Download Chapter 4: Characteristics of Prokaryotic and Eukaryotic Cells

Chapter 4: Characteristics of Prokaryotic and Eukaryotic Cells
Basic Cell Types
• Prokaryotic Cells
• Eukaryotic Cells
• Evolution of Endosymbiosis
• The Movement by Substances Across Membranes
Basic Cell Types
• Prokaryote: single-celled organisms, and all are bacteria.
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Eukaryote: single-celled or multi-cellular organisms
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Pro = before
Eu = true
Karyon = nucleus
Similarities: Plasma membrane, DNA and cell wall (plant cells)
Differences:
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Eukaryotic DNA is in a nucleus surrounded by a nuclear membrane
Prokaryotic DNA is in a nuclear region not surrounded by a membrane
Table 4.1:Similarities and Differences Between Prokaryotic/Eukaryotic Cells
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Prokaryotic cells have a single circular chromosome; Eukaryotic cells have paired
chromosomes
Prokaryotic cells lack histone proteins; Eukaryotic cells have histone proteins
Prokaryotic cell wall has peptidoglycan; plant and fungal cells have both cellulose and chitin
Domains
• A relatively new concept in biological classification, domain is the highest category
• Three domains:
• Archaea (ancient)
• Bacteria (eubacteria)
• Eukarya
Size, Shape and Arrangement
• Prokaryotes are among the smallest of all organisms
• Prokaryotes range from 0.5 – 2.0 µm in diameter and from 1.0 – 60 µm in length
• Exception: 1991- Epulopiscium fishelsoni is a bacterial symbiont of sturgeon fish (80 µm
in diameter and 600 µm in length
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Arrangements of Bacteria
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Cocci in pairs (diplococci): Neisseria sp.
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Cocci in chains (streptococci): Streptococcus sp.
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Rods in chains: Lactobacillus sp.
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Cocci in clusters: Staphylococcus sp.
An Overview of Structure
• Structurally, bacterial cells consist of the following:
• Cell membrane, usually surrounded by a cell wall
• Internal cytoplasm with ribosomes, nuclear region, and in some cases, granules and/or
vesicles
• Capsules, flagella, and pili (external)
The Cell Wall
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Lies outside the cell membrane in nearly all bacteria
Two important functions:
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Maintains the characteristic shape
Prevents the cell from bursting when fluids flow into the cell by osmosis
Components of Bacterial Cell Walls
Peptidoglycan (murein): The single most important component
• This polymer is made up of two alternating sugar units:
• N-acetylglucosamine (NAG)
• N-acetylmuramic acid (NAM)
• The sugars are joined by short peptide chains that consist of four amino acids
(tetrapeptides)
Teichoic Acid
• An additional component found in cell walls of gram-positive bacteria
• Consists of glycerol, phosphates, and ribitol (sugar alcohol)
• This polymer extends beyond the rest of the cell wall
• Two functions:
• Attachment site for bacteriophages
• Passageway for movement of ions in/out of cell
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Outer Membrane (OM)
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A bilayer membrane found in gram-negative bacteria
Forms the outermost layer of the cell wall; is attached to the peptidoglycan by a continuous
layer of lipoprotein molecules
Proteins called porins form channels through the OM
OM has surface antigens and receptors
Lipopolysaccharide (LPS)
• An important component of the OM
• Also called endotoxin; used to ID gram-negative bacteria
• Released when the cell walls of bacteria are broken down
• Consists of polysaccharides and Lipid A
Periplasmic Space
• The area between the cytoplasmic membrane and the plasma membrane in gram-negative
bacteria
• Active area of cell metabolism
• Contains the cell wall, digestive enzymes and transport proteins
• Gram-positive bacteria lack both an OM and a periplasmic space
Distinguishing Bacteria by Cell Walls
• Gram-positive Bacteria have a relatively thick layer of peptidoglycan (60-90%)
• Gram-negative Bacteria have a more complex cell wall with a thin layer of peptidoglycan (1020%)
• Acid-fast Bacteria is thick, like that of gram-positive bacteria, but has much less peptidoglycan
and about 60% lipid
Acid-Fast Bacteria
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Found in bacteria that belong to the genus, Mycobacterium sp.
Cell wall is mainly composed of lipid
Lipid component is mycolic acid
Acid-fast bacteria stain gram-positive
Controlling Bacteria by Damaging Cell Walls
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The antibiotic penicillin blocks the final stages of peptidoglycan synthesis
The enzyme lysozyme, found in tears and other human body secretions, digests peptidoglycan
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Wall-Deficient Organisms
• Bacteria that belong to the genus Mycoplasma have no cell walls
• They are protected from osmotic swelling and bursting by a strengthened cell membrane that
contains sterols
• Wall deficient strains are called L-forms
Internal Structure
Bacterial cells typically contain (in their cytoplasm):
 Ribosomes
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Nucleoid region
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Vacuoles
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Certain bacteria sometimes contain endospores
Ribosomes
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Consist of ribonucleic acid and protein; serve as sites of protein synthesis
Abundant in the cytoplasm of bacteria
Often grouped in long chains called polyribosomes
70S in bacteria; 80S in eukaryotes
Streptomycin & Erythromycin bind specifically to 70S ribosomes and disrupt bacterial protein
synthesis
Nuclear Region (Nucleoid)
• This centrally located nuclear region consists mainly of DNA, but also contains RNA and
protein
• DNA: Usually one large, circular chromosome
• Vibrio cholerae: Two chromosomes, one large and one small
• Plasmids: Extrachromosomal pieces of smaller, circular DNA
Internal Membrane Systems
• Photosynthetic bacteria and cyanobacteria contain internal membrane systems
• Referred to as chromatophores (Fig. 4.10)
• Derived from the cell membrane and contain the photosynthetic pigments
• Nitrifying bacteria also have internal membranes
Inclusions
• Within the bacterial cytoplasm are a variety of small bodies:
• Granules: Not membrane bound and contain densely compacted substances (glycogen or
polyphosphate)
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Vesicles: Specialized membrane-enclosed structures that contain gas or poly-Bhydroxybutyrate (lipid)
Endospores
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A specialized resting structure found in bacteria such as Bacillus sp. and Clostridium sp.
Helps the bacterial cell survive when conditions become unfavorable
Highly resistant to heat, drying, acids, bases, certain disinfectants and radiation
External Structure
• Many bacteria have structures that extend beyond or surround the cell wall
• Flagella and pili extend from the cell membrane through the cell wall and beyond
• Capsules and slime layers surround the cell wall
Arrangements of Bacterial Flagella
 Monotrichous: Bacteria with a single polar flagellum located at one end (pole)
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Amphitrichous: Bacteria with two flagella, one at each end
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Lophotrichous: Bacteria with several flagella at each end
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Peritrichous: Bacteria with flagella all over the surface
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Atrichous: Bacteria without flagella
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Cocci shaped bacteria rarely have flagella
Chemotaxis
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Sometimes bacteria move toward or away from substances in their environment by this
nonrandom process
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Positive chemotaxis: net result is movement towards the attractant (nutrients)
Negative chemotaxis: net result is movement away from the repellent
Pili
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Pilus (singular)
Tiny, hollow projections
Used to attach bacteria to surfaces
Not involved in movement
• Long conjugation pili (F-pili)
• Short attachment pili (fimbriae)
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Glycocalyx
• Used to refer to all polysaccharide/polypeptide-containing substances found external to
the cell wall:
• Capsules
• Slime Layers
• All bacteria have at least a thin slime layer
Capsule
• Protective structure outside the cell wall of the organism that secretes it
• Only certain bacteria are capable of forming capsules
• Chemical composition of each capsule is unique to the strain of bacteria that secreted it
• Encapsulated bacteria are able to evade host defense mechanisms (phagocytosis)
Slime Layer
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Less tightly bound to the cell wall and is usually thinner than a capsule
Protects the cell against drying, traps nutrients and binds cells together (biofilm)
Movement of Substances Across Membranes
• Passive Transport: Cell expends no energy to move substances down a concentration gradient
(high to low concentration)
• Simple Diffusion
• Facilitated Diffusion
• Osmosis
• Active Transport: Cell expends energy from ATP, enabling it to transport substances against a
concentration gradient
Endocytosis and Exocytosis
• Eukaryotic cells move substances by forming membrane-enclosed vesicles
• Endocytosis: Form by invagination (poking in) and surrounding substances from outside the
cell
• Exocytosis: Vesicles inside the cell fuse with the plasma membrane and extrude contents from
the cell
The effect of Tonicity on Osmosis - Review
Isotonic
Hypotonic
Hypertonic
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