Post on 10-Mar-2018
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Chapter 6:
A Tour of the Cell
1. Studying Cells
2. Intracellular Structures
3. The Cytoskeleton
4. Extracellular Structures
1. Studying Cells
10 m
1 m
0.1 m
1 cm
1 mm
100 µm
10 µm
1 µm
100 nm
10 nm
1 nm
0.1 nm Atoms
Small molecules
Lipids
Proteins
Ribosomes
Viruses
Smallest bacteria
Mitochondrion
Nucleus
Most bacteria
Most plant and animal cells
Frog egg
Chicken egg
Length of some nerve and muscle cells
Human height
Un
aid
ed
eye
Lig
ht
mic
ros
co
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Ele
ctr
on
mic
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co
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Concepts of
Microscopy MAGNIFICATION
• factor by which the image
produced is larger than the
actual object (e.g. “100X”)
RESOLUTION
• minimum distance across
which 2 points can be resolved
or seen distinctly
(limited by wavelength)
CONTRAST
• degree to which objects differ
from background
Limits of Resolution
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Light Microscopy
a typical compound microscope such as used in your lab
(a) Brightfield (unstained specimen)
(b) Brightfield (stained specimen)
TECHNIQUE RESULTS
50 µm
Bright Field Microscopy
Standard form of Light
Microscopy, poor contrast
Staining increases contrast,
though the staining process
usually kills the specimen
(c) Phase-contrast
(d) Differential-interference- contrast (Nomarski)
TECHNIQUE RESULTS
Phase Contrast Microscopy
Enhances misalignment of light
waves to create contrast
A variation of phase-contrast
microscopy involving a more complex
combination of filters and prisms.
Reveals internal detail
without staining, useful
for viewing live specimens
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(e) Fluorescence
TECHNIQUE RESULTS
50 µm
Fluorescence Microscopy
Fluorescent dyes or
antibodies with a
fluorescent tag stick to
specific targets which then
fluoresce under UV light.
Only the objects or structures that fluoresce are visible.
• objects that bind the fluorescent stain or antibody
• objects that are naturally fluorescent
Confocal Fluorescence Microscopy
Only light from a given depth or plane is transmitted,
“out of focus” light is excluded
(a) Scanning electron
microscopy (SEM)
TECHNIQUE RESULTS
(b) Transmission electron
microscopy (TEM)
Cilia
Longitudinal
section of
cilium
Cross section
of cilium 1 µm
1 µm Electron
Microscopy
specimen cut in thin
sections, higher
resolution
view of whole
specimen, reveals
surface features
Electromagnetic
lenses focus
electron beam
onto heavy metal-
stained specimen.
• electron beams
have very short
wavelengths
• allows far greater
resolution than with
light microscopy
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Homogenization
TECHNIQUE
Homogenate Tissue cells
1,000 g (1,000 times the force of gravity)
10 min Differential centrifugation
Supernatant poured into next tube
20,000 g
20 min
80,000 g
60 min Pellet rich in nuclei and cellular debris
Pellet rich in mitochondria (and chloro- plasts if cells are from a plant)
Pellet rich in “microsomes” (pieces of plasma membranes and cells’ internal membranes)
150,000 g
3 hr
Pellet rich in ribosomes
Fractionation by
Centrifugation
In addition to microscopic
examination, cells and their
structures are also studied
biochemically:
• in order to study a cellular
compartment biochemically,
it must be separated from
the rest of the cell
• this is accomplished through
successive centrifugation
steps at increasing speeds
Surface area increases while
total volume remains constant
5
1
1
6 150 750
125 125 1
6 6 1.2
Total surface area
[Sum of the surface areas
(height width) of all boxes
sides number of boxes]
Total volume
[height width length
number of boxes]
Surface-to-volume
(S-to-V) ratio
[surface area ÷ volume]
Why are cells the
size they are?
Why aren’t cells bigger?
1) Cell size is limited by the rate of diffusion:
• if cells get too large, it takes too much time for
nutrients, wastes, etc, to disperse in the cell
2) And also by the surface to volume ratio (S/V):
surface area of sphere = 4pr2
volume of sphere = (4/3)pr3
*Surface Area increases by square of radius
*Volume increases by cube of radius
***The larger the cell, the smaller the S/V ratio***
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2. Intracellular Structures
Cells come in 2 basic types:
1. Prokaryotic cells
(“before” nucleus)
• lack a nucleus & other
organelles
• small, unicellular
• organisms in the
following domains:
Bacteria
diameter ~1-10 mm
Archaea
2. Eukaryotic cells (“true” nucleus)
• have a nucleus, subcellular organelles
• unicellular or multicellular
• Eukarya: Protists, Fungi , Plants & Animals
• “large” (diameter ~10 mm – 1 mm)
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Fimbriae
Nucleoid
Ribosomes
Plasma membrane
Cell wall
Capsule
Flagella
Bacterial chromosome
(a) A typical rod-shaped bacterium
(b) A thin section through the bacterium Bacillus coagulans (TEM)
0.5 µm
Prokaryotic Cells
Have intracellular organization despite no organelles.
ENDOPLASMIC RETICULUM (ER)
Smooth ER Rough ER Flagellum
Centrosome
CYTOSKELETON:
Microfilaments
Intermediate filaments
Microtubules
Microvilli
Peroxisome
Mitochondrion Lysosome
Golgi apparatus
Ribosomes
Plasma membrane
Nuclear envelope
Nucleolus
Chromatin
NUCLEUS
Animal Cell
* not in plant cells
*
*
*
*
Nucleolus
Nucleus
Rough ER
Nuclear lamina (TEM)
Close-up of nuclear envelope
1 µm
1 µm
0.25 µm
Ribosome
Pore complex
Nuclear pore
Outer membrane Inner membrane
Nuclear envelope:
Chromatin
Surface of nuclear envelope
Pore complexes (TEM)
The Nucleus Where genetic material (DNA) is stored, gene expression begins.
Nucleolus
where
ribosomal
subunits are
assembled
from rRNA
& proteins
Chromatin
complex
of DNA &
histone
proteins
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Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large subunit
Small subunit
Diagram of a ribosome TEM showing ER and ribosomes
0.5 µm
Ribosomes Carry out protein synthesis by the process of translation.
Smooth ER
Rough ER Nuclear envelope
Transitional ER
Rough ER Smooth ER
Transport vesicle
Ribosomes Cisternae
ER lumen
200 nm
Endoplasmic
Reticulum
Rough ER (RER)
• ribosomes on cytoplasmic
face of ER membrane
synthesize proteins
across ER membrane into
lumen of ER
Smooth ER (SER) • has membrane-associated
enzymes that catalyze
new lipid synthesis
(also found in RER),
neutralizing toxins
• beginning of the secretory
pathway
• storage of calcium ions
cis face
(“receiving” side of Golgi apparatus)
Cisternae
trans face
(“shipping” side of Golgi apparatus)
TEM of Golgi apparatus
0.1 µm
The Golgi apparatus
• proteins destined to leave ER are transported to the Golgi where
they are modified, sorted and sent to various destinations.
• polysaccharides are produced in the Golgi apparatus as well
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Nucleus 1 µm
Lysosome
Digestive
enzymes Lysosome
Plasma
membrane
Food vacuole
(a) Phagocytosis
Digestion
(b) Autophagy
Peroxisome
Vesicle
Lysosome
Mitochondrion
Peroxisome
fragment
Mitochondrion
fragment
Vesicle containing
two damaged organelles 1 µm
Digestion
Lysosomes
• acidic compartments full of enzymes for the
breakdown or digestion of foreign or waste material
Smooth ER
Nucleus
Rough ER
Plasma membrane
cis Golgi
trans Golgi
The Endomembrane System
aka the
“Secretory
Pathway”
Free ribosomes in the mitochondrial matrix
Intermembrane space
Outer membrane
Inner membrane
Cristae
Matrix
0.1 µm
Mitochondria
ATP production via cellular respiration • convert energy from glucose, fatty acids, etc, to energy in ATP
Have
ribosomes
that resemble
those of
prokaryotes
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NUCLEUS
Nuclear envelope
Nucleolus
Chromatin
Rough endoplasmic reticulum
Smooth endoplasmic reticulum
Ribosomes
Central vacuole
Microfilaments
Intermediate filaments
Microtubules
CYTO- SKELETON
Chloroplast
Plasmodesmata
Wall of adjacent cell
Cell wall
Plasma membrane
Peroxisome
Mitochondrion
Golgi apparatus
Plant Cell *
*
*
*
* not in animal cells
Central vacuole
Cytosol
Central vacuole
Nucleus
Cell wall
Chloroplast
5 µm
Central Vacuole
Plant organelle
that stores water
and various ions
Source of “turgor
pressure” that
maintains rigidity
of plant cells
• swells when water
is plentiful due to
osmosis
• cell wall provides
support, prevents
lysis
Chloroplast
Stroma
Inner and outer membranes
Granum
Intermembrane space
Chloroplasts
Site of photosynthesis in plant cells.
• production of glucose from CO2 and H2O using sunlight
• the basis of essentially all ecosystems
Like mitochondria, have ribosomes and other
components that resemble those of prokaryotes
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1 µm
Chloroplast
Peroxisome
Mitochondrion
Peroxisomes Contain enzymes
that oxidize
(i.e., remove H)
various organic
molecules thus
forming H2O2
from O2
• H2O2 is then
converted to O2
and H2O
Involved in the
detoxification of
toxic substances,
breakdown of
fatty acids
3. The Cytoskeleton
Microtubule
Microfilaments 0.25 µm
The Cytoskeleton
A complex, highly dynamic intracellular network
of protein filaments largely responsible for:
• cell shape, rigidity • cell movement, motility
• localization of
organelles
• movement of vesicles (and organelles)
• dynamics of
cell division
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3 Basic Cytoskeletal Filaments Actin filaments/microfilaments (MF)
intermediate filaments (IF)
microtubules (MT)
IF
MT
MF
Actin subunit
10 µm
7 nm
Microfilaments
5 µm
Keratin proteins
Fibrous subunit (keratins coiled together)
8–12 nm
Intermediate Filaments
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10 µm
Column of tubulin dimers
Tubulin dimer
25 nm
Microtubules
Vesicle ATP
Receptor for
motor protein
Microtubule
of cytoskeleton
Motor protein
(ATP powered)
(a)
Microtubule Vesicles
(b)
0.25 µm
Vesicle
Transport
• a variety of
motor proteins
are involved in
binding and
transporting
vesicles along
cytoskeletal
fibers to their
destination
Centrosome
Microtubule
Centrioles
0.25 µm
Longitudinal section of one centriole
Microtubules Cross section of the other centriole
Centrosomes
• centrosomes contain
a pair of centrioles
(animal cells only)
• these structures
are involved in the
formation of the
mitotic spindle or
“spindle fibers”
that play such an
important role in
cell division
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5 µm
Direction of swimming
(a) Motion of flagella
Direction of organism’s movement
Power stroke
Recovery stroke
(b) Motion of cilia 15 µm
Flagella & Cilia FLAGELLA
are involved in
cell motility, are
very long, and
cells have
relatively few
(1 or several)
CILIA
are involved in
motility, moving
material across
the cell surface,
and are present
on the cell surface
in high numbers
4. Extracellular Structures
Secondary
cell wall
Primary
cell wall
Middle
lamella
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
Plasmodesmata
1 µm
Plant Cell Walls
Interior of cell
Interior of cell
0.5 µm Plasmodesmata Plasma membranes
Cell walls
Plant cell walls contain
fibers of cellulose and
other polysaccharides
as well as proteins
• one or more layers of
secondary cell wall may be
produced in some plant cells
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Tight junction
0.5 µm
1 µm Desmosome
Gap junction
Extracellular matrix
0.1 µm
Plasma membranes of adjacent cells
Space between cells
Gap
junctions
Desmosome
Intermediate filaments
Tight junction
Tight junctions prevent fluid from moving across a layer of cells
Intercellular
Junctions TIGHT JUNCTIONS
are impenetrable seals
connecting adjacent cells
that prevent fluid and other
materials from passing
between the cells
DESMOSOMES
are strong connections
between cells that create a
very strong sheet of cells
GAP JUNCTIONS (animal)
& PLASMODESMATA (plant)
provide channels through
which ions & other small
molecules can pass from
cell to cell
Polysaccharide molecule
Carbohydrates
Core protein
Proteoglycan molecule
Proteoglycan complex
Collagen
Fibronectin
Plasma membrane
Proteoglycan complex
Integrins
CYTOPLASM Micro-filaments
EXTRACELLULAR FLUID
The Extracellular Matrix A meshwork of protein fibers and polysaccharides
that retain fluid and produce gel-like matrix that
holds cells together in tissues.
Key Terms for Chapter 6
• Golgi apparatus, lysosome, peroxisome, vesicle
• mitochondria, chloroplasts
• endomembrane system, central vacuole
• prokaryotic vs eukaryotic
• nucleus, nucleolus, endoplasmic reticulum, ribosome
• cell wall, capsule, flagella, nucleoid, cytoplasm
• magnification, resolution, contrast
• bright field, phase contrast, fluorescent, confocal,
transmission & scanning electron microscopy
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• cytoskeleton, cilia, flagella, centrosome, centriole
• microtubules, microfilaments, intermediate filaments
• motor proteins
• tight junctions, gap junctions, desmosomes,
plasmodesmata
• extracellular matrix, proteoglycan
Relevant Chapter
Questions 1-9