Cytology Tế bào học

ENGLISH IN BIOLOGY I (ENGL: 311-I) UNIT ONE: CYTOLOGY (3 Class hours) 1.1 Cells and tissues The cell is the basic unit of life Microorganisms such as bacteria, yeast, and amoebae exist as single cells By contrast, the adult human is made up of about 30 trillion cells (1 trillion =1012) which are mostly organized into collectives called tissues Cells are, with a few notable exceptions, small with lengths measured in micrometers (µm, where 1000µm=1 mm) and their discovery stemmed from the conviction of a small group of seventeenth-century microscope makers that a new and undiscovered world lay beyond the limits of the human eye These pioneers set in motion a science and an industry that continues to the present day 1.1.1 Principles of Microscopy Microscopes make small objects appear bigger A light microscope will magnify an image up to 1500 times its original size Electron microscopes can achieve magnifications up to million times However, bigger is only better when more details are revealed The fineness of detail that a microscope can reveal is its resolving power This is defined as the smallest distance that two objects can approach one another yet still be recognized as being separate The resolution that a microscope achieves is mainly a function of the wavelength of the illumination source it employs The smaller the wavelength, the smaller the object that will cause diffraction, and the better the resolving power The light microscope, because it uses visible light of wavelength around 500 nanometers (nm, where 1000 nm=1µm), can distinguish objects as small as about half this: 250 nm It can therefore be used to visualize the smallest cells and the major intracellular structures or organelles The microscopic study of cell structure organization is known as cytology An electron microscope is required to reveal the ultrastructure (the fine detail) of the organelles and other cytoplasmic structures 1.1.2 Only Two Types of Cells Superficially at least, cells exhibit a staggering diversity Some lead a solitary existence; others live in communities; some have defined, geometric shapes; others have flexible boundaries; some swim, some crawl, and some are sedentary; many are green (some are even red, blue, or purple); others have no obvious coloration Given these differences, it is perhaps surprising that there are only two types of cell Bacterial cells are said to be prokaryotic (Greek for “before nucleus”) because they have very little visible internal organization so that, for instance, the genetic material is free within the cell They are also small, the vast majority being 1–2 μm in length The cells of all other organisms, from protists to mammals to fungi to plants, are eukaryotic (Greek for “with a nucleus”) These are generally larger (5–100μm, although some eukaryotic cells are large enough to be seen with the naked eye and structurally more complex Eukaryotic cells contain a variety of specialized structures known collectively as organelles, surrounded by a viscous substance called cytosol The largest organelle, the nucleus, contains the genetic information stored in the molecule deoxyribonucleic acid (DNA) The structure and function of organelles will be described in detail in subsequent chapters Table 1.1 provides a brief glossary of the major organelles and summarizes the differences between prokaryotic and eukaryotic cells Among eukaryotic cells the most striking difference is between those of animals and plants Plants have evolved a sedentary lifestyle and a mode of nutrition that means they must support a leaf canopy Their cells are enclosed within a rigid cell wall that gives shape to the cell and structural rigidity to the organism This is in contrast to the flexible boundaries of animal cells Plant cells frequently contain one or more vacuoles that can occupy up to 75% of the cell volume Vacuoles accumulate a high concentration of sugars and other soluble compounds Water enters the vacuole to dilute these sugars, generating hydrostatic pressure that is counterbalanced by the rigid wall In this way the cells of the plant become stiff or turgid, in the same way that when an inner tube is inflated inside a bicycle tire the combination becomes stiff Vacuoles are often pigmented, and the spectacular colors of petals and fruit reflect the presence of compounds such as the purple anthocyanins in the vacuole Cells of photosynthetic plant tissues contain a special organelle, the chloroplast, that houses the light-harvesting and carbohydrate-generating systems of photosynthesis Plant cells lack centrosomes although these are found in many algae Viruses occupy a unique space between the living and nonliving worlds On one hand they are made of the same molecules as living cells On the other hand they are incapable of independent existence, being completely dependent on a host cell to reproduce Almost all living organisms have viruses that infect them Human viruses include polio, influenza, herpes, rabies, ebola, smallpox, chickenpox, and the AIDS (acquired immunodeficiency syndrome) virus HIV (human immunodeficiency virus) Viruses are submicroscopic particles consisting of a core of genetic material enclosed within a protein coat called the capsid Some viruses have an extra membrane layer called the envelope Viruses are metabolically inert until they enter a host cell, whereupon the viral genetic material directs the host cell machinery to produce viral protein and viral genetic material Viruses often insert their genome into that of the host, an ability that is widely made use of in molecular genetics Bacterial viruses, called bacteriophages, are used by scientists to transfer genes between bacterial strains Human viruses are used as vehicles for gene therapy By exploiting the natural infection cycle of a virus such as adenovirus, it is possible to introduce a functional copy of a human gene into a patient suffering from a genetic disease such as cystic fibrosis 1.1.3 Summary All living organisms are made of cells Our understanding of cell structure and function has gone hand in hand with developments in microscopy and its associated techniques Light microscopy revealed the diversity of cell types and the existence of the major organelles: nucleus, mitochondrion and, in plants, the vacuole and chloroplast The electron microscope revealed the detailed structure of the larger organelles and resolved the cell ultrastructure, the fine detail at the nanometer scale There are two types of cells, prokaryotes and eukaryotes Prokaryotic cells have very little visible internal organization They usually measure 1–2µm across Eukaryotic cells usually measure 5–100µm across They contain a variety of specialized internal organelles, the largest of which, the nucleus, contains the genetic material 1.2 Membranes and organelles 1.2.1 Basic properties of cell membranes It is difficult to overstate the importance of membranes to living cells; without them life could not exist The plasma membrane, also known as the cell surface membrane or plasmalemma, defines the boundary of the cell It regulates the movement of materials into and out of the cell and facilitates electrical signaling between cells Other membranes define the boundaries of organelles and provide a matrix upon which complex chemical reactions can occur Some of these themes will be developed in subsequent chapters In the following section the basic structure of the cell membrane will be outlined The basic structure of a biological membrane is shown in Figure 3.1 Approximately half the mass is phospholipid, which spontaneously organizes to form a lipid bilayer All the membranes of the cell, including the plasma membrane, also contain proteins These may be tightly associated with the membrane and extracted from it only with great difficulty, in which case they are called integral proteins (e.g., the gap junction channel); or they may be separated with relative ease, in which case they are termed peripheral proteins (e.g., clathrin adaptor protein) Membrane proteins are free to move laterally, within the plane of the membrane Integral membrane proteins are often glycosylated that is, have sugar residues attached—on the side facing the extracellular medium Straight Through the Membrane: Diffusion Through the Bilayer Molecules of oxygen are uncharged Although they dissolve readily enough in water, they are also able to dissolve in the hydrophobic interior of lipid bilayers Oxygen molecules can therefore pass from the extracellular fluid into the interior of the plasma membrane, and from there pass on into the cytoplasm, in a simple diffusion process (Fig 3.2) Three other small molecules with important roles in biology— carbon dioxide, nitric oxide, and water itself—also pass across the plasma membrane by simple diffusion, as the uncharged hormones of the steroid family In contrast, charged ions cannot dissolve in hydrophobic regions and therefore cannot cross membranes by simple diffusion Beyond the Cell Membrane: The Extracellular Matrix In connective tissue, the spaces between cells are filled by an extracellular matrix consisting of polysaccharides and proteins such as collagen together with combinations of the two called proteoglycans The proteins provide tensile strength and elasticity while the polysaccharides form a hydrated gel that expands to fill the extracellular space The sugar-based extracellular matrix reaches its highest form of expression in the cell walls of plants (Fig 3.3) The plant cell wall consists primarily of cellulose microfibrils linked together by other polysaccharide molecules such as hemicellulose and pectin The thickness of the wall is determined largely by its pectin content In healthy plant cells the plasma membrane is constantly pressing against the cell wall In plants the extracellular medium is a much more dilute solution than the intracellular solution If no other forces operated, water would diffuse into plant cells down its concentration gradient, a process called osmosis However, plant cells are prevented from expanding by the inextensible cellulose microfibrils in their cell walls As water diffuses in, the hydrostatic pressure in the cells rises, making the combination of cell plus cell wall stiff The same effect can be seen when a bicycle inner tube is blown up inside the tire, and is called turgor If plants are starved of water, water will leave the cell interior, hydrostatic pressure drops, and the cells lose their turgor pressure, like a tire when the tube inside is deflated As a result the nonwoody parts of the plant wilt Figure 3.3 The cell wall directs plant cell growth Newly generated plant cells first lay down a primary cell wall of cellulose microfibrils that are orientated in one direction around the plant cell (Fig 3.3a) Hydrostatic pressure therefore tends to cause a growing plant cell to elongate in one direction, perpendicular to the axis of the microfibrils Once the growth of the cell is complete, more layers of cellulose and/or other compounds are added, most notably the polyphenolic compound lignin, to form the secondary cell wall (Fig 3.3b) Cell Junctions In multicellular organisms, and particularly in epithelia, it is often necessary for neighboring cells within a tissue to be connected together This function is provided by cell junctions In animal cells there are three types of junctions Those that form a tight seal between adjacent cells are known as tight junctions; those that allow communication between cells are known as gap junctions A third class of cell junction that anchors cells together, allowing the tissue to be stretched without tearing, are called anchoring junctions Plant cells not have tight junctions, gap junctions, or anchoring junctions but contain a unique class of communicating junction known as plasmodesmata Tight junctions are found wherever flow of extracellular medium is to be restricted and are particularly common in epithelial cells such as those lining the small intestine The plasma membranes of adjacent cells are pressed together so tightly that no intercellular space exists between them Tight junctions between the epithelial cells of the intestine ensure that the only way that molecules can get from the lumen of the intestine to the blood supply that lies beneath is by passing through the cells, a route that can be selective Gap junctions are specialized structures that allow cell-to-cell communication in animals (Fig 3.4) When two cells form a gap junction, ions and small molecules can pass directly from the cytosol of one cell to the cytosol of the other cell without going into the extracellular fluid Since ions can move through the junction, changes in electrical voltage are also rapidly transmitted from cell to cell by this route The structure that makes this possible is the gap junction channel Channels are water filled holes through membranes When two gap junction channels or connexons meet, they form a water-filled tube that runs all the way through the plasma membrane of the first cell, across the small gap between the cells, and through the plasma membrane of the second cell In the middle of the channel is a continuous hole about 1.5 nm in diameter This hole is large enough to allow small ions through (and therefore to pass electrical current) together with amino acids and nucleotides, but it is too small for proteins or nucleic acids Gap junctions are especially important in the heart, where they allow an electrical signal to pass rapidly between all the cardiac muscle cells, ensuring that they all contract at the proper time Each gap junction channel is composed of six protein subunits that can twist against each other to open and close the central channel in a process called gating that allows the cell to control the degree to which it shares solute with its neighbor The plasmodesmata that perforate the cell walls of many plant tissues serve much the same purpose as the gap junctions of animal cells but are much bigger and cannot shut quickly Some plant viruses use plasmodesmata to spread from cell to cell Figure 3.4 Gap junctions allow solute and electrical current to pass from the cytosol of one cell to the cytosol of its neighbor 1.2.2 Organelles Bounded by Double-Membrane Envelopes Three of the major cell organelles, the nucleus, mitochondrion, and, in plant cells, the chloroplast, share two distinctive features They are all enclosed within an envelope consisting of two parallel membranes and they all contain the genetic material DNA The nucleus is often the most prominent cell organelle It contains thegenome,the cell’s database, which is encoded in molecules of the nucleic acid, DNA The nucleus is bounded by a nuclear envelope composed of two membranes separated by an intermembrane space The inner membrane of the nuclear envelope is lined by a meshwork of proteins called the nuclear lamina which provides rigidity to the nucleus A two-way traffic of proteins and nucleic acids between the nucleus and the cytoplasm passes through holes in the nuclear envelope called nuclear pores The nucleus of a cell that is synthesizing proteins at a low level will have few nuclear pores In cells that are undergoing active protein synthesis, however, virtually the whole nuclear surface is perforated Mitochondria are among the most easily recognizable organelles due to the extensive folding of their inner membrane to form shelf like projections named cristae The number of cristae, like the number of mitochondria themselves, depends upon the energy budget of the cell in which they are found In striated muscle cells, which must contract and relax repeatedly over long periods of time, there are many mitochondria that contain numerous cristae; in fat cells, which generate little energy, there are few mitochondria and their cristae are less well developed This gives a clue as to the function of mitochondria: they are the cell’s power stations Mitochondria produce the molecule adenosine triphosphate (ATP), one of the cell’s energy currencies that provide the energy to drive a host of cellular reactions and mechanism The doublemembrane structure provides four distinct domains: the outer membrane, the inner membrane, the intermembrane space, and the matrix Each domain houses a distinct set of functions Chloroplasts are found only in photosynthetic protists and plant cells In addition to the two bounding membranes, they contain internal membranes called thylakoids which, in plants, form stacks called grana The thylakoids contain the proteins and other molecules responsible for light capture The dark reactions of photosynthesis, on the other hand, takes place in the matrix, called the stroma, which also contains the DNA and ribosomes 1.2.3 Organelles Bounded by Single-Membrane Envelopes Mitochondria and chloroplasts are frequently found close to another membranebound organelle, the peroxisome In human cells peroxisomes have a diameter of about 500 nm, and their dense matrix contains a heterogeneous collection of proteins concerned with a variety of metabolic functions, some of which are only now beginning to be understood Peroxisomes are so named because they are frequently responsible for the conversion of the highly reactive molecule hydrogen peroxide 10 - Osteichthyes or bony fish The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of 45 orders, and over 435 families and 28,000 species Unlike chondrichthyans, nearly all living osteichthyans have an ossified (bony) endoskeleton with a hard matrix of calcium phosphate Like many other taxonomic names, the name Osteichthyes (“bony fish”) was coined long before the advent of phylogenetic systematics When it was originally defined, the group excluded tetrapods, but we now know that such a taxon would be paraphyletic Therefore, systematists today include tetrapods along with bony fishes in the clade Osteichthyes Clearly, the name of the group does not accurately describe all of its members - Amphibia The amphibians are represented today by about 6,150 species of salamanders (order Urodela, “tailed ones”), frogs (order Anura, “tail-less ones”), and caecilians (order Apoda, “legless ones”) About 550 species of urodeles are known Some are entirely aquatic, but others live on land as adults or throughout life Most salamanders that live on land walk with a side-to-side bending of the body, a trait also found in early terrestrial tetrapods Paedomorphosis is common among aquatic salamanders; the axolotl, for instance, retains larval features even when it is sexually mature - Reptilia 59 The reptile clade includes tuataras, lizards, snakes, turtles, crocodilians, and birds, along with a number of extinct groups, such as plesiosaurs and ichthyosaurs Fossil evidence indicates that the earliest reptiles lived about 310 million years ago and resembled lizards Reptiles have diverged greatly since that time, but as a group they share several derived characters that distinguish them from other tetrapods For example, unlike amphibians, reptiles have scales that contain the protein keratin (as does a human nail) Scales help protect the animal’s skin from desiccation and abrasion In addition, most reptiles lay their shelled eggs on land Fertilization must occur internally, before the eggshell is secreted Many species of snakes and lizards are viviparous; in such species, the extraembryonic membranes form a kind of placenta that enables the embryo to obtain nutrients from its mother - Aves There are about 10,000 species of birds in the world Like crocodilians, birds are archosaurs, but almost every feature of their anatomy has been modified in their adaptation to flight Cladistic analyses of birds and reptilian fossils indicate that birds belong to the group of bipedal saurischian dinosaurs called theropods Several species of dinosaurs closely related to birds had feathers with vanes, and a wider range of species had filamentous feathers Such findings imply that feathers evolved long before powered flight Among the possible functions of these early feathers were insulation, camouflage, and courtship display Many of the characters of birds are adaptations that facilitate flight, including weight-saving modifications that make flying more efficient For example, birds lack a urinary bladder, and the females of most species have only one ovary The gonads of both females and males are usually small, except during the breeding season, when 60 they increase in size Living birds are also toothless, an adaptation that trims the weight of the head - Mammalia There are more than 5,300 known species of mammals on Earth The distinctive character from which mammals derive their name is their mammary glands, which produce milk for offspring All mammalian mothers nourish their young with milk, a balanced diet rich in fats, sugars, proteins, minerals, and vitamins Hair, another mammalian characteristic, and a fat layer under the skin help the body retain heat Like birds, mammals are endothermic, and most have a high metabolic rate Efficient respiratory and circulatory systems (including a four-chambered heart) support a mammal’s metabolism A sheet of muscle called the diaphragm helps ventilate the lungs Like birds, mammals generally have a larger brain than other vertebrates of equivalent size, and many species are capable learners And as in birds, the relatively long duration of parental care extends the time for offspring to learn important survival skills by observing their parents Differentiated teeth are another important mammalian trait Whereas the teeth of reptiles are generally uniform in size and shape, the jaws of mammals bear a variety of teeth with sizes and shapes adapted for chewing many kinds of foods Humans, like most mammals, have teeth modified for shearing (incisors and canine teeth) and for crushing and grinding (premolars and molars) 5.4 New words and biological terms 5.5 Grammar 5.6 Exercises and review questions 61 UNIT SIX: ANIMAL STRUCTURE AND FUNCTION (4 Class hours) 6.1 Tissues A tissue is composed of similarly specialized cells that perform a common function in the body The tissues of the human body can be categorized into four major types: epithelial tissue, which covers body surfaces and lines body cavities; connective tissue, which binds and supports body parts; muscular tissue, which moves body parts; and nervous tissue, which receives stimuli and conducts impulses from one body part to another - Epithelial tissue Epithelial tissue, also called epithelium, consists of tightly packed cells that form a continuous layer or sheet lining the entire body surface and most of the body’s inner cavities On the external surface, it protects the body from injury, drying out, and possible pathogen (virus and bacterium) invasion On internal surfaces, epithelial tissue may be specialized for other functions in addition to protection For example, epithelial tissue secretes mucus along the digestive tract and sweeps up impurities from the lungs by means of cilia (sing., cilium) It efficiently absorbs molecules from kidney tubules and from the intestine because of minute cellular extensions called microvilli 62 There are various types of epithelial tissue (Figure 6.1) Squamous epithelium is composed of flattened cells and is found lining the lungs and blood vessels Cuboidal epithelium contains cube-shaped cells and is found lining the kidney tubules Columnar epithelium has cells resembling rectangular pillars or columns, and nuclei are usually located near the bottom of each cell This epithelium is found lining the digestive tract Ciliated columnar epithelium is found lining the oviducts, where it propels the egg toward the uterus or womb An epithelium can be simple or stratified Simple means the tissue has a single layer of cells, and stratified means the tissue has layers of cells piled one on top of the other The walls of the smallest blood vessels, called capillaries, are composed of a single layer of epithelial cells The permeability of capillaries allows exchange of substances between the blood and tissue cells The nose, mouth, esophagus, anal canal, and vagina are all lined by stratified squamous epithelium Pseudostratified epithelium appears to be layered; however, true layers not exist because each cell touches the baseline The lining of the windpipe, or trachea, is called pseudostratified ciliated columnar epithelium A secreted covering of mucus traps foreign particles, and the upward motion of the cilia carries the mucus to the back of the throat, where it may either be swallowed or expectorated 63 Figure 6.1 Epithelial tissue An epithelium sometimes secretes a product, in which case it is described as glandular A gland can be a single epithelial cell, as in the case of mucus-secreting goblet cells found within the columnar epithelium lining the digestive tract, or a gland can contain many cells Glands that secrete their product into ducts are called exocrine glands, and those that secrete their product directly into the bloodstream are called endocrine glands The pancreas is both an exocrine gland, because it secretes digestive juices into the small intestine via ducts, and an endocrine gland, because it secretes insulin into the bloodstream - Connective tissue Connective tissue binds organs together, provides support and protection, fills spaces, produces blood cells, and stores fat (Figure 6.2) 64 + Loose fibrous connective tissue: This tissue supports epithelium and also many internal organs Its presence in lungs, arteries, and the urinary bladder allows these organs to expand It forms a protective covering enclosing many internal organs, such as muscles, blood vessels, and nerves + Dense fibrous connective tissue: This tissue contains many collagen fibers that are packed together This type of tissue has more specific functions than does loose connective tissue For example, dense fibrous connective tissue is found in tendons, which connect muscles to bones, and in ligaments, which connect bones to other bones at joints + Adipose tissue: In adipose tissue, the fibroblasts enlarge and store fat The body uses this stored fat for energy, insulation, and organ protection Adipose tissue is found beneath the skin, around the kidneys, and on the surface of the heart + Cartilage: In cartilage, the cells lie in small chambers called lacunae, separated by a matrix that is solid yet flexible Unfortunately, because this tissue lacks a direct blood supply, it heals very slowly There are three types of cartilage - hyaline cartilage, elastic cartilage, fibrocartilage - distinguished by the type of fiber in the matrix Figure 6.2 Connective tissues 65 + Bone: Bone is the most rigid connective tissue It consists of an extremely hard matrix of inorganic salts, notably calcium salts, deposited around protein fibers, especially collagen fibers The inorganic salts give bone rigidity, and the protein fibers provide elasticity and strength, much as steel rods in reinforced concrete + Blood: Blood is unlike other types of connective tissue in that the matrix (i.e., plasma) is not made by the cells Some people not classify blood as connective tissue; instead, they suggest a separate tissue category called vascular tissue If blood is transferred from a person’s vein to a test tube and prevented from clotting, it separates into two layers The upper liquid layer, called plasma, represents about 55% of the volume of whole blood and contains a variety of inorganic and organic substances dissolved or suspended in water The lower layer consists of red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets (thrombocytes) Collectively, these are called the formed elements and represent about 45% of the volume of whole blood Formed elements are manufactured in the red bone marrow of the skull, ribs, vertebrae, and ends of long bones (Figure 6.3) Figure 6.3 Blood components The red blood cells are small, biconcave, disk-shaped cells without nuclei The presence of the red pigment hemoglobin makes the cells red, and in turn, makes the blood red Hemoglobin is composed of four units; each is composed of the protein 66 globin and a complex iron-containing structure called heme The iron forms a loose association with oxygen, and in this way red blood cells transport oxygen White blood cells may be distinguished from red blood cells by the fact that they are usually larger, have a nucleus, and without staining would appear to be translucent White blood cells characteristically look bluish because they have been stained that color White blood cells, which fight infection, function primarily in two ways Some white blood cells are phagocytic and engulf infectious pathogens, while other white blood cells produce antibodies, molecules that combine with foreign substances to inactivate them Platelets are not complete cells; rather, they are fragments of giant cells present only in bone marrow When a blood vessel is damaged, platelets form a plug that seals the vessel and injured tissues release molecules that help the clotting process - Muscular tissue Muscular (contractile) tissue is composed of cells that are called muscle fibers Muscle fibers contain actin filaments and myosin filaments, whose interaction accounts for movement There are three types of vertebrate muscles: skeletal, smooth, and cardiac Skeletal muscle, also called voluntary muscle, is attached by tendons to the bones of the skeleton, and when it contracts, body parts move Contraction of skeletal muscle is under voluntary control and occurs faster than in the other muscle types Skeletal muscle fibers are cylindrical and quite long - sometimes they run the length of the muscle They arise during development when several cells fuse, resulting in one fiber with multiple nuclei The nuclei are located at the periphery of the cell, just inside the plasma membrane The fibers have alternating light and dark bands that give them a striated appearance These bands are due to the placement of actin filaments and myosin filaments in the cell (Figure 6.4) Smooth (visceral) muscle is so named because the cells lack striations The spindle-shaped cells form layers in which the thick middle portion of one cell is opposite the thin ends of adjacent cells (Figure 6.4) Consequently, the nuclei form an irregular pattern in the tissue Smooth muscle is not under voluntary control and therefore is said to be involuntary Smooth muscle, found in the walls of viscera (intestine, stomach, and other internal organs) and blood vessels, contracts more slowly than skeletal muscle but can remain contracted for a longer time When the smooth muscle of the intestine contracts, food moves along its lumen (central cavity) 67 Figure 6.4 Muscular tissues When the smooth muscle of the blood vessels contracts, blood vessels constrict, helping to raise blood pressure Cardiac muscle is found only in the walls of the heart Its contraction pumps blood and accounts for the heartbeat Cardiac muscle combines features of both smooth muscle and skeletal muscle It has striations like skeletal muscle, but the contraction of the heart is involuntary for the most part Cardiac muscle cells also differ from skeletal muscle cells in that they have a single, centrally placed nucleus The cells are branched and seemingly fused one with the other, and the heart appears 68 to be composed of one large interconnecting mass of muscle cells Actually, cardiac muscle cells are separate and individual, but they are bound end to end at intercalated disks, areas where folded plasma membranes between two cells contain adhesion junctions and gap junctions - Nervous Tissue Nervous tissue, which contains nerve cells called neurons, is present in the brain and spinal cord A neuron is a specialized cell that has three parts: dendrites, a cell body, and an axon (Figure 6.5) A dendrite is a process that conducts signals toward the cell body The cell body contains the major concentration of the cytoplasm and the nucleus of the neuron An axon is a process that typically conducts nerve impulses away from the cell body Long axons are covered by myelin, a white fatty substance The term fiber1 is used to refer to an axon along with its myelin sheath if it has one Outside the brain and spinal cord, fibers bound by connective tissue form nerves The nervous system has just three functions: sensory input, integration of data, and motor output Nerves conduct impulses from sensory receptors to the spinal cord and the brain where integration occurs The phenomenon called sensation occurs only in the brain, however Nerves also conduct nerve impulses away from the spinal cord and brain to the muscles and glands, causing them to contract and secrete, respectively In this way, a coordinated response to the stimulus is achieved In addition to neurons, nervous tissue contains neuroglial cells Figure 6.5 Nervous tissues 69 + Neuroglia: There are several different types of neuroglia in the brain, and much research is currently being conducted to determine how much “glia” contribute to the functioning of the brain Neuroglia outnumber neurons nine to one and take up more than half the volume of the brain, but until recently, they were thought to merely support and nourish neurons 6.2 Structures and functions of organ systems The body contains a number of systems that work together to maintain homeostasis - Maintenance of the body The internal environment of the body consists of the blood within the blood vessels and the tissue fluid that surrounds the cells Five systems (digestive, cardiovascular, lymphatic, respiratory, and urinary) add substances to and/or remove substances from the blood The digestive system consists of the mouth, esophagus, stomach, small intestine, and large intestine (colon) along with the associated organs: teeth, tongue, salivary glands, liver, gallbladder, and pancreas This system receives food and digests it into nutrient molecules, which can enter the cells of the body The cardiovascular system consists of the heart and the blood vessels that carry blood through the body Blood transports nutrients and oxygen to the cells, and removes their waste molecules that are to be excreted from the body Blood also contains cells produced by the lymphatic system The lymphatic system consists of lymphatic vessels, lymph fluid, lymph nodes, and other lymphoid organs This system protects the body from disease by purifying lymph and supporting lymphocytes, the white blood cells that produce antibodies Lymphatic vessels absorb fat from the digestive system and collect excess tissue fluid, which is returned to the blood circulatory system The respiratory system consists of the lungs and the tubes that take air to and from them The respiratory system brings oxygen into the body and takes carbon dioxide out of the body through the lungs The urinary system contains the kidneys and the urinary bladder This system rids the body of nitrogenous wastes and helps regulate the fluid level and chemical content of the blood Support and Movement The skeletal system and the muscular system allow the body and its parts to move They also protect and support the body The skeletal system, consisting of the bones of the skeleton, protects body parts For example, the skull forms a protective 70 encasement for the brain, as does the rib cage for the heart and lungs The skeleton, as a whole, serves as a place of attachment for the skeletal muscles Contraction of muscles in the muscular system accounts for movement of body parts - Coordination and regulation of body systems The nervous system consists of the brain, spinal cord, and associated nerves The nerves conduct nerve impulses from receptors to the brain and spinal cord They also conduct nerve impulses from the brain and spinal cord to the muscles and glands, allowing us to respond to both external and internal stimuli The endocrine system consists of the hormonal glands which secrete chemicals that serve as messengers between body parts Both the nervous and endocrine systems help maintain homeostasis by coordinating and regulating the functions of the body’s other systems The endocrine system also helps maintain the proper functioning of male and female reproductive organs - Continuance of the species The reproductive system involves different organs in the male and female The male reproductive system consists of the testes, other glands, and various ducts that conduct semen to and through the penis The testes produce sex cells called sperm The female reproductive system consists of the ovaries, oviducts, uterus, vagina, and external genitals The ovaries produce sex cells called eggs When a sperm fertilizes an egg, an offspring begins development - Integumentary System The skin and its accessory organs such as hair, nails, sweat glands, and sebaceous glands are collectively called the integumentary system Skin covers the body, protecting underlying tissues from physical trauma, pathogen invasion, and water loss; it also helps regulate body temperature Therefore, skin plays a significant role in homeostasis The skin even synthesizes certain chemicals such as vitamin D that affect the rest of the body Because skin contains sensory receptors, skin also helps us to be aware of our surroundings and to communicate through touch - Homeostasis and body systems Homeostasis is the relative constancy of the body’s internal environment Because of homeostasis, even though external conditions may change dramatically, internal conditions still stay within a narrow range The internal environment of the body consists of blood and tissue fluid Tissue fluid, which bathes all the cells of the body, is refreshed when molecules such as oxygen and nutrients move into tissue fluid from the blood, and when wastes move from tissue fluid into the blood Tissue fluid remains constant only as long as blood composition remains constant 71 All systems of the body contribute toward maintaining homeostasis and therefore a relatively constant internal environment The cardiovascular system conducts blood to and away from capillaries, where exchange occurs The heart pumps the blood and thereby keeps it moving toward the capillaries The formed elements also contribute to homeostasis Red blood cells transport oxygen and participate in the transport of carbon dioxide White blood cells fight infection, and platelets participate in the clotting process The lymphatic system is accessory to the cardiovascular system Lymphatic capillaries collect excess tissue fluid, and this is returned via lymphatic veins to the circulatory veins Lymph nodes help purify lymph and keep it free of pathogens The digestive system takes in and digests food, providing nutrient molecules that enter the blood and replace the nutrients that are constantly being used by the body cells The respiratory system adds oxygen to and removes carbon dioxide from the blood The chief regulators of blood composition are the liver and the kidneys They monitor the chemical composition of plasma and alter it as required Immediately after glucose enters the blood, it can be removed by the liver for storage as glycogen Later, the glycogen can be broken down to replace the glucose used by the body cells; in this way, the glucose composition of blood remains constant The hormone insulin, secreted by the pancreas, regulates glycogen storage The liver also removes toxic chemicals, such as ingested alcohol and other drugs The liver makes urea, a nitrogenous end product of protein metabolism Urea and other metabolic waste molecules are excreted by the kidneys Urine formation by the kidneys is extremely critical to the body, not only because it rids the body of unwanted substances, but also because it offers an opportunity to carefully regulate blood volume, salt balance, and the pH of the blood The nervous system and the endocrine systems regulate the other systems of the body They work together to control body systems so that homeostasis is maintained We have already seen that in negative feedback mechanisms, sensory receptors send nerve impulses to regulatory centers in the brain, which then direct effectors to become active Effectors can be muscles or glands Muscles bring about an immediate change Endocrine glands secrete hormones that bring about a slower, more lasting change that keeps the internal environment relatively stable 6.3 New words and biological terms 6.4 Grammar 6.5 Exercises and review questions 72 REFERECES Campbell, Neil A., Reece B Jane, Wasserman A Steven, Urry A Lisa, Minorsky V Peter, Cain L Michael, Jackson B Robery, 2014 Biology, 10th Ed, Pearson Education, Inc Jane Reece, Michael Cain, Stev Urry Biology, 9th Edition, 2011 Pearson Education, Inc., USA Gerard J Tortora, Berdell R Funke, Christine L 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