Inside the Cell
Chapter
4
Outline
	4.1	Cells Under the Microscope
	•	A microscope is usually needed to see a cell because most cells are quite small.49
	•	Cell surface-area-to-volume ratios explain why cells are so small.49
	4.2	The Two Main Types of Cells
	•	Prokaryotic cells do not have a membrane-bounded nucleus, but eukaryotic cells do.50 
	•	Bacteria always have a cell wall, a plasma membrane, a nucleoid, and a cytoplasm that contains many ribosomes.50–51
	4.3	The Plasma Membrane
	•	The plasma membrane is a phospholipid bilayer that regulates the passage of molecules into and out of the cell.52
	•	The embedded proteins in the membrane have numerous and diverse functions.53
	4.4	Eukaryotic Cells
	•	The membrane-bounded nucleus contains DNA within strands of chromatin, and communicates with ribosomes in the cytoplasm.56
	•	The endomembrane system consists of several organelles that cooperate with one another, often resulting in the secretion of proteins.58 
	•	The energy-related organelles are chloroplasts and mitochondria. In plant cells, chloroplasts use solar energy to produce carbohydrates. In both plant and animal cells, mitochondria break down these molecules to produce ATP.60–61
	•	The cytoskeleton, a complex system of filaments and tubules, gives the cell its shape and accounts for the movement of the cell and its organelles.62–63
	4.5	Outside the Eukaryotic Cell
	•	The surfaces of eukaryotic cells are modified in ways that facilitate communication between cells.64–65
Red blood cells are the most abundant cell type in your body, and they perform a critical job—delivering oxygen throughout your body and picking up carbon dioxide waste from its cells. Along with other types of blood cells, red blood cells are produced from precursor cells located in the marrow of your bones. Red blood cells do not resemble a typical animal cell in structure because they are chock full of hemoglobin, the pigment that makes them red and enables them to carry oxygen. Because red blood cells do not have the usual contents of an animal cell, they only live about 120 days before being destroyed by the liver and the spleen. Thus, the body continually faces the daunting task of replacing the three million red blood cells it loses every day. If this process of replacing the red blood cells is impaired, as occurs in a condition called aplastic anemia, a person can quickly die. 
Cells are the fundamental unit of all living things. Although the diversity of organisms is incredible, the cells of all organisms share many similarities. In fact, there are only a few structural differences between most types of cells. In this chapter you will learn about the major types of cells and how they are similar as well as different. Learning about the structure of cells will later help you understand their complex functioning.
4.1		Cells Under the Microscope
Cells are extremely diverse, but nearly all require a microscope to see them (Fig. 4.1). When we speak of “a cell” or “the cell,” we mean a cell that can be discussed in general terms. Our own bodies are composed of several hundred cell types, and each type is present billions of times over. There are nerve cells, muscle cells, gland cells, and bone cells, to name a few. Each type of cell is specialized to perform a particular function—a nerve cell conducts, a muscle cell contracts, a gland cell secretes, and a bone cell supports.
Cells are complex and yet tiny. The light microscope, invented in the seventeenth century, allows us to see cells but not much of their complexity. That’s because the properties of light limit the amount of detail a light microscope can reveal. Electron microscopes, invented in the 1930s, overcome this limit by using beams of electrons instead of beams of light as their source of illumination. An electron microscope enables us to see the fine details of cells and even some of the larger molecules within them. Figure 4.2 compares the visual ranges of the electron microscope, the light microscope, and the unaided eye.
Why are cells so small? To answer this question, consider that a cell needs a surface area large enough to allow adequate nutrients to enter it and to rid itself of wastes. If you were to cut a large cube into smaller cubes, the smaller cube would have a lot more surface area per volume than the large cube. For example, a 4-cm cube has a surface-area-to-volume ratio of only 1.5:1, but a 1-cm cube has a surface-area-to-volume ratio of 6:1. Therefore, small cells, not large cells, are most likely to have an adequate surface area for exchanging wastes and nutrients. 
A chicken’s egg is a cell several centimeters in diameter, but it is not actively metabolizing. If development begins, cell division occurs and provides the surface area needed for exchange of materials. Animal cells that specialize in absorption have ways to increase their surface-area-to-volume ratio. Often they have projections called microvilli (sing., microvillus), which increase the surface area.
4.2		The Two Main Types of Cells
All cells have an outer membrane called a plasma membrane. The plasma membrane encloses a semifluid substance called the cytoplasm and the cell’s genetic material. The plasma membrane regulates what enters and exits cells; the cytoplasm carries on chemical reactions; and the genetic material provides the information needed for growth and reproduction. When a cell reproduces, it produces more cells. The concepts that all organisms are composed of cells, and that cells come only from preexisting cells, are the two central tenets of the cell theory. 
Cells are divided into two main types according to the way their genetic material is organized. Prokaryotic cells are so named because they lack a membrane-bounded nucleus. Their DNA is located in a region of the cytoplasm called the nucleoid. The other type of cell, called a eukaryotic cell, has a nucleus that houses its DNA. 
Prokaryotic Cells
Prokaryotic cells are much smaller in size and simpler in structure than eukaryotic cells (Fig. 4.3). Their small size allows them to exist in great numbers in the air, in bodies of water, in the soil, and even on you. They are an extremely successful group of organisms, whose evolutionary history dates back to the first cells on Earth. 
Bacteria, a type of prokaryotic cell, are well known because they cause some serious diseases, including tuberculosis, throat infections, and gonorrhea. Even so, the biosphere would not long continue without bacteria. Many bacteria decompose dead remains and contribute to ecological cycles. Bacteria also assist humans in another way—we use them to manufacture all sorts of products, from industrial chemicals to foodstuffs and drugs. The active cultures found in a container of yogurt are bacteria that can be beneficial to us. Also, much of our knowledge about how DNA specifies the sequence of amino acids in proteins was learned by doing experiments utilizing E. coli, a bacterium that lives in the human intestine. The fact that this information applies to all prokaryotes and eukaryotes, even ourselves, reveals the remarkable unity of living things and gives evidence that all organisms share a common descent. 
Bacterial Structure 
In bacteria, the cytoplasm is surrounded by a plasma membrane, a cell wall, and possibly a capsule (Fig. 4.4). The cytoplasm contains a variety of different enzymes. These organic catalysts speed many types of chemical reactions that are required to maintain an organism. Both prokaryotes and eukaryotes have the same type of plasma membrane; its structure is discussed on page 52. The cell wall maintains the shape of the cell, even if the cytoplasm should happen to take up an abundance of water. The capsule is a protective layer of polysaccharides lying outside the cell wall. 
The DNA of a bacterium is located in a single coiled chromosome that resides in a region called the nucleoid. The many proteins specified by bacterial DNA are synthesized on tiny particles called ribosomes. A bacterial cell contains thousands of ribosomes. 
The appendages of a bacterium, namely flagella, fimbriae, and sex pili, are made of protein. Motile bacteria can propel themselves in water because they have flagella (sing., flagellum), which are usually about 20 nm in diameter and 1–70 nm long. The bacterial flagellum has a filament, a hook, and a basal body, which is a series of rings anchored in the cell wall and plasma membrane. The hook rotates 360 within the basal body, and this rotary motion propels bacteria—the bacterial flagellum does not move back and forth like a whip. Sometimes flagella occur only at the two ends of a cell, and other times they are dispersed randomly over the surface. 
Fimbriae are small, bristlelike fibers that sprout from the cell surface. They don’t have anything to do with motility, but they do help bacteria attach to a surface. Sex pili are rigid tubular structures used by bacteria to pass DNA from cell to cell. Bacteria can also take up DNA directly from the external medium or by way of viruses.
4.3		The Plasma Membrane 
The plasma membrane marks the boundary between the outside and inside of a cell. Its integrity and function are necessary to the life of the cell because it regulates the passage of molecules and ions into and out of the cell. 
In both prokaryotes and eukaryotes, the plasma membrane is a phospholipid bilayer (see Fig. 3.14). The polar heads of the phospholipids face toward the outside of the cell and toward the inside of the cell where there is a watery medium. The nonpolar tails face inward toward each other, where there is no water. Cholesterol molecules, if present, lend support to the membrane, which has the consistency of olive oil. Short chains of sugars are attached to the outer surface of some proteins, forming glycoproteins. The sugar chain helps a protein perform its particular function. 
The fluid-mosaic model states that the protein molecules embedded in the membrane have a pattern (form a mosaic) within the phospholipid -bilayer (Fig. 4.5). The pattern varies according to the particular membrane and also within the same membrane at different times. When you consider that the plasma membrane of a red blood cell contains over 50 different types of proteins, you can see why the membrane is said to be a mosaic.
Functions of Membrane Proteins
The proteins on the internal surface of the plasma membrane often help stabilize and shape the membrane. The embedded proteins also perform specific functions. There are several types of embedded proteins. 
Channel Proteins
Channel proteins permit the passage of molecules through the membrane. Via a channel, a specific substance can simply move across the membrane (Fig. 4.6a). For example, a channel protein allows hydrogen ions to flow across the inner mitochondrial membrane. Without this movement of hydrogen ions, ATP would never be produced.
Transport Proteins
Transport proteins are also involved in the passage of molecules through the membrane. They combine with a substance and help it move across the membrane (Fig. 4.6b). For example, a transport protein conveys sodium and potassium ions across a nerve cell membrane. Without this transport protein, nerve conduction would be impossible.
Cell Recognition Proteins
Cell recognition proteins are glycoproteins (Fig. 4.6c). Among other functions, these proteins enable our bodies to recognize a pathogen invasion so that an immune reaction can occur. Without this recognition, pathogens would be able to freely invade the body.
Receptor Proteins
A receptor protein has a shape that allows a specific molecule to bind to it (Fig. 4.6d). The binding of this molecule causes the protein to change its shape, and thereby bring about a cellular response. The coordination of the body’s organs is totally dependent on such signaling molecules. For example, the liver stores glucose after it is signaled to do so by insulin.
Enzymatic Proteins
Some plasma membrane proteins are enzymatic proteins that carry out metabolic reactions directly (Fig. 4.6e). Without enzymes, some of which are attached to the various membranes of the cell, a cell would never be able to perform the degradative and synthetic reactions that are important to its function. 
Junction Proteins
As discussed on page 65, proteins are also involved in forming various types of junctions between cells (Fig. 4.6f). The junctions assist cell-to-cell communication.
4.4		Eukaryotic Cells	 
As discussed in Chapter 1, protists, fungi, plants, and animals are all composed of eukaryotic cells. Unlike prokaryotic cells, eukaryotic cells have a membrane-bounded nucleus that houses their DNA. The DNA is located in several chromosomes. 
Eukaryotic cells are much larger than prokaryotic cells, and therefore, as you might expect from the previous discussion, they have less surface area per volume than prokaryotic cells. Wouldn’t this circumstance be a hindrance to their very existence? No, because their interior is compartmentalized into small structures called organelles that differ in structure and function. 
In this chapter, the organelles are divided into four categories: 
	1.	The nucleus and the ribosomes. The nucleus communicates with ribosomes in the cytoplasm. 
	2.	Organelles of the endomembrane system (ES). Each of these organelles has its own particular set of enzymes and produces its own products. The products are carried between the ES organelles by transport vesicles, small membranous sacs that keep the products from entering the cytoplasm. 
	3.	The energy-related organelles. The energy-related organelles—chloroplasts in plant cells and mitochondria in plant and animal cells—are self-contained. They even have their own genetic material, and their ribosomes resemble those of prokaryotic cells.
	4.	The cytoskeleton. The cytoskeleton is a lattice of protein filaments and tubules that maintains the shape of the cell and assists in the movement of organelles. Without an efficient means of moving organelles and their products, eukaryotic cells could not exist. The manner in which vesicles and other organelles move along the tracks provided by the cytoskeleton will be discussed in more detail later. 
Each structure in the typical animal and plant cells shown in Figures 4.7 and 4.8 has been given a particular color, and this color will be used to represent the same structure throughout this text.
Nucleus and Ribosomes
The nucleus stores genetic information, and the ribosomes in the cytoplasm use this information to carry out protein synthesis.
The Nucleus
Because of its large size, the nucleus is a noticeable structure in the eukaryotic cell (Fig. 4.9). The nucleus contains chromatin within a semifluid nucleoplasm. Chromatin looks grainy, but actually it is a network of strands. Just before the cell divides, the chromatin condenses and undergoes coiling into rodlike structures called chromosomes. All of the cells of an organism contain the same number of chromosomes except for the egg and sperm, which have half this number. 
Chromatin (and therefore chromosomes) is composed of DNA, protein, and some RNA. The genes are composed of DNA, and are located in the chromosomes. The nucleus therefore contains the genetic information that is passed on from cell to cell and from generation to generation.
RNA, which exists in several forms, is produced in the nucleus and has a sequence of bases that mirrors the sequence in DNA. A nucleolus is a dark region of chromatin where a type of RNA called ribosomal RNA (rRNA) is -produced, and where rRNA joins with proteins to form the subunits of ribosomes. Ribosomes are small bodies in the cytoplasm where protein synthesis occurs. Another type of RNA, called messenger RNA (mRNA), acts as an intermediary for DNA. It helps specify the sequence of amino acids during protein synthesis. The proteins of a cell determine its structure and functions, so the nucleus, which houses DNA, can be thought of as the command center for the cell.
The nucleus is separated from the cytoplasm by a double membrane known as the nuclear envelope. Even so, the nucleus communicates with the cytoplasm. The nuclear envelope has nuclear pores of sufficient size (100 nm) to permit the passage of ribosomal subunits and mRNA out of the nucleus into the cytoplasm, and the passage of proteins from the cytoplasm into the nucleus. 
Ribosomes 
Ribosomes are found in both prokaryotes and eukaryotes. In both types of cells, ribosomes are composed of two subunits, one large and one small. Each subunit has its own mix of proteins and rRNA. As mentioned, ribosomes are sites of protein synthesis. They receive mRNA from the nucleus, which carries a coded message from DNA. The mRNA encodes the correct sequence of amino acids in a polypeptide.
In eukaryotic cells, some ribosomes occur freely within the cytoplasm, either singly or in groups called polyribosomes. Polypeptides synthesized by these ribosomes are used in the cytoplasm. Many ribosomes are attached to the endoplasmic reticulum (ER), an organelle composed of many saccules and channels. After the ribosome binds to a receptor at the ER, the polypeptide being synthesized enters the lumen of the ER (Fig. 4.10).
Endomembrane System
The endomembrane system consists of the nuclear envelope, the membranes of the endoplasmic reticulum, the Golgi apparatus, and many small membranous sacs called vesicles. This system helps compartmentalize the cell, so that particular enzymatic reactions are restricted to specific regions. Transport vesicles carry molecules from one part of the system to -another. 
Endoplasmic Reticulum
The endoplasmic reticulum (ER) consists of a complicated system of membranous channels and saccules (flattened vesicles). It is physically continuous with the outer membrane of the nuclear envelope (Fig. 4.11).
Rough ER is studded with ribosomes on the side of the membrane that faces the cytoplasm; therefore, rough ER is able to synthesize proteins. It also modifies proteins after they have entered the central enclosed region of the ER, called the lumen. The rough ER forms vesicles in which large molecules are transported to other parts of the cell. Often these vesicles are on their way to the plasma membrane or the Golgi apparatus (described below).
Smooth ER, which is continuous with rough ER, does not have attached ribosomes. Smooth ER synthesizes lipids, such as phospholipids and steroids. The functions of smooth ER are dependent on the particular cell. In the testes, it produces testosterone, and in the liver, it helps detoxify drugs. Regardless of any specialized function, rough and smooth ER also form transport vesicles that carry molecules to other parts of the cell, notably the Golgi apparatus. 
Golgi Apparatus
The Golgi apparatus, named for its discoverer, Camillo Golgi, consists of a stack of three to twenty slightly curved, flattened saccules resembling pancakes (Fig. 4.12). The Golgi apparatus receives vesicles and alters their contents as molecules move through saccules in the Golgi. For example, the Golgi can substitute one sugar for another when a sugar chain is attached to a protein. This action can alter recognition of the protein and change its destination in the cell.
The Golgi apparatus packages its products in new vesicles. In animal cells, some of these vesicles are lysosomes, which are discussed next. Some transport vesicles proceed to the plasma membrane, where they discharge their contents during secretion. 
Lysosomes
Lysosomes are vesicles produced by the Golgi apparatus that digest molecules and even portions of the cell itself. Sometimes, after engulfing molecules outside the cell, a vesicle formed at the plasma membrane fuses with a lysosome. The contents of the vesicle are digested by lysosomal enzymes. In Tay-Sachs disease, a genetic disorder, lysosomes are missing an enzyme for a particular lipid molecule. The cells become so full of storage bodies that the individual dies, usually in childhood. Someday it may be possible to provide the missing enzyme for these children. 
Lysosomes also participate in apoptosis, or programmed cell death, which is a normal part of development. When a tadpole becomes a frog, lysosomes digest away the cells of the tail. In humans, the fingers of an embryo are at first webbed, but they are freed from one another as a result of lysosomal action.
Vacuoles
Vacuoles, like vesicles, are membranous sacs, but vacuoles are larger than vesicles. The vacuoles of some protists are quite specialized; they include contractile vacuoles for ridding the cell of excess water, and digestive vacuoles for breaking down nutrients (Fig. 4.13a). Vacuoles usually store substances, such as nutrients or ions. Plant vacuoles contain not only water, sugars, and salts, but also pigments and toxic molecules (Fig. 4.13b). The pigments are responsible for many of the red, blue, or purple colors of flowers and some leaves. The toxic substances help protect a plant from herbivorous animals. 
Energy-Related Organelles
Chloroplasts and mitochondria are the two eukaryotic membranous organelles that specialize in energy conversion. Chloroplasts use solar energy to synthesize carbohydrates. 
Mitochondria (sing., mitochondrion) break down carbohydrate-derived products to produce adenosine triphosphate (ATP) molecules. The production of ATP is of great importance because ATP serves as a carrier of energy in cells. The energy of ATP is used whenever a cell synthesizes molecules, transports molecules, or carries out a special function such as muscle contraction or nerve conduction. Without a constant supply of ATP, no cell could long exist. 
Chloroplasts
The chloroplast, a plant cell organelle, is quite large, being twice as wide and as much as five times the length of a mitochondrion. Chloroplasts have a three-membrane system. They are bounded by a double membrane, which includes an outer membrane and an inner membrane. The large inner space, called the stroma, contains a concentrated mixture of enzymes and thylakoids. The thylakoids are disklike sacs formed from the third membrane. A stack of thylakoids is called a granum. The lumens of thylakoid sacs are believed to form a large internal compartment called the thylakoid space (Fig. 4.14). The pigments that capture solar energy are located in the thylakoid membrane, and the enzymes that synthesize carbohydrates are in the stroma. The carbohydrates produced by chloroplasts serve as organic nutrient molecules for plants and indeed for all living things on Earth. 
The discovery that chloroplasts have their own DNA and ribosomes supports a hypothesis that chloroplasts are derived from single-celled algae that entered a eukaryotic cell in the distant past. As shown in Figure 4.8, plant cells contain both mitochondria and chloroplasts, as do algal cells.
Mitochondria
Even though mitochondria are smaller than chloroplasts, they can be seen with the light microscope in both plant and animal cells. We think of mitochondria as having a shape like that shown in Figure 4.15, but actually they often change shape, becoming longer and thinner or shorter and broader. Mitochondria can form long, moving chains, or they can remain fixed in one location-—often where energy is most needed. For example, they are packed between the contractile elements of cardiac cells and wrapped around the interior of a sperm’s flagellum.
Like chloroplasts, mitochondria are bounded by a double membrane. The inner membrane is highly convoluted into cristae that project into the matrix. These cristae increase the surface area of the inner membrane so much that in a liver cell they account for about one-third of the total membrane in the cell. The inner membrane encloses the matrix, which contains mitochondrial DNA and ribosomes. It has been suggested that mitochondria are derived from bacteria that took up residence in an early eukaryotic cell.
Mitochondria are often called the powerhouses of the cell because they produce most of the ATP the cell utilizes. (ATP is discussed on page 60.) Their matrix is a highly concentrated mixture of enzymes that break down carbohydrates and other nutrient molecules. These reactions supply the chemical energy that permits ATP synthesis to take place. The entire process, which also involves the cytoplasm, is called cellular respiration because oxygen is needed and carbon dioxide is given off.
The Cytoskeleton 
The cytoskeleton is a network of interconnected protein filaments and tubules that -extends from the nucleus to the plasma membrane in eukaryotic cells. The cytoskeleton can be compared to the bones and muscles of an animal. Bones and muscles give an animal structure and produce movement. Similarly, the elements of the cytoskeleton maintain cell shape and allow the cell and its organelles to move. The cytoskeleton is dynamic in that the elements can assemble and disassemble as appropriate. The cytoskeleton includes microtubules, intermediate filaments, and actin filaments. 
Microtubules
Each microtubule is a small, hollow cylinder (Fig. 4.16a). When microtubules assemble, tubulin molecules come together by twos that are arranged in rows. Microtubules have 13 rows of tubulin dimers surrounding what appears in electron micrographs to be an empty central core.
Microtubule assembly is controlled by a microtubule organizing center called the centrosome, which lies near the nucleus. Microtubules radiate from the centrosome, helping to maintain the shape of the cell and acting as tracks along which organelles can move. Whereas the motor molecule myosin is associated with actin filaments, the motor molecules kinesin and dynein are associated with microtubules. First, an organelle, such as a vesicle, combines with a motor molecule, and then the motor molecule attaches, detaches, and reattaches further along the cytoskeletal element, such as a microtubule. In this way, the organelle moves along the microtubular track (Fig. 4.16b).
Intermediate Filaments
Intermediate filaments are intermediate in size between actin filaments and microtubules. They are ropelike assemblies of fibrous polypeptides that typically run between the nuclear envelope and the plasma membrane. The network they form supports both the nucleus and the plasma membrane. 
The protein making up intermediate filaments is different from one cell type to another. In the skin, intermediate filaments made of the protein keratin give great mechanical strength to skin cells. 
Actin Filaments 
Each actin filament consists of two chains of globular actin monomers twisted about one another in a helical manner. Actin filaments support the cell and any projections, such as microvilli. They form a dense, complex web just under the plasma membrane (Fig. 4.17). 
When a muscle cell contracts, myosin acts as a motor molecule that pulls actin filaments toward the middle of the cell. Even in nonmuscle cells, myosin interacting with actin filaments produces movement as when cells move in an amoeboid fashion and/or engulf large particles. During animal cell division, the two new cells form when actin, in conjunction with myosin, pinches off the cells from one another. In plant cells, actin filaments apparently form the tracks along which chloroplasts circulate in the cytoplasm.
Centrioles
Centrioles are short cylinders with a 9  0 pattern of microtubule triplets—that is, nine sets of triplets occur in a ring, and none are in the middle of the cylinder (Fig. 4.18). In animal cells and most protists, two centrioles lie at right angles to one another in the middle of a centrosome. A centrosome, as mentioned previously, is the major microtubule organizing center for the cell. Therefore, it is possible that centrioles are also involved in the process by which microtubules assemble and disassemble.
Before an animal cell divides, the centrioles replicate, and the members of each pair are at right angles to one another. Then each pair becomes part of a separate centrosome. During cell division, the centrosomes move apart and most likely play a role in organizing a microtubular apparatus (the spindle), which assists the movement of chromosomes. In any case, each new cell eventually has its own centrosome and pair of centrioles. 
Plant and fungal cells have the equivalent of a centrosome, but this structure does not contain centrioles, suggesting that centrioles are not necessary for the assembly of cytoplasmic microtubules.
Cilia and Flagella
In eukaryotes, cilia and flagella are hairlike projections that can move either in an undulating fashion, like a whip, or stiffly, like an oar (Fig. 4.19a). Cells that have these organelles are capable of movement. Cilia are much shorter than flagella, but they have a similar construction. Both are membrane-bounded cylinders enclosing a matrix area. In the matrix are nine microtubule doublets arranged in a circle around two central microtubules, called the 9  2 pattern of -microtubules (Fig. 4.19b). Cilia and flagella move when the microtubule doublets slide past one another.
Each cilium and flagellum has a basal body lying in the cytoplasm at its base. Basal bodies have the same circular arrangement of microtubule triplets as centrioles and are believed to be derived from them. Whether centrioles are involved in organizing the microtubules in cilia and flagella is still a matter of study. 
4.5		Outside the Eukaryotic Cell
Now that we have completed our discussion of the plasma membrane and the cell contents, you might think we have finished our tour of the cell. However, most cells also have extracellular structures formed from materials the cell produces and transports across its plasma membrane. 
Plant Cell Walls
All plant cells have a cell wall. A primary cell wall contains cellulose fibrils and noncellulose substances, and these allow the wall to stretch when the cell is growing. Adhesive substances are abundant outside the cell wall in the middle lamella, a layer that holds two plant cells together. 
Some cells in woody plants have a secondary cell wall that forms inside the primary cell wall. The secondary wall has a greater quantity of cellulose fibrils, which are laid down at right angles to one another. Lignin, a substance that adds strength, is a common ingredient of secondary cell walls in woody plants.
In a plant, living cells are connected by plasmodesmata (sing., plasmodesma), numerous narrow, membrane-lined channels that pass through the cell wall (Fig. 4.20). Cytoplasmic strands within these channels allow direct exchange of some materials between adjacent plant cells and eventually among all the cells of a plant. The plasmodesmata allow only water and small solutes to pass freely from cell to cell. 
Cell Surfaces in Animals
Animal cells do not have a cell wall, but they do have two other exterior surface features of interest: (1) An extracellular matrix exists outside the cell, and (2) various junctions occur between some cell types. 
Extracellular Matrix
An extracellular matrix is a meshwork of polysaccharides and proteins in close association with the cell that produced them (Fig. 4.21). Collagen and elastic fibers are two well-known structural proteins in the extracellular matrix. Collagen gives the matrix strength, and elastic fibers give it resilience. Other proteins play a dynamic role by forming “highways” that direct the migration of cells during development. Sometimes the proteins bind to receptors in a cell’s plasma membrane, permitting communication between the extracellular matrix and the cytoplasm of the cell.
Polysaccharides in the extracellular matrix provide a rigid packing gel for the various matrix proteins. At present, we know that this gel permits rapid diffusion of nutrients, metabolites, and hormones between blood and tissue cells. Most likely, the polysaccharides regulate the activity of molecules that bind to receptors in the plasma membrane.
The extracellular matrix of various tissues may be quite flexible, as in cartilage, or rock solid, as in bone. The extracellular matrix of bone is hard because in addition to the components mentioned, mineral salts, notably calcium salts, are deposited outside the cell.
Junctions Between Cells
Three types of junctions are found between certain cells: adhesion junctions, tight junctions, and gap junctions. 
In adhesion junctions, internal cytoplasmic plaques, firmly attached to the cytoskeleton within each cell, are joined by intercellular filaments (Fig. 4.22a). The result is a sturdy but flexible sheet of cells. In some organs—such as the heart, stomach, and bladder, where tissues must stretch—adhesion junctions hold the cells together.
Adjacent cells are even more closely joined by tight junctions, in which plasma membrane proteins actually attach to each other, producing a zipperlike fastening (Fig. 4.22b). The cells of tissues that serve as barriers are held together by tight junctions; for example, the urine stays within kidney tubules because the tubules’ cells are joined by tight junctions.
A gap junction allows cells to communicate. A gap junction is formed when two identical plasma membrane channels join (Fig. 4.22c). The channel of each cell is lined by six plasma membrane proteins that allow the junction to open and close. A gap junction lends strength to the cells, but it also allows small molecules and ions to pass between them. Gap junctions are important in heart muscle and smooth muscle because they permit the flow of ions that is required for the cells to contract as a unit.-
The Chapter In Review
Summary
4.1		Cells Under the Microscope
	•	Cells are microscopic in size. Although a light microscope allows you to see cells, it cannot make out the detail that an electron microscope can. 
	•	Cells must remain small in order to have an adequate amount of surface area per cell volume.
4.2		The Two Main Types of Cells
	•	All cells have a plasma membrane, cytoplasm, and genetic material. 
	•	Prokaryotic cells do not have a membrane-bounded nucleus. Eukaryotic cells have a membrane-bounded nucleus as well as various membranous organelles. 
	•	Bacteria are representative of the prokaryotes. They have a cell wall and capsule, in addition to a plasma membrane. Their DNA is in the nucleoid. They have many ribosomes and three possible appendages.
4.3		The Plasma Membrane
The plasma membrane of both prokaryotes and eukaryotes is a phospholipid bilayer.
	•	The phospholipid bilayer regulates the passage of molecules and ions into and out of the cell.
	•	The fluid-mosaic model of membrane structure shows that the embedded proteins form a mosaic (varying) pattern.
	•	The types of embedded proteins include channel, transport, cell recognition, receptor, and enzymatic proteins.
4.4		Eukaryotic Cells
Eukaryotic cells, which are much larger than prokaryotic cells, have the following organelles:
Nucleus and Ribosomes
	•	The nucleus houses chromatin, which contains DNA, the genetic material. During division, chromatin becomes condensed into chromosomes. In the nucleolus, ribosomal RNA is produced, and it joins with proteins to form the subunits of ribosomes. These subunits exit the nucleus at nuclear pores.
	•	Ribosomes in the cytoplasm synthesize proteins with the help of mRNA, a DNA intermediary.
Endomembrane System
	•	Rough ER produces proteins, which are modified in the ER before they are packaged in transport vesicles. 
	•	Smooth ER has various metabolic functions depending on the cell type, but it also forms vesicles.
	•	The Golgi apparatus receives transport vesicles and modifies, sorts, and repackages proteins into vesicles that fuse with the plasma membrane as secretion occurs.
	•	Lysosomes are produced by the Golgi apparatus. They contain enzymes that carry out intracellular digestion.
Vacuoles and Vesicles
	•	Vacuoles are large membranous sacs specialized for storage, contraction, digestion, and other functions, depending on the type of organism and cells in which they occur. 
	•	Vesicles are small membranous sacs.
Energy-Related Organelles
	•	Chloroplasts capture the energy of the sun and carry on photosynthesis, which produces carbohydrates. 
	•	Mitochondria break down carbohydrate-derived products as ATP is produced during cellular respiration.
Cytoskeleton
	•	The cytoskeleton maintains cell shape and allows the cell and the organelles to move. 
	•	Microtubules radiate from the centrosome and are present in centrioles, cilia, and flagella.
	•	Intermediate filaments support the nuclear envelope and the plasma membrane and probably participate in cell-to-cell junctions.
	•	Actin filaments, the thinnest filaments, interact with the motor molecule myosin to bring about contraction.
Centrioles
	•	Centrioles are short cylinders with a 9  0 pattern of microtubule triplets. 
	•	In animal cells and those of most protists, two centrioles lie at right angles just outside the nucleus. 
	•	Centrioles are not present in plant cells.
Cilia and Flagella
	•	Cilia and flagella are hairlike projections that allow some cells to move.
	•	Both have a cylindrical construction with an internal 9  2 pattern of microtubules.
	•	Their basal bodies resemble centrioles.
The following table summarizes the contents of bacterial, animal, and plant cells: 
4.5		Outside the Eukaryotic Cell
Plant cells have a freely permeable cell wall, with cellulose as its main component. Small membrane-lined channels called plasmodesmata span the cell wall and contain strands of cytoplasm that allow materials to pass from one cell to another.
Animal cells typically have an extracellular matrix that contains proteins and polysaccharides produced by the cell. The matrix helps support cells and aids in communication between cells. Some animal cells have junctions. Adhesion junctions and tight junctions help hold cells together; gap junctions allow passage of small molecules between cells.
Thinking Scientifically
	1.	The 1992 movie Lorenzo’s Oil recounts the story of six-year-old Lorenzo Odone and his battle with adrenoleukodystrophy (ALD), which is caused by a malfunctioning organelle called a peroxisome. A peroxisome is an organelle that ordinarily contains enzymes capable of breaking down long-chain fatty acids. In ALD, the enzyme is missing and when long-chain fatty acids accumulate muscle weakness and loss of control result. a. How might a dietitian be able to help a person with ALD? b. What organelle discussed in this chapter is most similar in structure and function to a peroxisome?
	2.	Protists of the phylum Apicomplexa cause malaria and contribute to infections associated with AIDS. These parasites are unusual because they contain plastids. (Chloroplasts are a type of plastid but most plastids store materials.) Scientists have discovered that an effective antibiotic that kills the parasite inhibits the functioning of the plastid. Can you conclude that the plastid is necessary to the life of the parasite? Why or why not? 
Testing Yourself
Choose the best answer for each question.
	1.	Which of the following is not found in a prokaryotic cell?
	a. cytoplasm	d. ribosome
	b. plasma membrane	e. mitochondrion
	c. nucleoid
	2.	Which of the following can only be viewed with an electron microscope?
	a. virus	c. bacteria
	b. chloroplast	d. human egg
	3.	The plasma membrane has the consistency of
	a. butter.	c. olive oil.
	b. peanut butter.	d. water.
	4.	The plasma membrane is a mosaic of
	a. phospholipids and waxes.
	b. waxes and polysaccharides.
	c. polysaccharides and proteins.
	d. proteins and phospholipids.
For questions 5–8, match the items to those in the key.
Key:
	a. channel proteins	d. receptor proteins
	b. transport proteins	e. enzymatic proteins
	c. cell recognition proteins	f. junction proteins
	5.	Important component of the immune system.
	6.	Assist with cell-to-cell communication.
	7.	Combine with molecules to carry them across plasma membranes. 
	8.	Act as signaling molecules. 
	9.	Eukaryotic cells compensate for a low surface-to-volume ratio by
	a. taking up materials from the environment more efficiently.
	b. lowering their rate of metabolism.
	c. compartmentalizing their activities into organelles.
	d. reducing the number of activities in each cell.
	10.	What is synthesized by the nucleolus?
	a. mitochondria	c. transfer RNA
	b. ribosomal subunits	d. DNA
For questions 11–14, match the items to those in the key.
Key:
	a. single membrane
	b. double membrane
	c. no membrane
	11.	Ribosome 
	12.	Nucleus 
	13.	Vacuole 
	14.	Mitochondrion 
	15.	The organelle that can modify the sugars on a protein, determining the protein’s destination in the cell is the
	a. ribosome.	c. Golgi apparatus.
	b. vacuole.	d. lysosome.
	16.	Plant vacuoles may contain
	a. flower color pigments.
	b. toxins that protect plants against herbivorous animals.
	c. sugars.
	d. All of these are correct.
	17.	The nonmembrane component of a mitochondrion is called the
	a. cristae.	c. matrix.
	b. thylakoid.	d. granum.
	18.	Label the parts of the following cell.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
It is common to grow large cultures of bacterial cells for the purpose of extracting commercially important compounds produced naturally by the bacteria. However, we can now introduce human genes into bacteria, allowing them to produce human proteins. These proteins can then be extracted, purified, and used to treat human disorders. For example, human growth hormone is produced in this way and used to treat dwarfism. 
Do you believe it is right to transfer human genes into bacterial cells in order to produce large quantities of human proteins for medical purposes? Would your opinion change if the genes were put into the cells of a mammal such as a goat or a pig?
Understanding the Terms
actin filament62	
adenosine triphosphate 
(ATP)60
adhesion junction65
apoptosis58
capsule50
cell48
cell theory50
cellular respiration61
cell wall50, 64
centriole62
centrosome62
chloroplast60
chromatin56
chromosome56
cilium (pl., cilia)63
cristae61
cytoplasm50
cytoskeleton55, 62
endomembrane system58
endoplasmic reticulum (ER)57
eukaryotic cell50
extracellular matrix64
fimbriae51
flagellum (pl., flagella)51, 63
gap junction65
Golgi apparatus58
granum60
lysosome58
matrix61
microtubule62
mitochondrion60
nuclear envelope57
nuclear pore57
nucleoid51
nucleolus56
nucleus54
organelle54
plasma membrane52
plasmodesmata64
polyribosome57
prokaryotic cell50
ribosome51, 56
rough ER58
secretion58
sex pili51
smooth ER58
stroma60
surface-area-to-volume 
ratio49
thylakoid60
tight junction65
vacuole59
vesicle58 
Match the terms to these definitions:
a.	_______________	The concepts that all organisms are composed of cells, and cells only come from preexisting cells.
b.	_______________	The collection of membranes that compartmentalize the cell so that enzymatic reactions are confined to specific regions.
c.	_______________	Programmed cell death.
d.	_______________	A stack of thylakoids. 
e.	_______________	Composed of a stack of flattened saccules resembling pancakes.
f.	_______________	A lattice of protein fiber that maintains the shape of the cell and assists in the movement of organelles.
g.	_______________	A protective layer of polysaccharides outside the cell wall of bacteria.
The human body is composed of up to 100 trillion cells. 
Figure 4.1Diversity of cells.
All organisms are composed of cells, but it takes a microscope to see most of them. Today, three basic types of microscopes are in use: the light microscope, the transmission electron microscope, and the scanning electron microscope. The light microscope and the transmission electron microscope reveal the insides of cells, while the scanning electron microscope shows three-dimensional surface features. Pictures resulting from the use of the light microscope are called light micrographs (LM), and those that result from the use of the transmission electron microscope are called transmission electron micrographs (TEM). Use of the scanning electron microscope results in scanning electron micrographs (SEM).
Figure 4.2Relative sizes of some living things and their components. 
This diagram not only gives you an idea of the relative sizes of organisms, cells, and their components, but it also shows which of them can be seen with an electron microscope, a light microscope, and the unaided eye. These sizes are given in metric units, with each higher unit ten times greater than the lower unit. See Appendix B for the complete metric system.
Check Your Progress
Explain why a large surface-to-volume ratio is needed for the proper functioning of cells.
Answer:A cell needs a relatively large surface area for absorption of nutrients and secretion of wastes.
Figure 4.3Comparison of prokaryotic cells and  eukaryotic cells.
Prokaryotic cells are much smaller than eukaryotic cells. They are also less complex in structure, as we shall see.
Check Your Progress
	1.	What is the major distinction between a prokaryotic cell and a eukaryotic cell?
	2.	 List the three types of bacterial appendages. What do they have in common?
Answers:1. A prokaryotic cell lacks a membrane-bounded nucleus, while a eukaryotic cell has one.2. The three bacterial appendages—flagella, fimbriae, and sex pili—are all made of protein.
Figure 4.4Prokaryotic cell.
Bacterial anatomy is representative of prokaryotic cell structure. Prokaryotic cells have a nucleoid but no membrane-bounded nucleus.
Figure 4.5A model of the plasma membrane. 
The plasma membrane is composed of a phospholipid bilayer. The polar heads of the phospholipids are at the surfaces of the membrane; the nonpolar tails make up the interior of the membrane. Proteins embedded in the membrane have various functions (see Fig. 4.6).
Figure 4.6Membrane protein diversity.
These are some of the types of proteins embedded in the plasma membrane.
Check Your Progress
	1.	Briefly describe the structure of the plasma membrane.
	2.	 List six types of proteins found in the plasma membrane.
Answers:1. The plasma membrane is composed of a phospholipid bilayer, with the fatty acid tails pointing inward and many embedded proteins. 2. Channel, transport, cell recognition, receptor, enzymatic, and junction proteins.
Figure 4.7Structure of a typical animal cell.
a. False-colored TEM of an animal cell. b. Generalized drawing.
Figure 4.8Structure of a typical plant cell.
Figure 4.9Structure of the nucleus.
The nuclear envelope contains pores that allow substances to pass from the nucleus to the cytoplasm. 
a.	False-colored TEM of a plant cell. b. Generalized drawing.
Figure 4.10The nucleus, ribosomes, and endoplasmic reticulum (ER).
After mRNA leaves the nucleus, it attaches itself to a ribosome, and polypeptide synthesis begins. When a ribosome combines with a receptor at the endoplasmic reticulum (ER), the polypeptide enters the lumen of the ER through a channel in the receptor. Exterior to the ER, the ribosome splits, releasing the mRNA while a protein takes shape inside the ER lumen.
Figure 4.11Endoplasmic reticulum (ER).
Ribosomes are present on rough ER, which consists of flattened saccules, but not on smooth ER, which is more tubular. Proteins are synthesized and modified by rough ER; lipids are synthesized by smooth ER, which can have several other functions as well.
Figure 4.12Endomembrane system.
The organelles in the endomembrane system work together to carry out the functions noted. Plant cells do not have lysosomes.
Check Your Progress
Compare and contrast the structure and function of chloroplasts with those of mitochondria.
Answer:Structure: The two main parts of a chloroplast are the thylakoids and the stroma; the two main parts of a mitochondrion are the cristae and the matrix. Function: Chloroplasts are larger than mitochondria and capture energy from the sun to build carbohydrates. Mitochondria break down carbohydrates to release energy for ATP production.
Figure 4.13Vacuoles.
a. Contractile vacuoles of a protist. b. Plant vacuole. 
Figure 4.14Chloroplast structure.
a. Electron micrograph of a chloroplast, which carries out photosynthesis. b. Generalized drawing in which the outer and inner membranes have been cut away to reveal the grana, each of which is composed of a stack of membranous sacs called thylakoids. In some grana, but not all, it is obvious that the thylakoid spaces are interconnected.
Figure 4.15Mitochondrion structure.
a. Generalized drawing in which the outer membrane and portions of the inner membrane have been cut away to reveal the cristae. b. Electron micrograph of a mitochondrion, which is involved in cellular respiration. 
Figure 4.16Microtubules. 
a. Microtubules (tubulin dimer cylinders) radiate out from the centrosome. b. Motor molecules, such as kinesin shown here, change the location of an organelle by moving it along a microtubule. One end of kinesin binds to an organelle, and the other end attaches, detaches, and reattaches to the microtubule. ATP supplies the energy for movement.
Figure 4.17Actin filaments.
Actin filaments (helical actin polymers) are organized into bundles or networks just under the plasma membrane, where they lend support to the shape of a cell.
Check Your Progress
	1.	List the components of the cytoskeleton.
	2.	Explain the structure of cilia and flagella.
	3.	Give an example of a cell that has cilia and one that has flagella. Describe the functions of these cells.
Answers:1. Microtubules, intermediate filaments, and actin filaments. 2. Cilia and flagella are both composed of microtubules arranged in a particular pattern and enclosed by the plasma membrane.3. Cells lining the respiratory tract have cilia that sweep mucus and debris back up into the throat where it can be swallowed or ejected; sperm have flagella that allow them to swim to the egg.
Figure 4.18Centrioles.
A pair of centrioles lies to one side of the nucleus. Their exact function is unknown, however possibilities do exist.
Figure 4.19Cilia and flagella.
a. Cilia in the bronchial wall and the flagella of sperm are organelles capable of movement. The cilia in our bronchi sweep mucus and debris back up into the throat where it can be swallowed or ejected. The flagella of sperm allow them to swim to the egg. b. Cilia and flagella have a distinct pattern of microtubules bounded by a plasma membrane. Both have a basal body with the same pattern of microtubules as a centriole. Therefore, it is believed that centrioles give rise to basal bodies. 
Figure 4.20Plasmodesmata.
Plant cells are joined by membrane-lined channels called plasmodesmata that contain cytoplasm. Through these channels, water and small molecules can pass from cell to cell.
Figure 4.21Animal cell extracellular matrix.
The extracellular matrix supports an animal cell and also affects its behavior. 
Figure 4.22Junctions between cells of the  intestinal wall.
a. In adhesion junctions, intercellular filaments run between two cells. b. Tight junctions between cells form an impermeable barrier because their adjacent plasma membranes are joined. c. Gap junctions allow communication between two cells because adjacent plasma membrane channels are joined.
Check Your Progress
	1.	List the components of the nucleus and give a function  for each.
	2.	 Where are ribosomes found in the cell, and what do they do? 
Answers:1. nuclear envelope—defines the nucleus; nuclear pore—allows substances to move into and out of nucleus; nucleolus—formation of ribosomal RNA (rRNA); chromatin—becomes chromosomes and contains DNA.2. Ribosomes are found attached to the ER and in the cytoplasm. In the cytoplasm, they occur either singly or as polyribosomes. Ribosomes carry out protein synthesis. 
Check Your Progress
	1.	Contrast a plant’s primary cell wall with its secondary cell wall.
	2.	 Describe the composition of the extracellular matrix of an animal cell.
	3.	Compare and contrast adhesion junctions with tight junctions.
	4.	Describe the functions of gap junctions.
Answers:1. All plant cells have a primary wall, which is capable of stretching during growth and is composed of cellulose and other compounds. The secondary cell wall is formed in woody tissues and contains lignin in addition to cellulose. The lignin adds rigidity and strength to the cell wall.2. The extracellular matrix is composed of polysaccharides and proteins. In addition, the matrix in bone contains mineral salts.3. Both types of junctions hold adjacent animal cells together. Adhesion junctions are more flexible than tight junctions.4. Gap junctions provide strength to adjacent cells, but also allow molecules and ions to pass between them.
Comparison of Bacterial, Animal, 
and Plant Cells
	Bacteria	Animal	Plant
Cell wall	Yes	No	Yes 
Plasma membrane	Yes	Yes	Yes
Nucleus	No	Yes	Yes
Nucleolus	No	Yes	Yes
Ribosomes	Yes	Yes	Yes
Endoplasmic reticulum	No	Yes	Yes
Golgi apparatus	No	Yes	Yes
Lysosomes	No	Yes	No
Mitochondria	No	Yes	Yes
Chloroplasts	No	No	Yes
Cytoskeleton	No	Yes	Yes
Centrioles	No	Yes	No
9  2 cilia or flagella	No	Often	Some
Red blood cells coursing through a blood vessel.
A cell caught in the act of committing suicide, a necessary part of development.
A bone cell encased within bone.
An egg cell protected within an ovarian follicle.
A nerve cell within the brain.
Cone cell in eye accounts for color vision.
Check Your Progress
	1.	 Contrast rough endoplasmic reticulum with smooth endoplasmic reticulum.
	2.	Describe the relationship between the components of the endomembrane system.
Answers:1. Rough ER contains ribosomes, while smooth ER does not. Rough ER synthesizes proteins and modifies them, while smooth ER synthesizes lipids, among other activities.2. Transport vesicles from the ER proceed to the Golgi apparatus. The Golgi apparatus modifies their contents and repackages them in new vesicles, some of which carry out secretion, and some of which are lysosomes.