Download Chapter 4: Cellular Structure

Chapter 4:
Cellular Structure
1. Eukaryotic Cell Structure
2. Prokaryotic Cell Structure
1. Eukaryotic Cell Structure
Eukaryotic Organelles
Nucleus
Holds Genetic Material
• DNA associated with
histone proteins
Chromosomes when
condensed in M phase
Chromatin in G1, S & G2
when uncondensed
Ribosome Assembly
• assembled in nucleolus
• rRNA + ribosomal prot.
• carry out protein
synthesis in cytoplasm
*all gene expression begins in nucleus (transcription)*
Endoplasmic Reticulum (ER)
Rough ER (RER)
• ribosomes on cytoplasmic
face of ER membrane
• synthesize proteins across
ER membrane into lumen
• beginning of the
“secretory pathway”
Smooth ER (SER)
• no ribosomes
• has membrane-associated
enzymes that catalyze
new lipid synthesis
(also found in RER)
The Golgi Complex
Proteins destined to leave ER next go to the Golgi
• transported in vesicles, next stop in “secretory pathway”
• undergo any necessary modifications or processing
• then sent via vesicles to various destinations
• e.g., plasma membrane, exterior of cell, lysosomes
Mitochondria
ATP production
via respiration
• Krebs cycle
• e- transport
• chemiosmosis
Mitochondrial
structure is key
for H+ gradient
*H+ gradient fuels ATP synthesis *
• high [H+] in the
intermembrane
space produced
by e- transport
in inner membr.
Chloroplasts
Organelle of
photosynthesis:
• “light” reactions
occur in the
thylakoids
• convert sunlight
to energy in ATP
and NADPH
• “dark” reactions
occur in stroma
• energy from ATP
& NADPH used to
make sugars from
CO2 and H2O
Flagella & Cilia
Microbial structures used for locomotion:
Flagella
• long & “few”
• wave-like motion
Cilia
• short & “many”
Other Organelles
Lysosomes
• acidic compartments for the breakdown or
“digestion” of foreign or waste material
Vacuoles
• large storage compartments
Peroxisomes
• metabolize fats for heat production, degrade toxins
• H2O2 byproduct is “neutralized” by catalase
Centrosomes
• region containing centrioles and other proteins
• “organizing center” for mitotic spindle fibers
2. Prokaryotic Cell Structure
A. Cell Shape
B. “External” Structures
C. “Internal” Structures
A. Cell Shape
Prokaryotic Cell Shape
One convenient characteristic with which to
identify and classify prokaryotes is their
size and shape as seen in the microscope.
• the diameter of prokaryotic cells ranges from
~0.2 to 2.0 μm
• prokaryotes are essentially unicellular and more
or less maintain a constant shape (monomorphic)
• most prokaryotes have a spherical, rod-shaped
or spiral appearance though other shapes exist
as well…
Spherical or “Round” Cells
• spherical prokaryotes are
referred to as cocci
(singular = coccus)
• different kinds of cocci
exhibit characteristic
“arrangements”:
diplo- = found in pairs
strepto- = found in “chains”
staphylo- = irregular clusters
tetrad = group of 4
sarcina = cube structure
“Rod-Shaped” Cells
• rod-shaped prokaryotes
are referred to as bacilli
(singular = bacillus)
• also found in various
“arrangements”:
diplo- = length-wise pairs
strepto- = length-wise chains”
cocco- = “rounded” bacilli
Curved or Spiral Cells
vibrio = “curved rod”
spirillum = “twisted rod”
spirochete = “corkscrew
rod”
B. External Structures
Prokaryotic Cell Structures
Plasma Membrane
true barrier between
“internal” & “external”
• a phospholipid bilayer like any other membrane though
phospholipid content a bit different from eukaryotes
Diffusion & Osmosis
Plasma membrane
is a semi-permeable
barrier across which
some substances
can diffuse:
diffusion = movement
from high to low conc.
osmosis =
diffusion of
water
lysis prevented
by cell wall…
Concentration Gradients
Different concentrations of various substances
(i.e., ions) inside vs outside the cell are set up &
maintained by various membrane proteins:
protein pumps
• move substances from lower to higher conc.
• active transport – requires energy (ATP)
protein channels, transport proteins
• facilitated diffusion of specific molecules
The overall result of all these gradients is a net negative
charge inside the plasma membrane relative to outside.
Bacterial Cell Wall
The bacterial cell wall provides
structure & support:
• general component of cell wall is
a structure called peptidoglycan
• chains of a repeating disaccharide
connected by polypeptides
• structure
varies
among
different
bacteria
*protects cell from osmotic lysis*
Gram-positive Cell Wall
• multi-layered peptidoglycan cell wall w/teichoic acids
• NO outer membrane
• Gram-positive since 1o stain/mordant trapped by the
thick peptidoglycan layer (i.e., not removed by wash step)
Gram-negative Cell Wall
*
• single layer of peptidoglycan
• outer membrane containing lipopolysaccharide (LPS)
• LPS contains Lipid A, also referred to as endotoxin
• Gram-negative since 1o stain/mordant lost with wash
Some Features of Gram-negative
vs Gram-positive Bacteria
*
Bacterial Glycocalyx (“sugar coat”)
Outermost layer
that surrounds
the bacterium
Made of protein,
polysaccharide,
or both
• varies greatly
among bacteria
• called a capsule if compact, tightly attached to cell wall
• called a slime layer if loosely attached, water soluble
• mediates adhesion, biofilm formation
• protects from dessication, phagocytosis
Bacterial Flagella
Some bacteria “get around” via 1 or more flagella:
1 = monotrichous
0 = atrichous
>1 @ ea end = lophotrichous
1 @ ea end = amphitrichous
“all over” = peritrichous
Flagellum Structure
• consists of basal body, hook & filament
• basal body anchors flagellum in PM & cell wall,
rotates hook & filament to propel bacterium
• different from “wave-like” motion of eukaryotic flagellum
Flagella & Bacterial Motility
(RUN = flagella rotate
counterclockwise)
Bacteria can undergo
movement toward or away
from something (taxis):
e.g., chemotaxis
• toward or away from
a chemical substance
(TUMBLE =
clockwise)
Involves random “runs”
& “tumbles”:
• longer runs, less tumbles
to move toward “good stuff”
• shorter runs, more tumbles
to avoid “bad stuff”
Axial Filaments
“Endoflagella” found in spirochaetes.
• anchored at
one end and
rotate
• propel cell like
a“corkscrew”
Fimbriae and Pili
Non-motile appendages that are chemically and
functionally different than flagella.
Fimbriae
• many in number (usu.)
• involved in adhesion
to specific targets
Pili (or singular pilus)
• >1 used in conjugation
C. Internal Structures
Prokaryotic Ribosomes
Carry out protein synthesis (i.e., translation of mRNA).
Ribosomes consist of 1 large and 1 small subunit.
• both subunits are made of rRNA & ribosomal proteins
• smaller, somewhat different from eukaryotic ribosomes
• specifically targeted by some antibiotics
Endospores
When conditions are
bad, some Gram+
bacteria can form
endospores:
• inactive or “resting”
cells enclosed in a
highly resistant
spore coat
• remain dormant until
conditions are good
(can be 1000’s of yrs)
(“active” cells are called vegetative)
• very resistant to
heating, freezing,
dessication
The Genetic Material
A region called the nucleoid contains the circular
bacterial chromosome (DNA + non-histone proteins):
• usually several million
base pairs (bp) in size
e.g.
the E. coli genome is
~4 mega-bp’s (4 Mbp)
• contains all bacterial
genes plus an origin
of replication (Ori)
• Ori is where DNA
replication starts,
essential to copy
the chromosome
Plasmids
Some bacteria have >1
extrachromosomal,
non-essential circular
DNA molecules called
plasmids:
plasmid
map
• much smaller than
bacterial chromosome
• several kilo-base pairs
(usu. 3-6 Kb)
• have own Ori so it is
copied when cell divides
What’s the Role of Plasmids?
Plasmids generally contain genes that confer
some sort of advantage for survival and
reproduction:
1) genes providing protection from toxic substances
• including antibiotic resistance
2) genes enabling the metabolism of additional
sources of energy
3) genes for toxins to kill microbial competitors,
enhance pathogenicity
4) genes involved in gene transfer by conjugation
Inclusions & Chromatophores
Inclusions are deposits of various materials found in
certain types of bacteria (e.g., magnetosomes).
Chromatophores are pigment-containing infoldings of
the plasma membrane in some photosynthetic bacteria.
Key Terms for Chapter 4
• coccus, bacillus, vibrio, spirillum, spirochete
• diplo-, strepto-, staphylo-, tetrad, sarcinae
• peptidoglycan, teichoic acid, LPS, endotoxin
• glycocalyx, capsule, fimbriae, pili
• a-, mono-, amphi-, lopho-, peritrichous flagella
• chemotaxis, endospores, plasmids, nucleoid
• inclusions, chromatophores, vegetative
• periplasmic space (periplasm)
Relevant Chapter Questions
rvw: 1, 3-7, 9, 11
MC: 1-7, 9