Chapter 2. Cells, Tissues, Organs, And Organ Systems Of Animals

2
Cells, Tissues,
Organs, and Organ
Systems of Animals

Chapter Outline

1. 2.1 What Are Cells?
2. 2.2 Why Are Most Cells Small?
3. 2.3 Cell Membranes
   • Structure of Cell Membranes
   • Functions of Cell Membranes
4. 2.4 Movement across Membranes
   • Simple Diffusion
   • Facilitated Diffusion
   • Osmosis
   • Filtration
   • Active Transport
   • Bulk Transport
5. 2.5 Cytoplasm, Organelles, and Cellular Components
   • Cytoplasm
   • Ribosomes: Protein Workbenches
   • Endoplasmic Reticulum: Production and Transport
   • Golgi Apparatus: Packaging, Sorting, and Export
   • Lysosomes: Digestion and Degradation
   • Microbodies: A Diverse Category of Organelles
   • Mitochondria: Power Generators
   • Cytoskeleton: Microtubules, Intermediate Filaments, and Microfilaments
   • Cilia and Flagella: Movement
   • Centrioles and Microtubule-Organizing Centers
   • Vacuoles: Cell Maintenance
   • Vaults: A Newly Discovered Organelle
6. 2.6 The Nucleus: Information Center
   • Nuclear Envelope: Gateway to the Nucleus
   • Chromosomes: Genetic Containers
   • Nucleolus: Preassembly Point for Ribosomes
7. 2.7 Levels of Organization in Various Animals
8. 2.8 Tissues
   • Epithelial Tissue: Many Forms and Functions
   • Connective Tissue: Connection and Support
   • Nervous Tissue: Communication
   • Muscle Tissue: Movement
9. 2.9 Organs
10. 2.10 Organ Systems

Because all organisms are made of cells, the cell is as fundamental to an understanding of zoology as the atom is to an understanding of chemistry. In the hierarchy of biological organization, the cell is the simplest organization of matter that exhibits all of the properties of life (figure 2.1). Some organisms are single celled; others are multicellular. An animal has a body composed of many kinds of specialized cells. A division of labor among cells allows specialization into higher levels of organization (tissues, organs, and organ systems). Yet, everything that an animal does is ultimately happening at the cellular level.

FIGURE 2.1

Structural Hierarchy in a Multicellular Animal.

At each level, function depends on the structural organization of that level and those below it.

2.1 WHAT ARE CELLS?

LEARNING OUTCOMES

1. Differentiate between a prokaryotic and eukaryotic cell.
2. Describe the three parts of a eukaryotic cell.

Cells are the functional units of life, in which all of the chemical reactions necessary for the maintenance and reproduction of life take place. They are the smallest independent units of life. There are two basic types of cells: prokaryotes and eukaryotes. The prokaryotes lack nuclei and other membrane-bound organelles. These simpler (prokaryotic or prokaryotes; “before nucleus”) cells are classified into two domains: Archaea and Eubacteria. The Archaea have unique characteristics but also share features with Eubacteria and the third domain, Eukarya. Eukaryotic cells are larger and more complex than prokaryotic cells. Since animals and protista are composed of eukaryotic cells, this cell type will be emphasized in this chapter. Table 2.1 compares prokaryotic and eukaryotic cells.

TABLE 2.1
COMPARISON OF PROKARYOTIC ANDEUKARYOTIC CELLS

COMPONENT	PROKARYOTE	EUKARYOTE
Organization of genetic material
True membrane-bound nucleus	Absent	Present
DNA complexed with histones	No	Yes
Number of chromosomes	One	More than one
Nucleolus	Absent	Present
Mitosis occurs	No	Yes
Genetic recombination	Partial, unidirectional transfer of DNA	Meiosis and fusion of gametes
Mitochondria	Absent	Present
Chloroplasts	Absent	Present
Plasma membrane with sterols	Usually no	Yes
Flagella	Submicroscopic in size; composed of only one fiber	Microscopic in size; membrane bound; usually 20 microtubules in 9 + 2 pattern
Endoplasmic reticulum	Absent	Present
Golgi apparatus	Absent	Present
Cell walls	Usually chemically complex	Chemically simpler
Simpler organelles
Ribosomes	70S	80S (except in mitochondria and chloroplasts)
Lysosomes and peroxisomes	Absent	Present
Microtubules	Absent or rare	Present
Cytoskeleton	May be absent	Present
Vacuoles	Present	Present
Vesicles	Present	Present
Differentiation	Rudimentary	Tissues and organs

All eukaryotes (“true nucleus”) have cells with a membrane-bound nucleus containing DNA. In addition, eukaryotic cells contain many other structures called organelles (“little organs”) that perform specific functions. Eukaryotic cells also have a network of specialized structures called microfilaments and microtubules organized into the cytoskeleton, which gives shape to the cell and allows intracellular movement.

All eukaryotic cells have three basic parts (table 2.1):

1. The plasma membrane is the outer boundary of the cell. It separates the internal metabolic events from the environment and allows them to proceed in organized, controlled ways. The plasma membrane also has specific receptors for external molecules that alter the cell's function.
2. Cytoplasm (Gr. kytos, hollow vessel + plasm, fluid) is the portion of the cell outside the nucleus. The semifluid portion of the cytoplasm is called the cytosol. Suspended within the cytosol are the organelles.
3. The nucleus (pl., nuclei) is the cell control center. It contains the chromosomes and is separated from the cytoplasm by its own nuclear envelope. The nucleoplasm is the semifluid material in the nucleus.

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Because cells vary so much in form and function, no “typical” cell exists. However, to help you learn as much as possible about cells, figure 2.2 shows an idealized version of a eukaryotic cell and most of its component parts.

FIGURE 2.2

A Generalized Animal Cell.

Understanding of the structures in this cell is based mainly on electron microscopy. The sizes of some organelles and structures are exaggerated to show detail.

SECTION REVIEW 2.1

Prokaryotes are small cells that lack complex internal organization. The two domains are Archaea and Eubacteria. Eukaryotic cells exhibit compartmentalization and various organelles that carry out specific functions. The three parts of a Eukaryotic cell are the plasma membrane, cytoplasm, and nucleus.

What are some similarities between the Eukaryotes and Eubacteria?

2.2 WHY ARE MOST CELLS SMALL?

LEARNING OUTCOMES

1. Explain why most cells are small.
2. Determine how surface area changes as a function of volume.

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Most cells are small and can be seen only with the aid of a ­microscope. (Exceptions include the eggs of most vertebrates [fishes, amphibians, reptiles, and birds] and some long nerve cells.) One reason for the small size of cells is that the ratio of the volume of the cell's nucleus to the volume of its cytoplasm must not be so small that the nucleus, the cell's major control center, cannot control the cytoplasm.

Another aspect of cell volume works to limit cell size. As the radius of a cell lengthens, cell volume increases more rapidly than cell surface area (figure 2.3). The need for nutrients and the rate of waste production are proportional to cell volume. The cell takes up nutrients and eliminates wastes through its surface plasma membrane. If cell volume becomes too large, the surface-area-to-volume ratio is too small for an adequate exchange of ­nutrients and wastes.

FIGURE 2.3

The Relationship between Surface Area and Volume.

As the radius of a sphere increases, its volume increases more rapidly than its surface area. (SA/V = surface-area-to-volume ratio.)

SECTION REVIEW 2.2

A cell needs a surface area large enough to allow efficient movement of nutrients into the cell and waste material out of the cell. Small cells have a lot more surface area per volume than large cells. For example, a 4-cm cube has a surface-area-to-volume ratio of only 5.5:1, but a 1-cm cube has a ratio of 6:1.

If the cell radius of a cell increases 10 times, the surface area will increase by 100 times. How much will the volume increase?

2.3 CELL MEMBRANES

LEARNING OUTCOME

1. Relate the structure of the plasma membrane to the function of the membrane.

The plasma membrane surrounds the cell. Other membranes ­inside the cell enclose some organelles and have properties similar to those of the plasma membrane.

Structure of Cell Membranes

In 1972, S. Jonathan Singer and Garth Nicolson developed the fluid-mosaic model of membrane structure. According to this model, a membrane is a double layer (bilayer) of proteins and phospholipids, and is fluid rather than solid. The phospholipid bilayer forms a fluid “sea” in which specific proteins float like icebergs (figure 2.4). Being fluid, the membrane is in a constant state of flux—shifting and changing, while retaining its uniform structure. The word mosaic refers to the many different kinds of proteins dispersed in the phospholipid bilayer.

FIGURE 2.4

Fluid-Mosaic Model of Membrane Structure.

Intrinsic globular proteins may protrude above or below the lipid bilayer and may move about in the membrane. Peripheral proteins attach to either the inner or outer surfaces.

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The following are important points of the fluid-mosaic model:

1. The phospholipids have one polar end and one nonpolar end. The polar ends are oriented on one side toward the outside of the cell and into the fluid cytoplasm on the other side, and the nonpolar ends face each other in the middle of the bilayer. The “tails” of both layers of phospholipid molecules attract each other and are repelled by water (they are hydrophobic, “water dreading”). As a result, the polar spherical “heads” (the phosphate portion) are located over the cell surfaces (outer and inner) and are “water ­attracting” (they are hydrophilic).
2. Cholesterol is present in the plasma membrane and ­organelle membranes of eukaryotic cells. The cholesterol molecules are embedded in the interior of the membrane and help to make the membrane less permeable to water-soluble substances. In addition, the relatively rigid structure of the cholesterol molecules helps stabilize the membrane (figure 2.5).
   

   FIGURE 2.5

   The Arrangement of Cholesterol between Lipid Molecules of a Lipid Bilayer.
   

   Cholesterol stiffens the outer lipid bilayer and causes the inner region of the bilayer to become slightly more fluid. Only half the lipid bilayer is shown; the other half is a mirror image.
3. The membrane proteins are individual molecules attached to the inner or outer membrane surface (peripheral proteins) or embedded in it (intrinsic proteins) (seefigure 2.4). Some intrinsic proteins are links to sugar-protein markers on the cell surface. Other intrinsic proteins help to move ions or molecules across the membrane, and still others attach the membrane to the cell's inner scaffolding (the cytoskeleton) or to various molecules outside the cell.
4. When carbohydrates unite with proteins, they form ­glycoproteins, and when they unite with lipids, they form glycolipids on the surface of a plasma membrane. ­Surface carbohydrates and portions of the proteins and lipids make up the glycocalyx (“cell coat”) (figure 2.6). This arrangement of distinctively shaped groups of sugar molecules of the glycocalyx acts as a molecular “fingerprint” for each cell type. The glycocalyx is necessary for cell-to-cell recognition and the ­behavior of certain cells, and it is a key component in coordinating cell ­behavior in animals.

FIGURE 2.6

The Glycocalyx, Showing the Glycoproteins and Glycolipids.

Note that all of the attached carbohydrates are on the outside of the plasma membrane.

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How Do Zoologists Investigate the Inner Workings of the Tiny Structures within a Cell?

T

he small size of cells is the greatest obstacle to discovering their nature and the anatomy of the tiny structures within cells. The evolution of science often parallels the invention of instruments that extend human senses to new limits. Cells were discovered after microscopes were invented, and high-magnification microscopes are needed to see the smallest structures within a cell. Most commonly used are the light microscope, the transmission electron microscope (TEM), the scanning electron microscope, the fluorescence ­microscope, the scanning­ ­tunneling microscope, and the atomic force microscope.

Microscopes are the most important tools of cytology, the study of cell structure. But simply describing the diverse structures within a cell reveals little about their function. Today's modern cell biology developed from an integration of cytology with biochemistry, the study of ­molecules and the chemical processes of metabolism. Throughout this book, many photographs are presented using various microscopes to show different types of cells and the various tiny structures within. From these photographs it will become apparent that similarities among cells reveal the evolutionary unity of life.

Functions of Cell Membranes

Cell membranes (1) regulate material moving into and out of the cell, and from one part of the cell to another; (2) separate the inside of the cell from the outside; (3) separate various organelles within the cell; (4) provide a large surface area on which specific chemical reactions can occur; (5) separate cells from one another; and (6) are a site for receptors containing specific cell identification markers that differentiate one cell type from another.

The ability of the plasma membrane to let some substances in and keep others out is called selective permeability (L. permeare or per, through + meare, pass) and is essential for maintaining a “steady state” within the cell. However, before you can fully understand how substances pass into and out of cells and organelles, you must know how the molecules of those substances move from one place to another.

SECTION REVIEW 2.3

The major components of the plasma membrane are as follows: a phospholipid bilayer, cholesterol, membrane proteins, and the glycocalyx. This structure creates the outer boundary of the cell, it separates the internal metabolic events from the environment, and it allows the events to proceed in an organized, controlled way. The plasma membrane also has specific structures for movement of materials into and out of the cell and receptors for external molecules that alter the cell's function.

If the plasma membrane of a cell were just a single layer of phospholipids, how would this affect its function?

2.4 MOVEMENT ACROSS MEMBRANES

LEARNING OUTCOMES

1. Differentiate the different processes by which material can move into and out of the cell through the plasma membrane.
2. Explain the movement of water by osmosis.

Molecules can cross membranes in a number of ways, both by ­using their own energy and by relying on an outside energy source. Table 2.2 summarizes the various kinds of transmembrane movement, and the sections that follow discuss them in more detail.

TABLE 2.2
DIFFERENT TYPES OF MOVEMENT ACROSSPLASMA MEMBRANES

TYPE OF MOVEMENT	DESCRIPTION	EXAMPLE IN THE BODY OF A FROG
Simple diffusion	No cell energy is needed. Molecules move “down” a concentration gradient. Molecules spread out randomly from areas of higher concentration to areas of lower concentration until they are distributed evenly in a state of dynamic equilibrium.	A frog inhales air containing oxygen, which moves into the lungs and then diffuses into the bloodstream.
Facilitated diffusion	Carrier (transport) proteins in a plasma membrane temporarily bind with molecules and help them pass across the membrane. Other proteins form channels through which molecules move across the membrane.	Glucose in the gut of a frog combines with carrier proteins to pass through the gut cells into the bloodstream.
Osmosis	Water molecules diffuse across selectively permeable membranes from areas of higher concentration to areas of lower concentration.	Water molecules move into a frog's red blood cell when the concentration of water molecules outside the blood cell is greater than it is inside.
Filtration	Essentially protein-free plasma moves across capillary walls due to a pressure gradient across the wall.	A frog's blood pressure forces water and dissolved wastes into the kidney tubules during urine formation.
Active transport	Specific carrier proteins in the plasma membrane bind with molecules or ions to help them cross the membrane against a concentration gradient. Cellular energy is required.	Sodium ions move from inside the neurons of the sciatic nerve of a frog (the sodium-potassium pump) to the outside of the neurons.
Endocytosis	The bulk movement of material into a cell by the formation of a vesicle.
Pinocytosis	The plasma membrane encloses small amounts of fluid droplets (in a vesicle) and takes them into the cell.	The kidney cells of a frog take in fluid to maintain fluid balance.
Phagocytosis	The plasma membrane forms a vesicle around a solid particle or other cell and draws it into the phagocytic cell.	The white blood cells of a frog engulf and digest harmful bacteria.
Receptor-mediated endocytosis	Extracellular molecules bind with specific receptor proteins on a plasma membrane, causing the membrane to invaginate and draw molecules into the cell.	The intestinal cells of a frog take up large molecules from the inside of the gut.
Exocytosis	The bulk movement of material out of a cell. A vesicle (with particles) fuses with the plasma membrane and expels particles or fluids from the cell across the plasma membrane. The reverse of endocytosis.	The sciatic nerve of a frog releases a chemical (neurotransmitter).

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Simple Diffusion

Molecules move randomly at all temperatures above absolute zero (−273° C) (due to spontaneous molecular motion) from areas where they are highly concentrated to areas of lower concentration, until they are evenly distributed in a state of dynamic equilibrium. This process is simple diffusion (L. diffundere, to spread). Simple diffusion accounts for most of the short-distance transport of substances moving into and out of cells. Figure 2.7 shows the diffusion of sugar particles away from a sugar cube placed in water.

FIGURE 2.7

Simple Diffusion.

When a sugar cube is placed in water (a), it slowly dissolves (b) and disappears. As this happens, the sugar molecules diffuse from a region where they are more concentrated to a region (c) where they are less concentrated. Even distribution of the sugar molecules throughout the water is diffusion equilibrium (d).

Facilitated Diffusion

Polar molecules (not soluble in lipids) may diffuse through protein channels (pores) in the lipid bilayer (figure 2.8). The protein channels offer a continuous pathway for specific molecules to move across the plasma membrane so that they never come into contact with the hydrophobic layer or the membrane's polar surface.

FIGURE 2.8

Transport Proteins.

Molecules can move into and out of cells through integrated protein channels (pores) in the plasma membrane without using energy.

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Large molecules and some of those not soluble in lipids ­require assistance in passing across the plasma membrane. These molecules use facilitated diffusion, which, like simple diffusion, requires no energy input. To pass across the membrane, a molecule temporarily binds with a carrier (transport) protein in the plasma membrane and is transported from an area of higher concentration to one of lower concentration (figure 2.9).

FIGURE 2.9

Facilitated Diffusion and Carrier (Transport) Proteins.

Some molecules move across the plasma membrane with the assistance of carrier proteins that transport the molecules down their concentration gradient, from a region of higher concentration to one of lower concentration. A carrier protein alternates between two configurations, moving a molecule across a membrane as the shape of the protein changes. The rate of facilitated diffusion depends on how many carrier proteins are available in the membrane and how fast they can move their specific molecules.

Osmosis

The diffusion of water across a selectively permeable membrane from an area of higher concentration to an area of lower concentration is osmosis (Gr. osmos, pushing). Osmosis is just a special type of diffusion, not a different method (figure 2.10).

FIGURE 2.10

Osmosis.

(a) A selectively permeable membrane separates the beaker into two compartments. Initially, compartment 1 contains sugar and water molecules, and compartment 2 contains only water molecules. Due to molecular motion, water moves down the concentration gradient (from compartment 2 to compartment 1) by osmosis. The sugar molecules remain in compartment 1 because they are too large to pass across the membrane. (b) At osmotic equilibrium, the number of sugar molecules in compartment 1 does not increase, but the number of water molecules does.

Recent studies show that water, despite its polarity, can cross cell membranes, but this flow is limited. Water flow in living cells is facilitated by specialized water channels called aquaporins. Aquaporins fall into two general classes: those that are specific only for water, and others that allow small hydrophilic molecules (e.g., urea, glycerol) to cross the membrane.

The term tonicity (Gr. tonus, tension) refers to the relative concentration of solutes in the water inside and outside the cell. For example, in an isotonic (Gr. isos, equal +tonus, tension) solution, the solute concentration is the same inside and outside a red blood cell (figure 2.11a). The concentration of water molecules is also the same inside and outside the cell. Thus, water molecules move across the plasma membrane at the same rate in both directions, and there is no net movement of water in either direction.

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In a hypertonic (Gr. hyper, above) solution, the solute concentration is higher outside the red blood cell than inside. Because the concentration of water molecules inside the cell is higher than outside, water moves out of the cell, which shrinks (figure 2.11b). This condition is called crenation in red blood cells.

FIGURE 2.11

Effect of Salt Concentration on Cell Volumes.

(a) An isotonic solution with the same salt concentration inside and outside the cell has no effect on the size of the red blood cell. (b) A hypertonic (high-salt) solution causes water to leave the red blood cell, which shrinks. (c) A hypotonic (low-salt) solution results in an inflow of water, causing the red blood cell to swell. Arrows indicate direction of water movement.

In a hypotonic (Gr. hypo, under) solution, the solute concentration is lower outside the red blood cell than inside. Conversely, the concentration of water molecules is higher outside the cell than inside. As a result, water moves into the cell, which swells and may burst (figure 2.11c).

Filtration

Filtration is a process that forces small molecules across selectively permeable membranes with the aid of hydrostatic (water) pressure (or some other externally applied force, such as blood pressure). For example, in the body of an animal such as a frog, filtration is evident when blood pressure forces water and dissolved molecules through the permeable walls of small blood vessels called capillaries (figure 2.12). In filtration, large molecules, such as proteins, do not pass through the smaller membrane pores. Filtration also takes place in the kidneys when blood pressure forces water and dissolved wastes out of the blood vessels and into the kidney tubules in the first step in urine formation.

FIGURE 2.12

Filtration.

The high blood pressure in the capillary forces small molecules through the capillary membrane. Larger molecules cannot pass through the small openings in the capillary membrane and remain in the capillary. Arrows indicate the direction of small molecule movement.

Active Transport

Active-transport processes move molecules across a selectively permeable membrane against a concentration gradient—that is, from an area of lower concentration to one of higher concentration. This movement against the concentration gradient requires ATP energy.

The active-transport process is similar to facilitated diffusion, except that the carrier protein in the plasma membrane must use energy to move the molecules against their concentration gradient (figure 2.13). These carrier proteins are called uniporters if they transport a single type of molecule or ion, symporters if they transport two molecules or ions in the same direction, and antiporters if they transport two molecules or ions in the opposite direction.

FIGURE 2.13

Active Transport.

During active transport, a molecule combines with a carrier protein whose shape is altered as a result of the combination. This change in configuration, along with ATP energy, helps move the molecule across the plasma membrane against a concentration gradient.

One active-transport mechanism, the sodium-potassium pump, helps maintain the high concentrations of potassium ions and low concentrations of sodium ions inside nerve cells that are necessary for the transmission of electrical impulses. Another active-transport mechanism, the calcium pump, keeps the calcium concentration hundreds of times lower ­inside the cell than outside.

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Bulk Transport

Large molecules cannot be transported through the plasma membrane by the processes described in the previous sections. Endocytosis and exocytosis together provide bulk transport into and out of the cell, respectively. (The term “bulk” is used because many molecules are moved at the same time.)

In endocytosis (Gr. endon, within), the plasma membrane envelops large particles and molecules (figure 2.14) and moves them in bulk across the membrane. The three forms of endocytosis are pinocytosis, phagocytosis, and receptor-mediated endocytosis.

FIGURE 2.14

Endocytosis and Exocytosis.

Endocytosis and exocytosis are responsible for the bulk transport of molecules into and out of a cell.

Pinocytosis (Gr. pinein, to drink + cyto, cell) is the nonspecific uptake of small droplets of extracellular fluid. Phagocytosis (Gr. phagein, to eat + cyto, cell) is similar to pinocytosis except takes in solid material rather than liquid. Receptor-mediated endocytosis involves a specific receptor protein on the plasma membrane that “recognizes” an extracellular molecule and binds with it. The reaction stimulates the membrane to indent and create a vesicle containing the selected molecule.

In the process of exocytosis (Gr. exo, outside), the secretory vesicles fuse with the plasma membrane and release their contents into the extracellular environment (figure 2.14). This process adds new membrane material, which replaces the plasma membrane lost during exocytosis.

Cellular Organelles

SECTION REVIEW 2.4

The different processes by which material moves into and out of the cell through the plasma membrane include: simple diffusion, facilitated diffusion, osmosis, filtration, active transport, bulk transport, endocytosis, and exocytosis (pinocytosis, phagocytosis, and receptor-mediated endocytosis). Water passes through the plasma membrane and through aqua­porins in response to solute concentration differences inside and outside the cell. This transport process is called osmosis.

If you require that drugs be given to you by an intravenous (IV) process, what should the concentration of solutes in the IV solution be relative to your red blood cells?

2.5 CYTOPLASM, ORGANELLES, ANDCELLULAR COMPONENTS

LEARNING OUTCOMES

1. Relate the structure of the major cellular organelles to their function.
2. Explain the function of the cytoskeleton “cell skeleton.”

Many cell functions that are performed in the cytoplasmic compartment result from the activity of specific structures called organelles. Organelles effectively compartmentalize a cell's activities, ­improving efficiency and protecting cell contents from harsh chemicals. Organelles also enable cells to secrete various substances, derive energy from nutrients, degrade debris and waste materials, and reproduce. Table 2.3 summarizes the structure and function of these organelles, and the sections that follow discuss them in more detail.

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TABLE 2.3
STRUCTURE AND FUNCTION OF EUKARYOTICCELLULAR COMPONENTS

COMPONENT	STRUCTURE/DESCRIPTION	FUNCTION
Centriole	Located within microtuble-organizing center; contains nine triple microtubules	Forms basal body of cilia and flagella; functions in mitotic spindle formation
Chloroplast	Organelle that contains chlorophyll and is involved in photosynthesis	Traps, transforms, and uses light energy to convert carbon dioxide and water into glucose and oxygen
Chromosome	Made up of nucleic acid (DNA) and protein	Controls heredity and cellular activities
Cilia, flagella	Threadlike processes	Cilia and flagella move small particles past fixed cells and are a major form of locomotion in some cells
Cytomembrane system	The endoplasmic reticulum, Golgi apparatus, vacuoles, vesicles	Organelles, functioning as a system, modify, package, and distribute newly formed proteins and lipids
Cytoplasm	Semifluid enclosed within plasma membrane; consists of fluid cytosol, organelles, and other structures	Dissolves substances; houses organelles and vesicles
Cytoskeleton	Interconnecting microfilaments and microtubules; flexible cellular framework	Assists in cell movement; provides support; site for binding of specific enzymes
Cytosol	Fluid part of cytoplasm; enclosed within plasma membrane; surrounds nucleus	Houses organelles; serves as fluid medium for metabolic reactions
Endoplasmic reticulum (ER)	Extensive membrane system extending throughout the cytoplasm from the plasma membrane to the nuclear envelope	Storage and internal transport; rough ER is a site for attachment of ribosomes; smooth ER makes lipids
Golgi apparatus	Stacks of disklike membranes	Sorts, packages, and routes cell's synthesized products
Lysosome	Membrane-bound sphere	Digests materials
Microbodies	Vesicles that are formed from the incorporation of lipids and proteins and that contain oxidative and other enzymes; e.g., peroxisomes	Isolate particular chemical activities from the rest of the cell
Microfilament	Rodlike structure containing the protein actin	Gives structural support and assists in cell movement
Microtubule	Hollow, cylindrical structure	Assists in movement of cilia, flagella, and chromosomes; transport system
Microtubule-organizing center	Cloud of cytoplasmic material that contains centrioles	Dense site in the cytoplasm that gives rise to large numbers of microtubules with different functions in the cytoskeleton
Mitochondrion	Organelle with double, folded membranes	Converts energy into a form the cell can use
Nucleolus	Rounded mass within nucleus; contains RNA and protein	Preassembly point for ribosomes
Nucleus	Spherical structure surrounded by a nuclear envelope; contains nucleolus and DNA	Contains DNA that controls cell's genetic program and metabolic activities
Plasma membrane	The outer bilayered boundary of the cell; composed of protein, cholesterol, and phospholipids	Protection; regulation of material movement; cell-to-cell recognition
Ribosome	Contains RNA and protein; some are free, and some attach to ER	Site of protein synthesis
Vacuole	Membrane-surrounded, often large, sac in the cytoplasm	Storage site of food and other compounds; also pumps water out of a cell (e.g., contractile vacuole)
Vaults	Cytoplasmic ribonucleoproteins shaped like octagonal barrels	Dock at nuclear pores, pick up molecules synthesized in the nucleus, and deliver their load to various places within the cell
Vesicle	Small, membrane-surrounded sac; contains enzymes or secretory products	Site of intracellular digestion, storage, or transport

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Cytoplasm

The cytoplasm of a cell has two distinct parts: (1) The cytomembrane system consists of well-defined structures, such as the ­endoplasmic reticulum, Golgi apparatus, vacuoles, and vesicles. (2) The fluid cytosol suspends the structures of the cytomembrane system and contains various dissolved molecules.

Ribosomes: Protein Workbenches

Ribosomes are non-membrane-bound structures that are the sites for protein synthesis. They contain almost equal amounts of protein and a special kind of ribonucleic acid called ribosomal RNA (rRNA). Some ribosomes attach to the endoplasmic reticulum (see next section), and some float freely in the cytoplasm. Whether ribosomes are free or attached, they usually cluster in groups connected by a strand of another kind of ribonucleic acid called messenger RNA (mRNA). These clusters are called poly­ribosomes or polysomes (see figure 2.2).

Endoplasmic Reticulum: Production and Transport

The endoplasmic reticulum (ER) is a complex, membrane-bound labyrinth of flattened sheets, sacs, and tubules that branches and spreads throughout the cytoplasm. The ER is continuous from the nuclear envelope to the plasma membrane (see figure 2.2) and is a series of channels that helps various materials to circulate throughout the cytoplasm. It also is a storage unit for enzymes and other proteins and a point of attachment for ribosomes. ER with attached ribosomes is rough ER (figure 2.15a), and ER without attached ribosomes is smooth ER (figure 2.15b). Smooth ER is the site for lipid production, detoxification of a wide variety of organic molecules, and storage of calcium ions in muscle cells. Most cells contain both types of ER, although the relative proportion varies among cells.

FIGURE 2.15

Endoplasmic Reticulum (ER).

(a) Ribosomes coat rough ER. Notice the double membrane and the lumen (space) within it. (b) Smooth ER lacks ribosomes.

Golgi Apparatus: Packaging, Sorting, and Export

The Golgi apparatus or complex (named for Camillo Golgi, who discovered it in 1898) is a collection of membranes associated physically and functionally with the ER in the cytoplasm (figure 2.16a; see also figure 2.2). It is composed of flattened stacks of membranebound cisternae (sing., cisterna; closed spaces serving as fluid reservoirs). The Golgi apparatus sorts, packages, and secretes proteins and lipids.

FIGURE 2.16

Golgi Apparatus.

(a) The Golgi apparatus consists of a stack of cisternae. Notice the curved nature of the cisternae. (b) The Golgi apparatus stores, sorts, packages, and secretes cell products. Secretory vesicles move from the Golgi apparatus to the plasma membrane and fuse with it, releasing their contents to the outside of the cell via exocytosis.

Proteins that ribosomes synthesize are sealed off in little packets called transfer vesicles. Transfer vesicles pass from the ER to the Golgi apparatus and fuse with it (figure 2.16b). In the Golgi apparatus, the proteins are concentrated and chemically modified. One function of this chemical modification seems to be to mark and sort the proteins into different batches for different destinations. Eventually, the proteins are packaged into secretory vesicles, which are released into the cytoplasm close to the plasma membrane. When the vesicles reach the plasma membrane, they fuse with it and release their contents to the outside of the cell by exocytosis. Golgi apparatuses are most abundant in cells that secrete chemical substances (e.g., pancreatic cells secreting digestive enzymes and nerve cells secreting neurotransmitters). As noted in the next section, the Golgi apparatus also produces lysosomes.

Lysosomes: Digestion and Degradation

Lysosomes (Gr. lyso, dissolving + soma, body) are membrane­bound spherical organelles that contain enzymes called acid hydrolases, which are capable of digesting organic molecules (lipids, proteins, nucleic acids, and polysaccharides) under acidic conditions. The enzymes are synthesized in the ER, transported to the Golgi apparatus for processing, and then secreted by the Golgi apparatus in the form of lysosomes or as vesicles that fuse with lysosomes (figure 2.17). Lysosomes fuse with phagocytic vesicles, thus exposing the vesicle's contents to lysosomal enzymes.

FIGURE 2.17

Lysosome Formation and Function.

Lysosomes arise from the Golgi apparatus and fuse with vesicles that have engulfed foreign material to form digestive vesicles (phagolysosomes). These vesicles function in the normal recycling of cell constituents.

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Microbodies: A Diverse Category of Organelles

Eukaryotic cells contain a variety of enzyme-bearing, membraneenclosed vesicles calledmicrobodies. The distribution of enzymes into microbodies is one main way eukaryotic cells organize their metabolism.

One specific type of microbody is the peroxisome. Peroxisomes contain enzymes that catalyze the removal of electrons and associated hydrogen atoms from, for example, hydrogen peroxide. (If these oxidative enzymes were not isolated within microbodies, they would disrupt metabolic pathways.) Hydrogen peroxide is dangerous to cells because of its violent chemical ­reactivity. The enzyme in the peroxisome is catalase, which breaks down hydrogen peroxide to water and oxygen, which are both beneficial to cells.

Mitochondria: Power Generators

Mitochondria (sing., mitochondrion) are double-membrane-bound organelles that are spherical to elongate in shape. A small space separates the outer membrane from the inner membrane. The inner membrane folds and doubles in on itself to form incomplete partitions called cristae (sing., crista; figure 2.18). The cristae increase the surface area available for the chemical reactions that trap usable energy for the cell. The space between the cristae is the matrix. The matrix contains ribosomes, circular DNA, and other material. Because they convert energy to a usable form, mitochondria are frequently called the “power generators” of the cell. Mitochondria usually multiply when a cell needs to produce more energy.

FIGURE 2.18

Mitochondrion.

Mitochondrial membranes, cristae, and matrix. The matrix contains DNA, ribosomes, and enzymes.

Cytoskeleton: Microtubules, Intermediate Filaments, and Microfilaments

In most cells, the microtubules, intermediate filaments, and microfilaments form the flexible cellular framework called the cytoskeleton (“cell skeleton”) (figure 2.19). This latticed framework extends throughout the cytoplasm, connecting the various organelles and cellular components.

FIGURE 2.19

The Cytoskeleton.

Model of the cytoskeleton, showing the three-dimensional arrangement of the microtubules, intermediate filaments, and microfilaments.

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Microtubules are hollow, slender, cylindrical structures in animal cells. Each microtubule is made of spiraling subunits of globular proteins called tubulin subunits (figure 2.20a). Microtubules function in the movement of organelles, such as secretory vesicles, and in chromosome movement during division of the cell nucleus. They are also part of a transport system within the cell. For example, in nerve cells, they help move materials through the long nerve processes. Microtubules are an important part of the cytoskeleton in the cytoplasm, and they are involved in the overall shape changes that cells undergo during periods of specialization.

Intermediate filaments are a chemically heterogeneous group of protein fibers, the specific proteins of which can vary with cell type (figure 2.20b). These filaments help maintain cell shape and the spatial organization of organelles, as well as promote mechanical activities within the cytoplasm.

FIGURE 2.20

Three Major Classes of Protein Fibers Making Up the Cytoskeleton of Eukaryotic Cells.

(a) Microtubules consist of globular protein subunits (tubulins) linked in parallel rows. (b) Intermediate filaments in different cell types are composed of different protein subunits. (c) The protein actin is the key subunit in microfilaments.

Microfilaments are solid strings of protein (actin) molecules (figure 2.20c). Actin microfilaments are most highly developed in muscle cells as myofibrils, which help muscle cells to shorten or contract. Actin microfilaments in nonmuscle cells provide mechanical support for various cellular structures and help form contractile systems responsible for some cellular movements (e.g., amoeboid movement in some protozoa).

Cilia and Flagella: Movement

Cilia (sing., cilium; L. “eyelashes”) and flagella (sing., flagellum; L. “small whips”) are elongated appendages on the surface of some cells by which the cells, including many unicellular organisms, propel themselves. In stationary cells, cilia or flagella move ­material over the cell's surface.

It has been recently discovered that a cilium may also act as a signal-receiving “antenna” for the cell. In vertebrates, almost all cells seem to have one per cell. It is the membrane proteins on this single cilium (a primary cilium) that transmit molecular signals from the cell's external environment to its internal environment (the cytoplasm). When the molecular signal gets to the cytoplasm, it leads to changes in the cell's activities. Cilia-based signaling appears to be a necessity for brain functioning and ­embryonic development.

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Although flagella are 5 to 20 times as long as cilia and move somewhat differently, cilia and flagella have a similar structure. Both are membrane-bound cylinders that enclose a matrix. In this matrix is an axoneme or axial filament, which consists of nine pairs of microtubules arranged in a circle around two central tubules ­(figure 2.21). This is called a 9 + 2 pattern of microtubules. Each microtubule pair (a doublet) also has pairs of dynein (protein) arms projecting toward a neighboring doublet and spokes extending toward the central pair of microtubules. Cilia and flagella move as a result of the microtubule doublets sliding along one another.

FIGURE 2.21

Internal Structure of Cilia and Flagella.

In cross section, the arms extend from each microtubule doublet toward a neighboring doublet, and spokes extend toward the central paired microtubules. The dynein arms push against the adjacent microtubule doublet to bring about movement.

In the cytoplasm at the base of each cilium or flagellum lies a short, cylindrical basal body, also made up of microtubules and structurally identical to the centriole. The basal body controls the growth of microtubules in cilia or flagella. The microtubules in the basal body form a 9 + 0 pattern: nine sets of three with none in the middle.

Centrioles and Microtubule-Organizing Centers

The specialized nonmembranous regions of cytoplasm near the nucleus are themicrotubule-organizing centers. These centers of dense material give rise to a large number of microtubules with different functions in the cytoskeleton. For example, one type of center gives rise to the centrioles (see figure 2.2) that lie at right angles to each other. Each centriole is composed of nine triplet microtubules that radiate from the center like the spokes of a wheel. The centrioles are duplicated before cell division, are involved with chromosome movement, and help to organize the cytoskeleton.

Vacuoles: Cell Maintenance

Vacuoles are membranous sacs that are part of the cytomembrane system. Vacuoles occur in different shapes and sizes and have various functions. For example, some freshwater ­singlecelled organisms (e.g., protozoa) and sponges have ­contractile vacuoles that collect water and pump it to the ­outside to maintain the organism's internal environment. Other vacuoles store food.

Vaults: A Newly Discovered Organelle

In the 1990s, cell biologists discovered another organelle—the vault. Vaults are cytoplasmic ribonucleoproteins shaped like octagonal barrels (figure 2.22). Their name is derived from their multiple arches that look like vaulted cathedral ceilings. One cell may contain thousands of vaults. The function of vaults may be related to their octagonal shape. Similarly, the nuclear pores (see figure 2.23) are also octagonally shaped and the same size as vaults, leading to speculation that vaults may be cellular “trucks.” Vaults can dock at nuclear pores, pick up molecules synthesized in the nucleus, and deliver their load to various places within the cell. Because many vaults are always located near the nucleus, it is thought that they are picking up messenger RNA from the nucleus and transporting it to the ribosomes for protein synthesis.

FIGURE 2.22

Vaults.

(a) A three-dimensional drawing of the octagonal barrel-shaped organelle believed to transport messenger RNA from the nucleus to the ribosomes. (b) A vault opened to show its octagonal structure.

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SECTION REVIEW 2.5

Lysosomes function in the digestion of material within the cell; mitochondria convert energy into a form (ATP) the cell can use; ribosomes are the sites for protein synthesis; and vesicles are the site of intracellular digestion, storage, and secretion. The microtubules, intermediate filaments, and microfilaments make up the “cell skeleton” and function in connecting the various organelles and cellular components, and also transport through microtubules.

How do ribosomes of the rough endoplasmic reticulum differ from those free in the cytoplasm?

2.6 THE NUCLEUS: INFORMATIONCENTER

LEARNING OUTCOME

1. Categorize the functions of the nucleus in terms of the structure of the nucleus.

The nucleus (L. kernel or nut) contains the DNA and is the control and information center for the eukaryotic cell. It has two major functions. The nucleus directs chemical reactions in cells by transcribing genetic information from DNA into RNA, which then translates this specific information into proteins (e.g., enzymes) that determine the cell's specific activities (functions). The nucleus also stores ­genetic information and transfers it during cell division from one cell to the next, and from one generation of organisms to the next.

Nuclear Envelope: Gateway to the Nucleus

The nuclear envelope is a membrane that separates the nucleus from the cytoplasm and is continuous with the endoplasmic reticulum at a number of points. More than 3,000 nuclear pores penetrate the surface of the nuclear envelope (figure 2.23). These pores allow materials to enter and leave the nucleus, and they give the nucleus direct contact with the endoplasmic reticulum (see figure 2.2). ­Nuclear pores are not simply holes in the nuclear envelope; each is composed of an ordered array of globular and filamentous granules, probably proteins. The size of the pores prevents DNA from leaving the nucleus but permits RNA to be moved out.

FIGURE 2.23

The Nuclear Envelope.

A color-enhanced electron micrograph of a section through the nuclear envelope showing the double membrane and nuclear pores (arrows).

Chromosomes: Genetic Containers

The nucleoplasm is the inner mass of the nucleus. In a nondividing cell, it contains genetic material called chromatin. Chromatin consists of a combination of DNA and protein and is the uncoiled, tangled mass of chromosomes (“colored bodies”) containing the hereditary information in segments of DNA called genes. During cell division, each chromosome coils tightly, which makes the chromosome visible when viewed through a light microscope.

Nucleolus: Preassembly Point for Ribosomes

The nucleolus (pl., nucleoli) is a non-membrane-bound structure in the nucleoplasm that is present in nondividing cells (figure 2.24). Two or three nucleoli form in most cells, but some cells (e.g., ­amphibian eggs) have thousands. Nucleoli are preassembly points for ribosomes and usually contain ­proteins and RNA in many stages of synthesis and assembly. Assembly of ribosomes is completed after they leave the nucleus through the pores of the nuclear envelope.

FIGURE 2.24

Nucleus.

The nucleolus, chromatin, and nuclear envelope are visible in this nucleus (TEM × 16,000).

SECTION REVIEW 2.6

The nucleus is surrounded by an envelope of two phospholipid bilayers. The outer layer is continuous with the ER. Pores allow the passage of small molecules. The nucleolus is where rRNA is transcribed and ribosomes are assembled. Numerous chromosomes are present in eukaryotes.

Would you expect the pores in the nuclear envelope to have a function? If so, what is it?

2.7 LEVELS OF ORGANIZATION INVARIOUS ANIMALS

LEARNING OUTCOME

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1. Describe, from the simplest to the most complex, the five levels of organization in a higher animal cell.

Animals exhibit five major levels of organization. Each level is more complex than the one before and builds on it in a hierarchical fashion (see figure 2.1).

The first level exhibits protoplasmic organization. This level is found in unicellular organisms such as the protozoa where all life functions occur within the boundaries of a single cell. The second level exhibits cellular organization. Flagellates such as Volvoxand some sponges can be placed at this level, where there is an aggregation of cells that are functionally differentiated and exhibit a division of labor. The third level is the tissue level. Jellyfishes have aggregations of cells organized into definite patterns or layers, which form a tissue. The fourth level is the organ level. Organs are composed of one or more tissues and have more specialized functions than tissues. This level first appears in the flatworms, where specific structures such as reproductive organs, eyespots, and feeding structures are present. The fifth and highest level of organization is the system level. At this level, organs work together to form systems such as the circulatory, digestive, reproductive, and respiratory systems. This level first appears in the nemertean worms. Most animal phyla exhibit this level of organization.

SECTION REVIEW 2.7

The first level of organization in a higher animal is the protoplasmic level, followed by the cellular level, tissue level, and organ level, and the highest and most complex is the system level.

Do most organs have more than one type of tissue? Explain.

2.8 TISSUES

LEARNING OUTCOMES

1. Explain the structure and function of different epithelia.
2. Identify the different types of connective tissue.
3. Identify a unique feature of muscle cells.
4. Describe the basic function of neurons.

In an animal, individual cells differentiate during development to perform special functions as aggregates called tissues. A tissue (Fr. tissu, woven) is a group of similar cells specialized for the performance of a common function. The study of tissues is calledhistology (Gr. histos, tissue + logos, discourse). Animal tissues are classified as epithelial, connective, muscle, or nervous.

Epithelial Tissue: Many Forms and Functions

Epithelial tissue exists in many structural forms. In general, it either covers or lines something and typically consists of renewable sheets of cells that have surface specializations adapted for their specific roles. Usually, a basement membrane separates epithelial tissues from underlying, adjacent tissues. Epithelial tissues absorb (e.g., the lining of the small intestine), transport (e.g., kidney tubules), excrete (e.g., sweat and endocrine glands), protect (e.g., the skin), and contain nerve cells for sensory reception (e.g., the taste buds in the tongue). The size, shape, and arrangement of epithelial cells are directly related to these specific functions.

 

Epithelial tissues are classified on the basis of shape and the number of layers present. Epithelium can be simple, consisting of only one layer of cells, or stratified, consisting of two or more stacked layers (figure 2.25e). Individual epithelial cells can be flat (squamous epithelium; figure 2.25a), cube shaped (cuboidal epithelium; figure 2.25b), or columnlike (columnar epithelium; figure 2.25c). The cells of pseudostratified ciliated columnar epithelium possess cilia and appear stratified or layered, but they are not, hence the prefix pseudo. They look layered because their nuclei are at two or more levels within cells of the tissues (figure 2.25d) and they grow in height as old cells are replaced by new ones.

FIGURE 2.25

Tissue Types.

(a) Simple squamous epithelium consists of a single layer of tightly packed, flattened cells with a disk-shaped central nucleus (LM ×1000). Location: Air sacs of the lungs, kidney glomeruli, lining of heart, blood vessels, and lymphatic vessels. Function: Allows passage of materials by diffusion and filtration.

(b) Simple cuboidal epithelium consists of a single layer of tightly packed, cube-shaped cells. Notice the cell layer indicated by the arrow (LM ×1000).Location: Kidney tubules, ducts and small glands, and surface of ovary.Function: Secretion and absorption.

(c) Simple columnar epithelium consists of a single layer of elongated cells. The arrow points to a specialized goblet cell that secretes mucus (LM ×410). Location: Lines digestive tract, gallbladder, and excretory ducts of some glands. Function: Absorption, enzyme secretion.

(d) Pseudostratified ciliated columnar epithelium. A tuft of cilia tops each columnar cell, except for goblet cells (LM ×600). Location: Lines the bronchi, uterine tubes, and some regions of the uterus. Function: Propels mucus or reproductive cells by ciliary action.

(e) Stratified squamous epithelium consists of many layers of cells (LM ×120). Location: Lines the esophagus, mouth, and vagina. Keratinized variety lines the surface of the skin. Function: Protects underlying tissues in areas subject to abrasion.

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Connective Tissue: Connection and Support

Connective tissues support and bind. Unlike epithelial tissues, connective tissues are distributed throughout an extracellular matrix. This matrix frequently contains fibers that are embedded in a ground substance with a consistency anywhere from liquid to solid. To a large extent, the nature of this extracellular material determines the functional properties of the various connective tissues.

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Connective tissues have two general types of fiber arrangement. In loose connective tissue, strong, flexible fibers of the protein collagen are interwoven with fine, elastic, and reticular fibers, giving loose connective tissue its elastic consistency and making it an excellent binding tissue (e.g., binding the skin to underlying muscle tissue) (figure 2.25g). In fibrous connective tissue, the collagen fibers are densely packed and may lie parallel to one another, creating very strong cords, such as tendons (which connect muscles to bones or to other muscles) and ligaments (which connect bones to bones) (figure 2.25h).

(g) Loose connective tissue contains numerous fibroblasts (arrows) that produce collagenous and elastic fibers (LM ×280). Location: Widely distributed under the epithelia of the human body. Function: Wraps and cushions organs.

(h) Fibrous connective tissue consists largely of tightly packed collagenous fibers (LM ×250). The arrow points to a fibroblast. Location: Dermis of the skin, submucosa of the digestive tract, and fibrous capsules of organs and joints. Function: Provides structural strength.

Adipose tissue is a type of loose connective tissue that consists of large cells that store lipid (figure 2.25f). Most often, the cells accumulate in large numbers to form what is commonly called fat.

(f) Adipose tissue cells (adipocytes) contain large fat droplets that push the nuclei close to the plasma membrane. The arrow points to a nucleus (LM ×200). Location: Around kidneys, under skin, in bones, within abdomen, and in breasts. Function: Provides reserve fuel (lipids), insulates against heat loss, and supports and protects organs.

Cartilage is a hard yet flexible tissue that supports such structures as the outer ear and forms the entire skeleton of such animals as sharks and rays (figure 2.25i–k). Cells called chondrocytes lie within spaces called lacunae that are surrounded by a rubbery matrix that chondroblasts secrete. This matrix, along with the collagen and/or elastin fibers, gives cartilage its strength and elasticity.

(i) Hyaline cartilage cells are located in lacunae (arrow) surrounded by intercellular material containing fine collagenous fibers (LM ×160).Location: Forms embryonic skeleton; covers ends of long bones; and forms cartilage of nose, trachea, and larynx. Function: Support and reinforcement.

(j) Elastic cartilage contains fine collagenous fibers and many elastic fibers in its intercellular material (LM ×200). Location: External ear, epiglottis.Function: Maintains a structure's shape while allowing great flexibility.

(k) Fibrocartilage contains many large, collagenous fibers in its intercellular material (LM ×195). The arrow points to a fibroblast. Location:Intervertebral disks, pubic symphysis, and disks of knee joint. Function:Absorbs compression shock.

 

Bone cells (osteocytes) also lie within lacunae, but the matrix around them is heavily impregnated with calcium phosphate and calcium carbonate, making this kind of tissue hard and ideally suited for its functions of support and protection (figure 2.25l).Chapter 23 covers the structure and function of bone in more detail.

(l) Bone (osseous) tissue. Bone matrix is deposited in concentric layers around osteonic canals (LM ×160). Location: Bones. Function: Supports, protects, provides lever system for muscles to act on, stores calcium and fat, and forms blood cells.

Overview of Tissues

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Blood is a connective tissue in which a fluid called plasma suspends specialized red and white blood cells plus platelets (figure 2.25m). Blood transports various substances throughout the bodies of animals. Chapter 26 covers blood in more detail.

(m) Blood is a type of connective tissue. It consists of red blood cells, white blood cells, and platelets suspended in an intercellular fluid (plasma) (LM ×1250). Location: Within blood vessels. Function: Transports oxygen, carbon dioxide, nutrients, wastes, hormones, minerals, vitamins, and other substances.

Nervous Tissue: Communication

Nervous tissue is composed of several different types of cells: Impulse-conducting cells are called neurons (figure 2.25n); cells involved with protection, support, and nourishment are called neuroglia; and cells that form sheaths and help protect, nourish, and maintain cells of the peripheral nervous system are called peripheral glial cells. Chapter 24discusses nervous tissue in more detail.

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(n) Nervous tissue. Neurons in nervous tissue transmit electrical signals to other neurons, muscles, or glands (LM ×450). Location: Brain, spinal cord, and nerves. Function: Transmits electrical signals from sensory receptors to the spinal cord or brain, and from the spinal cord or brain to effectors (muscles and glands).

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Muscle Tissue: Movement

 

Muscle tissue allows movement. The three kinds of muscle tissue are skeletal, smooth, and cardiac (figure 2.25o–q). Skeletal muscle is attached to bones and makes body movement possible in vertebrates. The rhythmic contractions of smooth muscle create a churning action (as in the stomach), help propel material through a tubular structure (as in the intestines), and control size changes in hollow organs such as the urinary bladder and uterus. The contractions of cardiac muscle result in the heart beating. Chapter 23discusses the details of the contractile process in muscle tissue.

(o) Skeletal muscle tissue is composed of striated muscle fibers (cells) that are long and cylindrical and contain many peripheral nuclei (LM ×250).Location: In skeletal muscles attached to bones. Function: Voluntary movement, locomotion.

(p) Smooth muscle tissue is formed of spindle-shaped cells, each containing a single centrally located nucleus (arrow) (LM ×1000). Cells are arranged closely to form sheets. Smooth muscle tissue is not striated.Location: Mostly in the walls of hollow organs. Function: Moves substances or objects (foodstuffs, urine, a baby) along internal passageways; involuntary control.

 

(q)Cardiac muscle tissue consists of branched striated cells, each containing a single nucleus and specialized cell junctions called intercalated disks (arrow) that allow ions (action potentials) to move quickly from cell to cell (LM ×500). Location: The walls of the heart.Function: As the walls of the heart contract, cardiac muscle tissue propels blood into the circulation; involuntary control.

SECTION REVIEW 2.8

Simple epithelium can be classified as squamous, cuboidal, columnar, and pseudostratified. Connective tissue is classified as either loose or dense. Muscle cells are able to shorten or contract and change their length, which accomplishes movement. Nervous tissue is composed of neurons and neuroglia.

Why is blood considered a type of connective tissue?

2.9 ORGANS

LEARNING OUTCOME

1. Describe an organ as found in a mammal.

Organs (Gr. organnon, an independent part of the body) are the functional units of an animal's body that are made up of more than one type of tissue. Examples include the heart, lungs, liver, spleen, and kidneys.

SECTION REVIEW 2.9

An organ is the functional unit of a mammal's body; it is made up of more than one tissue type and usually has multiple functions.

What is the largest organ system in an animal's body?

2.10 ORGAN SYSTEMS

LEARNING OUTCOME

1. Identify the different organ systems of a vertebrate.

The next higher level of structural organization in animals is the organ system. An organ system (Gr. systema, being together) is an association of organs that together performs an overall function. The organ systems in higher vertebrate animals are the integumentary, skeletal, muscular, nervous, endocrine, circulatory, lymphatic, respiratory, digestive, urinary, and reproductive systems (table 2.4). Chapters 23 through 29 discuss these systems in detail.

TABLE 2.4
ORGAN SYSTEMS OF THE BODY

SYSTEM	MAJOR ORGANS	PRIMARY FUNCTIONS
Integumentary	Skin, hair, nails	Protection, thermoregulation
Nervous	Brain, spinal cord, nerves	Regulation of other body systems
Endocrine	Hormone-secreting glands, such as the pituitary, thyroid, and adrenals	Secretion of regulatory molecules called hormones
Skeletal	Bones, cartilages	Movement and support
Muscular	Skeletal muscles	Movements of the skeleton
Circulatory	Heart, blood vessels, lymphatic vessels	Movement of blood and lymph
Immune	Bone marrow, lymphoid organs	Defense of the body against invading pathogens
Respiratory	Lungs, airways	Gas exchange
Urinary	Kidneys, ureters, urethra	Regulation of blood volume and composition
Digestive	Mouth, stomach, intestine, liver, gall­bladder, pancreas	Breakdown of food into molecules that enter the body
Reproductive	Gonads, external genitalia, asso­ciated glands and ducts	Continuation of the human species

The highest level of organization in an animal body is the organismic level. All parts of the animal body function with one another to contribute to the total organism—a living entity or individual. Control and regulatory mechanisms within an animal maintain a constant internal environment. This constant state is called homeostasis (Gr. homeo,always the same + stasis, standing). In a constantly changing external environment, every animal must be able to maintain this “steady state.”

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Animals need energy in order to survive. Many of the chemical reactions that produce energy are regulated by enzymes. ­Together, energy and enzymes are the driving and controlling forces in animals. All animals harvest energy from nutrients to fuel their metabolism with energy from ATP.

SECTION REVIEW 2.10

The different organ systems of a vertebrate are the integumentary, nervous, endocrine, skeletal, muscular, circulatory, immune, respiratory, urinary, digestive, and reproductive.

Is there overlap between the different organ systems in a vertebrate?

EVOLUTIONARY INSIGHTS

The Origin of Eukaryotic Cells

T

he first cells were most likely very simple forms. The fossil record indicates that the earth is approximately 4 to 5 billion years old, and that the first cells may have arisen more than 3.5 billion years ago, whereas the eukaryotes are thought to have first appeared about 1.5 billion years ago.

The endosymbiont theory was first proposed by Lynn Margulis, a biologist at the University of Massachusetts at Amherst. She proposed that eukaryotes formed when large, nonnucleated cells engulfed smaller and simpler cells. (An endosymbiont is an organism that can live only inside another organism, forming a relationship that benefits both partners. Symbiosis is an intimate association between two organisms of different species. The merging of these different species to produce evolutionarily new forms is called symbiogenesis.)

More recently, DNA evidence indicates that both Archaea and Eubacteria contributed to the origin of eukaryotic cells (box figure 2.1) as follows. About 2.5 billion years ago, bacteria and cyanobacteria occurred together in water environments. Over millions of years, the cyanobacteria pumped oxygen into the primitive atmosphere as a result of photosynthesis. As a result, those cellular organisms that could tolerate free oxygen began to flourish.

We now know that free oxygen reacts with other molecules, producing harmful by-products (free radicals) that can disrupt normal biological functions. One way for a large cell, such as an archaeon, to survive in an oxygen-rich environment would be to engulf an aerobic (oxygen-utilizing) bacterium in an inward-budding vesicle of its plasma membrane. This captured bacterium would then contribute biological reactions to detoxify the free oxygen and radicals. Eventually the membrane of the enveloped vesicle became the outer membrane of the mitochondrion. The outer membrane of the engulfed aerobic bacterium became the inner membrane system of the mitochondrion. The small bacterium thus found a new home in the larger cell and as a result, the host cell could survive in the newly oxygenated atmosphere.

In a similar fashion, archaean cells that picked up cyanobacteria or a photosynthetic bacterium obtained the forerunners of chloroplasts and became the ancestors of the green plants. Once these ancient cells acquired their endosymbiont organelles, genetic changes impaired the ability of the captured cells to live on their own outside the host cells. Over many millions of years, the larger cells and the captured cells came to depend on one another for survival. The result of this interdependence is the compartmentalization in modern eukaryotic cells.

Although the exact mechanism for the evolution of the eukaryotic cell will never be known with certainty, the emergence of the eukaryotic cell led to a dramatic increase in the complexity and diversity of life-forms on the earth. At first, these newly formed eukaryotic cells existed only by themselves. Later, however, some probably evolved into multicellular organisms in which various cells became specialized into tissues, which in turn led to the potential for many different functions. These multicellular forms would then be able to adapt to life in a greater variety of environments.

BOX FIGURE 2.1

THE ORIGIN OF EUKARYOTIC CELLS.

According to the endosymbiont theory, eukaryotic cells may have originated many millions of years ago from a joining of eubacterial cells with archaean cells. The captured bacteria eventually became the organelles called mitochondria and chloroplasts. The archaean host contributed the membranes and cytoskeleton elements, which enabled the new cell to move and engulf smaller cells in the watery environment in which they were living. When some of the genetic material of the cap­tured cells moved to the nucleus-to-be (the archaeon's DNA), the smaller cells became dependent on their host.

SUMMARY

1. 2.1   What Are Cells?
   • All animal cells have three basic parts: the nucleus, cytoplasm, and plasma membrane.
2. 2.2   Why Are Most Cells Small?
   • The cell is small because the ratio of the volume of the cell's nucleus to the volume of its cytoplasm must not be so small that the nucleus cannot control the cytoplasm.
3. 2.3   Cell Membranes
   • Cell membranes, composed mainly of phospholipids and proteins, allow certain materials to move across them. This quality is called selective permeability. The fluid-mosaic model is based on knowledge of the plasma membrane.
3. 2.4   Cell Membranes
   • Some molecules use their own energy to pass across a cell membrane from areas of higher concentration to areas of lower concentration. Examples of these passive processes are simple diffusion, facilitated diffusion, osmosis, and filtration.
   • Active transport across cell membranes requires energy from the cell to move substances from areas of lower concentration to areas of higher concentration. Additional processes that move molecules across membranes are endocytosis and exocytosis. Three types of endocytosis are pinocytosis, phagocytosis, and receptor-mediated endocytosis.
5. 2.5   Cytoplasm, Organelles, and Cellular Components
   • The cytoplasm of a cell has two parts. The cytomembrane system consists of well-defined structures, such as the endoplasmic reticulum, Golgi apparatus, vacuoles, and vesicles. The aqueous part consists of the fluid cytosol.
   • Ribosomes are the sites of protein synthesis.
   • The endoplasmic reticulum (ER) is a series of channels that transports, stores enzymes and proteins, and provides a point of attachment for ribosomes. The two types of ER are smooth and rough.
   • The Golgi apparatus aids in the synthesis and secretion of glycoproteins, as well as processing and modifying other materials (e.g., enzymes).
   • Lysosomes containing digestive enzymes digest nutrients and clean away dead or damaged cell parts.
   • Microbodies are membrane-enclosed vesicles that contain a variety of enzymes. One specific microbody is the peroxisome.
   • Mitochondria convert energy in food molecules to ATP, a form the cell can use.
   • Microtubules, intermediate filaments, and microfilaments make up the cytoskeleton of the cell. The cytoskeleton functions in transport, support, and the movement of structures in the cell, such as organelles and chromosomes.
   • Cilia and flagella are appendages on the surfaces of some cells and function in movement.
   • Centrioles assist in cell division and help move chromosomes ­during cell division.
   • Vacuoles are membranous sacs that are part of the cytomembrane system.
   • Vaults are octagonal, barrel-shaped organelles believed to transport messenger RNA from the nucleus to the ribosomes.
6. 2.6   The Nucleus: Information Center
   • The nucleus of a cell contains DNA, which controls the cell's ­genetic program and other metabolic activities.
   • The nuclear envelope contains many pores that allow material to enter and leave the nucleus.
   • The chromosomes in the nucleus have DNA organized into genes, which are specific DNA sequences that control and regulate cell activities.
   • The nucleolus is a preassembly point for ribosomes.
7. 2.7   Levels of Organization in Various Animals
   • The first level of organization in a higher animal is the protoplasmic level, followed by the cellular level, tissue level, and organ level, and the highest and most complex is the system level.
8. 2.8   Tissues
   • Tissues are groups of cells with a common structure and function. The four types of tissues are epithelial, connective, muscle, and nervous.
9. 2.9   Organs
   • An organ is composed of more than one type of tissue.
10. 2.10   Organ Systems
    • An organ system is an association of organs.

CONCEPT REVIEW QUESTIONS

1. Which of the following is the smallest unit of life that can exist as a separate unit?
   1. An organ system
   2. Organs
   3. Tissues
   4. Cells
   5. Organelles
2. The ability of each cell to maintain a constant internal environment is called
   1. evolution.
   2. homeostasis.
   3. metabolism.
   4. adaptation.
   5. its physiology.
3. If the volume of a cell increases, its surface area relative to volume will
   1. decrease.
   2. increase to a lesser degree.
   3. remain the same.
   4. increase proportionally.
   5. increase to a large degree.
4. A chemical analysis of the plasma membrane or nuclear envelope would indicate the presence of
   1. microtubules and microfilaments.
   2. just proteins.
   3. just lipids.
   4. cellulose.
   5. both proteins and phospholipids.
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5. Which of the following contain(s) enzymes and play(s) a role in intracellular digestion?
   1. Ribosomes
   2. Golgi apparatus
   3. Mitochondria
   4. Lysosomes
   5. Microfilaments

ANALYSIS AND APPLICATION QUESTIONS

1. Why is the mitochondrion called the “power generator” of the cell?
2. One of the larger facets of modern zoology can be described as “membrane biology.” What common principles unite the diverse functions of membranes?
3. Why is the current model of the plasma membrane called the “fluid-mosaic” model? What is the fluid, and in what sense is it fluid? What makes up the mosaic?
4. If you could visualize osmosis, seeing the solute and solvent particles as individual entities, what would an osmotic gradient look like?
5. Why can some animal cells transport materials against a concentration gradient? Could animals survive without this capability?

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