Transcript A TOUR OF THE CELL

A TOUR OF THE
CELL
the fundamental unit of life
Microscopes - windows to the
world of the cell
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The discovery and early study of cells
progressed with the invention and improvement
of microscopes in the 17th century.
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Robert Hooke
Anton van Leeuwenhoek
Microscopes are a major tool in cytology, the
study of cell structures
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Cytology coupled with biochemistry, the study of molecules and
chemical processes in metabolism, developed modern cell biology.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
From Monk’s Room to Unit of
Life
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Robert Hooke
thought what he was
looking at resembled
the rooms monk’s
occupied…..
Cells
The Instruments
Compound Light Microscope
Electron Microscopes
Darkfield Microscopy
Scanning Electron Microscopy
Phase Contrast Microscopy
Transmission Electron Microscopy
Differential Interference Contrast Microscopy
Atomic Force Electron Microscopy
Fluorescence Microscopy
Confocal Microscopy
Compound Light Microscope
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Uses visible light
Has at least 2 sets of lenses
Can achieve maximum 2000X
magnification
Resolution of objects as small as
0.2 m
Light Microscopy
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In a light microscope visible light
passes through the specimen and then
through glass lenses.
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The lenses refract light such that the
image is magnified into the eye or a video
screen.
Brightfield Illumination
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Usual operations
Specimens must be stained
for viewing
Best magnification and
resolution with the oil
immersion objective
Oil has same refractive index
as glass
Light Microscopes
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Microscopes vary in magnification and
resolving power.
Magnification is the ratio of an object’s image
to its real size.
Resolving power is a measure of image clarity.
 It is the minimum distance two points can be
separated and still viewed as two separate
points.
 Resolution is limited by the shortest
wavelength of the source, in this case light.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Resolution of Light Microscopes
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The minimum resolution of
a light microscope is about
2 microns, the size of a
small bacterium
Light microscopes can
magnify effectively to
about 1,000 times the size
of the actual specimen.
 At higher magnifications,
the image blurs. Fig. 7.1
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Electron Microscopy
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Beam of electrons has shorter  so gives better resolution
than visible light
Electromagnetic lenses rather than glass
Done in a vacuum
Can resolve to 0.5nm and magnify up to 100,000 times.
Specimen must be dry….dead
Electron micrographs
Cell Fractionation
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The goal of cell fractionation is to separate the
major organelles of the cells so that their
individual functions can be studied.
The Procedure
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Fractionation begins with homogenization, gently
disrupting the cell
Then, the homogenate is spun in a centrifuge to
separate heavier pieces into the pellet while lighter
particles remain in the supernatant.
As the process is repeated at higher speeds and
longer durations, smaller and smaller organelles can
be collected in subsequent pellets
 ultracentrifuge
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can spin at up to 130,000 revolutions per minute and
apply forces more than 1 million times gravity
(1,000,000 g).
Why Look at the Parts rather
than the Whole
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Cell fractionation prepares quantities of
specific cell components.
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enables the functions of these organelles to be isolated,
especially by the reactions or processes catalyzed by their
proteins.
For example, one cellular fraction is enriched in
enzymes that function in cellular respiration.
Electron microscopy reveals that this fraction is rich
in the organelles called mitochondria.
Cytology and biochemistry complement each
other in connecting cellular structure and
function.
Cell Structure and Function
Fluorescent stain of cell
Early Discoveries
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Mid 1600s - Robert Hooke observed and
described cells in cork
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Late 1600s - Antony van Leeuwenhoek
observed sperm, microorganisms
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1820s - Robert Brown observed and
named nucleus in plant cells
Cell Theory
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Schleiden and Schwann
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Every organism is composed of one or more
cells
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Cell is smallest unit having properties of
life
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Virchow
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All exisiting cells arise from pre-existing
cells.
Cell
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Smallest unit of life
Can survive on its own or has potential
to do so
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Is highly organized for metabolism
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Senses and responds to environment
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Has potential to reproduce
Measuring
Cells Vary in Size
Why Are Cells So Small?
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Surface-to-volume ratio
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The bigger a cell is, the less surface
area there is per unit volume
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Above a certain size, material cannot be
moved in or out of cell fast enough
Size is Limited
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Metabolic requirements also set an upper limit to the size of a
single cell.
As a cell increases in size its volume increases faster than its
surface area.
 Smaller objects have a greater
ratio of surface area to volume.
Fig. 7.5
Structure of Cells
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Two or Three types of cells
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Archeo - cell type
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There is much evidence that this is a third cell type
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Prokaryotic
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Eukaryotic
All cells have:
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Plasma membrane
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Region where DNA is stored
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Cytoplasm
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ribosomes
Archeo-cell type and
Prokaryotes
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No nucleus
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No membrane bound organelles.
70s ribosomes
Cell walls contain petidoglycan Prokaryotic
Organisms
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Nucleoid area where DNA resides
Eubacteria
Cyanobacteria
Archeo-cell type
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Pseudomurein rather than peptidoglycan
Organisms belong to the Archeobacter
A prokaryotic cell
E. coli
Eukaryotic Cells
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Have a nucleus and other
organelles
Eukaryotic organisms
Protistans
 Fungi
 Plants
 Animals
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Overview of a plant cell
Overview of an animal cell
The nucleus contains a eukaryotic
cell’s genetic library
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contains most of the genes in a eukaryotic
cell.
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The nucleus averages about 5 microns in
diameter.
The nucleus is separated from the
cytoplasm by a double membrane.
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Some genes are located in mitochondria and
chloroplasts.
These are separated by 20-40 nm.
Where the double membranes are fused, a
pore allows large macromolecules and
particles to pass through.
Nucleolus
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In the nucleus is a region of densely stained
fibers and granules adjoining chromatin, the
nucleolus.
 ribosomal RNA (rRNA) is synthesized
 assembled with proteins from the cytoplasm
to form ribosomal subunits.
 The subunits pass from the nuclear pores to
the cytoplasm where they combine to form
ribosomes.
The nucleus and its envelope
Functions of Nucleus
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Keeps the DNA molecules of eukaryotic
cells separated from metabolic
machinery of cytoplasm
Makes it easier to organize DNA and to
copy it before parent cells divide into
daughter cells
The nucleus and its envelope
Cytomembrane System
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Group of related organelles in which lipids are
assembled and new polypeptide chains are
modified
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Products are sorted and shipped to various
destinations
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Components of the cytomembrane system
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Endoplasmic reticulum
Golgi apparatus
vesicles
Endoplasmic Reticulum
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In animal cells, continuous with nuclear
membrane
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Extends throughout cytoplasm
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Two regions - rough and smooth
Rough ER
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Arranged into flattened sacs
Ribosomes on surface give it a rough
appearance
Some polypeptide chains enter rough ER
and are modified
Cells that specialize in secreting
proteins have lots of rough ER
Smooth ER
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A series of interconnected tubules
No ribosomes on surface
Lipids assembled inside tubules
Smooth ER of liver inactivates wastes,
drugs
Sarcoplasmic reticulum of muscle is a
specialized form
Endoplasmic reticulum (ER)
Golgi Bodies
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Put finishing touches on proteins and
lipids that arrive from ER
Package finished material for shipment
to final destinations
Material arrives and leaves in vesicles
The Golgi apparatus
Vesicles
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Membranous sacs that
move through the
cytoplasm
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Lysosomes
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Peroxisomes
Lysosomes
Review: relationships among organelles of the
endomembrane system
The mitochondrion, site of cellular respiration
Specialized Plant Organelles
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Plastids
Central Vacuole
The chloroplast, site of photosynthesis
Other Plastids
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Chromoplasts
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No chlorophyll
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Abundance of carotenoids
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Color fruits and flowers red-to-yellow
Amyloplasts
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No pigments
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Store starch
The plant cell vacuole
Organelles with no
Membranes
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Ribosomes
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Cytoskeleton
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Function in protein synthesis
Function in maintenance of cell shape and
positioning of organelles
Centrioles (animals only)
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Function during cell division
Figure 7.10 Ribosomes
Ribosomes build a cell’s proteins
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Ribosomes contain rRNA and protein.
A ribosome is composed of two subunits that
combine to carry out protein synthesis.
Fig. 7.10
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Cytoskeleton
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Present in all eukaryotic cells
Basis for cell shape and internal
organization
Allows organelle movement within cells
and, in some cases, cell motility
Cytoskeletal Elements
intermediate
filament
microtubule
microfilament
Microtubules
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Largest elements
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Composed of the protein tubulin
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Arise from microtubule organizing
centers (MTOCs)
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Polar and dynamic
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Involved in shape, motility, cell division
Microfilaments
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Thinnest cytoskeletal elements
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Composed of the protein actin
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Polar and dynamic
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Take part in movement, formation and
maintenance of cell shape
Intermediate Filaments
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Present only in animal cells of certain
tissues
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Most stable cytoskeletal elements
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Six known groups
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Different cell types usually have 1-2
different kinds
Cell-to-Cell Junctions
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Plants
 Plasmodesmata
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Animals
 Tight
junctions
 Adhering
junctions
 Gap
junctions
plasmodesma
Animal Cell Junctions
tight
junctions
adhering
junction
gap
junction