Download Eukaryote and Prokaryote

Chapter 6
Chapter 6: A Tour of the
Cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Importance of Cells
• All organisms are made of cells
• The cell is the simplest collection of matter
that can live
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• Cell structure is correlated to cellular function
Figure 6.1
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10 µm
Microscopy
• Scientists use microscopes to visualize cells too small to see
with the naked eye
• Light microscopes (LMs)
– Pass visible light through a specimen
– Magnify cellular structures with lenses
• Electron microscopes (EMs)
– Focus a beam of electrons through a specimen (TEM)
or onto its surface (SEM)
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• The scanning electron microscope (SEM)
– Provides for detailed study of the surface of
a specimen
TECHNIQUE
RESULTS
1 µm
Cilia
(a) Scanning electron microscopy (SEM). Micrographs taken
with a scanning electron microscope show a 3D image of the
surface of a specimen. This SEM
shows the surface of a cell from a
rabbit trachea (windpipe) covered
with motile organelles called cilia.
Beating of the cilia helps move
inhaled debris upward toward
the throat.
Figure 6.4 (a)
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• The transmission electron microscope (TEM)
– Provides for detailed study of the internal
ultrastructure of cells
Longitudinal
section of
cilium
(b) Transmission electron microscopy (TEM). A transmission electron
microscope profiles a thin section of a
specimen. Here we see a section through
a tracheal cell, revealing its ultrastructure.
In preparing the TEM, some cilia were cut
along their lengths, creating longitudinal
sections, while other cilia were cut straight
across, creating cross sections.
Figure 6.4 (b)
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Cross section
of cilium
1 µm
Isolating Organelles by Cell Fractionation
Homogenization
Tissue
cells
1000 g
Homogenate
(1000 times the
force of gravity)
Differential centrifugation
10 min
- The centrifuge: Is used to
fractionate cells into their
component parts
Supernatant poured
into next tube
20,000 g
20 min
Pellet rich in
nuclei and
cellular debris
-Cell fractionation: Takes cells
apart and separates the major
organelles from one another
80,000 g
60 min
150,000 g
3 hr
Pellet rich in
mitochondria
(and chloroplasts if cells
are from a Pellet rich in
“microsomes”
plant)
(pieces of
plasma memPellet rich in
branes and
cells’ internal ribosomes
membranes)
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Eukaryotic cells have internal membranes that
compartmentalize their functions
• Two types of cells make up every organism
– Prokaryotic
– Eukaryotic
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Comparing Prokaryotic and Eukaryotic Cells
• All cells have several basic features in common
– They are bounded by a plasma membrane
– They contain a semifluid substance called the
cytosol
– They contain chromosomes
– They all have ribosomes
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• Prokaryotic cells
– Do not contain a nucleus
– Have their DNA located in a region called
the nucleoid
• Eukaryotic cells
– Contain a true nucleus, bounded by a
membranous nuclear envelope
– Are generally quite a bit bigger than
prokaryotic cells
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Pili: attachment structures on
the surface of some prokaryotes
Nucleoid: region where the
cell’s DNA is located (not
enclosed by a membrane)
Ribosomes: organelles that
synthesize proteins
Bacterial
chromosome
(a) A typical
rod-shaped bacterium
Plasma membrane: membrane
enclosing the cytoplasm
Cell wall: rigid structure outside
the plasma membrane
Capsule: jelly-like outer coating
of many prokaryotes
0.5 µm
Flagella: locomotion
organelles of
some bacteria
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(b) A thin section through the
bacterium Bacillus coagulans
(TEM)
• A smaller cell
– Has a higher surface to volume ratio, which
facilitates the exchange of materials into and
out of the cell
Surface area increases while
total volume remains constant
5
1
1
Total surface area
(height  width 
number of sides 
number of boxes)
6
150
750
Total volume
(height  width  length
 number of boxes)
1
125
125
Surface-to-volume
ratio
(surface area  volume)
6
12
6
Figure 6.7
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• The plasma membrane
– Functions as a selective barrier
– Allows sufficient passage of nutrients
and waste
Outside of cell
Carbohydrate side chain
Hydrophilic
region
Inside of cell
0.1 µm
Hydrophobic
region
Figure 6.8 A, B
(a) TEM of a plasma
membrane. The
plasma membrane,
here in a red blood
cell, appears as a
pair of dark bands
separated by a
light band.
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Hydrophilic
region
Phospholipid
Proteins
(b) Structure of the plasma membrane
A Panoramic View of the Eukaryotic Cell
• Eukaryotic cells
– Have extensive and elaborately arranged
internal membranes, which form organelles
• Plant and animal cells
– Have most of the same organelles
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• A animal cell
ENDOPLASMIC RETICULUM (ER)
Rough ER
Smooth ER
Nuclear envelope
Nucleolus
NUCLEUS
Chromatin
Flagelium
Plasma membrane
Centrosome
CYTOSKELETON
Microfilaments
Intermediate filaments
Ribosomes
Microtubules
Microvilli
Golgi apparatus
Peroxisome
Figure 6.9
Mitochondrion
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Lysosome
In animal cells but not plant cells:
Lysosomes
Centrioles
Flagella (in some plant sperm)
• A plant cell
Nuclear envelope
Nucleolus
Chromatin
NUCLEUS
Centrosome
Rough
endoplasmic
reticulum Smooth
endoplasmic
reticulum
Ribosomes (small brwon dots)
Central vacuole
Tonoplast
Golgi apparatus
Microfilaments
Intermediate
filaments
CYTOSKELETON
Microtubules
Mitochondrion
Peroxisome
Plasma membrane
Chloroplast
Cell wall
Plasmodesmata
Wall of adjacent cell
Figure 6.9
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In plant cells but not animal cells:
Chloroplasts
Central vacuole and tonoplast
Cell wall
Plasmodesmata
The Nucleus: Genetic Library of the Cell
• The nucleus
–
Contains most of the genes in the
eukaryotic cell
• The nuclear envelope
–
Encloses the nucleus, separating its contents from the cytoplasm
Nucleus
1 µm
Nucleolus
Chromatin
Nucleus
Nuclear envelope:
Inner membrane
Outer membrane
Nuclear pore
Pore
complex
Rough ER
Surface of nuclear
envelope.
1 µm
Ribosome
0.25 µm
Close-up of
nuclear
envelope
Figure 6.10
Pore complexes (TEM).
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Nuclear lamina (TEM).
Ribosomes: Protein Factories in the Cell
– Are particles made of ribosomal RNA
and protein
ER
– Carry out protein synthesis
Endoplasmic reticulum (ER)
Ribosomes
Cytosol
Free ribosomes
Bound ribosomes
Large
subunit
0.5 µm
TEM showing ER and ribosomes
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Small
subunit
Diagram of a ribosome
The endomembrane system regulates protein
traffic and performs metabolic functions in the
cell
• The endomembrane system
– Includes many different structures
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The Endoplasmic Reticulum: Biosynthetic Factory
• The endoplasmic
reticulum (ER)
Smooth ER
– Accounts for more
than half the total
membrane in many
eukaryotic cells
– Is continuous with
the nuclear envelope
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Rough ER
Nuclear
envelope
ER lumen
Cisternae
Ribosomes
Transitional ER
Transport vesicle
Smooth ER
Rough ER 200 µm
• There are two distinct regions of ER
– Smooth ER, which lacks ribosomes
• Synthesizes lipids
• Metabolizes carbohydrates
• Stores calcium
• Detoxifies poison
– Rough ER, which contains ribosomes
• Produces proteins and membranes, which are distributed by
transport vesicles
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The Golgi Apparatus: Shipping and
Receiving Center
• The Golgi apparatus
– Receives many of the transport vesicles
produced in the rough ER
– Consists of flattened membranous sacs called
cisternae
• Functions of the Golgi apparatus include
– Modification of the products of the rough ER
– Manufacture of certain macromolecules
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• Functions of the Golgi apparatus
Golgi
apparatus
cis face
(“receiving” side of
Golgi apparatus)
1 Vesicles move
2 Vesicles coalesce to
6 Vesicles also
form new cis Golgi cisternae
from ER to Golgi
transport certain
Cisternae
proteins back to ER
3 Cisternal
maturation:
Golgi cisternae
move in a cisto-trans
direction
Figure 6.13
5 Vesicles transport specific
proteins backward to newer
Golgi cisternae
4 Vesicles form and
leave Golgi, carrying
specific proteins to
other locations or to
the plasma membrane for secretion
trans face
(“shipping” side of
Golgi apparatus)
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0.1 0 µm
TEM of Golgi apparatus
- Is a membranous sac of
hydrolytic enzymes
Nucleus
1 µm
- Can digest all kinds of
macromolecules
- Phagocytosis: intracellular
digestion
Lysosome
Lysosome contains
active hydrolytic
enzymes
Food vacuole
fuses with
lysosome
Hydrolytic
enzymes digest
food particles
Digestive
enzymes
Lysosome
Plasma membrane
Digestion
Food vacuole
(a) Phagocytosis: lysosome digesting food
Figure 6.14 A
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Vacuoles: Diverse Maintenance Compartments
• A plant or fungal cell
– May have one or several vacuoles
• Food vacuoles
– Are formed by phagocytosis
• Contractile vacuoles
– Pump excess water out of protist cells
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• Central vacuoles
– Are found in plant cells
– Hold reserves of important organic
compounds and water
Central vacuole
Cytosol
Tonoplast
Nucleus
Central
vacuole
Cell wall
Chloroplast
Figure 6.15
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5 µm
• Relationships among organelles of the
endomembrane system
1 Nuclear envelope is
connected to rough ER,
which is also continuous
with smooth ER
Nucleus
Rough ER
2
Membranes and proteins
produced by the ER flow in
the form of transport vesicles
to the Golgi
Smooth ER
cis Golgi
Nuclear envelop
3
Golgi pinches off transport
Vesicles and other vesicles
that give rise to lysosomes and
Vacuoles
Plasma
membrane
trans Golgi
4
Lysosome available
for fusion with another
vesicle for digestion
Figure 6.16
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5 Transport vesicle carries 6
proteins to plasma
membrane for secretion
Plasma membrane expands
by fusion of vesicles; proteins
are secreted from cell
• Mitochondria and chloroplasts change energy
from one form to another
• Mitochondria
– Are the sites of cellular respiration
• Chloroplasts
– Found only in plants, are the sites of
photosynthesis
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• Mitochondria are enclosed by two membranes
– A smooth outer membrane
– An inner membrane folded into cristae
Mitochondrion
Intermembrane space
Outer
membrane
Free
ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
Figure 6.17
Mitochondrial
DNA
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100 µm
Chloroplasts: Capture of Light Energy
• Chloroplasts
–
Are found in leaves and other green organs of plants and in algae
–
Thylakoids, membranous sacs
–
Stroma, the internal fluid
Chloroplast
Ribosomes
Stroma
Chloroplast
DNA
Inner and outer
membranes
Granum
1 µm
Figure 6.18
Thylakoid
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Peroxisomes: Oxidation
• Peroxisomes
– Produce hydrogen peroxide and convert it to
water
Chloroplast
Peroxisome
Mitochondrion
Figure 6.19
1 µm
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• The cytoskeleton
– Is a network of fibers extending throughout the
cytoplasm
Microtubule
Figure 6.20
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0.25 µm
Microfilaments
– Is involved in cell motility, which utilizes motor
proteins
ATP
Vesicle
Receptor for
motor protein
Motor protein
Microtubule
(ATP powered)
of cytoskeleton
(a) Motor proteins that attach to receptors on organelles can “walk”
the organelles along microtubules or, in some cases, microfilaments.
Vesicles
Microtubule
0.25 µm
Figure 6.21 A, B
(b) Vesicles containing neurotransmitters migrate to the tips of nerve cell
axons via the mechanism in (a). In this SEM of a squid giant axon, two
vesicles can be seen moving along a microtubule. (A separate part of the
experiment provided the evidence that they were in fact moving.)
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• There are three main types of fibers that make
up the cytoskeleton
Table 6.1
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Microtubules
• Microtubules
– Shape the cell
– Guide movement of organelles
– Help separate the chromosome copies in
dividing cells
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Centrosomes and Centrioles
• The centrosome
–
Is considered to be a “microtubule-organizing center”
–
Contains a pair of centrioles
Centrosome
Microtubule
Centrioles
0.25 µm
Figure 6.22
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Longitudinal section
of one centriole
Microtubules
Cross section
of the other centriole
Cilia and Flagella
• Flagella beating pattern
(a) Motion of flagella. A flagellum
usually undulates, its snakelike
motion driving a cell in the same
direction as the axis of the
flagellum. Propulsion of a human
sperm cell is an example of
flagellatelocomotion (LM).
Direction of swimming
Figure 6.23 A
1 µm
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• Ciliary motion
(b) Motion of cilia. Cilia have a backand-forth motion that moves the
cell in a direction perpendicular
to the axis of the cilium. A dense
nap of cilia, beating at a rate of
about 40 to 60 strokes a second,
covers this Colpidium, a
freshwater protozoan (SEM).
Figure 6.23 B
15 µm
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• Cilia and flagella share a common
ultrastructure
Outer microtubule
doublet
Dynein arms
0.1 µm
Central
microtubule
Outer doublets
cross-linking
proteins inside
Microtubules
Radial
spoke
Plasma
membrane
Basal body
(b)
0.5 µm
(a)
0.1 µm
Triplet
(c)
Figure 6.24 A-C
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Cross section of basal body
Plasma
membrane
• The protein dynein
– Is responsible for the bending movement of
cilia and flagella
Microtubule
doublets
ATP
Dynein arm
(a) Powered by ATP, the dynein arms of one microtubule doublet
grip the adjacent doublet, push it up, release, and then grip again.
If the two microtubule doublets were not attached, they would slide
relative to each other.
Figure 6.25 A
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ATP
Outer doublets
cross-linking
proteins
Anchorage
in cell
(b) In a cilium or flagellum, two adjacent doublets cannot slide far because
they are physically restrained by proteins, so they bend. (Only two of
the nine outer doublets in Figure 6.24b are shown here.)
Figure 6.25 B
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1
3
2
(c) Localized, synchronized activation of many dynein arms
probably causes a bend to begin at the base of the Cilium or
flagellum and move outward toward the tip. Many successive
bends, such as the ones shown here to the left and right,
result in a wavelike motion. In this diagram, the two central
microtubules and the cross-linking proteins are not shown.
Figure 6.25 C
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Microfilaments (Actin Filaments)
• Microfilaments
– Are built from molecules of the protein actin
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– Are found in microvilli
Microvillus
Plasma membrane
Microfilaments (actin
filaments)
Intermediate filaments
Figure 6.26
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0.25 µm
• Microfilaments that function in cellular motility
– Contain the protein myosin in addition to actin
Muscle cell
Actin filament
Myosin filament
Myosin arm
Figure 6.27 A
(a) Myosin motors in muscle cell contraction.
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• Amoeboid movement
– Involves the contraction of actin and myosin
filaments
Cortex (outer cytoplasm):
gel with actin network
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
Figure 6.27 B
(b) Amoeboid movement
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• Cytoplasmic streaming
– Is another form of locomotion created by
microfilaments
Nonmoving
cytoplasm (gel)
Chloroplast
Streaming
cytoplasm
(sol)
Parallel actin
filaments
Figure 6.27 C
(b) Cytoplasmic streaming in plant cells
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Cell wall
Intermediate Filaments
• Intermediate filaments
– Support cell shape
– Fix organelles in place
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Extracellular components and connections
between cells help coordinate cellular activities
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Cell Walls of Plants
• Plant cell walls
– Are made of cellulose fibers embedded in
other polysaccharides and protein
– May have multiple layers
Central
vacuole
of cell
Plasma
membrane
Secondary
cell wall
Primary
cell wall
Central
vacuole
of cell
Middle
lamella
1 µm
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
Figure 6.28
Plasmodesmata
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The Extracellular Matrix (ECM) of Animal Cells
• Animal cells
– Lack cell walls
– Are covered by an elaborate matrix, the ECM
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• The ECM
– Is made up of glycoproteins and other
macromolecules
EXTRACELLULAR FLUID
Collagen
A proteoglycan
complex
Polysaccharide
molecule
Carbohydrates
Core
protein
Fibronectin
Plasma
membrane
Integrin
Integrins
Microfilaments
Figure 6.29
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CYTOPLASM
Proteoglycan
molecule
• Functions of the ECM include
– Support
– Adhesion
– Movement
– Regulation
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Intercellular Junctions: Plants= Plasmodesmata
• Plasmodesmata
– Are channels that perforate plant cell walls
Cell walls
Interior
of cell
Interior
of cell
Figure 6.30
0.5 µm
Plasmodesmata
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Plasma membranes
Animals: Tight Junctions, Desmosomes, and Gap Junctions
• In animals, there are three types of intercellular
junctions
– Tight junctions
– Desmosomes
– Gap junctions
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• Types of intercellular junctions in animals
TIGHT JUNCTIONS
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells
0.5 µm
At tight junctions, the membranes of
neighboring cells are very tightly pressed
against each other, bound together by
specific proteins (purple). Forming continuous seals around the cells, tight junctions
prevent leakage of extracellular fluid across
A layer of epithelial cells.
DESMOSOMES
Desmosomes (also called anchoring
junctions) function like rivets, fastening cells
Together into strong sheets. Intermediate
Filaments made of sturdy keratin proteins
Anchor desmosomes in the cytoplasm.
Tight junctions
Intermediate
filaments
Desmosome
Gap
junctions
Space
between Plasma membranes
cells
of adjacent cells
1 µm
Extracellular
matrix
Gap junction
Figure 6.31
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0.1 µm
GAP JUNCTIONS
Gap junctions (also called communicating
junctions) provide cytoplasmic channels from
one cell to an adjacent cell. Gap junctions
consist of special membrane proteins that
surround a pore through which ions, sugars,
amino acids, and other small molecules may
pass. Gap junctions are necessary for communication between cells in many types of tissues,
including heart muscle and animal embryos.
The Cell: A Living Unit Greater Than the Sum of Its Parts
5 µm
• Cells rely on the integration of structures and
organelles in order to function
Figure 6.32
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