The Cell Theory
And how it
developed
Problem: Cells are small
 Most cells are too small to be seen
without magnification.
– Look at your hand? Do you see cells?
Sizes of cellular
components
Solution: Magnification

Simple magnifiers: drop of water or glass
bead.
Compound magnification
 First
microscope invented by
Zacharias and Hans Janssen in the
1590's
– Uses a series of lenses to make larger
images
Microscopes
Various styles of
microscope
were developed
by mid 1600’s,
mostly to
examine cloth
Robert Hooke - 1665
First recorded examination of
biological material with
microscope
 Looked at slices of cork
 Described structure as
“cells”

Hooke’s observation
Hooke’s observations


Cork was long-dead
material – saw remains of
cell walls only.
Later examined fresh plant
tissue – saw fluids inside.
Impact:
 Cells are the basic structural unit of
living things.
Anton Von Leeuwenhoek
 Good amateur microscope
maker.
 1668 - Examined pond water with
microscope:
saw freely moving “beasties” or
“animalcules”.
Von Leeuwenhoek
 Also examined
blood.
 First to observe
single-celled
organisms in tooth
debris.
Von Leeuwenhoek

Considered
the father
of
microbiology
Impact:
 A single cell can perform all life
functions.
Henri Dutrochet - 1824
 Proposed that all living things were
made of cells.
– based on observations of many
organisms.
Technology – 1820’s on:
 Development of staining techniques
improves visibility of cellular
structure.
– unstained tissue mostly clear.
– Special chemical reactions can show
structures, enzymes, whole cell shape,
etc.

Staining makes
bacterial cells stand
out.
Robert Brown - 1831
 Realizes
that all
cells have a dense
region in the
center.
 Calls it a nucleus.
 Function unknown.
Matthias Schleiden - 1838
 Observed many
plant samples
 Concluded that
all plants are
made of cells
Theodor Schwann - 1839
 Observed many
animal samples
 Concluded that all
animals are made of
cells.
Johannes Purkinje - 1839
 Reported that all cells he observed
were filled with jelly-like substance.
 Named this substance protoplasm.
– assumed to have special life-giving
chracteristics.
Impact:
 All living things are made of cells
Rudolph Virchow - 1855
 Observed
cells
dividing.
 True in plants
and animals
Impact:
 All cells come from pre-existing cells
Cell Theory – late 1800’s
1. All living things are made of cells
and cell products.
2. All cells carry on life activities. Life
activities of multicelled organisms
are combined product of all cells.
3. New cells arise only from other cells
by the process of cell division.
Technology improves
 1939 – Electron microscope allows
much greater magnification.
– Uses electrons, rather than light.
– Allows much greater resolution, too.
– Specimens must be exposed to
vacuum, so not good for living things.
Getting information from an
image:
Magnification tells how many times bigger
the object appears. Bigger is easier to
see.
 Resolution tells how much detail is
available in the image.

– A bigger image with poor resolution is
useless!
Electron Microscope
Scanning Electron Microscope
Bounces electrons off
surfaces without
penetrating.
 Produces very
detailed 3-D view of
surface.

– No internal structure!
SEM Images
Transmission Electron Microscope
Maximum magnification – can see large
molecules!
 Electrons pass through very thin (0.2mm)
slices of an object and are detected on a
screen.
 Allows examination of structures within
cells.

TEM Image
Compound Light Microscope
Light passes up from the
base and is focused
through a diaphragm. It
passes through the
specimen, and a series of
lenses in the objective
magnify the specimen.
 Lenses in eyepiece provide
a magnified view of the first
image.

The specimen

The object to be observed
is placed on a glass slide.
– Must be thin and clear
enough for light to pass
through.
It must be covered with
water or another clear
liquid.
 A thin cover glass is
placed over the object
and liquid.

Illumination
Different types of illumination can
emphasize different features of the
specimen.
 Can be combined with special stains – UV
light with fluroescent stains or labels for
particular types of materials.

Types of Light Microscopy
Stereomicroscope
A stereo or dissection microscope is a low power
light microscope. Good for magnifying objects
and getting a three dimensional view of
surfaces.
 Light can bounce off the surface of the
specimen, be transmitted through it, or both.
 You cannot typically see individual cells because
of the low magnification, and because the
objects are too thick for good resolution.

A stereo microscope
Note the 2 light
sources:
1. From above, or
epi-illumination
2. From below, or
transillumination
Stereomicroscope

Also called a dissecting microscope, it is
most useful for magnifying objects while
they are being manipulated.
What to use?
What type of microscope you use depends
on what information you need.
 May use a stereomicroscope to obtain a
tissue sample, a light microscope to look
at the tissue, and transmission electron
microscope to learn about the details
within the cells.
