Chapter 2--Cystology

name the stages of mitosis and describe the events that occur in each one

Cells divide by two mechanisms called mitosis and meiosis. Meiosis, however, is restricted to one purpose, the production of eggs and sperm, and is therefore treated in chapter 26 on reproduction. Mitosis serves all the other functions of cell division: the development of an individual, composed of some 40 trillion cells, from a one-celled fertilized egg; continued growth of all the organs after birth; the replacement of cells that die; and the repair of damaged tissues. Four phases ofmitosis are recognizable—prophase, metaphase, anaphase, and telophase (fig. 2.23). In prophase,41 at the outset of mitosis, the chromosomes coil into short, compact rods that are easier to distribute to daughter cells than the long, delicate chromatin of interphase. At this stage, a chromosome consists of two genetically identical bodies called chromatids, joined together at a pinched spot called the centromere (fig. 2.24). There are 46 chromosomes, two chromatids per chromosome, and one molecule of DNA in each chromatid. The nuclear envelope disintegrates during prophase and releases the chromosomes into the cytosol. The centrioles begin to sprout elongated microtubules called spindle fibers, which push the centrioles apart as they grow. Eventually, a pair of centrioles comes to lie at each pole of the cell. Spindle fibers grow toward the chromosomes, where some of them become attached to a platelike protein complex called the kinetochore42 (kih-NEE-toe-core) on each side of the centromere. The spindle fibers then tug the chromosomes back and forth until they line up along the midline of the cell.In metaphase,43 the chromosomes are aligned on the cell equator, oscillating slightly and awaiting a signal that stimulates each chromosome to split in two at the centromere. The spindle fibers now form a lemon-shaped array called the mitotic spindle. Long microtubules reach out from each centriole to the chromosomes, and shorter microtubules form a starlike aster44 that anchors the assembly to the inside of the plasma membrane at each end of the cell. Anaphase45 begins with activation of an enzyme that cleaves the two sister chromatids from each other at the centromere. Each chromatid is now regarded as a separate, single-stranded daughter chromosome. One daughter chromosome migrates to each pole of the cell, with its centromere leading the way and the arms trailing behind. Migration is achieved by means of motor proteins in the kinetochore crawling along the spindle fiber as the fiber itself is "chewed up" and disassembled at the chromosomal end. Since sister chromatids are genetically identical and since each daughter cell receives one chromatidfrom each chromosome, the daughter cells of mitosis are genetically identical. In telophase,46 the chromatids cluster on each side of the cell. The rough ER produces a new nuclear envelope around each cluster, and the chromatids begin to uncoil and return to the thinly dispersed chromatin form. The mitotic spindle breaks up and vanishes. Each new nucleus forms nucleoli, indicating it has already begun making RNA and preparing for protein synthesis. Telophase is the end of nuclear division but overlaps with cytokinesis47 (SY-toe-kih-NEE-sis), division of the cytoplasm into two cells. Early traces of cytokinesis appear even in anaphase. It is achieved by the motor protein myosin pulling on microfilaments of actin in the terminal web. This creates a crease called the cleavage furrow around the equator of the cell, and the cell eventually pinches in two. Interphase has now begun for these new cells.

interior structures of the cell

classified into three groups—cytoskeleton, organelles, and inclusions—all embedded in the clear, gelatinous cytosol. If we think of a cell as being like an office building, the cytoskeleton would be like the steel beams and girders that hold it up and define its shape and size; the plasma membrane and its channels would be the building's exterior walls and doors; and organelles would be the interior rooms that divide the building into compartments with different functions.

describe the processes for moving material into and out of a cell

three methods of movement through plasma membranes, as well as filtration, an important mode of transport across the walls of certain blood vessels. -Filtration = process in which a physical pressure forces fluid through a membrane, like the weight of water forcing it through the paper filter in a drip coffeemaker. In the body, the prime example of filtration is blood pressure forcing fluid to seep through the walls of the blood capillaries into the tissue fluid. This is how water, salts, organic nutrients, and other solutes pass from the bloodstream to the tissue fluid, where they can get to the cells surrounding a blood vessel. This is also how the kidneys filter wastes from the blood. Capillaries hold back large particles such as blood cells and proteins. In most cases, water and solutes filter through narrow gaps between the capillary cells. In some cases, however, the cells have large filtration pores through them, like the holes in a slice of Swiss cheese, allowing for more rapid filtration of large solutes such as protein hormones. -Simple diffusion = net movement of particles from a place of high concentration to a place of lower concentration—in other words, down a concentration gradient. Diffusion is how oxygen and steroid hormones enter cells and potassium ions leave, for example. The cell does not have to expend any energy to achieve this; all molecules are in spontaneous random motion, and this alone provides the energy for their diffusion through space. Molecules diffuse through air, liquids, and solids. They can penetrate both living membranes (the plasma membrane) and nonliving ones (such as dialysis tubing and cellophane) if the membrane has large enough gaps or pores. We say that the plasma membrane is selectively permeable because it lets some particles through but holds back larger ones. Nonpolar solutes such as oxygen and carbon dioxide, and hydrophobic substances suchas steroids, diffuse through the lipid regions of the plasma membrane; hydrophilic solutes such as salts, however, can diffuse only through the water-filled protein channels of the membrane. -facilitated diffusion and active transport are called carrier-mediated transport because they employ transport proteins in the plasma membrane. -Facilitated diffusion = the movement of a solute through a membrane, down its concentration gradient, with the aid of a carrier. The carrier transports solutes such as glucose that cannot pass through the membrane unaided. It binds to a particle on one side of a membrane, where the solute is more concentrated, and releases it on the other side, where it is less concentrated. The process requires no expenditure of metabolic energy by the cell. One use of facilitated diffusion is to absorb the sugars and amino acids from digested food. -active transport = the carrier-mediated transport of a solute through a unit membrane up its concentration gradient, with the expenditure of energy provided by adenosine triphosphate (ATP). ATP is essential to this process because moving particles up a gradient requires an energy input, like getting a wagon to roll uphill. If a cell dies and stops producing ATP, active transport ceases immediately. One use of active transport is to pump calcium out of cells. Calcium is already more concentrated in the ECF than in the ICF, so pumping even more calcium into the ECF is an "uphill" movement. An especially well-known active transport process is the sodium-potassium (Na+−K+) pump, which binds three sodium ions from the ICF and ejects them from the cell, then binds two potassium ions from the ECF and releases these into the cell. The Na+−K+ pump plays roles in controlling cell volume; generating body heat; maintaining the electrical excitability of your nerves, muscles, and heart; and providing energy for other transport pumps to draw upon in moving such solutes as glucose through the plasma membrane. About half of the calories that you "burn" every day are used just to operate your Na+−K+ pumps. -Osmosis = net flow of water through a selectively permeable membrane from the "more watery" side (the one with less dissolved matter) to the "less watery" side (with more dissolved matter). Water molecules tend to cling to particles of dissolved matter and resist going back through the membrane in the opposite direction—hence the net accumulation of water on the side with more solute. An exception to this is reverse osmosis, in which a physical force drives water back to the more dilute side of the membrane. This principle is used to desalinate seawater, converting it to drinkable freshwater, in arid regions and ships at sea. In the human body, the force generated by the heartbeat causes reverse osmosis at the blood capillaries, driving water out of the blood and into the tissue fluid; but where the blood pressure is lower, capillaries absorb tissue fluid by osmosis. Many cells have membrane channel proteins called aquaporins that allow water to pass easily through the membrane. Imbalances in osmosis underlie such problems as diarrhea, constipation, and edema. Osmosis is also a vital consideration in intravenous fluid therapy and kidney dialysis. -vesicular transport = All of the processes discussed up to this point move molecules or ions individually through the plasma membrane. In vesicular transport, however, cells move much larger particles or droplets of fluid through the membrane in bubblelike vesicles. Vesicular processes that bring matter into a cell are called endocytosis12 (EN-doe-sy-TOE-sis), and those that release material from a cell are called exocytosis13 (EC-so-sy-TOE-sis). Like active transport, all forms of vesicular transport require ATP. There are three forms of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis (fig. 2.11). Pinocytosis14 (PIN-oh-sy-TOE-sis), or "cell drinking," occurs in all human cells. In this process, dimples form in the plasma membrane and progressively sink in until they pinch off as pinocytotic vesicles containing droplets of ECF (fig. 2.11a). Kidney tubule cells use this method to reclaim the small amount of protein that filters out of the blood, thus preventing the protein from being lost in the urine. Receptor-mediated endocytosis (fig. 2.11b) is more selective. It enables a cell to take in specific molecules from the ECF with a minimum of unnecessary fluid. Molecules in the ECF bind to specific receptor proteins on the plasma membrane. The receptors then cluster together and the membrane sinks in at this point, creating a pit. The pit soon pinches off to form a vesicle in the cytoplasm. Cells use receptor-mediated endocytosis to absorb cholesterol and insulin from the blood. Hepatitis, polio, and AIDS viruses trick our cells into admitting them by receptor-mediated endocytosis. In phagocytosis15 (FAG-oh-sy-TOE-sis), or "cell eating," a cell reaches out with footlike extensions called pseudopods (see fig. 2.14), surrounds a particle such as a bacterium or a bit of cell debris, and engulfs it, taking it into a cytoplasmic vesicle called a phagosome to be digested. Phagocytosis is carried out especially by white blood cells and macrophages, which are described in chapter 3. Exocytosis (fig. 2.11c) is the process of discharging material from a cell. It is used, for example, by digestive glands to secrete enzymes, by breast cells to secrete milk, and by sperm cells to release enzymes for penetrating an egg. It resembles endocytosis in reverse. A secretory vesicle in the cell migrates to the surface and fuses with the plasma membrane. A pore opens up that releases the products from the cell, and the empty vesicle usually becomes part of the plasma membrane. In addition to releasing cell products, exocytosis is the cell's way of replacing the bits of membrane removed by endocytosis.

state some tenets of the cell theory

-every living organism is made of cells -cells arise only through the division of preexisting cells rather than springing spontaneously from nonliving matter -all cells have the same basic chemical components, such as carbohydrates, lipids, proteins, and nucleic acids. -Cells are the simplest entities considered to be alive; no one molecule such as DNA or an enzyme is alive in itself.

identify cell shapes from their descriptive terms

• Squamous2 (SQUAY-mus)—a thin, flat, scaly shape, often with a bulge where the nucleus is—much like the shape of a fried egg "sunny side up." Squamous cells line the esophagus and form the surface layer (epidermis) of the skin. • Cuboidal3 (cue-BOY-dul)—squarish-looking in frontal tissue sections and about equal in height and width; liver cells are a good example. • Columnar—distinctly taller than wide, such as the inner lining cells of the stomach and intestines. • Polygonal4—having irregularly angular shapes with four, five, or more sides. Cells that look cuboidal or columnar in frontal view are commonly polygonal in an end view, like a quartz crystal. • Stellate5—having multiple pointed processes projecting from the body of a cell, giving it a somewhat starlike shape. The cell bodies of many nerve cells are stellate. • Spheroidal to ovoid—round to oval, as in egg cells and white blood cells. • Discoid—disc-shaped, as in red blood cells. • Fusiform6 (FEW-zih-form)—spindle- or toothpick-shaped; elongated, with a thick middle and tapered ends, as in smooth muscle cells. • Fibrous—long, slender, and threadlike, as in skeletal muscle cells and the axons (nerve fibers) of nerve cells. In some cells, it is important to distinguish one surface from another, because cell surfaces may differ in function and membrane composition.

discuss the types and clinical uses of stem cells

One of the most controversial scientific issues in the last several years has been stem-cell research. Stem cells are immature cells with the ability to develop into one or more types of mature, specialized cells. Their ability to give rise to a diversity of mature cell types is called developmental plasticity. Adult stem (AS) cells exist in most of the body's organs. Despite the name, they are not limited to adults, but are found also in fetuses, infants, and children. They multiply and replace older cells that are lost to damage or normal cellular turnover. Some AS cells are unipotent, able to develop into only one mature cell type, such as the cells that develop into sperm or epidermal squamous cells. Some are multipotent, able to differentiate into multiple mature cell types, such as certain bone marrow cells that give rise to multiple types of white blood cells. Embryonic stem (ES) cells comprise human embryos (technically, preembryos; see chapter 4) of up to 150 cells. They are pluripotent, able to develop into any type of embryonic or adult cell. ES cells are easily obtained from the excess embryos created in fertility clinics when a couple attempts to conceive a child by in vitro fertilization (IVF). In IVF, eggs are fertilized in glassware and allowed to develop to about 8 to 16 cells. Some of these are then transplanted into the mother's uterus. Excess embryos are created to compensate for the low probability of success. Those that are not transplanted to the uterus are usually destroyed, but they present a potential source of stem cells for research and therapy. Skin and bone marrow adult stem cells have been used in therapy for many years. There is hope that stem cells can be manipulated to replace a broader range of tissues, such as injured spinal cords, cardiac muscle damaged by a heart attack, brain tissue lost in Parkinson and Alzheimer diseases, or the insulin-secreting cells needed by people with diabetes mellitus. Adult stem cells, however, seem to have limited developmental potential and to be unable to produce all cell types needed to treat a broad range of diseases. In addition, they are present in very small numbers and are difficult to isolate and culture in the quantities needed for therapy. Embryonic stem cells are easier to obtain and culture and have more developmental flexibility, but their use remains embroiled in political, religious, and ethical debate. Some would argue that if the excess embryos from IVF are destined to be destroyed, it would seem sensible to use them for beneficial purposes. Others argue, however, that potential medical benefits cannot justify the destruction of a human embryo or even a preembryo of scarcely more than 100 cells. Further scientific advances, however, are beginning to defuse the controversy to some degree. Cell biologists have developed methods to make adult stem cells reverse course on their developmental paths, going back to a pluripotent state that would enable them to go down new roads. In principle, an AS cell that was originally destined to become smooth muscle, for example, might be made to revert to a pluripotent state, then chemically guided down a path leading to nervous tissue for Alzheimer patients or persons paralyzed by spinal cord injury. Such modified adult cells, now called induced pluripotent stem (iPS) cells, have begun to show great promise for both medical benefit and reduced controversy.

list the main organelles of a cell and explain their functions

The minute, metabolically active structures within a cell are called organelles (literally, "little organs") because they are to the cell what organs are to the body—structures that play individual physiological roles in the survival of the whole (see fig. 2.5). A cell may have 10 billion protein molecules, some of which are powerful enzymes with the potential to destroy the cell if they are not contained and isolated from other cellular components. You can imagine the enormous problem of keeping track of all this material, directing molecules to the correct destinations, and maintaining order against the incessant tug of disorder. Cells maintain order partly by compartmentalizing their contents in organelles. -nucleus = The nucleus (fig. 2.18) is the largest organelle and usually the only one clearly visible with the light microscope. It contains the cell's chromosomes and is therefore the genetic control center of cellular activity. Granular organelles called ribosomes are produced here, and the early steps in protein synthesis occur here under the direction of the genes. Most cells have only one nucleus,but there are exceptions. Mature red blood cells have none; they are anuclear.25 A few cell types are multinucleate—having 2 to 50 nuclei—including some liver cells, skeletal muscle cells, and certain bone-dissolving cells. The nucleus is usually spheroidal to elliptical in shape and averages about 5 µm in diameter. It is surrounded by a nuclear envelope consisting of two parallel membranes. The envelope is perforated with nuclear pores, about 30 to 100 nm in diameter, formed by a ring-shaped nuclear pore complex of eight proteins. These proteins regulate molecular traffic into and out of the nucleus and bind the two membranes together. The inside of the nuclear envelope is lined by a web of intermediate filaments, called the nuclear lamina, like a cage enclosing the DNA. The material within the nucleus is called the nucleoplasm. Most of it consists of finely dispersed, granular filaments called chromatin (CRO-muh-tin), visible only with the TEM. Chromatin is composed of a complex of proteins and DNA, the latter constituting the cell's genes. When cells prepare to divide, the chromatin coils and condenses into short thick rods visible with the LM, and we can see that it consists of (usually) 46 separate bodies, the chromosomes26 (see fig. 2.24). The nuclei of nondividing cells also usually exhibit one or more dense masses called nucleoli (singular, nucleolus), where subunits of the ribosomes are made before they are transported out to the cytoplasm. -ER = The term endoplasmic reticulum (ER) literally means "little network within the cytoplasm." It is a system of interconnected channels called cisternae27 (sis-TUR-nee) enclosed by a membrane (fig. 2.19a). In areas called rough endoplasmic reticulum, the network consists of parallel, flattened cisternae covered with ribosomes, which give it its rough or granular appearance. The rough ER is continuous with the outer membrane of the nuclear envelope (see fig. 2.18), and adjacent cisternae are connected by perpendicular bridges. In areas called smooth endoplasmic reticulum, the membrane lacks ribosomes, the cisternae are more tubular in shape, and they branch more extensively. The cisternae of the smooth ER are continuous with those of the rough ER, so the two are different parts of the same cytoplasmic network. The endoplasmic reticulum synthesizes steroids and other lipids, detoxifies alcohol and other drugs, and manufactures all of the membranes of the cell. Rough ER produces the phospholipids and proteins of the plasma membrane. It also synthesizes proteins that are either secreted from the cell or packaged in organelles called lysosomes. Rough ER is most abundant in cells that synthesize large amounts of protein, such as antibody-producing cells and cells of the digestive glands. Most cells have only a scanty smooth ER, but it is relatively abundant in cells that engage extensively in detoxification, such as liver and kidney cells. Long-term abuse of alcohol, barbiturates, and other drugs leads to tolerance partly because the smooth ER proliferates and detoxifies the drugs more quickly. Smooth ER is also abundant in cells that synthesize steroid hormones, for example in the testes and ovaries. Skeletal and cardiac muscle contain extensive networks of modified smooth ER called sarcoplasmic reticulum, which releases calcium to trigger muscle contraction and stores the calcium between contractions. -ribosomes = Ribosomes are small granules of protein and ribonucleic acid (RNA) found in the cytosol, on the outer surfaces of the rough ER and nuclear envelope, in the nucleoli, and in mitochondria. Ribosomes "read" coded genetic messages (messenger RNA) from the nucleus and assemble amino acids into proteins specified by the code. The unattached ribosomes found scattered throughout the cytoplasm make enzymes and other proteins for use within the cell. The ribosomes attached to the rough ER make proteins that will either be packaged in lysosomes or, in cases such as digestive enzymes, secreted from the cell. -golgi complex = The Golgi28 (GOAL-jee) complex is a small system of cisternae that synthesizes carbohydrates and certain lipids and puts the finishing touches on protein and glycoprotein synthesis (fig. 2.19b). The complex resembles a stack of pita bread. Typically, it consists of about six cisternae, slightly separated from each other, each of them a flattened, slightly curved sac with a swollen rim. Figure 2.20 shows the functional interaction between the ribosomes, endoplasmic reticulum, and Golgi complex. Ribosomes link amino acids together in a genetically specified order to make a particular protein. This new protein threads its way into the cisterna of the rough ER, where enzymes trim and modify it. The altered protein is then shuffled into a little transport vesicle, a spheroidal organelle that buds from the ER and carries the protein to the nearest cisterna of the Golgi complex. The Golgi complex sorts these proteins, passes them along from one cisterna to the next, cuts and splices some of them, adds carbohydrates to some of them, and finally packages the proteins in membrane-bounded Golgi vesicles. These vesicles bud off the rim of the cisterna farthest from the ER, like the warm wax globules of a lava lamp. They are seen in abundance in the neighborhood of the Golgi complex. Some Golgi vesicles become lysosomes, discussed shortly; some migrate to the plasma membrane and fuse with it, contributing fresh protein and phospholipid to the membrane; and some become secretory vesicles that store a cell product, such as breast milk protein, mucus, or digestive enzymes, for later release by exocytosis. -proteasomes = Ribosomes manufacture numerous proteins for intracellular use, but these proteins cannot linger in the cell forever. Whentheir job is done, they must be disposed of. Cells also need to rid themselves of damaged and nonfunctional proteins and foreign proteins introduced by such means as viral infections. The job of protein disposal is handled by organelles called proteasomes, which are themselves composed of hollow cylindrical complexes of protein (fig. 2.19c). The cell tags undesirable proteins for destruction and transports them to a proteasome; enzymes of the proteasome unwind them and break them down into amino acids. Proteasomes degrade more than 80% of a cell's proteins. -lysosomes = Another organelle designed for destroying things is the lysosome29 (LY-so-some) (fig. 2.19d), a package of enzymes enclosed in a membrane. Although often round or oval, lysosomes are extremely variable in shape. When viewed with the TEM, they often exhibit dark gray contents devoid of structure, but sometimes show crystals or parallel layers of protein. At least 50 lysosomal enzymes have been identified. They break down proteins, nucleic acids, carbohydrates, phospholipids, and other substances. White blood cells called neutrophils phagocytize bacteria and digest them with the enzymes of their lysosomes. Lysosomes also digest and dispose of surplus, nonvital, and worn-out organelles; this process is called autophagy30 (aw-TOFF-uh-jee). They also aid in a process of "cell suicide" called apoptosis (AP-op-TOE-sis), or programmed cell death, in which cells that are no longer needed undergo a prearranged death. The uterus, for example, weighs about 900 g at full-term pregnancy and, through apoptosis, shrinks to 60 g within 5 or 6 weeks after birth. -Peroxisomes = resemble lysosomes but contain different enzymes and are not produced by the Golgi complex. Their general function is to use molecular oxygen (O2) to oxidize organic molecules, especially to break down fatty acids into two-carbon molecules that can be used as an energy source for ATP synthesis. Such reactions produce hydrogen peroxide (H2O2), hence the name of the organelle. H2O2 is then used to oxidize other molecules, and the excess is broken down to water and oxygen by an enzyme called catalase. Peroxisomes also neutralize free radicals and detoxify alcohol, other drugs, and a variety of blood-borne toxins. Peroxisomes occur in nearly all cells but are especially abundant in the liver and kidneys. -Mitochondria = (singular, mitochondrion) are organelles specialized for a process called aerobic respiration, which synthesizes most of the body's ATP. They have a variety of shapes: spheroidal, rod-shaped, bean-shaped, or threadlike (fig. 2.19e), and they constantly move, squirm, and change shape. Like the nucleus, a mitochondrion is surrounded by a double membrane. The inner membrane usually has folds called cristae32 (CRIS-tee), which project like shelves across the organelle and bear the enzymes that produce most of the ATP. The space between the cristae is called the mitochondrial matrix. It contains enzymes, ribosomes, and a small, circular DNA molecule called mitochondrial DNA (mtDNA), which is genetically different from the DNA in the cell's nucleus (see Deeper Insight 2.2). Mutations in mtDNA are responsible for some muscle, heart, and eye diseases. -Centrioles = A centriole (SEN-tree-ole) is a short cylindrical assembly of microtubules arranged in nine groups of three microtubules each (fig. 2.19f). Near the nucleus, most cells have a small, clear patch of cytoplasm called the centrosome33 containing a pair of mutually perpendicular centrioles (see fig. 2.5). These centrioles play a role in cell division described later. In ciliated cells, each cilium also has a basal body composed of a single centriole perpendicular to the plasma membrane. Two microtubules of each triplet form the peripheral microtubules of the axoneme of the cilium.

describe the structure and function of microvilli, cilia, flagella, pseudopods, and cell junctions (these all are cell surface extensions that aid in absorption, movement, and sensory processes)

-microvilli:Microvilli16 (MY-cro-VIL-eye; singular, microvillus) are extensions of the plasma membrane that serve primarily to increase its surface area (fig. 2.12). They are best developed in cells specialized for absorption, such as the epithelial cells of the intestines and kidney tubules. The small intestine has about 200 million microvilli per square millimeter, with about 3,000 on the surface of each absorptive cell. They give such cells far more absorptive surface area than they would have if their apical surfaces were flat. On many cells, microvilli are little more than tiny bumps on the plasma membrane. On cells of the taste buds and inner ear, they are well developed but serve sensory rather than absorptive functions. Individual microvilli cannot be distinguished very well with the light microscope because they are only 1 to 2 µm long. On some cells, they are very dense and appear as a fringe called the brush border. With the scanning electron microscope, they resemble a deep-pile carpet. With the transmission electron microscope, microvilli typically look like finger-shaped projections of the cell surface. They show little internal structure, but often have a bundle of stiff supportive filaments of a protein called actin. Actin filaments attach to the inside of the plasma membrane at the tip of the microvillus; at its base, they extend a little way into the cell and anchor the microvillus to a protein mesh called the terminal web. When tugged by another protein in the cytoplasm, actin can shorten a microvillus to milk its absorbed contents downward into the cell. -cilia:Cilia (SIL-ee-uh; singular, cilium17) are hairlike processes about 7 to 10 µm long. Nearly every cell has a solitary, nonmotile primary cilium a few micrometers long. Its function in some cases is still a mystery, but apparently many of them are sensory, serving as the cell's "antenna" for monitoring nearby conditions. The light-absorbing parts of the retinal cells in the eye are modified primary cilia; in the inner ear, they play a role in the senses of motion and balance; and in kidney tubules, they are thought to monitor fluid flow. Odor molecules bind to nonmotile cilia on the sensory cells of the nose. Defects in the development, structure, or function of cilia—especially the nonmotile primary cilia—are responsible for several hereditary diseases called ciliopathies.Motile cilia occur in only a few organs, mainly in the respiratory tract, uterine (fallopian) tubes, internal cavities of the brain and spinal cord, and some male reproductive ducts. However, they are very abundant where they do occur; ciliated cells typically have 50 to 200 cilia each (fig. 2.13a). These cilia beat in synchronized waves that sweep across the surface of an epithelium, always in the same direction, moving substances such as fluid, mucus, and egg cells. Cilia possess a central core called the axoneme18 (ACK-so-neem), an orderly array of thin protein cylinders called microtubules. In motile cilia, there are two central microtubules surrounded by a ring of nine microtubule pairs arranged like a Ferris wheel (fig. 2.13b-d). The central microtubules stop at the cell surface, but the peripheral microtubules continue a short distance into the cell as part of a basal body that anchors the cilium. In each pair of peripheral microtubules, one tubule has paired dynein (DINE-een) arms along its length. Dynein,19 a motor protein, uses energy from ATP to "crawl" up the adjacent pair of microtubules. When microtubules on the front of the cilium crawl up the microtubules behind them, the cilium bends toward the front. Nonmotile primary cilia lack the two central microtubules and the dynein arms. -flagella: There is only one functional flagellum20 (fla-JEL-um) in humans—the whiplike tail of a sperm. It is much longer than a cilium and has an identical axoneme, but between the axoneme and plasma membrane it also has a complex sheath of coarse cytoskeletal filaments that stiffen the tail and give it more propulsive power. -pseudopods:Pseudopods21 (SOO-do-pods) are cytoplasm-filled extensions of the cell varying in shape from fine, filamentous to blunt, fingerlike processes (fig. 2.14). Unlike the other three kinds of surface extensions, they change continually. Some form anew as the cell surface bubbles outward and cytoplasm flows into a lengthening pseudopod, while others are retracted into the cell and disappear. The freshwater organism Amoeba furnishes a familiar example of pseudopods, which it uses for locomotion and food capture. White blood cells called neutrophils crawl about like amebae by means of fingerlike pseudopods, and when they encounter a bacterium or other foreign particle, they reach out with their pseudopods to surround and engulf it. Macrophages—tissue cells derived from certain white blood cells—reach out with thin filamentous pseudopods to snare bacteria and "reel them in" to be digested by the cell. Like little janitors, macrophages therefore keep our tissues cleaned up. Blood platelets reach out with pseudopods to adhere to each other and to the walls of damaged blood vessels, forming plugs that temporarily halt bleeding. -cell junctions:Also at the cell surface are certain cell junctions that link cells together and attach them to the extracellular material. Such attachments enable cells to grow and divide normally, resist stress, communicate with each other, and control the movement of substances through the gaps between cells. Without them, cardiac muscle cells would pull apart when they contracted, and every swallow of food would scrape away the lining of the esophagus. We will examine three types of junctions—tight junctions, desmosomes, and gap junctions (fig. 2.15). Each type serves a different purpose, and two or more types often occur in a single cell.

types of cell junctions

-tight junction = A tight junction completely encircles an epithelial cell near its apical surface and joins it tightly to the neighboring cells like the plastic harness on a six-pack of soda cans. At a tight junction, the plasma membranes of two adjacent cells come very close together and are linked by transmembrane cell adhesion proteins. These proteins seal off the intercellular space and make it difficult for substances to pass between the cells. In the stomach and intestines, for example, tight junctions prevent digestive juices from seeping between epithelial cells and digesting the underlying connective tissue. They also help to prevent intestinal bacteria from invading the tissues, and they ensure that most digested nutrients pass through the epithelial cells and not between them. -desmosome = A desmosome23 (DEZ-mo-some) is a protein patch that holds cells tightly together at a specific point. We can compare a tight junction and a desmosome, respectively, to a zipper and a snap on a pair of jeans. Desmosomes are not continuous and therefore cannot prevent substances from passing around them and going between cells. Theyserve to keep cells from pulling apart and thus enable a tissue to resist mechanical stress. Desmosomes are common in the epidermis, the epithelium of the uterine cervix, other epithelia, and cardiac muscle. Hooklike J-shaped proteins approach the cell surface from within and penetrate into a thick protein plaque on the inner face of the plasma membrane, and then the short arm of the J turns back into the cell—thus anchoring the plaque to the cytoskeleton. Proteins of the plaque are linked to transmembrane proteins which, in turn, are linked to transmembrane proteins of the next cell, forming a zone of strong cell adhesion. Each cell mirrors the other and contributes half of the desmosome. Such connections among neighboring cells create a strong structural network that binds cells together throughout the tissue (see Deeper Insight 2.1). The basal cells of an epithelium are similarly linked to the underlying basement membrane by half desmosomes called hemidesmosomes, so an epithelium cannot easily peel away from the underlying tissue. -gap junction = A gap (communicating) junction is formed by a connexon, which consists of six transmembrane proteins arranged in a ring, somewhat like the segments of an orange, surrounding a central water-filled channel. Ions, glucose, amino acids, and other small solutes can diffuse through the channel directly from the cytoplasm of one cell into the next. In the human embryo, nutrients pass from cell to cell through gap junctions until the circulatory system forms and takes over the role of nutrient distribution. In cardiac muscle, gap junctions allow electrical excitation to pass directly from cell to cell so that the cells contract in near unison.

glycocalyx

All of our cells are essentially sugar-coated. They have a fuzzy surface coat called the glycocalyx22 (GLY-co-CAY-licks), composed of short chains of sugars belonging to the membrane glycolipids and glycoproteins. The glycocalyx has multiple functions. It cushions the plasma membrane and protects it from physical and chemical injury, somewhat like the Styrofoam "peanuts" in a shipping carton. It functions in cell identity and thus in the body's ability to distinguish its own healthy cells from diseased cells, invading organisms, and transplanted tissues. Human blood types and transfusion compatibility are determined by the glycocalyx. The glycocalyx also includes the cell-adhesion molecules described earlier and, thus, helps to bind tissues together and enables a sperm to bind to an egg and fertilize it.

give some examples of cell inclusions and explain how inclusions differ from organelles

Inclusions are of two kinds: accumulated cell products such as pigments, fat droplets, and granules of glycogen (a starchlike carbohydrate); and internalized foreign matter such as dust, viruses, and bacteria. Inclusions are never enclosed in a membrane, and unlike the organelles and cytoskeleton, they are not essential to cell survival.

describe the life cycle of a cell

Most cells periodically divide into two daughter cells, so a cell has a life cycle extending from one division to the next. This cell cycle is divided into four main phases: G1, S, G2, and M (fig. 2.21). The first gap (G1) phase is an interval between cell division and DNA replication. During this time, a cell synthesizes proteins, grows, and carries out its predestined tasks for the body. Most human physiology pertains to what cells do in the G1 phase. Cells in G1 also accumulate the materials needed in the next phase to replicate their DNA. The synthesis (S) phase is the period in which a cell makes duplicate copies of its centrioles and all of its nuclear DNA. Each of its DNA molecules uncoils into two separate strands, and each strand acts as a template for the synthesis of the missing strand. A cell begins the S phase with 46 molecules of DNA and ends it with 92. The cell then has two identical sets of DNA molecules, which are available to be divided up between daughter cells at the next cell division. The second gap (G2) phase is a relatively brief interval between DNA replication and cell division. In G2, a cell finishes replicating its centrioles and synthesizes enzymes that control cell division. It also checks the accuracy of its DNA replication and usually repairs any errors that are detected. The mitotic (M) phase is the period in which a cell replicates its nucleus, divides its DNA into two identical sets (one per nucleus), and pinches in two to form two genetically identical daughter cells. The details of this phase are considered in the next section. Phases G1, S, and G2 are collectively called interphase—the time between M phases. The length of the cell cycle varies greatly from one cell type to another. Cultured connective tissue cells called fibroblasts divide about once a day and spend 8 to 10 hours in G1, 6 to 8 hours in S, 4 to 6 hours in G2, and 1 to 2 hours in M. Stomach and skin cells divide rapidly, bone and cartilage cells slowly, and skeletal muscle and nerve cells not at all. Some cells leave the cell cycle for a "rest" and cease to divide for days, years, or the rest of one's life. Such cells are said to be in the G0 (G-zero) phase. The balance between cells that are actively cycling and those standing by in G0 is an important factor in determining the number of cells in the body. An inability to stop cycling and enter G0 is characteristic of cancer cells (see Deeper Insight 2.3).

describe the cytoskeleton and its functions

The cytoskeleton is a network of protein filaments and tubules that structurally support a cell, determine its shape, organize its contents, direct the movement of substances within the cell, and contribute to movements of the cell as a whole. It can form a very dense supportive web in the cytoplasm (fig. 2.16). It is connected to transmembrane proteins of the plasma membrane and they, in turn, are connected to proteins external to the cell, so there is a strong structural continuity from extracellular material to the cytoplasm. Cytoskeletal elements may even connect to chromosomes in the nucleus, enabling physical tension on a cell to move nuclear contents and mechanically stimulate genetic function. The cytoskeleton is composed of microfilaments, intermediate filaments, and microtubules. Microfilaments (thin filaments) are about 6 nm thick and are made of the protein actin. They form a fibrous terminal web (membrane skeleton) on the cytoplasmic side of the plasma membrane. The lipids of the plasma membrane are spread out over the terminal web like butter on a slice of bread. The web, like the bread, provides physical support, whereas the lipids, like butter, provide a permeability barrier. It is thought that, without this support by the terminal web, the lipids would break up into little droplets and the plasma membrane would not be able to hold together. As described earlier, actin microfilaments also form the supportive cores of the microvilli and play a role in cell movement. Muscle cells especially are packed with actin, which is pulled upon by the motor protein myosin to make muscles contract. Intermediate filaments (8-10 nm in diameter) are thicker and stiffer than microfilaments. They give a cell its shape, resist stress, and participate in the junctions that attach cells to their neighbors. In epidermal cells, they are made of the tough protein keratin and occupy most of the cytoplasm. They are responsible for the strength of hair and fingernails. A microtubule (25 nm in diameter) is a hollow cylinder made of 13 parallel strands called protofilaments. Each protofilament is a long chain of globular proteins called tubulin (fig. 2.17). Microtubules radiate from the centrosome (see p. 46) and hold organelles in place, form bundles that maintain cell shape and rigidity, and act somewhat like railroad tracks to guide organelles and molecules to specific destinations in a cell. They form the ciliary and flagellar basal bodies and axonemes described earlier, and as discussed later in relation to organelles and mitosis, they form the centrioles and mitotic spindle involved in cell division. Microtubules are not permanent structures. They appear and disappear moment by moment as tubulin molecules assemble into a tubule and then suddenly break apart again to be used somewhere else in the cell (fig. 2.16a). The double and triple sets of microtubules in cilia, flagella, basal bodies, and centrioles, however, are more stable.

discuss the way that developments in microscopy have changed our view of cell structure

The most important thing about a good microscope is not magnification but resolution—the ability to reveal detail. types of microscopes: -LM/light microscope = uses visible light to produce its images. most limited in the amount of useful magnification it can produce. Light microscopes today magnify up to 1,200 times. There are several varieties of light microscopes, including the fluorescence microscope -TEM/transmission electron microscope = Resolution improves when objects are viewed with radiation of shorter wavelengths. Electron microscopes achieve higher resolution by using not visible light but beams of electrons with very short wavelength (0.005 nm). The transmission electron microscope (TEM), invented in the mid-twentieth century, is usually used to study specimens that have been sliced ultrathin with diamond knives and stained with heavy metals such as osmium, which absorbs electrons. The TEM resolves details as small as 0.5 nm and attains useful magnifications of biological material up to 600,000 times. This is good enough to see even things as small as proteins, nucleic acids, and other large molecules. Such fine detail is called cell ultrastructure. Even at the same magnifications as the LM, the TEM reveals far more detail (fig. 2.1). It usually produces two-dimensional black-and-white images, but electron photomicrographs are often colorized for instructional purposes. -SEM/scanning electron microscope = uses a specimen coated with vaporized metal (usually gold). The electron beam strikes the specimen and discharges secondary electrons from the metal coating. These electrons then strike a fluorescent screen and produce an image. The SEM yields less resolution than the TEM and is used at lower magnification, but it produces dramatic three-dimensional images that are sometimes more informative than TEM images, and it does not require that the specimen be cut into thin slices. The SEM can view only the surfaces of specimens; it does not see through an object as the LM or TEM does. Cell interiors can be viewed, however, by a freeze-fracture method in which a cell is frozen, cracked open, coated with gold vapor, and then viewed by either TEM or SEM. Figure 2.2 compares red blood cells photographed with the LM, TEM, and SEM.

describe the structure of the plasma membrane

The plasma membrane defines the boundaries of the cell, governs its interactions with other cells, and controls the passage of materials into and out of the cell. The side that faces the cytoplasm is the intracellular face of the membane, and the side that faces outward is the extracellular face. A great deal of human physiology takes place at the cell surface—for example, the binding of signaling molecules such as hormones, the stimulation of cellular activity, the attachment of cells to each other, and the transport of materials into and out of cells. -an oily, two-layered lipid film with proteins embedded in it -By weight, it is about half lipid and half protein. Since the lipid molecules are smaller and lighter, however, they constitute about 90% to 99% of the molecules in the membrane -About 75% of the membrane lipid molecules are phospholipids = consists of a three-carbon backbone called glycerol, with fatty acid tails attached to two of the carbons and a phosphate-containing head attached to the third. The two fatty acid tails are hydrophobic8 (water-repellent) and the head is hydrophilic9 (attracted to water)The phospholipids are not stationary but highly fluid—drifting laterally from place to place, spinning on their axes, and flexing their tails. -About 20% of the lipid molecules are cholesterol. Cholesterol has an important impact on the fluidity of the membrane. If there is too little cholesterol, plasma membranes become excessively fragile. People with abnormally low cholesterol levels suffer an increased incidence of strokes because of the rupture of fragile blood vessels. On the other hand, excessively high concentrations of cholesterol in the membrane can inhibit the action of its enzymes and other proteins. -The remaining 5% of the lipids are glycolipids = phospholipids with short carbohydrate chains bound to them. Glycolipids occur only on the extracellular face of the membrane. They contribute to the glycocalyx, a sugary cell coating discussed later. -An important quality of the plasma membrane is its capacity for self-repair.

state the size range of human cells and explain why cell size is limited

The smallest objects most people can see with the naked eye are about 100 µm, which is about one-quarter the size of the period at the end of this sentence. A few human cells fall within this range, such as the egg cell and some fat cells, but most human cells are about 10 to 15 µm wide. The longest human cells are nerve cells (sometimes over a meter long) and muscle cells (up to 30 cm long), but both are usually too slender to be seen with the naked eye. There are several factors that limit the size of cells. If a cell swells to excessive size, it ruptures like an overfilled water balloon. In addition, cell size is limited by the relationship between its volume and surface area. The surface area of a cell is proportional to the square of its diameter, while volume is proportional to the cube of diameter. Thus, for a given increase in diameter, cell volume increases much faster than surface area. In short, a cell that is too big cannot support itself.Also, if a cell were too large, molecules could not diffuse from place to place fast enough to support its metabolism. Having organs composed of many small cells instead of fewer large ones has another advantage: The death of one or a few cells is of less consequence to the structure and function of the whole organ.

outline the major structural components of a cell

There are about 200 kinds of cells in the human body, with a variety of shapes, sizes, and functions. -plasma membrane = (cell membrane) forms the cell's surface boundary. The material enclosed by the plasma membrane is the cytoplasm,7 and the material within the nucleus (usually the cell's largest organelle) is the nucleoplasm. -cytoplasm (containing the following):contains the cytoskeleton, a supportive framework of protein filaments and tubules; an abundance of organelles, diverse structures that perform various metabolic tasks for the cell; and inclusions, which are foreign matter or stored cell products. The cytoskeleton, organelles, and inclusions are embedded in a clear gel called the cytosol. -cytoskeleton -organelles & nucleus -inclusions -cytosol The cytosol is also called the intracellular fluid (ICF). All body fluids not contained in the cells are collectively called the extracellular fluid (ECF). The ECF located between the cells is also called tissue (interstitial) fluid. Some other extracellular fluids include blood plasma, lymph, and cerebrospinal fluid.

explain the functions of the lipid, protein, and carbohydrate components of the plasma membrane

membrane proteins: -Proteins constitute from 1% to 10% of the membrane molecules. They fall into two broad classes called integral and peripheral proteins. Integral proteins penetrate at least partially into the phospholipid bilayer, and if they pass all the way through, they are also called transmembrane proteins. They have hydrophilic regions in contact with the cytoplasm and extracellular fluid, and hydrophobic regions that pass back and forth through the membrane lipid like a thread through fabric (fig. 2.8). Most of the transmembrane proteins are glycoproteins, which, like glycolipids, have carbohydrate chains linked to them and help form the glycocalyx. Peripheral proteins are those that do not protrude into the phospholipid layer but adhere to either face of the membrane, usually the intracellular face. Some transmembrane proteins drift about freely in the plasma membrane, while others are anchored to the cytoskeleton and thus held in one place. Most peripheral proteins are anchored to the cytoskeleton and associated with transmembrane proteins. • Receptors (fig. 2.9a). Cells communicate with each other by chemical signals such as hormones and neurotransmitters. Some of these messengers (epinephrine, for example) cannot enter their target cells but can only "knock on the door" with their message. They bind to a membrane protein called a receptor, and the receptor triggers physiological changes inside the cell. Some of these have a dual function as receptor and transport proteins—they bind chemicals from the extracellular fluid and transport them into the cell. Others function as both receptors and gated channels, binding a chemical messenger and opening to allow ions into or out of the cell through the receptor protein itself. This is the mechanism by which nerve cells stimulate muscle fibers to contract. • Enzymes (fig. 2.9b). Some membrane proteins are enzymes that carry out chemical reactions at the cell surface. Some of these break down chemical messengers after the message has been received. Enzymes in the plasma membranes of intestinal cells carry out the final stages of starch and protein digestion. • Channel proteins (fig. 2.9c). Some membrane proteins have tunnels through them that allow water and hydrophilic solutes to enter or leave a cell. These are called channel proteins. Some channels are always open, whereas others, called gates or gated channels (fig. 2.9d), open or close when they are stimulated and thus allow things to enter or leave the cell only at appropriate times. Membrane gates are responsible for firing of the heart's pacemaker, muscle contraction, and most of our sensory processes, among other functions. • Transport proteins (see fig. 2.10c, d). Some membrane proteins, called transport proteins (carriers), don't merely open to allow substances through—they actively bind to a substance on one side of the membrane and release it on the other side. Carriers are responsible for transporting glucose, amino acids, sodium, potassium, calcium, and many other substances into and out of cells. • Cell-identity markers (fig. 2.9e). The glycoproteins and glycolipids of the membrane are like genetic identification tags, unique to an individual (or to identical twins). They enable the body to distinguish what belongs to it from what does not—especially from foreign invaders such as bacteria and parasites. • Cell-adhesion molecules (fig. 2.9f). Cells adhere to each other and to extracellular material through membrane proteins called cell-adhesion molecules (CAMs). With few exceptions (such as blood cells and metastasizing cancer cells), cells do not grow or survive normally unless they are mechanically linked to the extracellular material. Special events such as sperm-egg binding and the binding of an immune cell to a cancer cell also require CAMs.