Topic 1: Cells
The Cell
1.1 Cell Theory
•All living things
are made of
cells.
•Cells are the
basic units of
life.
•Cells come only
from other cells
Since the microscope was
introduced biologists have
examined the structure of
living things. In most
cases they have been
found to be compose of
cells. There are however
some strange exceptions
to the general cell
pattern.
Muscle Cells: very large with many
nuclei
•Muscle Cells called fibers
can be very long
(300mm).
•They are surrounded by a
single plasma membrane
but they are multinucleated.(many nuclei).
•This does not conform to
the standard view of a
small single nuclei within
a cell
Fungal Hyphae: again very large with
many nuclei and a continuous cytoplasm
•The tubular system of hyphae
form dense networks called
mycelium.
•Like muscle cells they are multinucleated
•They have cell walls composed
of chitin
•The cytoplasm is continuous
along the hyphae with no end
cell wall or membrane
•Single celled organisms have
one region of cytoplasm
surrounded by a cell
membrane.
•They can perform all the
functions of a differentiated
Multicellular organism.
•This is an image of an amoeba. A single cell
protoctista capable of all essential functions.
What cell organelles can you see?
Acetabularia
•This is an example of an
algae
•The cells are some 7 cm
long!
•This seems to contradict
theory about cell size. We
would expect a large
surface area to volume ratio
which facilitates a rapid rate
of diffusion.
Bone: so much extra cellular material the
basic cell structure seems lost
•There are some cells that
secrete material outside of
the cell membrane
•The secretions solidify and
dominate the structure
•In this example the bone cell
has secreted concentric
layers of 'bone' largely
calcium phosphates.
•The living cell is difficult to
distinguish.
A virus is a non-cellular structure
•Composed of a protein coat
and containing either the
nucleic acid DNA or RNA
•Viruses are the smallest and
simplest of the microbes.
•They are a-cellular (not
made of cells) and, since
they cannot reproduce (or
do any of the seven
characteristics of life) on
their own, they are
considered non-living.
•They are in fact more like
complex chemicals than
simple living organisms.
•Viruses are obligate parasites
that can only reproduce
inside host cells which get
damaged in the process,
leading to disease.
•Viruses are thought to have
arisen from lengths of DNA
that became separated from
their cells.
Bacteriophage
•These are viruses that
infect bacteria rather than
eukaryotic cells
•They are an important tool
in genetic engineering
•Remember this is a protein
structure not a cell
Advantages of using a light microscope
•Light microscopy has a resolution of
about 200 nm, which is good enough
to see cells, but not the details of cell
organelles.
•Specimens can be seen in color
(unlike the monochrome electron
micrographs) which includes staining
and natural color's.
•There is a wide field of view to
observe the tissue structure (2mm)
•Living specimens can be observed
and therefore their movement can be
studied
Advantages of the Electron Microscope
The electron microscope has
greater resolution (detail) and
magnification than the light
microscope.
•Resolution approx = 0.25 nm
•Magnifications = x 500,000Rather
than shinning light through the
specimen the EM takes advantage
of the short wavelength of the
electron.
Rather than shinning light through the
specimen the EM takes advantage of the
short wavelength of the electron.
•A beam of electrons is fired from a hot metal wire and
focused on the specimen by a series of
electromagnets.
•The image is produced by the same principles as a TV
screen. Photographs are then produced (electron
micrographs)
•The Electron Microscope allows the study of the
organelles of cell structure.
•The transmission electron microscope (TEM) works
much like a light microscope, transmitting a beam of
electrons through a thin specimen and then focusing the
electrons to form an image on a screen or on film. This is
the most common form of electron microscope and has the
best resolution.
•The scanning electron microscope (SEM) scans a fine
beam of electron onto a specimen and collects the
electrons scattered by the surface. This has poorer
resolution, but gives excellent 3-dimensional images of
surfaces.
Disadvantages of the Electron Microscope
•Specimens are dead due to preparation technique such as
staining, dehydration and placement in a vacuum
•Artifacts (not natural/artificial feature)can be produced in the
specimens by the preparation techniques
•No movement can be studies as specimens are dead
•Field of view is very small
Light
Electron
Cheap to purchase
(100 –500)
Cheap to operate
Expensive to buy
(over 1,000,000)
Expensive to produce electron
beams
Small and portable
Large and requires special
rooms
Simple and easy preparations Lengthy and complex
preparations
Material rarely distorted by
Preparation distorts material
preparation
Vacuum is not required
Vacuum is required
Natural color maintained
Magnifies objects only up to
2000 times
All images in black and white
Magnifies over 500,000 times
Comparison of Size
•In cell Biology it is most important to be able to provide details of the
size of structure observed. SI units are used at all times to provide
this information.
•The following diagram provides an approximation of the sizes of
different structures