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Chapter 4: Cell Structure & Function (Outline) Cell Theory Cell Size Prokaryotic Cells Eukaryotic Cells Organelles Nucleus Endomembrane System Cytoskeleton Centrioles, Cilia and Flagella Development of Cell Theory In 1665, English Scientist Robert Hooke discovered cells while looking at a thin slice of cork In 1673, Anton van Leuwenhoek observed pond scum & discovered single-celled organisms using a handmade microscope In 1831, English botanist Robert Brown described the nucleus of cells In 1838, German Botanist, Matthias Schleiden, stated that all plant parts are made of cells In 1839, German physiologist Theodor Schwann stated that all animal tissues are composed of cells In 1858, Rudolf Virchow German physician concluded that cells must arise from preexisting cells Cell Theory A unifying concept in biology Originated from the work of biologists Schleiden, Schwann & Virchow States that: All organisms are composed of cells (Schleiden & Schwann, 1838-39) The cell is the basic unit of structure & function in organisms (Schleiden & Schwann, 1838-39) All cells come only from preexisting cells since cells are self-reproducing (Virchow, 1858) Cell Size Most much smaller than one millimeter (mm) Some as small as one micrometer (mm) Size restricted by Surface/Volume (S/V) ratio Surface is membrane, across which cell acquires nutrients and expels wastes Volume is living cytoplasm, which demands nutrients and produces wastes As cell grows, volume increases faster than surface Cells specialized in absorption modified to greatly increase surface area per unit volume Surface to Volume Ratio TotalSurfaceArea (Height Width Number Of Sides Number Of Cubes) 96 cm2 192 cm2 384 cm2 TotalVolume (Height Width Length x Number Of Cubes) 64 cm3 64 cm3 64 cm3 SurfaceAreaPerCube/VolumePerCube (Surface Area/ Volume) 1.5/1 3/1 6/1 Sizes of living things and their component Prokaryotic Cells Prokaryotes – lack a membrane-bounded nucleus and are structurally less complicated than the eukaryotes Prokaryotes are responsible for either all or significant portions of all of the following Nutrient recycling – mineralization; nitrogen fixing Decomposition of dead organisms Disease (infectious) – tuberculoses; anthrax Commercial uses – foodstuffs; antibiotics; insulin Prokaryotes are divided into two domains Domain Bacteria Domain Archaea Prokaryotic Cells Nuclear body is not bounded by a nuclear membrane Usually contains one circular chromosome composed of deoxyribonucleic acid (DNA) The nuclear body is called a nucleoid Extra chromosomal piece of DNA called plasmid Structurally simple Three basic shapes: Bacillus (rod) Coccus (spherical) Spirilla (spiral) Prokaryotic Cells: The Envelope Cell Envelopes include Glycocalyx Layer of polysaccharides outside cell wall May be slimy and easily removed, or Well organized and resistant to removal (capsule) Cell wall Consist of peptidoglycan (amino disaccharide & peptide) Maintains shape of the cell Plasma membrane Like in eukaryotes – a phospholipid bilayer with proteins Form internal pouches (mesosomes), why? Prokaryotic Cells: Cytoplasm Cytoplasm - semifluid solution bounded by a plasma membrane containing Nucleoid – location of the single bacterium chromosome (coiled) Plasmid – extrachromosomal piece of circular DNA Inclusion bodies – Stored granules of various substances Ribosomes – tiny particles where protein is synthesized (contain RNA & protein in 2 subunits) Thylakoids – extensive internal membranes found in cyanobacteria, function? Prokaryotic Cells: Appendages Appendages are made of protein that include Flagella – the most common form of bacterial motility (made up of a filament, hook & basal body) Fimbriae – small, bristle-like fibers that sprout from the cell surface (attach bacteria to a surface) Conjugation pili – rigid tubular structures used to pass DNA from cell to cell Prokaryotic Cells: Visual Summary Eukaryotic Cells Domain Eukarya Protists Fungi Plants Animals Eukaryotic cells contain: a true nucleus, bound by a double membrane a complex collection of organelles a plasma membrane Eukaryotic Cells : Organelles Compartmentalization: Allows eukaryotic cells to be larger than prokaryotic cells Isolates reactions from others Two classes: Endomembrane system: Organelles that communicate with one another via membrane channels and small vesicles Energy related organelles Mitochondria & chloroplasts Basically independent & self-sufficient Animal and Plant Cells Nucleus Command center of cell, why? Separated from cytoplasm by nuclear envelope Consists of double layer of membrane Nuclear pores permit exchange of ribosomal subunits & mRNA between nucleoplasm & cytoplasm Contains chromatin in semifluid nucleoplasm Chromatin contains DNA of genes Condenses to form chromosomes Dark nucleolus composed of ribosomal RNA (rRNA) Produces subunits of ribosomes Anatomy of the nucleus Messenger RNA (mRNA) carries information about a protein sequence to the ribosome Transfer RNA (tRNA) assembles the amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis Ribosomes Serve in protein synthesis Composed of rRNA Consists of a large subunit and a small subunit Each subunit is composed of protein and rRNA Subunits made in nucleolus Number of ribosomes in a cell varies depending on function (e.g. pancreatic cells) May be located: On the endoplasmic reticulum (ER), thereby making it “rough”, or Free in the cytoplasm, either singly or in groups called polyribosomes Ribosome Function Ribosome binding to the endoplasmic reticulum occurs through a signal peptide on the synthesized protein Signal peptide combines with a signal recognition particle (SRP) SRP attaches to SRP receptor, thus allowing protein to enter the lumen of the ER The signal peptide is removed from the protein (via signal peptidase) in the lumen of the ER Ribosomal subunits & mRNA break away and protein folds into its final shape Nucleus, Ribosomes, & ER Endomembrane System Restrict enzymatic reactions to specific compartments within cell Consists of: Nuclear envelope Membranes of endoplasmic reticulum Golgi apparatus Vesicles Several types Transport materials between organelles of the system Endomembrane System: The Endoplasmic Reticulum A membrane network within the cytoplasm of cells involved in the synthesis, modification and transport of cellular materials Rough ER Studded with ribosomes on cytoplasmic side Protein anabolism Synthesizes proteins Modifies proteins - adds sugar to protein (i.e. glycoproteins) Forms vesicles - transport of large molecules to other parts of cell (i.e. Plasma membrane or Golgi apparatus) Smooth ER Continuous with rough ER; No attached ribosomes Synthesis of lipids (i.e. phospholipids & steroids) Endoplasmic Reticulum Endomembrane System: The Golgi Apparatus Golgi Apparatus Consists of 3-20 flattened, curved membranebound saccules called cisternae Resembles stack of deflated balloons Modifies proteins (i.e. glycosylation) and lipids Packages them in vesicles Receives vesicles from ER on cis face Prepares for “export” in vesicles from trans face Within cell Export from cell (secretion, exocytosis) Golgi Apparatus Endomembrane System: Lysosomes Membrane-bound vesicles (common in animal cells but rare in plant cells) Produced by the Golgi apparatus Low pH Contain hydrolytic enzymes Digestion of large molecules Recycling of cellular resources Destroying nonfunctional organelles Lysosomes participate in apoptosis Normal part of development Example: tadpole → frog Peroxisomes Similar to lysosomes Membrane-bounded vesicles Enclose oxidative enzymes However Enzymes synthesized by free ribosomes in cytoplasm (instead of ER) Active in lipid metabolism Catalyze reactions that produce hydrogen peroxide H2O2 Toxic molecule Broken down to H2O and O2 by catalase enzyme Alcohol detoxification in liver Germinating seeds oxidize fatty acids to sugars → growth Peroxisomes & Vacuoles Energy-Related Organelles: Chloroplast Structure An organelle found within the cells of green plants & eukaryotic algae Bounded by a double membrane Inner membrane infolded Forms disc-like thylakoids, which are stacked to form grana Suspended in semi-fluid stroma Green due to chlorophyll Chlorophyll absorbs light between the red and blue spectrums and reflects green light, making leaves appear green Found ONLY in inner membranes of chloroplast Energy-Related Organelles: Chloroplasts Chloroplasts are a type of plastid & are considered to have originated as endosymbiotic cyanobacteria Has its own DNA and reproduces independently of the cell Captures light energy to drive cellular machinery Photosynthesis Synthesizes carbohydrates from CO2 and H2O Makes own food using CO2 as only carbon source Chloroplast Structure Other Plastids Different types of plastids are classified according to the kinds of pigments they contain Chromoplasts lack chlorophyll but contain carotenoids responsible for the yellow, orange, & red colors of some flowers and fruits Leucoplasts are colorless plastids, which synthesize and store a variety of energy sources in nonphotosynthetic tissues Amyloplasts (starch) Elaioplasts (lipids) Energy-Related Organelles: Mitochondria Mitochondria are rod-shaped organelles that can be considered the power generators of the cell Bounded by double membrane Cristae – Infoldings of inner membrane that encloses matrix, why? Matrix – Inner semifluid containing respiratory enzymes Involved in cellular respiration – process by which chemical energy of sugar is converted to ATP Produce most of ATP utilized by the cell Has its own DNA and reproduces independently of the cell Mitochondrial Structure The Cytoskeleton Maintains cell shape Assists in movement of cell and organelles Three types of macromolecular fibers Actin Filaments Intermediate Filaments Microtubules Dynamic, assemble and disassemble as needed Protein phosphorylation (e.g. protein kinases) Phosphorylation → disassembly Dephosphorylation → assembly Cytoskeleton Protein Fibers The Cytoskeleton: Actin Filaments Extremely thin filaments like a twisted pearl necklace Dense web just under plasma membrane maintains cell shape Support for microvilli in intestinal cells Intracellular traffic control For moving stuff around within cell Cytoplasmic streaming in plant cells Function in pseudopods of amoeboid cells Pinches off dividing animal cells apart during mitosis Important component in muscle contraction (other is myosin) Actin Filaments Actin Filament Operation Actin filaments interact with motor molecules (proteins that can attach, detach and reattach to the actin filament) Myosin pulls actin filaments in the presence of ATP In muscle cells, cytoplasmic myosin tails are bound to membranes, while heads interact with actin The Cytoskeleton: Intermediate Filaments Intermediate in size between actin filaments and microtubules Rope-like assembly of fibrous polypeptides Vary in nature (i.e. from tissue to tissue and from time to time) Functions: Mechanical stability of the plasma- and the nucleus-membranes Cell-cell interaction, like those holding skin cells tightly together (keratin) The Cytoskeleton: Microtubules Hollow cylinders made of two globular proteins called a and b tubulin giving rise to structures called dimers Dimers then arrange themselves into tubular spirals of 13 dimers around Assembly: Under control of Microtubule Organizing Center (MTOC) Most important MTOC is centrosome Interacts with proteins kinesin and dynein to cause movement of organelles Microtubule Operation Microtubules Microtubules disassemble and then reassemble into a spindle during cellular division Colchicine - a plant toxic (defense mechanism) that inhibits polymerization by binding to tubulin and preventing microtubule assembly The Cytoskeleton (Summary) Microfilaments regulate: Cell shape Cell movement Intermediate filaments effect: The mechanical stability of the plasma- & the nucleus-membranes Cell-cell interaction Microtubules effect: Localization and transport of organelles Cell division Microtubular Arrays: Centrioles Short, hollow cylinders Composed of 27 microtubules Microtubules are arranged in 9 sets of 3 each (9 + 0) pattern One pair per animal cell Located on centrosome of animal cells Oriented at right angles to each other Separate during mitosis (cell division) May give rise to basal bodies of cilia & flagella Plant cells do not have centrioles Centrioles Microtubular arrays: Cilia and Flagella Hair-like projections from cell surface that aid in cell movement Very different from prokaryote flagella Outer covering of plasma membrane Inside is a cylinder of 18 microtubules arranged in 9 pairs Two single microtubules run down the centre of the shaft (9 + 2) pattern found in cilia and flagella In eukaryotes, cilia are much shorter and numerous than flagella Cilia move in coordinated waves like oars Flagella move like a propeller or cork screw Cilia and Flagella The pairs of microtubules are connected by short arms of protein dymein Movement of the cilia or flagella is the result of sliding movements between microtubule pairs Beneath each cilium of flagellum in the cytoplasm of the cell is a basal body The two central microtubules of the cilia/flagellum do not extend into the basal boy. The nine pairs of microtubule do and they are joined by a third microtubule. Centrioles are needed to create basal bodies in order to produce cilia and/or flagella