Download A Tour of the Cell Chapter 6: 1. Studying Cells 2. Intracellular Structures

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