Cell Shapes
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Chapter 3 Cellular Form and Function Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. INTRODUCTION All organisms are composed of cells Single-cell organisms humans Cells are responsible for all structural and functional properties of a living organism Workings of the human body Mechanisms of disease Rationale of therapy 2-2 DEVELOPMENT OF THE CELL THEORY Cytology—scientific study of cells Began when Robert Hooke coined the word cellulae to describe empty cell walls of cork Theodor Schwann concluded, about two centuries later, that all animal tissues are made of cells Louis Pasteur established beyond any reasonable doubt that “cells arise only from other cells” Refutes the idea of spontaneous generation—living things arise from nonliving matter 3-3 DEVELOPMENT OF THE CELL THEORY Modern cell theory All organisms composed of cells and cell products The cell is the simplest structural and functional unit of life An organism’s structure and functions are due to the activities of its cells Cells come only from preexisting cells, not from nonliving matter Cells of all species have many fundamental similarities in their chemical composition and metabolic mechanisms. 3-4 CELL SHAPES AND SIZES About 200 types of cells in the human body Squamous Polygonal Stellate Squamous Cuboidal Cuboidal Columnar Spheroid to ovoid Polygonal Stellate Discoid Fusiform Discoid Fusiform (spindle-shaped) Fibrous Note: Some of these cell shapes appear as in tissue sections, but not their three-dimensional shape Columnar Spheroid Fibrous 3-5 CELL SHAPES AND SIZES Human cell size Most from 10–15 micrometers (µm) in diameter Egg cells (very large) 100 µm diameter Barely visible to the naked eye Nerve cell at 1 meter long Longest human cell Too slender to be seen with naked eye 2-6 CELL SHAPES AND SIZES Limitations on cell size Cell growth increases volume more than surface area Surface area of a cell is proportional to the square of its diameter Volume of a cell is proportional to the cube of its diameter Nutrient absorption and waste removal utilize surface area If cell becomes too large, may rupture like overfilled water balloon 20 m Growth 20 m 10 m 10 m Large cell Diameter = 20 mm Surface area = 20 mm ´ 20 mm ´ 6 = 2,400 mm2 Volume = 20 mm ´ 20 mm ´ 20 mm = 8,000 mm3 Small cell Diameter = 10 mm Surface area = 10 mm ´ 10 mm ´ 6 = 600 mm2 Volume = 10 mm ´ 10 mm ´ 10 mm = 1,000 mm3 Effect of cell growth: Diameter (D) increased by a factor of 2 Surface area increased by a factor of 4 (= D2) Volume increased by a factor of 8 (= D3) 3-7 BASIC COMPONENTS OF A CELL Light microscope reveals plasma membrane, nucleus, and cytoplasm Cytoplasm—fluid between the nucleus and surface membrane Resolution (ability to reveal detail) of electron microscopes reveals ultrastructure Organelles, cytoskeleton, and cytosol (ICF) Plasma membrane Nucleus Nuclear envelope Mitochondria Golgi vesicle Golgi complex Ribosomes 2.0 m 3-8 BASIC COMPONENTS OF A CELL Plasma (cell) membrane Surrounds cell, defines boundaries Made of proteins and lipids Composition and function can vary from one region of the cell to another Cytoplasm Organelles Cytoskeleton Cytosol (intracellular fluid, ICF) Apical cell surface Microfilaments Microvillus Terminal web Desmosome Secretory vesicle undergoing exocytosis Fat droplet Secretory vesicle Intercellular space Centrosome Centrioles Golgi vesicles Golgi complex Lateral cell surface Free ribosomes Intermediate filament Nucleus Lysosome Nucleolus Microtubule Nuclear envelope Rough endoplasmic reticulum Smooth endoplasmic reticulum Extracellular fluid (ECF) Fluid outside of cell Mitochondrion Plasma membranes Hemidesmosome Basal cell surface Basement membrane 3-9 THE PLASMA MEMBRANE Unit membrane—forms the border of the cell and many of its organelles - Appears as a pair of dark parallel lines around cell (viewed with the electron microscope) Plasma membrane—unit membrane at cell surface - Defines cell boundaries - Governs interactions with other cells - Controls passage of materials in and out of cell - Intracellular face—side that faces cytoplasm - Extracellular face—side that faces outward Plasma membrane of upper cell Intercellular space Plasma membrane of lower cell Nuclear envelope Nucleus (a) 100 nm THE PLASMA MEMBRANE Extracellular fluid Peripheral protein Glycolipid Glycoprotein Carbohydrate chains Extracellular face of membrane Phospholipid bilayer Channel Peripheral protein Cholesterol Transmembrane protein Intracellular fluid Intracellular face of membrane Proteins of cytoskeleton (b) Oily film of lipids with diverse proteins embedded 3-11 MEMBRANE LIPIDS 98% of molecules in plasma membrane are lipids Phospholipids 75% of membrane lipids are phospholipids Amphiphilic molecules arranged in a bilayer Hydrophilic phosphate heads face water on each side of membrane Hydrophobic tails—directed toward the center, avoiding water Drift laterally from place to place Movement keeps membrane fluid 3-12 MEMBRANE LIPIDS Cholesterol 20% of the membrane lipids Holds phospholipids still and can stiffen membrane Glycolipids 5% of the membrane lipids Phospholipids with short carbohydrate chains on extracellular face Contributes to glycocalyx—carbohydrate coating on the cell surface 3-13 MEMBRANE PROTEINS Membrane proteins Transmembrane (integral) proteins 2% of the molecules in plasma membrane 50% of its weight Can drift about freely in phospholipid film Some anchored to cytoskeleton Peripheral proteins Carbohydrate Adhere to one face of the membrane Usually tethered to the cytoskeleton Phospholipid bilayer Cytoskeletal protein 3-14 MEMBRANE PROTEINS Functions of membrane proteins include the following: Receptors, second-messenger systems, enzymes, ion channels, carriers, cell-identity markers, cell-adhesion molecules Chemical messenger Breakdown products CAM of another cell Ions (a) Receptor A receptor that binds to chemical messengers such as hormones sent by other cells (b) Enzyme An enzyme that breaks down a chemical messenger and terminates its effect (c) Ion Channel A channel protein that is constantly open and allows ions to pass into and out of the cell (d) Gated ion channel A gated channel that opens and closes to allow ions through only at certain times (e) Cell-identity marker A glycoprotein acting as a cellidentity marker distinguishing the body’s own cells from foreign cells (f) Cell-adhesion molecule (CAM) A cell-adhesion molecule (CAM) that binds one cell to another 3-15 RECEPTORS, SECOND-MESSANGER SYSTEMS AND ENZYMES Cell communication via chemical signals Receptors—surface proteins on plasma membrane of target cell Bind these chemicals (hormones, neurotransmitters) Receptor usually specific for one substrate Messenger (chemical) binds to a surface receptor Triggers changes within the cell that produce a second messenger in the cytoplasm Involves transmembrane proteins and enzymes 3-16 CHANNEL PROTEINS Transmembrane proteins with pores that allow water and dissolved ions to pass through membrane Some are constantly open Some are gated channels that open and close in response to stimuli Ligand (chemically)-regulated gates Voltage-regulated gates Mechanically regulated gates (stretch and pressure) 3-17 CARRIERS OR PUMPS Transmembrane proteins bind to glucose, electrolytes, and other solutes Transfer them across membrane Pumps consume ATP in the process 3-18 CELL-IDENTITY MARKERS Glycoproteins contribute to the glycocalyx Carbohydrate surface coating Acts like a cell’s “identification tag” Enables our bodies to identify which cells belong to it and which are foreign invaders 3-19 CELL-ADHESION MOLECULES (CAMS) Adhere cells to each other and to extracellular material Cells do not grow or survive normally unless they are mechanically linked to the extracellular material Special events: sperm–egg binding; binding of immune cell to a cancer cell requires CAMs 3-20 THE GLYCOCALYX Unique fuzzy coat external to the plasma membrane Carbohydrate moieties of membrane glycoproteins and glycolipids Unique in everyone but identical twins Functions Protection Immunity to infection Defense against cancer Transplant compatibility – Cell adhesion – Fertilization – Embryonic development 3-21 MICROVILLI Extensions of membrane (1–2 mm) Serves to increase cell’s surface area Best developed in cells specialized in absorption Gives 15 to 40 times more absorptive surface area On some cells they are very dense and appear as a fringe—“brush border” Milking action of actin Actin filaments shorten microvilli Pushing absorbed contents down into cell 3-22 MICROVILLI Glycocalyx Microvillus Actin microfilaments (a) 1.0 m (b) • Actin microfilaments are centered in each microvilli 0.1 m 2-23 CILIA Hairlike processes 7–10 mm long Single, nonmotile primary cilium found on nearly every cell “Antenna” for monitoring nearby conditions Sensory in inner ear, retina, nasal cavity, and kidney Motile cilia—respiratory tract, uterine tubes, ventricles of the brain, efferent ductules of testes Beat in waves Sweep substances across surface in same direction Power strokes followed by recovery strokes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mucus Saline layer Epithelial cells 1 2 3 Power stroke (a) (b) 4 5 6 7 Recovery stroke 3-24 CILIA Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cilia (a) 10 µm a: Custom Medical Stock Photo, Inc. 3-25 CILIA Axoneme: Peripheral microtubules Central microtubules Dynein arms Axoneme—core of cilia that is the structural basis for ciliary movement Has 9 + 2 structure of microtubules Shaft of cilium Basal body Nine pairs form basal body inside the cell membrane Anchors cilium Dynein arms “crawl” up adjacent microtubule, bending the cilia Uses energy from ATP Plasma membrane (b) Dynein arm Central microtubule Axoneme Peripheral microtubules (d) Cilia Saline layer Chloride pumps pump Cl- into ECF Na+ and H2O follows Cilia beat freely in saline layer Microvilli Dynein arm Central microtubule Peripheral microtubules (c) 0.15 m c: © Biophoto Associates/Photo Researchers, Inc. 3-26 CYSTIC FIBROSIS Saline layer at cell surface due to chloride pumps move Cl- out of cell. Na+ ions and H2O follow Cystic fibrosis—hereditary disease in which cells make chloride pumps, but fail to install them in the plasma membrane Chloride pumps fail to create adequate saline layer on cell surface Thick mucus plugs pancreatic ducts and respiratory tract Inadequate digestion of nutrients and absorption of oxygen Chronic respiratory infections Life expectancy of 30 Mucus Saline layer Epithelial cells 3-27 FLAGELLA Tail of a sperm—only functional flagellum Whiplike structure with axoneme identical to cilium Much longer than cilium Stiffened by coarse fibers that support the tail Movement is more undulating, snakelike No power stroke or recovery stroke as in cilia 3-28 THE CELL INTERIOR Structures in the cytoplasm Organelles, cytoskeleton, and inclusions All embedded in a clear gelatinous cytosol 3-29 THE CYTOSKELETON Cytoskeleton—collection of filaments and cylinders Determines shape of cell, lends structural support, organizes its contents, directs movement of substances through the cell, and contributes to the movements of the cell as a whole Composed of: Microfilaments: 6 nm thick, actin, forms terminal web Intermediate fibers: 8–10 nm, support, strength, and structure Microtubules: 25 nm, tubulin, movement 3-30 CYTOSKELETON Microvilli Microfilaments Secretory vesicle in transport Desmosome Terminal web Lysosome Kinesin Microtubule Intermediate filaments Centrosome Microtubule undergoing disassembly Intermediate filaments Microtubule in the process of assembly Nucleus Mitochondrion (a) Basement membrane Hemidesmosome 3-31 MICROTUBULES A microtubule is a cylinder of 13 parallel strands called protofilaments Each protofilament is a long chain of globular proteins called tubulin Microtubules radiate from centrosome and hold organelles in place, form bundles that maintain cell shape and rigidity, and act somewhat like railroad tracks Motor proteins “walk” along these tracks carrying organelles and other macromolecules to specific locations in the cell Not permanent structures, they come and go moment by moment (a) (b) (c) Microtubule Protofilaments Dynein arms Tubulin 3-32 ORGANELLES Internal structures, carry out specialized metabolic tasks Membranous organelles Nucleus, mitochondria, lysosomes, peroxisomes, endoplasmic reticulum, and Golgi complex Nonmembranous organelles Ribosomes, centrosomes, centrioles, basal bodies 3-33 THE NUCLEUS Nucleus—largest organelle (5 mm) Most cells have one nucleus A few cells are anuclear or multinucleate Nuclear envelope—two unit membranes surround nucleus Perforated by nuclear pores formed by rings of protein Regulate molecular traffic through envelope Hold two unit membranes together Nucleoplasm—material in nucleus Chromatin (threadlike matter) composed of DNA and protein Nucleoli—one or more dark masses where ribosomes are produced 3-34 ENDOPLASMIC RETICULUM Endoplasmic reticulum—system of interconnected channels called cisternae enclosed by unit membrane Rough endoplasmic reticulum— composed of parallel, flattened sacs covered with ribosomes Continuous with outer membrane of nuclear envelope Adjacent cisternae are often connected by perpendicular bridges Produces the phospholipids and proteins of the plasma membrane Synthesizes proteins that are packaged in other organelles or secreted from cell 3-35 ENDOPLASMIC RETICULUM Smooth endoplasmic reticulum Lack ribosomes Cisternae more tubular and branching Cisternae are thought to be continuous with those of rough ER Synthesizes steroids and other lipids Detoxifies alcohol and other drugs Manufactures all membranes of the cell Rough and smooth ER are functionally different parts of the same network 3-36 SMOOTH AND ROUGH ER Rough endoplasmic reticulum Ribosomes Cisternae (c) Smooth endoplasmic reticulum 3-37 GOLGI COMPLEX Golgi complex—a small system of cisternae that synthesize carbohydrates and put the finishing touches on protein and glycoprotein synthesis Receives newly synthesized proteins from rough ER Sorts them, cuts and splices some of them, adds carbohydrate moieties to some, and packages the protein into membrane-bound Golgi vesicles Some become lysosomes Some migrate to plasma membrane and fuse to it Some become secretory vesicles for later release 3-38 LYSOSOMES Lysosomes—package of enzymes bound by a single unit membrane Extremely variable in shape Functions Mitochondria Lysosomes Intracellular hydrolytic digestion of proteins, nucleic acids, complex carbohydrates, phospholipids, and other substances Autophagy—digest and dispose of Golgi complex worn out mitochondria and other organelles Autolysis—“cell suicide”: some cells are meant to do a certain job and then (a) Lysosomes destroy themselves 1 mm 3-39 PEROXISOMES Peroxisomes—resemble lysosomes but contain different enzymes and are not produced by the Golgi complex General function is to use molecular oxygen to oxidize organic molecules These reactions produce hydrogen peroxide (H2O2) Catalase breaks down excess peroxide to H2O and O2 Neutralize free radicals, detoxify alcohol, other drugs, and a variety of blood-borne toxins Break down fatty acids into acetyl groups for mitochondrial use in ATP synthesis In all cells, but abundant in liver and kidney Peroxisomes Smooth ER (b) Peroxisomes 0.3 mm 3-40 MITOCHONDRIA Mitochondria—organelles specialized for synthesizing ATP Variety of shapes: spheroid, rod-shaped, kidney-shaped, or threadlike Surrounded by a double unit membrane Inner membrane has folds called cristae Spaces between cristae called matrix Matrix contains ribosomes, enzymes used for ATP synthesis, small circular DNA molecule – Mitochondrial DNA (mtDNA) “Powerhouses” of the cell Matrix Outer membrane Inner membrane Mitochondrial ribosome Intermembrane space Crista 1 m Energy is extracted from organic molecules and transferred to ATP 3-41 RIBOSOMES Ribosomes—small granules of protein and RNA Found in nucleoli, in cytosol, and on outer surfaces of rough ER, and nuclear envelope They “read” coded genetic messages (messenger RNA) and assemble amino acids into proteins specified by the code 3-42 CENTRIOLES Centriole—a short cylindrical assembly of microtubules arranged in nine groups of three microtubules each Two centrioles lie perpendicular to each other within a small, clear area of cytoplasm called the centrosome Play role in cell division Cilia and flagella formation Each basal body of a cilium or flagellum is a single centriole oriented perpendicular to plasma membrane Basal bodies originate in centriolar organizing center Migrate to plasma membrane Two microtubules of each triplet elongate to form the nine pairs of peripheral microtubules of the axoneme Cilium reaches full length in less than an hour 3-43 INCLUSIONS Two kinds of inclusions Stored cellular products Foreign bodies Glycogen granules, pigments, and fat droplets Viruses, intracellular bacteria, dust particles, and other debris phagocytized by a cell Never enclosed in a unit membrane Not essential for cell survival 3-44 And now presenting: THE CELL- Live 3-45
