ANAPHY CHAP 3 NOTES PDF

ANAPHY CHAP 3 NOTES body chem- istry appears in Chapter 2.) The cell is the structural and functional unit Strange as it may seem, especially when we of the human body....

ANAPHY CHAP 3 NOTES body chem- istry appears in Chapter 2.) The cell is the structural and functional unit Strange as it may seem, especially when we of the human body. feel our rm muscles, living cells are about 60 fi The many types of tissues formed by cells percent water, which is one of the reasons allow for the division of labor among the water is essential for life. body systems. Cells carry out the chemical activities Did You Get It? needed to sustain life, and they divide to 1. De ne cell. A cell is the (living) unit of life. fi form or repair tissues. 2. According to the cell theory, what the organism can do depends on _______. (Fill in the blank.) What its CELLS cells can do. ⁃ cells are the structural units of all living things, from one-celled organisms such as amoebas to Anatomy of a Generalized Cell complex multicellular organisms such as ➔ Learning Objectives humans, dogs, and trees. De ne generalized cell. ⁃ fi The human body contains 50 to 100 trillion of Identify on a cell model or diagram the these tiny building blocks. three major cell regions (nucleus, cytoplasm, and plasma membrane). Overview of the Cellular Basis of Life List the structures of the nucleus, and Name and describe the four concepts of explain the function of chromatin and the cell theory. nucleoli. List four elements that make up the bulk of living matter. Although no one cell type is exactly like all others, cells do have the same basic parts, and there are certain In the late 1600s, Robert Hooke was looking through functions common to all cells. Here we will talk about a crude microscope at some plant tissue—cork. He the generalized cell, which demonstrates most of saw some cubelike structures that reminded him of the these typical features. long rows of monk’s rooms (or cells) at the monastery, ⁃ In general, all cells have three main regions or so he named these structures “cells.” The living cells parts—a nucleus (nu′kle-us), a plasma that had formed the cork were long since dead; only membrane, and the cytoplasm (si′to-plazm′ the plant cell walls remained. However, the name stuck ′) (Figure 3.1a). and is still used to describe the smallest unit of all ⁃ The nucleus is usually located near the center living things. of the cell. It is surrounded by the semi uid fl Since the late 1800s, cell research has been cytoplasm, which in turn is enclosed by the exceptionally fruitful and has provided us with the plasma mem- brane, which forms the outer cell following four concepts collectively known as the cell boundary. (If you look ahead to Figure 3.4 on p. theory: 68, you can see a more detailed illustration of A cell is the basic structural and generalized cell structure.) functional unit of living organisms. So, when you de ne cell properties, you are in The Nucleus fi fact de ning the properties of life. Anything that works, works best when it is controlled. fi The activity of an organism depends on For cells, “headquarters,” or the control center, is the the collective activities of its cells. nucleus (nucle = kernel). According to the principle of ⁃ The genetic material, or deoxyribonucleic acid complementarity, the activities of cells are (DNA), is a blueprint that contains all the dictated by their structure (anatomy), instructions needed for building the whole which determines function (physiology). body; so, as you might expect, human DNA Continuity of life has a cellular basis. di ers from frog DNA. More speci cally, DNA ff fi has genes, which carry the instructions for We will expand on all of these concepts as we building proteins. go along. Let’s begin with the idea that the cell is the ⁃ DNA is also absolutely necessary for cell smallest living unit. Whatever its form, however it reproduction. A cell that has lost or ejected behaves, the cell contains all the parts necessary to its nucleus (for whatever reason) is destined survive in a changing world. It follows, then, that loss to “self-destruct.” of cell homeostasis underlies virtually every Although the nucleus is most often oval or disease.Perhaps the most striking thing about a cell is spherical, its shape usually conforms to the shape of its organization. the cell. For example, if the cell is elongated, the ⁃ Yet if we chemically analyze cells, we nd that nucleus is usually elongated as well. The nucleus has fi they are made up primarily of the same four three recognizable regions or structures: the nuclear elements—carbon, oxygen, hydrogen, and envelope, nucleolus, and chromatin. nitrogen—plus much smaller amounts of several other elements. (A detailed account of Nuclear Envelope The nuclear boundary is a double membrane barrier called the nuclear envelope, or nuclear membrane The exible plasma membrane is a fragile, fl (Figure 3.1b). transparent barrier that contains the cell contents and ⁃ Between the two membranes is a uid- lled separates them from the surrounding environment. fl fi “moat,” or space. At various points, the two (The terms cell membrane or cytoplasmic membrane layers of the nuclear envelope fuse, generating are sometimes used instead, but because nearly all openings called nuclear pores. cellular organelles are composed of membranes, in this ⁃ Like other cellular membranes, the nuclear text we will always refer to the cell’s surface or outer envelope allows some but not all substances to limiting membrane as the plasma membrane.) Although pass through it, but substances pass through it the plasma membrane is important in de ning the fi much more freely than elsewhere because of its limits of the cell, it is much more than a passive relatively large pores. The nuclear membrane envelope, or “baggie.” As you will see, its unique encloses a jellylike uid called nucleoplasm structure allows it to play a dynamic role in many fl (nu′kle-o-plazm′′) in which other nuclear cellular activities. elements are suspended. The Fluid Mosaic Model The structure of the plasma membrane consists Nucleolus of two phospholipid (fat) layers arranged “tail to tail,” The nucleus contains one or more small, dark- with cholesterol and oating proteins scattered among fl staining, essentially round bodies called nucleoli (nu- them (Figure 3.2). kle′o-li; “little nuclei”). ⁃ Some phospholipids may also have sugar ⁃ Nucleoli (plural for nucleolus) are sites where groups attached, forming glycolipids. The cell structures called ribosomes are assembled. proteins, some of which are free to move and Most ribosomes eventually migrate into the bob in the lipid layer, form a constantly cytoplasm, where they serve as the actual sites changing pattern or mosaic, hence the name of of protein synthesis. the model that describes the plasma membrane. Chromatin When a cell is not dividing, its DNA is carefully wound around proteins called histones to form a loose Remember, phospholipids are polar molecules: The network of “beads on a string” called chromatin charged end interacts with water, and the fatty acid (kro′mah-tin) that is scattered throughout the nucleus. chains do not. It is this property of polarity that When a cell is dividing to form two daughter cells, the makes phospholipids a good foundation for cell chromatin threads coil and con- dense to form dense, membranes. rodlike bodies called chromosomes (chromo = colored, soma = body)— much the way a stretched The olive oil–like phospholipid bilayer forms the spring becomes shorter and thicker when allowed to basic “fabric” of the membrane. relax. We discuss the functions of DNA and the events ⁃ The polar “heads” of the lollipop-shaped of cell division in the Cell Physiology section (beginning phospholipid molecules are hydrophilic (“water on p. 76). loving”) and are attracted to water, the main component of both the intracellular and Did You Get It? extracellular uids, and so they lie on both the fl 3. Plasma membrane = external barrier that inner and outer surfaces of the membrane. regulates what enters and leaves the cell. ⁃ Their nonpolar fatty acid “tails,” being Cytoplasm = the area where most cell hydrophobic (“water fearing”), avoid water and activities occur. Nucleus = control center of line up in the center (interior) of the membrane. the cell. The self-orienting property of the phospholipids 4. The generalized cell is a concept that allows biological membranes to reseal describes organelles and functions common themselves quickly when torn. The to all cells. hydrophobic makeup of the membrane interior 5. Nucleoli are the sites of synthesis of makes the plasma membrane relatively ribosomes, which are important in protein impermeable to most water-soluble molecules. synthesis. The cholesterol helps to both stabilize the membrane and keep it exible. fl The Plasma Membrane ➔ Learning Objectives The proteins scattered in the lipid bilayer are Describe the chemical composition of the responsible for most of the specialized functions of plasma membrane, and relate it to the membrane. Some proteins are enzymes. Many of membrane functions. the proteins protruding from the cell exterior are Compare the structure and function of receptors for hormones or other chemical messengers tight junctions, desmosomes, and gap or are binding sites for anchoring the cell to bers or to fi junctions. other structures inside or outside the cell. Most proteins that span the membrane are involved in these junctions are buttonlike thickenings of transport. adjacent plasma membranes (plaques) that ⁃ For example, some cluster together to form are connected by ne protein laments. fi fi protein channels (tiny pores) through which Thicker protein la- ments extend from the fi water and small water-soluble molecules or plaques inside the cells to the plaques on the ions can move; others act as carriers that bind cells’ opposite sides, thus forming an to a substance and move it through the internal system of strong “guy wires.” membrane. Gap junctions, or communicating junc- ⁃ Branching sugar groups are attached to most tions, function mainly to allow communica- of the proteins abutting the extracellular space. tion. Such “sugar-proteins” are called glycoproteins, These junctions are commonly found in the and because of their presence, the cell surface heart and between embryonic cells. In gap is a fuzzy, sticky, sugar-rich area called the junctions, the neighboring cells are con- glycocalyx (gli-ko-ka′liks). (You can think of nected by hollow cylinders composed of your cells as being sugar coated.) proteins (called connexons) that span the ⁃ Among other things, these glycoproteins entire width of the abutting membranes determine your blood type, act as receptors (which are therefore called transmembrane that certain bac- teria, viruses, or toxins can proteins). Chemical molecules, such as nutri- bind to, and play a role in cell-to-cell ents or ions, can pass directly through the recognition and interactions. water- lled connexon channels from one cell ⁃ fi De nite changes in glycoproteins occur in cells to another. fi that are being transformed into cancer cells. (We discuss cancer in “A Closer Look” on pp. Did You Get It? 104–105.) 6. Why do phospholipids (which form the bulk of plasma membranes) organize into a bilayer, tail to tail, in a watery environment? The phospholipids have both polar (heads) and nonpolar (tails) regions. Cell Membrane Junctions Polar aligns with polar (water and other polar Although certain cell types—blood cells, sperm molecules inside and outside the cell). Nonpolar cells, and some phagocytic cells (which ingest bacteria aligns with nonpolar in the membrane interior. and foreign debris)—are “footloose” in the body, many 7. The external faces of some membrane proteins have other types, particularly epithelial cells, are knit into sugar groups attached to them. What are three roles tight communities. Typically, cells are bound together these sugar-coated proteins play in the life of a cell? in three ways: They act as receptors, determine blood type, and Glycoproteins in the glycocalyx act as an play a role in cell-to-cell interactions. adhesive or cellular glue. 8. What is the special function of gap junctions? Of Wavy contours of the membranes of tight junctions? Communication and binding adjacent cells t together in a tongue-and- together, respectively. fi groove fashion. Special cell membrane junctions are The Cytoplasm formed (Figure 3.3). These junctions vary ➔ Learning Objective structurally depending on their roles. Identify the organelles on a cell model or describe them, and indicate the major Because this last factor is the most important, function of each. let us look more closely at the main types of junctions: The cytoplasm is the cellular material outside tight junctions, desmosomes, and gap junctions. the nucleus and inside the plasma membrane. Tight junctions are impermeable junctions ⁃ It is the site of most cellular activities, so you that encircle the cells and bind them might think of the cytoplasm as the “factory together into leakproof sheets. oor” of the cell. Although early scientists fl In tight junctions, adjacent plasma believed that the cytoplasm was a structureless membranes fuse together tightly like a zipper gel, the electron microscope has revealed that and prevent substances from passing it has three major components: the cytosol, through the extracellular space be- tween inclusions, and organelles. Let’s take a look at cells. In the small intestine, for example, each of these. these junctions prevent digestive enzymes from seeping into the bloodstream. Cytosol and Inclusions Desmosomes (dez′mo-soˉmz) are anchoring The cytosol is semitransparent uid that fl junctions scattered like rivets along the sides suspends the other elements. Dissolved in the cytosol, of adjacent cells. which is largely water, are nutrients and a variety of They prevent cells subjected to mechanical other solutes (sol′yuˉtz; dissolved substances). stress (such as heart muscle cells and skin cells) from being pulled apart. Structurally,  Inclusions are chemical substances that may or ⁃ By contrast, cells that are relatively inactive (an may not be present, depending on the speci c cell unfertilized egg, for instance) have fewer. fi type. ⁃ Most inclusions are stored nutrients or cell Ribosomes products oating in the cytosol. They include Ribosomes (ri′bo-soˉ mz) are tiny, bilobed, dark fl the lipid droplets common in fat cells, glycogen bodies made of proteins and one variety of RNA called granules abundant in liver and muscle cells, ribosomal RNA. pigments such as melanin in skin and hair cells, ⁃ Ribosomes are the actual sites of protein mucus and other secretory products, and synthesis in the cell. various kinds of crystals. It may help to think of ⁃ Ribosomes that oat freely in the cytoplasm fl an inclusion as a cellular “pantry” where manufacture proteins that function inside the items are kept on hand until needed. cell, while others attach to membranes such as the rough ER, which produces proteins that Organelles function outside the cell. The organelles (or′′gah-nelz′; “little organs”) are specialized cellular compartments (Figure 3.4, p. Endoplasmic Reticulum 68) that are the metabolic machinery of the cell. Each The endoplasmic retic- ulum (en′′do-plas′mik type of organelle is specialized to carry out a speci c re ̆ -tik′u-lum; “network within the cytoplasm”), or ER, is fi function for the cell as a whole, much like the organs a system of uid- lled tunnels (or canals) that coil and fl fi carry out specialized functions for the whole body. twist through the cytoplasm. Some synthesize proteins, others package those ⁃ It is continuous with the nuclear envelope and proteins, and so on. accounts for about half of a cell’s mem- branes. ⁃ Many organelles are bounded by a membrane ⁃ It serves as a mini circulatory system for the similar to the plasma membrane. cell because it provides a network of channels ⁃ These membrane boundaries allow organelles for carrying substances (primarily proteins) from to maintain an internal environment quite one part of the cell to another. di erent from that of the surrounding cytosol. ⁃ There are two forms of ER, rough and smooth ff This compartmentalization is crucial to their (see Figure 3.4); a particular cell may have both ability to perform their specialized functions for forms or only one, depending on its speci c fi the cell. Let’s consider what goes on in each of functions. these workshops of our cellular factory. The rough endoplasmic reticulum is so called because it is studded with ribosomes. Mitochondria ⁃ Because essentially all of the building materials Mitochondria (mi′′to-kon′dre-ah; singular: of cellular membranes are formed either in it or mitochondrion) are usually depicted as tiny, lozenge- on it, you can think of the rough ER as the cell’s like or sausage-shaped organelles (see Figure 3.4), membrane factory. but in living cells they lengthen and change shape ⁃ The proteins made on its ribosomes migrate almost continuously. into the rough ER tunnels, where they fold into ⁃ The mitochondrial wall consists of a double their functional three-dimensional shapes. membrane, equal to two plasma membranes These proteins are then dispatched to other placed side by side. The outer membrane is areas of the cell in small “sacs” of membrane smooth and featureless, but the inner called transport vesicles (Figure 3.5) that membrane has shel ike protrusions called carry substances around the cell. ⁃ fl cristae (kris′te; “crests”). Rough ER is especially abundant in cells that ⁃ Enzymes dissolved in the uid within the make (synthesize) and export (secrete) proteins fl mitochondria, as well as enzymes that form —for example, pancreatic cells, which produce part of the cristae membranes, carry out the digestive enzymes to be delivered to the small reactions in which oxygen is used to break intestine. down foods. As the foods are broken down, ⁃ The enzymes that catalyze the synthesis of energy is released. Much of this energy membrane lipids reside on the external escapes as heat, but some is captured and (cytoplasmic) face of the rough ER, where used to form ATP molecules. the needed building blocks are readily ATP provides the energy for all cellular work, available. and every living cell requires a constant supply of ATP Although the smooth endoplasmic reticulum for its many activities. Because the mitochondria communicates with the rough variety, it plays no role in supply most of this ATP, they are the “power- houses” protein synthesis, because it lacks ribosomes. of the cell. ⁃ Instead it functions in lipid metabolism ⁃ Metabolically “busy” cells, such as liver and (cholesterol and fat synthesis and breakdown) muscle cells, use huge amounts of ATP and and detoxi cation of drugs and pesticides. fi have hundreds of mitochondria, which replicate themselves by pinching in half. ⁃ Hence it is not surprising that the liver cells are Peroxisomes chock-full of smooth ER. So too are body cells Peroxisomes (per-ok′sih-soˉ mz) are that produce steroid-based hormones—for membranous sacs containing powerful oxidase (ok′s ̆ ı- instance, cells of the male testes that daˉz) enzymes that use molecular oxygen (O2) to manufacture testosterone. detoxify a number of harmful or poisonous substances, including alcohol and formalde- hyde. However, their Golgi Apparatus most important function is to “disarm” dangerous The Golgi (gol′je) apparatus appears as a stack of free radicals. attened membranous sacs that are associated with ⁃ Free radicals are highly reactive chemicals fl swarms of tiny vesicles. with unpaired electrons that can damage the ⁃ It is generally found close to the ER and is the structure of proteins and nucleic acids. principal “tra c director” for cellular proteins. ⁃ Free radicals are normal by-products of cellular ⁃ ffi Its major function is to modify, package, and metabolism, but if allowed to accumulate, they ship proteins (sent to it by the rough ER via can have devastating e ects on cells. ff transport vesicles) in speci c ways, depending fi Peroxisomes convert free radicals to hydrogen on their nal destination (Figure 3.6). peroxide (H2O2), a function indicated in their naming ⁃ fi Initially, all proteins leaving the Golgi apparatus (peroxisomes = peroxide bodies). accumulate in sacs called Golgi vesicles. ⁃ The enzyme catalase (kat′ah-laˉs) then converts As proteins “tagged” for export accumulate in the excess hydrogen peroxide to water. Golgi apparatus, the sacs swell. Peroxisomes are especially numerous in liver ⁃ Then their swollen ends, lled with protein, and kidney cells, which are very active in fi pinch o and form secretory vesicles (ves′ ̆ ı- detoxi cation. ⁃ ff fi kulz), which travel to the plasma membrane. Although peroxisomes look like small ⁃ When the vesicles reach the plasma lysosomes (see Figure 3.4), they do not arise by membrane, they fuse with it, the membrane budding from the Golgi apparatus. ruptures, and the contents of the sac are ⁃ Instead, one way they replicate themselves is ejected to the outside of the cell (pathway 1 in by simply pinching in half, like mitochondria, Figure 3.6). but most peroxisomes appear to bud directly ⁃ Mucus is packaged this way, as are digestive from the ER. enzymes made by pancreatic cells. ⁃ In addition to its packaging-for-release Did You Get It? functions, the Golgi apparatus pinches o 9. How do the cytosol and the cytoplasm di er? The ff ff sacs containing proteins and phospholipids cytosol is the liquid portion of the cytoplasm. destined for a “home” in the plasma Cytoplasm includes cytosol, organelles, and membrane (pathway 2 in Figure 3.6) or other inclusions. cellular membranes. It also packages hydrolytic 10. Which two organelles are sacs of enzymes, and enzymes into membrane-bound organelles what is the function of each of these organelles? called lysosomes that remain in the cell Lysosomes break down ingested bacteria, worn- (pathway 3 in Figure 3.6 and discussed next). out organelles, and dead cells. Peroxisomes detoxify a number of harmful toxic substances and Lysosomes disarm free radicals. Lysosomes (li′so-soˉ mz; “breakdown bodies”), 11. Which organelle is the major site of ATP synthesis? which appear in di erent sizes, are membranous Which packages proteins? Mitochondria are the ff “bags” containing powerful digestive enzymes. major site of ATP synthesis. The Golgi apparatus is ⁃ Because lysosomal enzymes are capable of the packaging site. digesting worn-out or nonusable cell structures and most foreign substances that enter the cell, Cytoskeleton lysosomes function as cellular “stomachs.” An elaborate network of protein structures ⁃ Lysosomes are especially abundant in white extends throughout the cytoplasm. This network, or blood cells called phagocytes, the cells that cytoskeleton, acts as a cell’s “bones and muscles” by dispose of bacteria and cell debris. As we furnishing an internal framework that determines cell mentioned, the enzymes they contain are shape, supports other organelles, and provides the formed by ribosomes on the rough ER and machinery for intracellular transport and various types packaged by the Golgi apparatus. of cellular movements. ⁃ From its smallest to its largest elements, the Homeostatic Imbalance 3.1 cytoskeleton is made up of micro laments, fi The lysosomal membrane is ordinarily quite intermediate laments, and microtubules fi stable, but it becomes fragile when the cell is injured or (Figure 3.7). deprived of oxygen and when excessive amounts of ⁃ Although there is some overlap in roles, vitamin A are present. When lyso- somes rupture, the generally speaking micro laments (such as fi cell self-digests. ____________✚ actin and myosin) are most involved in cell motility and in producing changes in cell shape.  (You could say that cells move when they get Microvilli (mi′′kro-vil′i; “little shaggy hairs”) are tiny, their act(in) together.) ngerlike extensions of the plasma membrane that fi ⁃ The strong, stable, ropelike intermediate project from an exposed cell surface (see Figure 3.3). laments are made up of brous subunits. ⁃ They increase the cell’s surface area fi fi They help form desmosomes (see Figure 3.3) tremendously and so are usually found on the and provide internal guy wires to resist pulling surface of cells active in absorption such as forces on the cell. intestinal and kidney tubule cells. Microvilli ⁃ The tubelike microtubules are made up of have a core of actin laments that extend into fi repeating subunits of the protein tubulin. They the internal cytoskeleton of the cell and sti en ff determine the overall shape of a cell and the the microvillus. distribution of organelles. They are very ⁃ Note that microvilli are “alcoves” projecting o ff important during cell division (see pp. 82–85). one cell surface and do not involve microtubules. Centrioles The paired centrioles (sen′tre-oˉlz), collectively called Did You Get It? the centrosome, lie close to the nucleus (see Figure 12. Microtubules and micro laments are fi 3.4). involved in cell mobility. ⁃ They are rod-shaped bodies that lie at right 13. The basis of centrioles is microtubules; that angles to each other; internally they are made of microvilli is a core of actin laments. fi up of a pinwheel array of nine triplets of ne fi 14. Microvilli increase the cell surface area for microtubules. Centrioles are best known for absorption. their role in generating microtubules and also for directing the formation of the mitotic spindle during cell division (look ahead to Figure 3.15, Cell Diversity p. 84). ➔ Learning Objective Apply the principle of complementarity to di erent cell types by comparing overall Cell Extensions ff shapes, internal structures, and special In addition to the cell structures described functions. previously, some cells have obvious surface So far in this chapter, we have focused on a extensions. These come in two major “ avors,” or gener- alized human cell. However, the trillions of cells fl varieties, depending on whether they have a core of in the human body include over 200 di erent cell types micro- tubules or actin laments. ff that vary greatly in size, shape, and function. They fi include sphere-shaped fat cells, disc-shaped red blood Cilia and Flagella cells, branching nerve cells, and cube- shaped cells of Cilia (sil′e-ah; “eyelashes”) are whiplike cellular kidney tubules. extensions that move substances along the cell Depending on type, cells also vary greatly in surface. length—ranging from 1/12,000 of an inch in the ⁃ For example, mucus is carried up and away smallest cells to over a yard in the nerve cells that from the lungs by “crowd sur ng” on the cause you to wiggle your toes. fi ciliated cells lining the respiratory system. ⁃ A cell’s shape re ects its function. For example, Where cilia appear, there are usually many of fl the at, tilelike epithelial cells that line the them pro- jecting from the exposed cell fl inside of your cheek t closely together, surface. fi forming a living barrier that protects underlying ⁃ When a cell is about to make cilia, its centrioles tissues from bacterial invasion. multiply and then line up beneath the plasma ⁃ The shapes of cells and the relative numbers of membrane at the free cell surface. Microtubules the various organelles they contain relate to then begin to “sprout” from the centrioles and specialized cell functions (Figure 3.8). Let’s put pressure on the membrane, forming the take a look at some examples of specialized projections. cells. If the projections formed by the centrioles are Cells that connect body parts (Figure 3.8a) substantially longer, they are called agella ( ah- Fibroblast. This cell has an elongated fl fl jel′ah). shape, like the cable-like bers that it se- ⁃ The only example of a agellated cell in the fi cretes. It has an abundant rough ER and a fl human body is the sperm, which has a single large Golgi apparatus to make and secrete propulsive agellum called its tail (look ahead the protein building blocks of these bers. fl to Figure 3.8g). fi Erythrocyte (red blood cell). This cell car- ⁃ Notice that cilia propel other substances across ries oxygen in the blood. Its biconcave disc a cell’s surface, whereas a agellum propels the shape provides extra surface area for the fl cell itself. uptake of oxygen and streamlines the cell so it ows easily through the bloodstream. So Microvilli fl  much oxygen-carrying pigment is packed in 15. Name the two cell types involved in connecting erythrocytes that all other organ- elles have body parts or regions. Fibroblasts and erythrocytes. been shed to make room. 16. What is the main function of a neuron? Neurons gather information and control body functions. Cells that cover and line body organs (Figure 3.8b) Epithelial cell. The hexagonal shape of this Cell Physiology cell is exactly like a “cell” in a honeycomb of Each of the cell’s internal parts is designed to perform a beehive. This shape allows epithelial cells a speci c function for the cell. As mentioned earlier, fi to pack together in sheets. An epithe- lial cell most cells have the ability to metabolize (use nutrients has abundant intermediate laments and to build new cell material, break down substances, and fi desmosomes that resist tearing when the make ATP), digest foods, dispose of wastes, epithelium is rubbed or pulled. reproduce, grow, move, and respond to a stimulus (irritability). Cells that move organs and body parts ⁃ We consider most of these functions in detail in (Figure 3.8c) later chapters. (For example, we cover Skeletal, cardiac, and smooth muscle metabolism in Chapter 14, and the ability to cells. These cells are elongated and lled react to a stimulus in Chapter 7). fi with abundant contractile laments, so they ⁃ In this chapter, we consider only the functions fi can shorten forcefully and move the bones, of membrane transport (the means by which pump blood, or change the size of internal substances get through plasma membranes), organs to move substances around the body. protein synthesis, and cell reproduction (cell division). Cell that stores nutrients (Figure 3.8d) Fat cell. The huge spherical shape of a fat Membrane Transport cell is produced by a large lipid droplet in its ➔ Learning Objectives cytoplasm. De ne selective permeability, di usion fi ff (including simple and facilitated di usion ff Cell that ghts disease and osmosis), active transport, passive fi Cell that ghts disease (Figure 3.8e) transport, solute pumping, exocytosis, fi White blood cells such as the macrophage (a endocytosis, phagocytosis, pinocytosis, phagocytic cell). This cell extends long pseu- hypertonic, hypotonic, and isotonic. dopods (“false feet”) to crawl through tissue Describe plasma membrane structure, to reach infection sites. The many lysosomes and explain how the various transport within the cell digest the infectious microor- processes account for the directional ganisms (such as bacteria) that it “eats.” movements of speci c substances across fi the plasma membrane. Cell that gathers information and controls body functions (Figure 3.8f) The uid environment on both sides of the plasma fl Nerve cell (neuron). This cell has long pro- membrane is an example of a solution. It is important cesses (extensions) for receiving messages that you really understand solutions before we dive into and transmitting them to other structures an explanation of membrane transport. in the body. The processes are covered with ⁃ In the most basic sense, a solution is a an extensive plasma membrane, and homogeneous mixture of two or more a plentiful rough ER synthesizes membrane components. components and signaling molecules called ⁃ Examples include the air we breathe (a mixture neurotransmitters. of gases), seawater (a mixture of water and salts), and rubbing alcohol (a mixture of water Cells of reproduction (Figure 3.8g) and alcohol). Oocyte (female). The largest cell in the body, ⁃ The substance present in the largest amount this egg cell contains several cop- ies of all in a solution is called the solvent (or dissolving organelles, for distribution to the daughter medium). cells that arise when the fertilized egg ⁃ Water is the body’s chief solvent. Components divides to become an embryo. or substances present in smaller amounts are Sperm (male). This cell is long and stream- called solutes. lined, built for swimming to the egg for ⁃ The solutes in a solution are so tiny that the fertilization. Its agellum acts as a motile molecules cannot be seen with the naked eye fl whip to propel the sperm. and do not settle out. Intracellular uid (collectively, the nucleoplasm and fl the cytosol) is a solution containing small amounts of Did You Get It? gases (oxygen and carbon dioxide), nutrients, and high speeds, they collide and change direction salts, dissolved in water. with each collision. ⁃ So too is extracellular uid, or interstitial ⁃ The overall e ect of this erratic movement is fl ff uid, the uid that continuously bathes the that molecules move down their concentration fl fl exterior of our cells. gradient (spread out). ⁃ You can think of interstitial uid as a rich, ⁃ The greater the di erence in concentration ff fl nutritious, and rather unusual “soup.” It between the two areas, the faster di usion ff contains thousands of ingredients, including occurs. nutrients (amino acids, sugars, fatty acids, ⁃ Because the driving force (source of energy) is vitamins), regula- tory substances such as the kinetic energy of the molecules themselves, hormones and neurotransmitters, salts, and the speed of di usion is a ected by the size of ff ff waste products. the molecules (the smaller the faster) and ⁃ To remain healthy, each cell must extract from temperature (the warmer the faster). this soup the exact amounts of the substances An example should help you understand di usion. ff it needs at speci c times and reject the rest. Picture yourself dropping a tea bag into a cup of fi The plasma membrane is a selectively permeable boiling water, but not stirring the cup. The bag itself barrier. represents the semipermeable cell membrane, which ⁃ Selective permeability means that a barrier lets only some molecules leave the tea bag. As the tea allows some substances to pass through it molecules dissolve in the hot water and collide while excluding others. repeatedly, they begin to “spread out” from the tea bag ⁃ Thus, it allows nutrients to enter the cell but even though it was never stirred. Eventually, as a result keeps many undesirable or unnec- essary of their activity, the entire cup will have the same substances out. At the same time, valuable cell concentration of tea molecules and will appear uniform proteins and other substances are kept within in color. (A laboratory example that might be familiar to the cell, and wastes are allowed to pass out of some students is illustrated Figure 3.9). it. ⁃ The hydrophobic core of the plasma membrane is a physical barrier to di usion. However, ff Homeostatic Imbalance 3.2 molecules will di use through the plasma ff The property of selective permeability is typical only of membrane if any of the following are true: healthy, unharmed cells. When a cell dies or is badly The molecules are small enough to pass damaged, its plasma membrane can no longer be through the membrane’s pores (channels selective and becomes permeable to nearly everything. formed by membrane proteins). We see this problem when some- one has been The molecules are lipid-soluble. severely burned. Precious uids, pro- teins, and ions The molecules are assisted by a fl “weep” (leak out) from the dead and damaged cells at membrane carrier. the burn site. ⁃ The unassisted di usion of solutes through the ff plasma membrane (or any selectively perme- Substances move through the plasma membrane in able membrane) is called simple di usion ff basically two ways—passively or actively. (Figure 3.10a). Solutes transported this way ⁃ In passive processes, substances are are lipid-soluble (such as fats, fat-soluble transported across the membrane without any vitamins, oxygen, carbon dioxide). energy input from the cell. ⁃ In active processes, the cell provides the metabolic energy (ATP) that drives the trans- Di usion of water through a selectively permeable ff port process. membrane such as the plasma membrane is speci cally called osmosis (oz-mo′sis). ⁃ fi Passive Processes: Di usion and Filtration Because water is highly polar, it is repelled by ff Di usion (d ̆ ı-fu′zhun) is an important means of the (nonpo- lar) lipid core of the plasma ff passive membrane transport for every cell of the body. membrane, but it can and does pass easily ⁃ The other passive transport process, ltration, through special pores called aquaporins fi generally occurs only across capillary walls. (“water pores”) created by proteins in the Let’s examine how these two types of passive membrane (Figure 3.10b). transport di er. ⁃ Osmosis into and out of cells is occurring all ff the time as water moves down its Di usion concentration gradient. The movement of water ff Di usion is the process by which molecules (and ions) across the membrane occurs quickly. Anyone ff move away from areas where they are more administering an IV (intravenous, into the vein) concentrated (more numerous) to areas where they solution must use the correct solution to are less concentrated (with fewer of them). protect the patient’s cells from life-threatening ⁃ All molecules possess kinetic energy, or energy of motion (as described in Chapter 2), and as the molecules move about randomly at  dehydration or rupture (see “A Closer Look” on formed by membrane proteins for di usion of ff p. 79). certain small solutes. A carrier protein undergoes Still another example of di usion is facilitated shape changes that allow di usion of a speci c ff fi ff di usion. substance through the membrane. ⁃ ff Facilitated di usion provides passage for 20. Explain the phrase “move down the concentration ff certain needed substances (notably glucose) gradient.” “Down the concentration gradient” that are both lipid-insoluble and too large to means to move from where there is a lot of pass through the membrane pores, or charged, something (high concentration) to where there is as in the case of chloride ions passing through not a lot (low concentration). a membrane protein channel. ⁃ Although facilitated di usion follows the laws of Active Processes ff di usion—that is, the substances move down Whenever a cell uses ATP to move substances ff their own concentration gradients—a protein across the membrane, the process is active. membrane protein channel is used (Figure ⁃ Substances moved actively are usually unable 3.10c), or a membrane protein that acts as a to pass in the desired direction by di usion. ff carrier (Figure 3.10d) is needed to move ⁃ They may be too large to pass through glucose and certain other solutes passively membrane channels, the membrane may lack across the membrane into the cell. special protein carri- ers for their transport, Substances that pass into and out of cells by di usion they may not be able to dissolve in the fat core, ff save the cell a great deal of energy. When you consider or they may have to move “uphill” against their how vitally important water, glucose, and oxygen are to concentration gradients. cells, you can under- stand just how necessary these The two most important active processes are active passive transport processes really are. Glucose and transport and vesicular transport. oxygen continu- ally move into the cells (where they are in lower concentration because the cells keep Active Transport using them up), and carbon dioxide (a waste product Sometimes called solute pumping, active transport is of cel- lular activity) continually moves out of the cells similar to facilitated di usion in that both processes ff into the blood (where it is in lower concentration). require protein carriers that interact speci cally and fi reversibly with the substances to be transported Filtration across the membrane. Filtration is the process by which water and solutes ⁃ However, facilitated di usion is driven by the ff are forced through a membrane (or capillary wall) by kinetic energy of the di using molecules, ff uid, or hydrostatic, pres- sure. whereas active transport uses ATP to energize ⁃ fl In the body, hydrostatic pressure is usually its protein carriers, which are called solute exerted by the blood. pumps. ⁃ Like di usion, ltration is a passive process, ⁃ Amino acids, some sugars, and most ions are ff fi and a gradient is involved. transported by solute pumps, and in most ⁃ In ltration, however, the gradient is a pressure cases these substances move against fi gradient that actually pushes solute-containing concentration (or electrical) gradients. uid ( ltrate) from the higher-pressure area ⁃ This is opposite to the direction in which fl fi through the lter to the lower-pressure area. In substances would naturally ow by di usion, fi fl ff the kid- neys, water and small solutes lter out which explains the need for energy in the form fi of the capillaries into the kidney tubules of ATP. because the blood pressure in the capillaries is greater than the uid pressure in the tubules. The sodium-potassium (Na+-K+) pump alternately fl Part of the ltrate formed in this way eventually carries sodium ions (Na+) out of and potas- sium ions fi becomes urine. (K+) into the cell (Figure 3.11). ⁃ Filtration is not very selective. For the most ⁃ This process is absolutely necessary for normal part, only blood cells and protein molecules too trans- mission of nerve impulses. large to pass through the membrane pores are ⁃ There are more sodium ions outside the cells held back. than inside, so those inside tend to remain in the cell unless the cell uses ATP to force, or Did You Get It? “pump,” them out. 17. What is the energy source for all types of di usion? ⁃ ATP is split into ADP and Pi (inorganic ff Kinetic energy is the energy source for di usion. phosphate), and the phosphate is then ff 18. What determines the net direction of any di usion attached to the sodium-potassium pump in a ff process? The concentration gradient determines process called phosphorylation. the direction that water and solutes move by ⁃ Likewise, there are more potassium ions inside di usion. Movement is from high to low cells than in the extracellular uid, and ff fl concentration of a given substance. potassium ions that leak out of cells must be 19. What are the two types of facilitated di usion, and actively pumped back inside. ff how do they di er? A channel protein is an opening ff ⁃ Because each of the pumps in the plasma way to “clean house”—not a means of getting membrane trans- ports only speci c nutrients. fi substances, active transport pro- vides a way Cells eat by phagocytosis and drink by a form of for the cell to be very selective in cases where endocytosis called pinocytosis (p ̆ ı′′no-si-to′sis; “cell substances cannot pass by di usion. (No pump drinking”), during which the cell “gulps” droplets of ff —no transport.) extracellular uid. (It may help you to remember the fl name of this process to think of your mother drink- ing Vesicular Transport Some substances cannot get a glass of pinot noir wine.) through the plasma membrane by active or passive ⁃ The plasma membrane indents to form a tiny transport. pit, or “cup,” and then its edges fuse around Vesicular transport, which involves help from ATP to the droplet of extracellular uid containing fl fuse or separate membrane vesicles and the cell dissolved proteins or fats (see Figure 3.13a). membrane, moves substances into or out of cells “in Unlike phagocytosis, pinocytosis is a routine bulk” without their actually crossing the plasma activity of most cells. membrane directly. The two types of vesicular ⁃ It is especially important in cells that function in transport are exocytosis and endocytosis. absorption (for example, cells forming the lining of the small intestine). Exocytosis (ek′′so-si-to′sis; “out of the cell”) (Figure 3.12) is the mechanism that cells use Receptor-mediated endocytosis is the to actively secrete hormones, mucus, and main cellular mechanism for taking up other cell products or to eject certain cellular speci c target molecules (Figure 3.13c). fi wastes. In this process, receptor proteins on the The product to be released is rst plasma membrane bind exclusively with fi “packaged” (typically by the Golgi apparatus) certain substances. Both the receptors and into a secretory vesicle. high concentrations of the attached target The vesicle migrates to the plasma molecules are internalized in a vesicle, and membrane, fuses with it, and then ruptures, then the con- tents of the vesicle are dealt spilling its contents out of the cell (also look with in one of the ways shown in Figure back at pathway 1 of Figure 3.6). 3.13a. Exocytosis involves a “docking” process in Although phagocytosis and pinocytosis are which docking proteins on the vesicles important, they are not very selec- tive recognize plasma membrane docking compared to receptor-mediated proteins and bind with them. This binding endocytosis. Speci c substances taken in by fi causes the membranes to “cork- screw” receptor-mediated endocytosis include together and fuse (see Figure 3.12). enzymes, some hormones, cholesterol, and iron. Endocytosis (en′′do-si-to′sis; “into the cell”) Unfortunately, u viruses exploit this route to fl includes those ATP-requiring processes that enter and attack our cells. take up, or engulf, extracellular substances by enclosing them in a vesicle (Figure 3.13a, Did You Get It? p. 82). 21. What happens when the Na+-K+ pump is Once the vesicle is formed, it detaches from phosphorylated? When K+ binds to the pump protein? the plasma membrane and moves into the When the pump protein is phosphorylated, it cytoplasm, where it typically fuses with a changes shape. When K+ binds, phosphate is lysosome and its contents are digested (by released. lysosomal enzymes). 22. Which vesicular transport process moves large However, in some cases, the vesicle travels particles into the cell? Phagocytosis moves large to the opposite side of the cell and releases particles into the cell. its contents by exocytosis there. 23. Which process is more selective—pinocytosis or receptor-mediated endocytosis? Receptor- mediated If the engulfed substances are relatively large particles, endocytosis. such as bacteria or dead body cells, and the cell separates them from the external environment by pseudopods, the endocytosis process is more Cell Division speci cally called phagocytosis (fag′′o-si-to′sis), a ➔ Learning Objectives fi term that means “cell eating” (Figure 3.13b). Brie y describe the process of DNA ⁃ fl Certain white blood cells, such as the replication and of mitosis. Explain the macrophage, and other “professional” importance of mitotic cell division. phagocytes of the body act as scavenger cells Describe the roles of DNA and of the three that police and protect the body by ingesting major varieties of RNA in protein bacteria and other foreign debris. Hence, synthesis. phagocytosis is a protective mechanism—a The cell life cycle is the series of changes a cell goes consists of two events. Mitosis (mi-to′sis), or through from the time it is formed until it divides. division of the nucleus, occurs rst. ⁃ fi The cycle has two major periods: The second event is division of the ⁃ interphase, in which the cell grows and carries cytoplasm, cytokinesis (si′′to-k ̆ ı-ne′sis), on its usual metabolic activities, and which begins when mitosis is nearly ⁃ cell division, during which it reproduces itself. completed. Although the term interphase might lead you to believe that it is merely a resting time between Mitosis the phases of cell division, this is not the case. Mitosis is the process of dividing a nucleus into two During interphase, which is by far the longer phase of daughter nuclei with exactly the same genes as the the cell cycle, the cell is very active and is preparing for “mother” nucleus. cell division. A more accurate name for interphase ⁃ As explained previously, DNA replication would be metabolic phase. precedes mitosis, so that for a short time the cell nucleus contains a double dose of genes. Preparations: DNA Replication When the nucleus divides, each daughter The function of cell division is to produce more cells for nucleus ends up with exactly the same genetic growth and repair processes. information as the original mother cell. The ⁃ Because it is essential that all body cells have stages of mitosis include the following events the same genetic material, an important event (Figure 3.15): always precedes cell division: Prophase (pro′faˉz). As cell division begins, ⁃ The DNA molecule (the genetic material) is the chromatin threads coil and shorten so duplicated exactly in a process called DNA that the barlike chromosomes become replication. This occurs toward the end of visible under a microscope. interphase. Because DNA has already been replicated, each chromosome is actually made up of CONCEPTLINK two identical strands called sister Recall that DNA is a very complex molecule. It is chromatids (kro′mah-tidz), held together by composed of building blocks called nucleotides, each a small buttonlike body called a centromere consisting of a deoxyribose sugar, a phosphate group, (sen′tro-meˉr) (see Figure 3.15). and a nitrogen-containing base. Essentially, DNA is a The centrioles separate from each other and double helix, a ladderlike molecule that is coiled into a begin to move toward opposite sides of the spiral staircase shape. The upright parts of the DNA cell, directing the assembly of a mitotic “ladder,” or backbone, are alternating phosphate and spindle (composed of microtubules) sugar units, and the rungs of the ladder are made of between them as they move. The spindle pairs of nitrogen-containing bases. provides sca olding for the attachment and ff movement of the chromosomes during the The precise trigger for DNA synthesis is unknown, but later mitotic stages. once it starts, it continues until all the DNA has been By the end of prophase, the nuclear replicated. envelope and the nucleoli have broken down ⁃ The process begins as the DNA helix “unzips,” and temporarily disappeared, and the gradually separating into its two nucleotide chromosomes have attached randomly to chains (Figure 3.14). Each nucleo- tide strand the spindle bers by their centromeres. fi then serves as a template, or set of Metaphase (met′ah-faˉz). In this short stage, instructions, for building a new nucleotide the chromosomes line up at the metaphase strand. plate (the center of the spindle midway Remember that nucleotides join in a complementary between the centrioles) so that a straight line way: of chromosomes is seen. Adenine (A) always bonds to thy- mine (T), Anaphase (an′ah-faˉz). During anaphase, the and guanine (G) always bonds to cytosine centromeres that have held the chromatids (C). Hence, the order of the nucleotides on together split. the template strand also determines the The chromatids (now called chro- mosomes order on the new strand. For example, a again) begin to move slowly apart, drawn TACTGC sequence on a template strand toward opposite ends of the cell. The would generate a new strand with the order chromosomes seem to be pulled by their ATGACG. The end result is two DNA half- centromeres, with their “arms” dangling molecules that are identical to the original behind them. DNA helix, each consisting of one old and This careful division of sister chromatids one newly assembled nucleotide strand. ensures that each daughter cell gets one copy of every chromosome. Events of Cell Division Anaphase is over when the chromosomes In all cells other than bacteria and some cells stop moving. of the reproductive system, cell division  Telophase (tel′o-faˉz). Telophase is DNA’s information is encoded in the sequence essentially prophase in reverse. of bases. Each sequence of three bases (a triplet) calls The chromosomes at opposite ends of the for a particular amino acid. (Amino acids are the cell uncoil to become threadlike chromatin building blocks of proteins and are joined dur- ing again. protein synthesis; see Chapter 2.) The spindle breaks down and disappears, a ⁃ For example, a DNA base sequence of AAA nuclear envelope forms around each speci es an amino acid called phenylalanine, fi chromatin mass, and nucleoli appear in each and CCT calls for glycine. Just as di erent ff of the daughter nuclei. arrangements of notes on sheet music are played as di erent chords, variations in the ff Mitosis is basically the same in all animal cells. arrangements of A, C, T, and G in each gene Depending on the type of tissue, it takes from 5 allow cells to make all the di erent kinds of ff minutes to several hours to complete, but typi- cally it proteins needed. A single gene contains an lasts about 2 hours. Centriole replication is deferred estimated 300 to 3,000 base pairs in sequence. until late interphase of the next cell cycle, when DNA replication begins before the onset of mitosis. The Role of RNA By itself, DNA is rather like a coded message; its information is not useful until it is decoded. Cytokinesis ⁃ Furthermore, most ribosomes—the Cytokinesis, or the division of the cytoplasm, usually manufacturing sites for proteins—are in the begins during late anaphase and completes during cytoplasm, but DNA never leaves the nucleus telophase. during interphase. ⁃ A contractile ring made of micro laments forms ⁃ Thus, DNA requires not only a decoder but also fi a cleavage furrow over the midline of the a trusted messenger to carry the instructions spindle, and it eventually squeezes, or pinches, for building proteins to the ribosomes. the original cytoplasmic mass into two parts. ⁃ These messenger and decoder functions are Thus, at the end of cell division, two daughter carried out by a second type of nucleic acid, cells exist. called ribonucleic (ri0bo-nu- kle′ik) acid, or ⁃ Each is smaller with less cytoplasm than the RNA. mother cell had but is genetically identical to the mother cell. The daughter cells grow and RNA di ers from DNA: ⁃ ff carry out normal cell activities (interphase) until in being single-stranded, it is their turn to divide. ⁃ in having ribose sugar instead of deoxyribose, and Mitosis and cytokinesis usually go hand in ⁃ in having a uracil (U) base instead of thymine hand, but in some cases the cytoplasm is not divided. (T) (recall what you learned in Chapter 2). This condition leads to the formation of binucleate (two nuclei) or multinucleate cells. This is fairly common in Three varieties of RNA play a special role in protein the liver and in the formation of skeletal muscle. synthesis. ⁃ Ribosomal RNA (rRNA) helps form the Protein Synthesis ribosomes, where proteins are built. Proteins are key substances for all aspects of cell life. ⁃ Messenger RNA (mRNA) molecules are long, ⁃ Fibrous (structural) proteins are the major single nucleo- tide strands that resemble half of building materials for cells (see Chapter 2). a DNA molecule. They carry the “message” ⁃ Other proteins, the globular (functional) containing instructions for protein synthesis proteins, per- form functional roles in the body. from the DNA (gene) in the nucleus to the ⁃ For example, all enzymes, biological catalysts ribosomes in the cytoplasm. that speed up every chemical reaction that ⁃ Transfer RNA (tRNA) molecules are small, occurs in cells, are functional proteins. It cloverleaf-shaped molecules that escort amino follows, then, that every cell needs to produce acids to the ribosome. proteins, a process called protein synthesis. This is accomplished with the DNA blueprints The Process of Protein Synthesis known as genes and with the help of the Protein synthesis involves two major phases: nucleic acid RNA. Let’s look at this process and transcription, when complementary mRNA (the its components more closely. messenger) is made using the information in the DNA gene, and translation, when the information carried in Genes: The Blueprint for Protein Structure mRNA molecules is “decoded” and translated from In addition to replicating itself for cell division, DNA nucleic acids into proteins. serves as the master blueprint for protein syn- thesis. Traditionally, a gene is de ned as a DNA segment that Transcription fi carries the information for building one protein. The word transcription often refers to one of the jobs done by a secretary— converting notes from one form (shorthand notes or an audio recording) into another to pick up another amino acid ( 5 ). When the last form (a letter, for example). codon (the termination, or “stop,” codon) is read, the ⁃ In other words, the same information is protein is released. transformed from one form or format to another. Did You Get It? ⁃ In cells, transcription involves the trans- fer of 24. How do the terms template strand and information from the sequence of bases in a complementary relate to DNA synthesis? DNA DNA gene into the complementary sequence of is double-stranded. When it is replicated, mRNA by an enzyme (see Figure 3.16, step each strand serves as a template to build a ( 1 ). complementary strand. Thus, if the template ⁃ DNA is the template for transcription, and strand is ACT, the complementary strand mRNA is the product. formed at that site is TGA. ⁃ Each three-base sequence specifying a 25. What is cytokinesis? What results if cytokinesis particular amino acid on the DNA gene is called does not happen? Cytokinesis is the division a triplet, and the corresponding three-base of the cytoplasm. If cytokinesis does not sequences on mRNA are called codons. occur, the result is a binucleate cell. ⁃ The form is di erent, but the same information 26. What is the role of mRNA in protein synthesis? ff is being conveyed. Thus, if the (partial) What about tRNA? mRNA carries the coded sequence of DNA triplets is AAT-CGT-TCG, information for building proteins from the RNA base-pairing rules (A:U, G:C) tell us that DNA to the ribosome where protein the corresponding codons on mRNA would be synthesis occurs. tRNA delivers amino acids UUA-GCA-AGC. to the ribosome and “checks” the location by recognizing the mRNA codon with its Translation anticodon. A translator takes words in one language and restates 27. What are the two stages of protein synthesis, them in another language. and in which stage are proteins actually ⁃ In the translation phase of protein synthesis, synthesized? What occurs in the other stage? the language of nucleic acids (base sequence) Transcription and translation. Proteins are is “translated” into the language of proteins synthesized during translation. Transcription (amino acid sequence). Translation occurs in is the production of mRNA using DNA as a the cytoplasm and involves three major template. varieties of RNA. ⁃ Translation consists of the following series of events (see Figure 3.16, steps 2–5). ⁃ Once the mRNA attaches to the ribosome ( 2 ), tRNA transfers, or delivers, amino acids to the ribosome, where they are linked together by peptide bonds (formed by dehydration synthesis; see Chapter 2) in the exact sequence speci ed by the gene (and its fi mRNA). ⁃ There are about 45 common types of tRNAs, each capable of carrying one of the 20 types of amino acids. ⁃ But that is not the only job of the tRNAs. They also have to recognize the mRNA codons to “double check” that the amino acid they are toting will be added in the correct order. They can do this because they have a special three- base sequence called an anticodon on their “head” that can temporarily bind to the com- plementary codons ( 3 ). Once the rst tRNA has maneuvered itself into the fi correct position at the beginning of the mRNA message, the ribosome moves the mRNA strand along, bringing the next codon into position to be read by another tRNA. As amino acids are brought to their proper positions along the length of mRNA, they are joined together with peptide bonds cata- lyzed by the large ribosomal subunit ( 4 ). As each amino acid is added to the chain, its tRNA is released and moves away from the ribo- some