4 Eukaryotic Cell Structure 13

6.4 Eukaryotic Cell Structure 14 6.4 Eukaryotic Cell Structure (cont.) Animal cell Chloroplasts and mitochondria are double-membrane organelles involved in energy reactions. Mitochondrion Photosynthesis occurs in chloroplasts. Mitochondria are the major sites of ATP synthesis in most eukaryotic cells. Plant cell Chloroplast 6.4 Eukaryotic Cell Structure 14

6.4 Eukaryotic Cell Structure 15 6.4 Eukaryotic Cell Structure (cont.) Animal cell Centrioles are tubular structures in the cells of animals and some fungi and algae that participate in cell reproduction. Centrioles Centrioles consist of a pair of cylindrical bundles of microtubules. Plant cell 6.4 Eukaryotic Cell Structure 15

6.4 Eukaryotic Cell Structure 16 6.4 Eukaryotic Cell Structure (cont.) Some eukaryotic cells have flagella that are covered by the plasma membrane and consist of long bundles of microtubules. Enzymes associated with these microtubules provide energy for the motion of the flagellum by breaking down ATP. 6.4 Eukaryotic Cell Structure 16

6.4 Eukaryotic Cell Structure 17 Parallel bundles of microtubules make up the internal structure of the flagellum. 6.4 Eukaryotic Cell Structure 17

6.4 Eukaryotic Cell Structure 18 6.4 Eukaryotic Cell Structure (cont.) Cilia are short flagella. Eukaryotic flagella and cilia move cells along by whipping in an oarlike motion against the fluid surrounding a cell. Cilia can also help move material along a cell or tissue. Scanning electron micrograph of rows of cilia on a hamster’s inner ear cells. x6,700 6.4 Eukaryotic Cell Structure 18

6.4 Eukaryotic Cell Structure 19 6.4 Eukaryotic Cell Structure (cont.) 6.4 Eukaryotic Cell Structure 19

End of Section 2

6.5 Cooperation Among Cells 1 Multicellular Organization 6.5 Cooperation Among Cells When one-celled organisms divide, some new cells may remain together in a cluster. In a cluster of cells, each cell has an individual life and may break away from the cluster at some point. Though prokaryotes such as these exist in clusters, chains, and films and as isolated cells, each bacterium is still an individual organism Anabaena, color added. x600 6.5 Cooperation Among Cells 1

6.5 Cooperation Among Cells 2 Multicellular Organization 6.5 Cooperation Among Cells (cont.) Some unicellular microorganisms live in groups called colonies. Members of a colony may interact in ways that give them advantages over isolated organisms, but each member is still a separate organism. In some colonies, such as that of Volvox, a colonial algae, individual cells take on specialized roles. Bacteria in dental plaque biofilm on an unbrushed tooth. x2,000 6.5 Cooperation Among Cells 2

6.5 Cooperation Among Cells 3 Individual cells of a Volvox colony, (a), look like unicellular alga Chlamydomonas, (b). Volvox colonies, (c), are seen here through the light microscope. x125 a b c 6.5 Cooperation Among Cells 3

6.5 Cooperation Among Cells 4 Multicellular Organization 6.5 Cooperation Among Cells (cont.) Some types of Volvox have delicate strands of cytoplasm connecting the cells and can coordinate their motions. Volvox has some characteristics of a colony, but it also has some specialized reproductive cells, and the two ends of the colony are different. Volvox could be considered just barely a multicellular organism. 6.5 Cooperation Among Cells 4

Multicellular Organization 6.6 Division of Labor Organisms must have enough surface area for the living cells within to exchange food, wastes, and other substances with their environment. Large plants and animals use structures, such as blood vessels, lungs, and leaves, that add internal surface area. Structures that enable large organisms to survive require a number of specialized cell types. 6.6 Division of Labor 1

Multicellular Organization 6.6 Division of Labor (cont.) All cells must carry on the basic activities of life, but each type of cell often takes on a special job as well. A gland cell is specialized for making certain types of chemicals. A nerve cell is efficient in conducting electric signals. A muscle cell is specialized for movement. The cells that form an organism’s outer covering, or epidermis, may be specialized. 6.6 Division of Labor 2

Multicellular Organization 6.6 Division of Labor (cont.) Hydra is a small, threadlike freshwater animal with a ring of tentacles at one end. The animal’s cells look slightly different and are different in specialization. x15 6.6 Division of Labor 3

Multicellular Organization 6.6 Division of Labor (cont.) Cells of larger organisms are much more distinctive in appearance. Muscle cells, color added (a), and red blood cells, color added (b), are specialized cells in humans. a x124 b x288 6.6 Division of Labor 4

Multicellular Organization 6.6 Division of Labor (cont.) In multicellular organisms, a group of cells with the same specialization usually work together. Each specialized mass or layer of cells is called a tissue. Different tissues may be organized into organs. Organs may be incorporated into systems of organs. 6.6 Division of Labor 5

(b) The heart is composed mostly of cardiac muscle tissue. (a) The heart, blood, and blood vessels are the organs that make up an animal’s circulatory system. (b) The heart is composed mostly of cardiac muscle tissue. (c) Specialized cells form cardiac muscle tissue. 6.6 Division of Labor 6

Multicellular Organization 6.7 Systems In most multicellular organisms, the inner cells cannot obtain nutrients directly from the outside environment or pass their wastes directly to the outside environment. Specialized systems are required to handle deliveries between the environment and the cells. 6.7 Systems 1

Multicellular Organization 6.7 Systems (cont.) Most specialized systems are necessary for three reasons: 1. a division of labor occurs among cells, 2. many individual cells cannot work together without regulation and coordination, and 3. most cells are not in direct contact with the outside environment. 6.7 Systems 2

Multicellular Organization 6.7 Systems (cont.) In many organisms, additional specializations have developed within the organ systems. Specialized systems are required to handle deliveries between the environment and the cells. Cells are the lowest level of organization that truly can be considered living. 6.7 Systems 3

Levels of structure in the biosphere 6.7 Systems 4

End of Section 3

Summary Prokaryotic cells are smaller and less specialized than eukaryotic cells. The most distinguishing characteristic of eukaryotic cells is the presence of organelles, which include the nucleus, mitochondria, chloroplasts, ribosomes, vacuoles, endoplasmic reticulum, and other compartments with specific functions in the eukaryotic cell. Cells may exist alone as unicellular organisms. Cells may be clustered and form multicellular organisms. Tissues, organs, and systems become more complex in larger multicellular organisms. Chapter Highlights 1

Reviewing Key Terms ___ cilia ___ vacuoles e d ___ plasmid b Match the term on the left with the correct description. ___ cilia ___ vacuoles ___ plasmid ___ nucleoid ___ lysosome ___ tissue e d b c f a a. specialized mass of cells b. extrachromosomal elements that contain a few genes that help bacteria survive under specific conditions. c. sites of synthesis and assembly of rRNA and tRNA d. large vesicle that grows as a plant cell matures e. short flagella f. vesicle that may fuse with the plasma membrane and release digestive enzymes outside of the cell Chapter Highlights 2

Reviewing Ideas 1. What is the major drawback of the electron microscope? The major drawback of the electron microscope is that the steps needed to prepare samples for examination kill any living cells before they can be observed. Chapter Highlights 3

Reviewing Ideas 2. What is the lowest level of organization that truly can be considered living? Where does this fall on the continuum of biological organization? Cells, the basic unit of life, are about midway on the continuum of biological organization. They are also the lowest level of organization that truly can be considered living. Chapter Highlights 3

Using Concepts 3. Why are prokaryotes a vital part of every ecosystem? Some prokaryotes are bacterial decomposers that help recycle nutrients such as carbon, nitrogen, and sulfur compounds that otherwise would remain unavailable in wastes and dead organisms. Chapter Highlights 4

Using Concepts 4. How does a Hydra survive without a circulation system? In a Hydra, all cells are in close proximity to the outside environment and can directly receive nutrients and expel waste products. Chapter Highlights 4

Synthesize 5. What are the basic requirements to be considered a multicellular organism? Aside from having more than one cell, the cells of a multicellular organism should exhibit signs of specialization and coordination and division of labor in their activities. Chapter Highlights 5

End of Chapter Presentation

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Chapter Animations Menu The structure of a prokaryotic cell Internal membrane system of a eukaryotic cell Levels of structure in the biosphere Chapter Animations Menu

The structure of a prokaryotic cell Animation 1

Internal membrane system of a eukaryotic cell Animation 2

Levels of structure in the biosphere Animation 3

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