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Chapter 3 and 4 An Introduction to Prokaryotic and Eukaryotic Cell Structure and Function
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Characteristics of Cells and Life
All living things (single and multicellular) are made of cells that share some common characteristics: basic shape – spherical, cubical, cylindrical internal content – cytoplasm, surrounded by a membrane DNA chromosome(s), ribosomes, metabolic capabilities Two basic cell types: eukaryotic and prokaryotic
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Characteristics of Cells
Eukaryotic cells: animals, plants, fungi, and protists contain double-membrane bound nucleus with DNA chromosomes contain membrane-bound organelles that compartmentalize the cytoplasm and perform specific functions Prokaryotic cells: bacteria and archaea no nucleus or other membrane-bound organelles
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Prokaryotic cell
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External Structures Appendages Glycocalyx – surface coating
two major groups of appendages: motility – flagella and axial filaments (periplasmic flagella) attachment or channels – fimbriae and pili Glycocalyx – surface coating
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Flagellar Arrangements
Monotrichous – single flagellum at one end Lophotrichous – small bunches arising from one end of cell Amphitrichous – flagella at both ends of cell Peritrichous – flagella dispersed over surface of cell; slowest
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Fimbriae Fine, proteinaceous, hairlike bristles from the cell surface
Function in adhesion to other cells and surfaces
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Pili Rigid tubular structure made of pilin protein
Found only in Gram negative cells Function to join bacterial cells for partial DNA transfer called conjugation
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Glycocalyx Coating of molecules external to the cell wall, made of sugars and/or proteins Two types: slime layer - loosely organized and attached capsule - highly organized, tightly attached Functions: protect cells from dehydration and nutrient loss inhibit killing by white blood cells by phagocytosis contributing to pathogenicity attachment - formation of biofilms
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The Cell Envelope External covering outside the cytoplasm
Composed of two basic layers: cell wall and cell membrane Maintains cell integrity Two generally different groups of bacteria demonstrated by Gram stain: Gram-positive bacteria: thick cell wall composed primarily of peptidoglycan and cell membrane Gram-negative bacteria: outer membrane, thin peptidoglycan layer, and cell membrane
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Structure of Cell Walls
Determines cell shape, prevents lysis (bursting) or collapsing due to changing osmotic pressures Peptidoglycan is primary component: unique macromolecule composed of a repeating framework of long glycan chains cross-linked by short peptide fragments
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Gram-positive Cell Wall
Thick, homogeneous sheath of peptidoglycan 20-80 nm thick includes teichoic acid and lipoteichoic acid: function in cell wall maintenance and enlargement during cell division; move cations across the cell envelope; stimulate a specific immune response
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Gram-negative Cell Wall
Composed of an outer membrane and a thin peptidoglycan layer Outer membrane is similar to cell membrane bilayer structure outermost layer contains lipopolysaccharides and lipoproteins (LPS) endotoxin that may become toxic when released during infections may function as receptors and blocking immune response contains porin proteins in upper layer – regulate molecules entering and leaving cell Bottom layer composed of phospholipids and lipoproteins
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Gram-negative Cell Wall
Single, thin sheet of peptidoglycan Protective structure while providing some flexibility and sensitivity to lysis Periplasmic space surrounds peptidoglycan
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The Gram Stain Differential stain that distinguishes cells with a Gram-positive cell wall from those with a Gram-negative cell wall Gram-positive - retain crystal violet and stain purple Gram-negative-lose crystal violet and stain red from safranin counterstain Important basis of bacterial classification and identification Practical aid in diagnosing infection and guiding drug treatment
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Atypical Cell Walls Some bacterial groups lack typical cell wall structure i.e. Mycobacterium and Nocardia Gram-positive cell wall structure with lipid mycolic acid (cord factor) pathogenicity and high degree of resistance to certain chemicals and dyes basis for acid-fast stain used for diagnosis of infections caused by these microorganisms Some have no cell wall i.e. Mycoplasma cell wall is stabilized by sterols pleomorphic
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Bacterial Internal Structures
Cell cytoplasm: dense gelatinous solution of sugars, amino acids, and salts 70-80% water serves as solvent for materials used in all cell functions
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Bacterial Internal Structures
Plasmids small circular, double-stranded DNA free or integrated into the chromosome duplicated and passed on to offspring not essential to bacterial growth and metabolism may encode antibiotic resistance, tolerance to toxic metals, enzymes and toxins used in genetic engineering- readily manipulated and transferred from cell to cell
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Bacterial Internal Structures
Ribosomes made of 60% ribosomal RNA and 40% protein consist of two subunits: large and small Prokaryotic differ from Eukaryotic ribosomes in size and number of proteins site of protein synthesis present in all cells
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Bacterial Internal Structures
Inclusions and granules intracellular storage bodies vary in size, number and content bacterial cell can use them when environmental sources are depleted examples: glycogen, poly-b-hydroxybutyrate, gas vesicles for floating, sulfur and phosphate granules (metachromatic granules)
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Endospores Resistance linked to high levels of calcium and dipicolinic acid Dehydrated, metabolically inactive thick coat Longevity verges on immortality – 25 to 250 million years Resistant to ordinary cleaning methods and boiling Pressurized steam at 120oC for minutes will destroy
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Bacterial Shapes, Arrangements, and Sizes
Variety in shape, size, and arrangement but typically described by one of three basic shapes: coccus - spherical bacillus – rod coccobacillus – very short and plump vibrio – gently curved spirillum - helical, comma, twisted rod spirochete – spring-like
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Bacterial Shapes, Arrangements, and Sizes
Arrangement of cells is dependent on pattern of division and how cells remain attached after division: cocci: singles diplococci – in pairs tetrads – groups of four irregular clusters chains cubical packets bacilli: palisades
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Species and Subspecies
Species –a collection of bacterial cells which share an overall similar pattern of traits in contrast to other bacteria whose pattern differs significantly Strain or variety – a culture derived from a single parent that differs in structure or metabolism from other cultures of that species (biovars, morphovars) Type – a subspecies that can show differences in antigenic makeup (serotype or serovar), susceptibility to bacterial viruses (phage type) and in pathogenicity (pathotype)
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The History of Eukaryotes
They first appeared approximately 2 billion years ago Evidence suggests evolution from prokaryotic organisms by symbiosis Organelles originated from prokaryotic cells trapped inside them
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Eukaryotic
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External Structures Glycocalyx
an outermost boundary that comes into direct contact with environment usually composed of polysaccharides appears as a network of fibers, a slime layer or a capsule functions in adherence, protection, and signal reception beneath the glycocalyx fungi and most algae have a thick, rigid cell wall protozoa, a few algae, and all animal cells lack a cell wall and have only a membrane
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External Boundary Structures
Cell wall rigid, provides structural support and shape Fungi have thick inner layer of polysaccharide fibers composed of chitin or cellulose and a thin layer of mixed glycans algae – varies in chemical composition; substances commonly found include cellulose, pectin, mannans, silicon dioxide, and calcium carbonate
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External Boundary Structures
Cytoplasmic (cell) membrane typical bilayer of phospholipids and proteins sterols confer stability serves as selectively permeable barrier in transport Eukaryotic cells also contain membrane-bound organelles that account for 60-80% of their volume
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Internal Structures Nucleus
compact sphere, most prominent organelle of eukaryotic cell nuclear envelope composed of two parallel membranes separated by a narrow space and is perforated with pores contains chromosomes nucleolus – dark area for rRNA synthesis and ribosome assembly
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Internal Structures Endoplasmic reticulum – two types:
Rough endoplasmic reticulum (RER)– originates from the outer membrane of the nuclear envelop and extends in a continuous network through cytoplasm; rough due to ribosomes; proteins synthesized and shunted into the ER for packaging and transport; first step in secretory pathway Smooth endoplasmic reticulum (SER)– closed tubular network without ribosomes; functions in nutrient processing, synthesis and storage of lipids, etc.
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Internal Structures Golgi apparatus
consists of a stack of flattened sacs called cisternae closely associated with ER Transitional vesicles from the ER containing proteins go to the Golgi apparatus for modification and maturation Condensing vesicles transport proteins to organelles or secretory proteins to the outside
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Internal Structures Lysosomes
vesicles containing enzymes that originate from Golgi apparatus involved in intracellular digestion of food particles and in protection against invading microbes
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Internal Structures Mitochondria
consists of an outer membrane and an inner membrane with folds called cristae Cristae hold the enzymes and electron carriers of aerobic respiration divide independently of cell contain DNA and prokaryotic ribosomes function in energy production
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Internal Structures Ribosomes composed of rRNA and proteins
40S and 60S subunits form 80S ribosomes larger than prokaryotic ribosomes function in protein synthesis
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Kingdom Fungi 100,000 species divided into 2 groups:
macroscopic fungi (mushrooms, puffballs, gill fungi) microscopic fungi (molds, yeasts) Majority are unicellular or colonial; a few have cellular specialization
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Microscopic Fungi Exist in two morphologies:
yeast – round ovoid shape, asexual reproduction hyphae – long filamentous fungi or molds Some exist in either form – dimorphic – characteristic of some pathogenic molds
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Fungal Nutrition All are heterotrophic
Majority are harmless saprobes living off dead plants and animals Some are parasites, living on the tissues of other organisms, but none are obligate; mycoses – fungal infections Growth temperature 20o-40oC Extremely widespread distribution in many habitats
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Fungal Reproduction Primarily through spores formed on reproductive hyphae Asexual reproduction – spores are formed through budding or mitosis; conidia or sporangiospores Sexual reproduction – spores are formed following fusion of male and female strains and formation of sexual structure Sexual spores and spore-forming structures are one basis for classification
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Fungal Classification
The Fungi kingdom is subdivided into two subkingdoms Amastigomycota - common inhabitants of terrestrial habitats. Several of them are human pathogens Mastigomycota - primitive filamentous fungi that live primarily in water and may cause disease in potatoes and grapes
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Amastigomycota Fungi that produce sexual and asexual spores (perfect)
Zygomycota – zygospores; sporangiospores and some conidia Ascomycota – ascospores; conidia Basidiomycota – basidiospores; conidia Fungi that produce only asexual spores Deuteromycota – majority are yeasts and molds; no sexual spores known; conidia
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Roles of Fungi Adverse impact Beneficial impact
mycoses, allergies, toxin production destruction of crops and food storages Beneficial impact decomposers of dead plants and animals sources of antibiotics, alcohol, organic acids, vitamins used in making foods and in genetic studies
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Distribution and Importance of Parasitic Worms
Approximately 50 species parasitize humans Afflict billions of humans Distributed worldwide; some restricted to certain geographic regions with higher incidence in tropics
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