THE CELL
Cell - from the Latin word cella meaning small room
Robert Hooke (about 1670) - first to use term in a biological sense.
Used it to describe the small, organized compartments he observed
in dead cork tissue
Not empty - contained "jellylike" material - cell cytoplasm
In animals the cell consists of plasmalemma (cell membrane) +
cytoplasm containing various organelles and other inclusions
CELL - may be defined as the smallest structural unit of a living
organism that can function independently if placed in an
appropriate environment.
Cell Theory - developed independently by both Schleiden
and Schwann in 1832. States that all living organisms are
constructed of small sub-units called cells.
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Composition of mammalian tissues
1. cells
2. intercellular substance
3. tissue fluid
However, items 2 and 3 would not be present if the tissue did
not contain cells.
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So, where do the trillions of cells that compose your body come from?
Egg + sperm -> Fertilization -> Zygote
Cleavage + subsequent cell division
Gastrulation - establishes primary germ layers
ectoderm
mesoderm
endoderm
Human egg
Organogenesis - establishes organ primordia
Histogenesis - establishes specific tissue types - this
involves
Cellular differentiation - specific function
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HISTOGENESIS/CELLULAR
DIFFERENTIATION -> MAJOR
SPECIALIZATIONS OF CELLS
1. contraction - muscle cells
2. conduction of electrical
impulses - nerve cells
3. storage of lipids - adipose cells
4. synthesis of enzymes and other
substances - secretory cells
5. support of tissues, cushioning connective tissue cells
6. protection - immune system
cells, skin cells
7. gas transport - erythrocytes
ETC.
Cellular structure and
function are linked!
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CELL STRUCTURE
3 major cell components
1. Plasmalemma = plasma membrane = cell membrane
2. Cytoplasm (fluid, soluble molecules, various organelles, other nonmembranous inclusions)
3. Nucleus
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THE PLASMALEMMA - CELL MEMBRANE
Barrier between the cell and
outside world.
Determines what enters and
leaves the cell.
Classical model was a
phospho-lipid bilayer with the
hydrophilic phosphate radicals
forming the outer surfaces and
the hydrophobic fatty acid
chains in between.
http://www.cbc.umn.edu/~mwd/cell_www/chapter2/membrane.html
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THE PLASMALEMMA - CELL MEMBRANE
Fluid mosaic model - proteins
embedded in or associated with lipid
bilayer, some on one side or the other
and others extending across bilayer.
Most proteins that do not extend across
are on inner surface adjacent to
cytoplasm. Cholesterol is present in
external surface.
Proteins are not static - may move
about in or on lipid bilayer. Movement
mediated by cytoskeleton.
Oligosaccharide components of
glycoproteins extend from external
surface forming the glycocalyx functions in recognition and adhesion.
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THE PLASMALEMMA - CELL MEMBRANE
Molecular components of the plasmalemma:
1. phospholipids
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http://www.jdaross.mcmail.com/cell2.htm
Molecular components of the plasmalemma:
2. proteins - integral (transmembrane)
and peripheral (mostly on inner surface)
3. cholesterol and glycolipids - between
some phospholipids in the external
surface
4. glycocalyx - carbohydrate
components of glycoproteins and
glycolipids
http://www.jdaross.mcmail.com/cell2.htm
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CYTOPLASM
Major constituents:
1. Cytoskeleton - microtubules and various filaments
2. Organelles - membrane bound structures
3. Cytoplasmic inclusions - non-membrane
bound, non-filament structures, may be
temporary.
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Cytoskeleton
Microtubules - 18 - 30 nm diameter:
1. Major skeletal component - “stiff” rods
composed of 13 protofilaments - each 4-5 nm
diameter
2. Function in support for cell structure, e.g.
“hold-up” microvilli, pseudopodia - can be
demonstrated with colchicine
3. May provide “skeletal” structure for contractile
elements of cell to work against - necessary for
movement
4. Provide pathways for transport of molecules,
e.g. axonal transport
5. Form framework for flagella and cilia - 9+2
axoneme structure
6. Basis for mitotic spindle in conjunction with
centrioles - important in movement of replicated
chomosomes to opposite poles of mitotic
apparatus.
http://cellbio.utmb.edu/cellbio/microtub.htm
http://www.physorg.com/news4689.html
http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/kinesin.htm
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Epithelial cell
with labeled
microtubules
(red). Nucleus
is yellow.
Microtubule labeling rhodamine conjugated to
tubulin antibodies
Nucleus - sytox green stain
(DNA specific stain)
http://www.itg.uiuc.edu/technology/atlas/structures/nucleus/nucleus_07.htm
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CYTOPLASMIC FILAMENTS
Microfilaments - 5-7 nm diameter
1. Composed of actin
2. Responsible for cell movements
e.g. bending of microvilli, contraction of
pseudopodia, formation of cell division furrow
3. Actin is a component of sarcomere in striated
muscle
http://images.google.com/imgres?imgurl=http://www.arn.org/docs/glicksman/090104%2
520fig2%2520actin.jpg&imgrefurl=http://www.arn.org/docs/glicksman/eyw_040901.ht
m&usg=__Wc8TIhLWWpddPqpUrbbhRzzvqV0=&h=389&w=480&sz=46&hl=en&star
t=11&um=1&tbnid=zYOy2Ggbp3yxPM:&tbnh=105&tbnw=129&prev=/images%3Fq%
3Dsarcomere%2Bpicture%26hl%3Den%26client%3Dsafari%26rls%3Den%26sa%3DX
%26um%3D1
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Microfilaments - rhodamine conjugated phalloidin
http://micro.magnet.fsu.edu/cells/microfilaments/microfilaments.html
http://www.itg.uiuc.edu/technology/atlas/structures/
Nucleus - sytox green (DNA specific stain)
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http://www.bio.miami.edu/~cmallery/150/cells/c8.6x26.microfilaments.jpg
Intermediate filaments - 10 - 15 nm diameter
Comprise a variety of other filaments found in
the cell, e.g.
1. Myosin filaments of sarcomere
2. Tonofilaments of desmosomes
3. Neurofilaments found in neurons
4. Components of terminal web at apical end
of epithelial cells
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Intermediate filaments neurofilaments
Confocal maximum
intensity projection of
guinea pig embryo
labeled with neurofilament
antibody visualized with a
fluorescent probe.
Labeling in brain, spinal
cord, cranial nerves, and
spinal nerves
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http://microscopy.bio-rad.com/moviesandimages/Neuroscience.htm
CELLULAR JUNCTIONS
1. Zonula occludens
2. Zonula adherens
3. Desmosome structures
a. macula adherens - desmosome
b. hemidesmosome
c. septate desmosome
4. Gap junction
5. Interdigitating membrane
We will discuss these further when
we talk about epithelia.
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ORGANELLES
The Mitochondrion (sgl.), Mitochondria (pl.)
1. Organelles specialized for the production of ATP
2. Recent cellular and molecular evidence -> leaves no doubt
that mitochondria evolved from primitive bacteria that were
symbionts of the ancestral precursor of the eukaryotic cell.
http://cellbio.utmb.edu/cellbio/mitoch2.htm
http://micro.magnet.fsu.edu/cells/mitochondria/mitochondria.html
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The Mitochondrion (sgl.), Mitochondria (pl.)
3. Length - 0.5 - 1.0  m wide, up to 10  m
4. Characterized by a double (inner and outer) membrane.
The inner membrane forms folds called cristae. intercristal space (matrix) between cristae.
5. Elementary particles on stalks that arise from cristae.
http://micro.magnet.fsu.edu/cells/mitochondria/mitochondria.html
http://cellbio.utmb.edu/cellbio/mitochondria_1.htm#inner%20membrane
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5. ATP production:
a. Glycolysis, which occurs in the cytoplasm outside
the mitochondrion -- glucose converted to 2
pyruvate + 2 ATP
b. Pyruvate used to make acetyl co-enzyme A
c. In the mitochondrion acetyl co-enzyme A enters
the Krebs cycle -> Krebs cycle reactions occur in
matrix (intercristal space) -- results in free H+
and 2 ATP (one for each pyruvate molecule)
d. H+ processed through electron transport chain
proteins on the cristal membranes and finally,
the ATP synthase in the elementary particles
gives rise to additional ATP. (oxidative
phosphorylation)
e. Oxidative phosphorylation -> 30 - 36 ATP are
produced for each molecule of glucose converted
to carbon dioxide and water.
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The Mitochondrion
“Three-dimensional reconstruction of
a dendritic mitochondrion from chick
cerebellum. Created tomographically
from a tilt series of a 0.5 micron
section. This movie steps through
every fourth slice of the tomogram, a
three-dimensional image of the
mitochondrion.”
http://www.sci.sdsu.edu/TFrey/MitoMovie.htm
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The Mitochondrion
Another way of rendering tomographic
data to reveal mitochondrial structure.
Surface rendering.
http://www.sci.sdsu.edu/TFrey/MitoMovie.htm
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Endoplasmic Reticulum (ER)
1. A layered membrane bound organelle that consists of a
network of tubular and vesicular cisternae that ramify
throughout large volumes of cytoplasm.
2. Membrane of ER surrounds cavities that are within the
ER.
3. Membrane of ER is continuous with outer nuclear
membrane.
4. Site of processing of lipid and protein for cellular
functions.
5. Rough ER -> ribosomes associated with outer surface ->
processing of proteins
6. Smooth ER -> no ribosomes -> processing of lipids (e.g.
steroids)
7. Sarcoplasmic reticulum - specialized smooth ER
associated with striated muscle. Specialized for storage
and release of Ca+.
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The Golgi Body/Apparatus (called the
dictyosome in plants)
1. An organelle consisting of interconnected stacks
of flattened, membrane bound saccules that are
called cisternae.
2. Plays role in packaging and processing of
materials synthesized within cell,
a. lysosomal enzymes
b. secretory products
3. Convex side of stack ->
forming (cis) face
4. Concave side of stack ->
maturing (trans) face
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http://library.thinkquest.org/C004535/golgi_apparatus.html
The Golgi Body
5. Materials synthesiszed and processed in ER > carried in transfer vesicles to forming face ->
vesicle contents emptied into cisternae -> further
processing
6. Materials move through cisternae to maturing
face -> packaged into vesicles that bud off
maturing face.
a. lysosomes
b. secretory vesicles
7. These vesicles may fuse with each other in
cytoplasm to form larger vesicles.
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Visualizing the Golgi Apparatus
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http://www.itg.uiuc.edu/technology/atlas/structures/
Electron tomogram - Golgi Apparatus
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http://bio3d.colorado.edu/pubs/Golgi/Gset1.html
The Golgi Body/Apparatus
Green fluorescent protein (GFP) was attached
to a viral glycoprotein (VSVG) that moves
through the secretory pathway to the cell
surface.
Time lapse movie shows the movement of this
labeled protein (in transfer vesicles) from the
endoplasmic reticulum to the golgi apparatus.
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http://dir.nichd.nih.gov/cbmb/pb1labob.html
The cytoplasm is a crowded place. Tomographic 3-D reconstruction of a volume of
cytoplasm
“Figure 5. Stereo views of a model generated by tomographic reconstruction of a slab of cytoplasm from a cultured cell.
The model displays the Golgi complex in the context of surrounding organelles: ER, yellow; membrane-bound ribosomes,
blue; free ribosomes, orange; MTs, bright green; dense core vesicles, bright blue; clathrin-negative vesicles, white;
clathrin-positive compartments and vesicles, bright red; clathrin-negative compartments and vesicles, purple;
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mitochondria, dark green. “
http://www.jcb.org/cgi/content/full/153/6/F25
The Nucleus
Largest and most conspicuous organelle of
eukaryotic cells.
Location of genetic material - DNA - chromosomes
Site of DNA and RNA synthesis
Molecular content (mainly the DNA) is basophilic - so
nucleus takes up a dark/dense stain in many tissue
preparations since stains are often basophilic
At ultrastructural level - DNA is of two types in
nucleus of non-dividing cell - unraveled,
euchromatin, involved in synthetic activities.
Condensed and obvious - heterochromatin
DNA in dividing cell is highly condensed into typical
chromosomes
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General Nuclear Structure
Nuclear pores allow
molecules to get in and out
of the nucleus, particularly
mRNA, a very large
molecule.
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Nuclear pores
Nuclear pores are formed at
sites where the inner and outer
membranes of the nuclear
envelope are joined. Also, the
heterochromatin, which carries
the genetic material, is
organized so that a space or
"pathway" is created to the
nuclear pore.
The upper figure above
shows a frontal view of the
This electron micrograph shown
nuclear pore as though you
in the figure above depicts a
are looking down on the
transverse view of a nuclear pore
pore. It contains 8 subunits
complex seen with the
that "clamp" over region of
transmission electron
the inner and outer
microscope.
membrane where they join.
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http://cellbio.utmb.edu/cellbio/nuclear_envelope.htm#Pore
Nuclear pores
In order to see the structure of the nuclear pore as diagramed in the preceding figure,
special preparative techniques for electron microscopy have to be used.
Freeze etching
Negative staining
How nuclear pores function is not well understood.
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Nuclear pore complexes (NPCs) are embedded in the
nuclear membrane and are about 9 nm in diameter. They
regulate the movement of molecules and ions in and out of
the nucleus.
Small molecules and ions pass though the pores by
passive diffusion.
Large molecules like RNA and ribonuceloproteins move
through the pores via active transport mechanisms that
require ATP.
To some extent movement through the pore appears to be
regulated by opening and closing of the distal ring
structure, an event that is caused by the presence or
absence of calcium ions.
Atomic force microscopy
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http://www.scripps.edu/~stoffler/proj/NPC/npc.html
The Nucleolus
Where ribosomal RNA is
synthesized
Composed of the nucleolar
organizing center, pars
granulosa, pars fibrosa, and
a supporting matrix
Nucleolar organizing center - location
of rRNA genes
Pars fibrosa = fibrillar zone surrounds
the nucleolar organizing center,
composed of newly formed rRNA
Pars granulosa = granular zone
outside pars fibrosa, composed of
large and small ribosomal
subunits.
rRNA is synthesized from
DNA in the nucleolus that is
derived from a number of
different chromosomes.
The ribonucleoprotein components of ribosomes
move out of the nucleus through the nuclear
pores. They are assembled into ribosomes in the
cytoplasm. Whole ribosomes cannot
36 pass through
the nuclear pores.
The Chromosomes
Humans - 46 chromosomes, 23 pairs
A fiber of chromatin consists of a single, long DNA double helix to which is
added two kinds of proteins - histones and nonhistone chromosomal proteins.
The chromatin fibers are dispersed within the nucleus, anchored at various
places to the inner membrane of the nuclear envelope.
http://www.csc.fi/jpr/emt/movies/chromosome.html
http://life.nthu.edu.tw/~d867415/chromatin.html
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http://www.people.virginia.edu/~js6s/zsfig/chromatin.html
Chromosomes
Euchromatin - dispersed chromatin in
nucleus of non-dividing cell - active in
synthesis of mRNA
Heterochromatin - condensed chromatin in
nucleus of non-dividing cell
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The inner life of a cell
http://multimedia.mcb.harvard.edu/
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Cell division
1. Interphase - period between divisions, 3
major components, G1, S, G2. Synthesis of
DNA takes place during the “S” stage.
2. Prophase - Replicated chromosomes
condense (each chromatid is connected to its
duplicate at the centromere), nuclear
envelope breaks down near end of prophase,
the nucleolus disappears, and the mitotic
apparatus (mitotic spindle) forms.
3. Metaphase - Replicated chromosomes
align on the “equator” of the mitotic
apparatus. The chromotids are attached to
the microtubules of the mitotic spindle at the
centromere.
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Cell division
4. Anaphase - Duplicate chromatids
separate at the centromere and move
toward opposite poles of the mitotic
spindle.
5. Telophase - Chromosomes reach the
poles of the mitotic spindle, nuclear
envelope (membrane) reforms, cell
completes division.
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Mitosis and
Meiosis
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Cytoplasmic Inclusions
1. Lipid droplets
2. Glycogen
3. Melanin (pigment)
4. Lipofuscin (pigment)
5. Crystals of various types
6. Hemosiderin (pigment)
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Lipid droplets
http://www.medinfo.ufl.edu/year1/histo/images/d14a.jpg
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Glycogen granules
“Ewing's sarcoma of bone with extensive cytoplasmic
glycogen deposits, electron micrograph.”
http://www.medlib.med.utah.edu/WebPath/HISTHTML/EM/EM027.html
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Lipofuscin
“Easily seen at the poles of the cardiac nuclei, brown-red in color. ... As one ages, the
amount of lipofuscin will continue to increase, especially in the heart. One can determine
whether a heart is relatively young or old, as a direct reflection of the amount of lipofuscin
present.”
http://erl.pathology.iupui.edu/C603/LABEL49.HTM
http://uhsweb.edu/tdemark/0052.htm
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Hemosiderin
“The brown coarsely granular material in macrophages in
this alveolus is hemosiderin (iron oxide) that has
accumulated as a result of the breakdown of RBC's and
release of the iron in hemoglobin. The macrophages clear
up this debris, which is eventually recycled.”
http://www-medlib.med.utah.edu/WebPath/CINJHTML/CINJ041.html47