Download Cell Structure and Function Chapter 3

Chapter 3
Cell Structure and Function
How do you define life?
Growth
Reproduction
Response to stimulus
Metabolism
Prokaryotic and Eukaryotic Cells: An Overview
Cells
Prokaryotes
Eukaryotes
Figure 3.1
Prokaryotes
Composed of bacteria and archaea
Lack nucleus
Lack various internal structures bound with phospholipid membranes
Are typically 1.0 µm in diameter or smaller
Have a simple structure
Eukaryotes
Have nucleus
Have internal membrane-bound organelles
Are larger: 10–100 µm in diameter
Have more complex structure
Composed of algae, protozoa, fungi, animals, and plants
Will be covered in detail in Chapter 12
Approximate size of various types of cells.
Figure 3.4
Structure plan of a prokaryote
External
Cell envelope
Glycocalyx
Appendages
Capsule
Slime layer
Flagella
Fimbriae
Pili
Cell wall
Cell membrane
Internal
Cytoplasm
Ribosomes
Inclusions
Nucleoid
Cytoskeleton
Endospore
Cross section of a typical prokaryotic cell
Inclusions
Ribosome
Cytoplasm
Nucleoid
Glycocalyx
Flagellum
Cell wall
Cytoplasmic membrane
Figure 3.2
Model of a bacterial cell
Cytoplasm = consists of a gel-like network
Cell membrane = encloses the cytoplasm
Cell wall = covers the cell membrane
Nucleoid = area of the cytoplasm that contains the chromosome in
the form of looped coils
Flagellum = external helical filament whose rotary motor propels the
cell
External structures
Appendages
Motility
Flagella
Attachment
Fimbriae
Axial filaments
(periplasmic flagella)
Surface coating- Glycocalyx
Channels
Pili
Glycocalyx
Gelatinous, sticky substance surrounding the outside of the cell
Composed of polysaccharides or polypeptides
Bacillus anthracis = polypeptide capsule
Streptococcus pneumoniae = polysaccharide capsule
Functions
Protect cells from dehydration and nutrient loss
Inhibit killing by white blood cells by phagocytosis, contributing to
pathogenicity
Attachment - formation of biofilms
Glycocalyx
Slime layer
Loosely attached to cell surface
Capsule
Firmly attached to cell surface
Water soluble
Composed of organized repeating
Sticky layer allows prokaryotes to units of organic chemicals
attach to surfaces
May prevent bacteria from being
recognized by host immune system
(capsule)
(slime layer)
Figure 3.5
Flagella
Are responsible for movement (motility)
Are not present on all bacteria
Composed of protein flagellin
Three parts make up a flagellum: filament, hook, and basal body
Basal body anchors the filament and hook to cell wall
Rotation propels bacterium through environment
Bacteria move in response to stimuli (taxis): Runs or tumbles
Flagella: Structure
Embed movie ?
PLAY
Flagella: Structure
http://www.rowland.harvard.edu/labs/bacteria/movies/misc.php
Flagella: Movement
Guide bacteria in a direction in response to external stimulus:
Chemical stimuli – chemotaxis; positive and negative
Light stimuli – phototaxis ; positive
Signal sets flagella into motion clockwise or counterclockwise:
Counterclockwise – results in smooth linear direction – run
Clockwise – tumbles
Embed movie ?
Flagellar arrangements
Monotrichous – single
flagellum at one end
Amphitrichous – flagella at both
ends of cell
Vibrio cholerae
Aquaspirillum sp.
Lophotrichous – small bunches
emerging from the same site
Peritrichous – flagella dispersed
all over surface of cell
Proteus vulgaris
Spirillum serpens
www.microbeworld.org
Periplasmic flagella (Axial filament)
Internal flagella, enclosed in the
space between the outer sheath and
the cell wall peptidoglycan
Produce cellular motility by
contracting and imparting twisting or
flexing motion
Examples: Spirochetes like Borrelia
burgdorferi
Endoflagella
rotate
Axial filament
Axial filament
rotates around
cell
Outer
membrane
Cytoplasmic
membrane
Figure 3.8
Spirochete
corkscrews
and moves
forward
Axial filament
Fimbriae
Sticky, bristle like projections
Flagellum
Used by bacteria to adhere to one
another and to substances in
environment
Shorter than flagella
Serve an important function in
biofilms
Figure 3.10
Fimbria
Pili (Pilus)
Rigid tubular structure made of pilin
protein
Pilus
Mostly found in Gram-negative bacteria
Longer than fimbriae but shorter than
flagella
Function to join bacterial cells for partial
DNA transfer called conjugation
Figure 3.11
Pili involved in conjugation are
called sex pilus or F-pilus
Bacterial Cell Walls
Provide structure and shape and protect cell from osmotic forces
Assist some cells in attaching to other cells or in resisting antimicrobial
drugs
Can target cell wall of bacteria with antibiotics
Composed of peptidoglycan
Scientists describe two basic types of bacterial cell walls
Gram-positive and Gram-negative
Bacterial shapes, arrangements, and sizes
Vary 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
Bacterial arrangements
Arrangement of cells is dependent on pattern of division and how cells
remain attached after division:
–Cocci:
•Singles
• in pairs (diplococci)
•groups of four (tetrads)
• Irregular clusters (staphylococci)
•Chains (streptococci)
•Cubical packets- groups of 8 (sarcina)
–Bacilli:
•Diplobacilli
•Chains
•Palisades
Peptidoglycan (Murein)
Peptidoglycan is unique to bacteria
Thus, the enzymes responsible for its biosynthesis make excellent
targets for antibiotics
Penicillin inhibits the transpeptidase that cross-links the peptides
Vancomycin prevents cross-bridge formation by binding to the
terminal D-Ala-D-Ala dipeptide
Unfortunately, the widespread use of such antibiotics selects for
evolution of resistant strains
Possible structure of peptidoglycan
Sugar
backbone
Tetrapeptide
(amino acid)
crossbridge
Connecting chain
of amino acids
Figure 3.13
Gram-Positive Bacterial Cell Walls
Relatively thick layer of peptidoglycan
Contain unique polyalcohols called teichoic acids and lipoteichoic acids
Up to 60% mycolic acid in acid-fast bacteria helps cells survive
desiccation
Peptidoglycan layer
(cell wall)
Cytoplasmic
membrane
Gram-positive cell wall
Lipoteichoic acid
Teichoic acid
Integral
protein
Figure 3.14a
Gram-Negative Bacterial Cell Walls
Have only a thin layer of peptidoglycan
Bilayer membrane outside the peptidoglycan contains phospholipids,
proteins, and lipopolysaccharide (LPS)
Lipid A portion of LPS can cause fever, vasodilation, inflammation,
shock, and blood clotting
Contain porin proteins in upper layer – regulate molecules entering
and leaving cell
Porin
Outer
membrane
of cell wall
Peptidoglycan
layer of cell wall
Gram-negative cell wall
Porin
(sectioned)
Periplasmic space
Cytoplasmic
membrane
Phospholipid layers
Lipopolysaccharide
(LPS) layer, containing
lipid A
Integral
proteins
Structures of Gram-positive and Gram-negative bacterial cell
walls
Nontypical 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, e.g. Mycoplasma sp.
Cell wall is stabilized by sterols
Pleomorphic
Unusual forms of medically significant bacteria
Mycoplasmas
Lack cell wall
Cell membrane has sterols- prevents
lysis of cells
Pleomorphic
Mycoplasma pneumoniae: primary
atypical pneumonia
Colony shows “fried egg” morphology
G.A. Meloni et al. J. Gen Microbiol. 1980
Bacterial Cytoplasmic Membranes
Structure
Referred to as phospholipid bilayer
Composed of lipids and associated proteins
Integral proteins
Peripheral proteins
Fluid mosaic model describes current understanding of membrane
structure
The structure of a prokaryotic cytoplasmic membrane: a
phospholipid bilayer
Head, which
contains phosphate
(hydrophilic)
Phospholipid
Tail
(hydrophobic)
Integral
proteins
Cytoplasm
Integral
protein
Phospholipid
bilayer
Peripheral protein
Integral protein
Figure 3.15
Bacterial Cytoplasmic Membranes
Function
Energy storage
Harvest light energy in photosynthetic bacteria
Selectively permeable
Naturally impermeable to most substances
Proteins allow substances to cross membrane
Maintain concentration and electrical gradient
Transport across cell membrane (Passive processes)
Figure 3.19 Effects of isotonic, hypertonic, and hypotonic solutions on cells.
Cells without a wall
(e.g., mycoplasmas,
animal cells)
H2 O
H2 O
H2 O
Cell wall
Cells with a wall
(e.g., plants, fungal
and bacterial cells)
Cell wall
Cell membrane
Isotonic
solution
H2 O
H2 O
H2 O
Cell membrane
Hypertonic
solution
Hypotonic
solution
Transport across cell membrane (Active processes)
Extracellular fluid
Uniport
Cytoplasmic
membrane
ATP
ATP
ADP
ADP
P
P
Symport
Cytoplasm
Uniport
Antiport
Energy in the form of ATP is used for transport
Carrier proteins involved in transport
Coupled transport:
uniport and symport
Group translocation across cell membrane (Active processes)
Glucose
Extracellular
fluid
PO4
Cytoplasm
Glucose 6-PO4
Cytoplasm of Bacteria
Cytosol
Liquid portion of cytoplasm
Mostly water
Contains cell's DNA in region
called the nucleoid
Ribosomes
Cytoskeleton
Inclusions
May include reserve deposits of
chemicals
Organization and segregation of bacterial chromosomes
Xindan Wang, Paula Montero Llopis & David Z. Rudner
Nature Reviews Genetics 14, 191-203 (March 2013)
Nucleoid
• Chromosome
– Single, circular, double-stranded
DNA molecule that contains all the
genetic information required by a
cell
• Plasmids
– Free small circular, double-stranded
DNA
– Not essential to bacterial growth
and metabolism
– Used in genetic engineering readily manipulated and
transferred from cell to cell
Bacterial ribosome
Ribosomes (70S) where S= Svedberg units of sedimentation
Made of 60% ribosomal RNA and 40% protein
Consist of two subunits: large (50S) and small (30S)
Prokaryotic differ from eukaryotic ribosomes in size and number of
proteins
Site of protein synthesis
Found in all cells
A portion of an E. coli chromosome being transcribed (left to right) and
being simultaneously translated. The arrow points to the putative site
where RNA polymerase is first bound to the DNA. The chains of dark
bodies are polysomes, that is, several ribosomes on the same mRNA
molecule.
Miller OL Jr, Hamkalo BA, Thomas CA Jr. Visualization of bacterial genes in action. Science. 1970 Jul 24;169(3943):392-5
http://schaechter.asmblog.org/schaechter/
poly β hydroxyl butyrate (PHB) granules
Intracellular storage bodies
Vary in size, number, and content
Bacterial cell can use them when
environmental sources are depleted
Ralstonia eutropha
Wahl et al. BMC Microbiology 2012, 12:262
Cytoskeleton
Composed of three or four types of protein fibers
Can play different roles in the cell
Cell division
Cell shape
Segregate DNA molecules
Move through the environment
Figure 3.24
Endospores
Unique structures produced by some bacteria
Defensive strategy against unfavorable conditions
Vegetative cells transform into endospores when multiple nutrients
are limited. Dehydrated, metabolically inactive
Longevity verges on immortality, 250 million years
Resistant to extreme conditions such as heat, radiation, chemicals
Resistant to ordinary cleaning methods and boiling
Pressurized steam at 120oC for 20-30 minutes will destroy them
The formation of an endospore
Cell wall
1
Cytoplasmic
membrane
DNA is replicated.
DNA
Vegetative cell
2 DNA aligns along
the cell’s long axis.
3
Cytoplasmic membrane
invaginates to form
forespore.
Cytoplasmic membrane
4 grows and engulfs
forespore within a
second membrane.
Vegetative cell’s DNA
disintegrates.
A cortex of peptidoglycan
is deposited between the
5 membranes; meanwhile,
dipicolinic acid and
calcium ions accumulate
within the center of the
endospore.
6 Spore coat forms
around endospore.
Forespore
First
membrane
Second
membrane
Endospore matures:
7 completion of spore coat
and increase in resistance
to heat and chemicals by
unknown process.
Cortex
Spore coat
Outer
spore coat
Endospore
8 Endospore is released from
original cell.
Figure 3.23
External Structures of Archaea
Hamus
Glycocalyces
Grappling
hook
Flagella
Fimbriae and hami
Prickles
Many archaea have fimbriae
Some make fimbria-like structures called
hami
Function to attach archaea to surfaces
Figure 3.25
Archaeal Cell Walls and Cytoplasmic Membranes
Most archaea have cell walls
Do not have peptidoglycan
Contain variety of specialized polysaccharides and proteins
All archaea have cytoplasmic membranes
Maintain electrical and chemical gradients
Control import and export of substances from the cell
Figure 3.26
Cytoplasm of Archaea
Archaeal cytoplasm similar to bacterial cytoplasm
70S ribosomes
Fibrous cytoskeleton
Circular DNA
Archaeal cytoplasm also differs from bacterial cytoplasm
Different ribosomal proteins
Different metabolic enzymes to make RNA
Genetic code more similar to eukaryotes