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Chapter 3: Cellular Form and Function 3.1 Concepts of Cellular Structure Development of Cell Theory Cytology—the scientific study of cells (Robert Hooke) Theodor Schwann—studied a wide range of animal cells and concluded that all animals are made of cells Spontaneous Generation—idea that living things arise from non-living matter Cell Theory—generalization of how cells are formed and how living things are made Cell Shapes and Sizes Squamous—thin, flat, scaly shape that looks like the shape of a sunny side up eggLines the esophagus and form the epidermis of the skin Cuboidal—squarish-looking in frontal sections and about equal in height and width Liver cells Columnar—distinctly taller than wide Inner lining cells of the stomach and intestines Polygonal—having irregularly angular shapes with 4, 5, or more sides Stellate—having multiple pointed processes projecting from the body (starlike) Cell bodies of many nerve cells Spheroidal to Ovoid—round to oval Egg cells and White blood cells Discoidal—disc shaped; ovoid and flat Red blood cells Fusiform—spindle-shaped; elongated, with a thick middle and tapered ends Smooth muscle cells Fibrous—long, slender, and threadlike Skeletal muscle cells and the axons of nerve cells Micrometer (μm)—most useful unit of measure for designating cell sizes

Several factors that limit the size of cells: If cells swelled to excessive size, it could rupture Cell size limited by the relationship between its volume and surface area Surface area is proportional to the square of its diameter, Volume is proportional to the cube of its diameter If a cell were too large, molecules couldn’t diffuse from place to place fast enough Time required for diffusion is proportional to the square of distance (diameter doubled, travel time quadruples) Basic Components of a Cell Cytoplasm—fluid between the nucleus and surface membrane Transmission Electron Microscope (TEM)—uses a beam of electrons in place of light to see a cell’s ultrastructure A fine degree of detail extending even to the molecular level Scanning Electron Microscope (SEM)—produces dramatic three-dimensional images at high magnification and resolution Can only view surface features of cells Vascular Corrosion Cast—technique for visualizing the blood vessels of an organ Vessels are drained and flushed with saline, then carefully filled with resin After the resin solidifies, the tissue is dissolved with a corrosive agent, leaving a resin cast of the vessels, which is photographed by SEM Gives insights into the blood supply to an organ from macro- to microscopic levels Plasma (Cell) Membrane—proteins and lipids that surrounds the cell Composition and functions can differ from one region of a cell to another (ex. basal, lateral, and apical surfaces) Cytoplasm is crowded with fibers, tubules, passages, compartments, and contains: Cytoskeleton—supportive framework of protein filaments and tubules Organelles—diverse structures that perform various metabolic tasks for the cell

Inclusions—foreign matter or stored cell products Cytosol/Intracellular Fluid (ICF)—embeds the cytoskeleton, organelles, and inclusions Extracellular Fluid (ECF) can also be called Tissue (Interstitial) Fluid Some other extracellular fluids include blood plasma, lymph, and cerebrospinal fluid 3.2 The Cell Surface The Plasma Membrane—defines boundaries of the cell, governs its interactions with other cells, and controls the passage of materials in/out Intracellular Face of the membrane—side that faces the cytoplasm Extraceullar Face of the membrane—side that faces outward Membrane Lipids 75% of membrane molecules are phospholipids, amphipathic molecules that arrange themselves into a bilayer Cholesterol molecules, found near membrane surfaces amid the phospholipids, constitute about 20% of the membrane lipids 5% of the membrane lipids are glycolipids—phospholipids with short carbohydrate chains on the extracellular face of the membrane Contributes to the glycocalyx, a carbohydrate coating on the cell surface with multiple functions

Membrane Proteins Two broad classes of membrane proteins: Transmembrane Proteins—passes completely through the phospholipid bilayer Most are glycoproteins bound to oligosaccharides on the extracellular side of the membrane

Many of these float freely in the phospholipid film, but others are anchored to the cytoskeleton Peripheral Proteins—does not protrude into the phospholipid layer, but adhere to either the inner or outer face of the membrane Functions of membrane proteins include: Receptors—usually specific for one particular messenger Second-Messenger Systems—produce when a messenger binds to a surface receptor and triggers changes within the cell Process involves both transmembrane proteins and peripheral proteins Enzymes—carries out the final stages of starch and protein digestion in the small intestine, help produce second messengers, and break down hormones and other signaling molecules whose job is done, stopping them from excessively stimulating a cell Channel Proteins—passages that allow water and hydrophilic solutes to move through the membrane Leak Channels—always open and allow materials to pass through continually Gates (Gated Channels)—opens and closes under different circumstances and allow solutes through at some times Responds to three types of stimuli: Ligand-Gated Channels—responds to chemical messengers Voltage-Gated Channels—responds to changes in electrical potential across plasma membrane Mechanically Gated Channels—responds to physical stress on a cell (stress/pressure) Channelopathies—family of diseases caused by defects in channel proteins Carriers—transmembrane proteins that bind to glucose, electrolytes, and other solutes and transfers other side of membrane Pumps—carriers that consume ATP in the process

Cell-Identity Markers—glycoproteins contribute to the glycocalyx which acts like an “identification tag” Cell-Adhesion Molecules—extracellular material through membrane proteins which cells adhere to Some cells don’t grow or survive normally unless they’re mechanically linked to the extracellular material Second Messengers—essential for understanding hormone and neurotransmitter action Epinephrine (first messenger) cannot pass through plasma membrane so it binds to a surface receptor Receptor is linked to a peripheral G protein (named for ATP-like chemical (GTP) from which they get their energy) When activated, G proteins relays the signal to another membrane protein (Adenylate Cyclase) Removes two phosphate groups from ATP and converts it to cyclic AMP (cAMP) (second messenger) cAMP activates cytoplasmic enzymes (kinases) which adds phosphate groups to other cellular enzymes The Glycocalyx—external fuzzy coat composed of carbohydrate moieties of membrane glycolipids and glycoproteinsExtensions of the Cell Surface—aids in absorption, movement, and sensory processes Microvilli—extensions of the plasma membrane that serve primarily to increase a cell’s surface area Best developed in cells specialized for absorption (epithelial cells of intestines and kidneys) Gives cells 15 to 40 times as much absorptive surface area as they would have if their apical surface were flat

Individual microvilli cannot be distinguished, but on some cells they appear as a fringe (brush border) at the apical cell surface Cilia and Flagella Cilia—harlike processes Many are sensory, serving as the cell’s antenna for monitoring nearby conditions Motile cilia occur in the respiratory tract, uterine (fallopian) rubes, internal cavities (ventricles) of the brain, and short ducts (efferent ductules) associated with the testes Beats in waves that sweep across the surface of an epithelium in the same direction, propelling materials Axoneme—structural basis for ciliary movement that consists of an array of thin protein cylinders (microtubules) Central microtubules stop at the cell surface, but peripheral microtubules continue a short distance into the cell as a part of a basal body that anchors the cilium In each pair of peripheral microtubules, one tubule has two little dynein arms Dynein, motor protein, uses energy from ATP to crawl up the adjacent pair of microtubules Flagellum—has an axoneme surrounded by a sheath of coarse fibers that stiffen the tail and give it more propulsive power Pseudopods—cytoplasm-filled extensions of the cell varying in shape from fine, filamentous processes to blunt fingerlike ones 3.3 Membrane Transportmem Selectively Permeable—allows some things through but usually prevents other things from entering or leaving the cells Methods of moving substances can be classified in two ways: Passive Mechanisms—requires no ATP expenditure by the cell (filtration, diffusion, osmosis) Active Mechanisms—consumes ATP (active transport, vesicular transport)

Carrier-Mediated Mechanisms—uses a membrane protein to transport substances from one side of the membrane to the other Filtration—a process in which physical pressure forces fluids through a selectively permeable membrane Simple Diffusion— net movement of particles from a place of high concentration to a place of lower concentration Diffusion rates are important because they determine how quickly a cell can acquire nutrients or rid itself of wastes Some factors that affect the rate of diffusion through a membrane are: Temperature—the warmer the substance, the more readily its particles diffuse Molecular Weight—heavy molecules move sluggishly and diffuse more slowly than small molecules Steepness of Concentration Gradient—rapid diffusion if there is a greater concentration difference Membrane Surface Area—more surface area allows for more membrane available for particles to diffuse through Membrane Permeability—how easily a particle can move through the membrane Osmosis—the net flow of water from one side of a selectively permeable membrane to another Imbalances in osmosis underlie problems such as diarrhea, constipation, hypertension, and edema (tissue swelling) Can easily move through channel proteins (aquaporins) specialized for water Reverse Osmosis—process in which a mechanical pressure applied to one side of the system can override osmotic pressure and drive water through a membrane against its concentration gradient Capillary Filtration—the body’s principal pump (heart) drives water out of the smallest blood vessels by reverse osmosis

Osmolarity and Tonicity Osmolarity (osmotic concentration)—milliosmoles per liter (mOsm/L)—expresses quantity of non permeating particles per liter of sol’n Tonicity—the ability of a solution to affect the fluid volume and pressure in a cell Hypotonic solution—has a lower concentration of nonpermeating solutes than the intracellular fluid (ICF) Cells in a hypotonic solution absorb water, swell, and may burst (lyst) Hypertonic solution—has a higher concentration of nonpermeating solutes than the ICF Cells in a hypertonic solution lose water and shrivel (crenate) Isotonic solution— total concentration of nonpermeating solutes is the same as in the ICF, causing no change in cell volume or shape Carrier-Mediated Transport—a solute binds to a carrier in plasma membrane, which changes shape and releases the solute to the other side Carriers exhibits specificity for its ligand and saturation As the solute concentration rises, its rate of transport increases to a certain point Carriers saturated, no more are available to handle the demand, and transport levels off at a rate called transport maximum (Tm) Three types of carriers: Uniports—carries only one type of solute Symports—carrier proteins that carries two or more solutes through a membrane simultaneously in the same direction (cotransport) Antiports—carrier proteins that carries two or more solutes in opposite directions (countertransport) Three mechanisms of carrier-mediated transport: Facilitated Diffusion—carrier-mediated transport of a solute through a membrane down its concentration gradient

Primary Active Transport—process where a carrier moves a substance through cell up its concentration gradient using ATP (NaK Pump) Secondary Active Transport—requires energy input, but depends only indirectly on ATP

Vesicular Transport—moves large particles, droplets of fluids, or numerous molecules at once through the membrane in vesicles of membraneEndocytosis—process that bring matter into a cell Three forms of endocytosis: Phagocytosis—process of engulfing particles (few specialized cells) Phagosome—a vesicle in the cytoplasm surrounded by a unit membrane Pinocytosis—process of taking in droplets of ECF containing molecules of some use to the cell (all human cells) Pinocytotic Vesicles—membrane-bound pits formed when plasma membrane becomes dimpled or caved in Receptor-Mediated—more selective form of either phagocytosis or pinocytosis Enables a cell to take in specific molecules from ECF with a minimum of unnecessary matter Particles bind to specific receptors, which then cluster together and the membrane sinks in to create a pit coated with peripheral membrane proteins (clathrin) Transcytosis—transport of material across a cell (capture on one side and release on the other) Exocytosis—process that release material from a cell

3.4 The Cell Interior The Cytoskeleton—a network of protein filaments and cylinders that structurally support a cell, determine its shape, organize its contents, direct the movement of materials within a cell, and contribute to movements of the cell as a whole Forms a dense supportive scaffold in the cytoplasm; is connected to the transmembrane proteins of the plasma membrane, and in turn are connected to protein fibers external to the cell, creating a strong structural continuity from extracellular material to the cytoplasm; and may even connect to chromosomes in the nucleus, enabling physical tension on a cell to move nuclear contents and mechanically stimulate genetic function Are composed of: Microfilaments (thin filaments)—made of protein actin; widespread throughout the cell, but concentrated in fibrous mat called the terminal web (membrane skeleton) on the cytoplasmic side of the plasma membrane Intermediate Filaments—gives the cell its shape, resist stress, and participate in junctions that attach cells to their neighbors In epidermal cells, they’re made of keratin and occupy most of the cytoplasm Microtubules—cylinders made of 13 parallel strands (protofilaments) Each protofilament is a long chain of globular proteins (tubulin) Radiate from an area called the centrosome Holds organelles in place, form bundles that maintain cell shape and rigidity, and act somewhat like monorail tracks Organelles—internal structures of a cell that carry out specialized metabolic tasks Some are surrounded by membranes and are referred to as membranous organelles Examples are: Nucleus, Mitochondria, Lysosomes, Peroxisomes, Endoplasmic Reticulum, and Golgi Complex Organelles without membranes includes: Ribosomes, Proteasomes, Centrosomes, Centrioles, and Basal Bodies

The Nucleus—usually the largest organelle and the only one clearly visible with a light microscope Contains the cell’s chromosomes and is the genetic control center of cellular activity Most cells have a singular nucleus, but.. Mature red blood cells have none (anucellular) Skeletal muscle cells, some liver cells, and certain bone-dissolving cells have 2-50 nuclei (multinuclear) Nuclear Envelope—encloses the nucleus in a double membrane Perforated with nuclear pores formed by a ring of proteins (nuclear pore complex) Nuclear Lamina—narrow, dense fibrous zone, inside nuclear envelope, composed of web of intermediate filaments Supports the nuclear envelope and pores, provides points of attachment and organization for the chromosomes, and plays a role in regulating DNA replication and the cell life cycle Abnormalities of its structure or function are associated with certain genetic diseases and premature cell death Nucleoplasm—material in the nucleus Includes chromatin (fine threadlike matter composed of DNA and protein) and nucleoli (where ribosomes are produced) Endoplasmic Reticulum (ER)—system of interconnected channels (cisterns) enclosed by a unit membrane Rough ER—cisterns are parallel, flattened sacs covered with ribosomes Smooth ER—cisterns are tubular, branch more extensively, and lack ribosomes Synthesizes steroids and other lipids, detoxifies alcohol and other drugs, and manufactures nearly all membranes of the cells

Rough ER produces phospholipids and proteins of the plasma membrane and synthesizes proteins that are either secreted from the cell or packaged in organelles such as lysosomes Smooth ER stores calcium and releases it to trigger muscle contractions Ribosomes—small granules of proteins and RNA found in the nucleoli, in the cytosol, in the mitochondria, and on the outer surfaces of the rough ER and nuclear envelope The “read” coded genetic messages (mRNA) and assemble amino acids into proteins specified by the code Unattached ribosomes scattered throughout the cytoplasm make enzymes and other proteins for use within the cell Ribosomes attach to rough ER when they make proteins destined to be packaged in lysosomes or to be secreted from the cell Golgi Complex—small system of cisterns that synthesize carbohydrates; adds finishing touches to protein/glycoprotein synthesis Receives newly synthesized proteins, sorts them, cuts and splices some of them, and adds carbohydrate moieties to some Golgi Vesicles—most mature cistern with finish cell product Some vesicles become lysosomes, some migrate to plasma membrane and fuse with it, some become secretory vesicles that store a cell product for later release Lysosomes—a package of enzymes bound by a membrane Autophagy—when lysosomes digest and dispose of surplus or nonmetal organelles and other cell components in order to recycle their nutrients to more important cell needs Autolysis—the digestion of surplus cells by their own lysosomal enzymes Peroxisomes—resembles lysosomes but contain different enzymes Produced by collaboration between ER, mitochondria, and by fission of preexisting peroxisomes General function is to use molecular oxygen (O2) to oxidize organic molecules

Produces hydrogen peroxide (H2O2) Used to oxidize other molecules, and the excess is broken down to water and oxygen via catalase Neutralizes free radicals and detoxify alcohol, other drugs, and a variety of blood-borne toxins Also decomposes fatty acids into two-carbon fragments that the mitochondria use as an energy source for ATP synthesis Proteasomes—structurally simple organelle that functions as protein disposal Hallow, cylindrical complexes of proteins located in both the cytoplasm and nucleus A cell tags undesirable proteins for destruction and transports them to a proteasome The proteasome’s enzymes unfold the undesirable protein and breaks it down into short peptides and free amino acids Can be used to synthesize new proteins or be presented to the immune system for further degradation Mitochondria—organelles specialized for synthesizing ATP Surrounded by a double membrane Inner membrane has folds called cristae which project like shelves across the organelle The matrix contains ribosomes; enzymes used in ATP synthesis; and small circular DNA molecules (mtDNA) Centrioles—a short cylindrical assembly of microtubules, arranged in nine groups of three microtubules each Near nucleus, most cells have a clear patch of cytoplasm (centrosome) containing a pair of mutually perpendicular centrioles Plays a role in cell division Inclusions Are of two kinds: Accumulated cell products such as glycogen granules, pigments, and oil droplets

Foreign bodies such as viruses, bacteria, and dust particles and other debris phagocytized by a cell---...