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Burton's Microbiologyfor the Health Sciences
Chapter 3. Cell Structure and Taxonomy
Chapter 3 Outline
• Introduction
• Eucaryotic Cell Structure
• Procaryotic Cell Structure
• Summary of Structural Differences Between Procaryoticand Eucaryotic Cells
• Reproduction of Organisms and Their Cells
• Taxonomy
• Determining Relatedness Among Organisms
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Introduction
• The cell is the fundamental unit of any living organism because it exhibits the basic characteristics of life.
• There are two categories of cells: eucaryotic and procaryotic.
• Some microbes are procaryotes (bacteria and archaea), some are eucaryotes (algae, protozoa, fungi), and some are not composed of cells (viruses, prions, viroids).
Acellular and Cellular Microbes
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Introduction, cont.
• Eucaryotic cells contain a “true” nucleus, whereas procaryotic cells do not. A true nucleus consists of nucleoplasm, chromosomes, and a nuclear membrane.
• Eucaryotic cells have membrane-bound organelles, whereas procaryotic cells do not.
• Both eucaryotic and procaryotic cells possess a cell membrane. Cell membranes have selective permeability, allowing only certain substances to pass through them.
Eucaryotic Cell Structure
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The Eucaryotic Nucleus
• The “command center” of the cell.
• 3 components: nucleoplasm, chromosomes, and nuclear membrane.
• Chromosomes are embedded in the nucleoplasm.
• Eucaryotic chromosomes consist of linear DNA molecules and proteins.
• Genes are located along chromosomes. An organism's complete collection of genes is referred to as its genotype or genome.
• Each gene contains the information to produce one or more gene products (usually proteins).
The Eucaryotic Nucleus, cont.
• Although most genes code for proteins, some code for 2 types of ribonucleic acid (RNA)
– Ribosomal ribonucleic acid (rRNA)
– Transfer ribonucleic acid (tRNA)
• The number and composition of chromosomes and the number of genes on each chromosome are characteristic of the particular species of organism.
• Human diploid cells have 46 chromosomes (23 pairs).
• It has been estimated that the human genome consists of between 20,000 and 30,000 genes.
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Other Eucaryotic Cell Structures
• Cytoplasm
– A semi-fluid, gelatinous, nutrient
matrix
– Contains storage granules and a
variety of organelles
– Each organelle has a specific
function
– The cytoplasm is where most
metabolic reactions occur
– Onion Cytoplasmic Streaming
http://www.youtube.com/watch?
v=iNX9fP1yfT8
Other Eucaryotic Cell Structures• Endoplasmic Reticulum (ER)
– A highly convoluted system of membranes arranged to form a transport network in the cytoplasm
– Rough ER has ribosomes attached to it; smooth ER does not
• Ribosomes
– Consist of ribosomal RNA and protein
– The sites of protein synthesis
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Ribosomes
RibosomeProkaryotic vs. Eukaryotic Ribosome
Other Eucaryotic Cell Structures, cont.• Golgi Complex
– Also called the Golgi apparatus or Golgi body
– Connects or communicates with ER
– Completes the transformation of newly synthesized proteins and packages them for storage or export (“packaging plants”)
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Other Eucaryotic Cell Structures, cont.
• Lysosomes and Peroxisomes
– Originate in the Golgi complex
– Lysosomes contain lysozyme and other digestive enzymes
– Peroxisomes are membrane-bound vesicles where H2O2 is generated and broken down
Other Eucaryotic Cell Structures, cont.
• Mitochondria
– “Power plants,” “powerhouses,” or “energy factories”
– ATP molecules are produced within mitochondria by cellular respiration
– Number of mitochondria varies depending on activities of the cell
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Bacterium and Mitochondria
Other Eucaryotic Cell Structures, cont.
• Plastids
– Membrane-bound structures containing photosynthetic pigments
– They are sites of photosynthesis
– Chloroplasts are a type of plastid; they contain chlorophyll
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Other Eucaryotic Cell Structures, cont.
• Cytoskeleton
– A system of fibers throughout the cytoplasm
– 3 types of fibers: microtubules, microfilaments and intermediate filaments
– Microtubules and microfilaments are essential for a variety of activities
Other Eucaryotic Cell Structures, cont.
• Cell wall
– Some eucaryotic cells contain cell walls – an external structure to provide shape, protection, and rigidity
– Simpler in structure than procaryotic cell walls
– Chitin found in cell walls of fungi; cellulose in cell walls of algae and plants
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Plant vs. Zombies…I mean Bacteria
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Other Eucaryotic Cell Structures, cont.
• Flagella and Cilia (contain microtubules)
– Some eucaryotic cells (e.g., spermatozoa and certain protozoa) possess long, thin, whiplike organelles of locomotion called flagella.
– Flagellated cells may possess one or more flagella.
– Some cells move by means of cilia, which are shorter, thinner, and more numerous than flagella; described as being “hair-like.”
– Cilia can be found on some species of protozoa and certain types of cells in our bodies (e.g., ciliated epithelial cells in the respiratory tract).
Cilia
Cross sections of cilia showing the 9 + 2 arrangement of microtubules.
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Procaryotic Cell Structure
• Procaryotic cells are about 10 times smaller than eucaryotic cells.
• Procaryotic cells are simple compared to eucaryotic cells.
• Procaryotic cells reproduce by binary fission.
• All bacteria are procaryotes, as are archaea.
• Unlike eucaryotic cells, the cytoplasm of eucaryotic cells is not filled with internal membranes.
• The cytoplasm of procaryotic cells is surrounded by a cell membrane, a cell wall (usually), and sometimes a capsule or slime layer.
Procaryotic Cell
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Procaryotic Cell Structure, cont.
• Chromosome
– Procaryotic chromosome usually consists of a single, long, supercoiled, circular DNA molecule –serves as the control center of the cell
– Plasmids are small circular molecules of DNA that are not part of the chromosome (extra-chromosomal)
Procaryotic Cell Structure, cont.
• Cell membrane
– Similar in structure and function to the eucaryotic cell membrane
– Selectively permeable
– Many enzymes are attached to the cell membrane and metabolic reactions take place there
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Procaryotic Cell Structure, cont.
• Cytoplasm
– Semi-liquid that consists of water, enzymes, waste products, nutrients, proteins, carbohydrates and lipids – materials required for metabolic functions
• Cytoplasmic particles
– Most are ribosomes, some of which occur in clusters
– Eucaryotic ribosomes are smaller than procaryotic ribosomes, but their function is the same – they are the sites of protein synthesis
Procaryotic Cell Structure, cont.
• Bacterial Cell Wall
– A rigid exterior that defines the shape of bacterial cells – chemically complex
– Main constituent of most bacterial cell walls is peptidoglycan (only found in bacteria)
– Gram-positive bacteria have a thick layer of peptidoglycan; Gram-negative bacteria have a much thinner layer
– Mycoplasma spp. do not have a cell wall; they are pleomorphic
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Gram-negative and Gram-positive Cell Walls
Bacterial Cell Walls
A. Gram-positive bacterium B. Gram-negative bacterium
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Gram + vs. Gram -
Gram’s Stain
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Procaryotic Cell Structure, cont.• Glycocalyx (Slime Layers and
Capsules)
– Some bacteria possess glycocalyx, a slimy, gelatinous material produced by the cell membrane and secreted outside the cell wall
– 2 types of glycocalyx – slime layer (loosely connected to the cell wall) and capsule (highly organized and firmly connected)
• Pseudomonas spp. produces a slime layer
• K. pneumoniae, N. meningitidis and S. pneumoniae possess a capsule, which serves an antiphagocytic function
Capsule Stain (Example of a negative staining technique)
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Procaryotic Cell Structure, cont.
• Flagella
– Motile bacteria possess flagella – whiplike appendages composed of threads of protein called flagellin
– Number and arrangement of flagella are characteristic of a particular species:
• Peritrichous bacteria – flagella over entire surface
• Lophotrichous bacteria – flagella at one end
• Amphitrichous bacteria – flagella at both ends
• Monotrichous bacteria – single polar flagellum
Four Basic Types of Flagellar Arrangement on Bacteria
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A Peritrichous Salmonella Cell
Procaryotic Cell Structure (continued)
• Pili (also called fimbriae)
– Hair-like structures, most often observed on Gram-negative bacteria
– Composed of polymerized protein molecules called pilin
– Pili are thinner than flagella, have a rigid structure and are not associated with motility
– Pili enable bacteria to anchor themselves to surfaces
– Some bacteria possess a sex pilus for conjugation
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Proteus vulgaris cell, showing pili and several flagella
Pili
Flagella
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Procaryotic Cell Structure, cont.
• Spores (Endospores)
– A few genera (e.g., Bacillus and Clostridium) are capable of forming thick-walled spores as a means of survival
– The process of spore formation is called sporulation – it is not reproduction
– Spores have been shown to survive for many years and are resistant to heat, cold, drying, and most chemicals
– Usually one spore is produced in a bacterial cell and generates into one vegetative bacterium
– Endospores can be visualized using a spore stain
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A Bacillus Cell With a Well-Defined Endospore
Endospore
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Recap of Structural Differences Between Procaryotic and Eucaryotic Cells
• Eucaryotic cells contain a true nucleus; procaryotic cells do not.
• Eucaryotic cells are divided into plant and animal types
– Animal cells do not have a cell wall, plant cells have a simple cell wall.
• Eucaryotic cells contain membranous structures and many membrane-bound organelles; procaryotic cells possess no membranes other than the cell membrane that encloses the cytoplasm
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Reproduction of Organisms and Their Cells
• Procaryotic Cell Reproduction
– Procaryotic cells reproduce by a process known as binary fission – one cell splits in half to become two daughter cells.
• Before a procaryotic cell divides in half, the chromosome must be duplicated.
– The time it takes for binary fission to occur is called the generation time.
• Generation time varies from one species to another and depends on growth conditions (under ideal conditions, E. coli has a generation time of about 20 minutes).
Binary Fission of a Bacterial Cell
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Taxonomy
• Taxonomy is the science of classification of living organisms.
• Taxonomy consists of classification, nomenclature, and identification.
• Classification is the arrangement of organisms into taxonomic groups (known as taxa).
• Tool for remembering the sequence of Taxa
– “King David Came Over for Good Spaghetti” KDCOFGS, K for Kingdom, D for Division, C for Class, O for Order, F for Family, G for Genus and S for species.
Microbial Classification
• The science of taxonomy was established based on the binomial system of nomenclature.
• In the binomial system, each organism is given 2 names –genus and the specific epithet. Taken together, both names constitute the species.
– For example, Escherichia coli; Escherichia is the genus and coli is the specific epithet.
– The genus is frequently abbreviated with just a single letter, (e.g., E for Escherichia).
• The abbreviation “sp.” is used to designate a single species and “spp.” for more than one species.
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Microbial Classification
• Organisms are categorized into larger groups based on their similarities and differences.
• The Five-Kingdom System of Classification
1. Bacteria and archaea – Kingdom Procaryotae
2. Algae and protozoa – Kingdom Protista
3. Fungi – Kingdom Fungi
4. Plants – Kingdom Plantae
5. Animals – Kingdom Animalia
• Viruses are not included because they are acellular.
• Other systems of classification do exist.
Microbial Classification (continued)
• The Three-Domain System of Classification
1. Archaea (procaryotic)
2. Bacteria (procaryotic)
3. Eucarya (all eucaryotic organisms)
• The Three-Domain System is based on differences in the structure of certain ribosomal RNA (rRNA) molecules among organisms in the 3 domains.
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Determining Relatedness Among Organisms
• The most widely used technique for gauging diversity or “relatedness” of organisms is called ribosomal RNA (rRNA) sequencing.
• Ribosomes are composed of two subunits; a small subunit and a large subunit.
• The small subunit is composed of only one rRNA molecule, which is coded for by a gene called the 16S rRNA gene in procaryotes and the 18S rRNA gene in eucaryotes.
Determining Relatedness Among Organisms, cont.
• To determine how closely related one procaryoticorganism is to another, scientists compare the sequence of nucleotide base pairs in the 16S rRNA gene from one of the organisms to the sequence of base pairs in the 16S rRNA gene from the other organisms.
• The more similar the sequence of base pairs, the more closely related are the organisms.
