CELL THEORY
DR. A. TARAB
DEPT. OF BIOCHEMISTRY
HKMU
• In biology, cell theory is a scientific theory
which describes the properties of cells
• These cells are the basic unit of structure in all
organisms and also the basic unit of
reproduction
• The discovery of the cell was made possible
through the invention of the microscope
• In the first century BC, Romans were able to
make glass, discovering that objects appeared to
be larger under the glass
• In Italy during the 12th century, Salvino D’Armate
made a piece of glass to fit over one eye, allowing
for a magnification effect to that eye
• It was not until the 1590s when a Dutch spectacle
maker Zacharias Jansen began to test lenses that
progress had been made to microscopes
• Jansen was able to obtain about 9x magnification,
but the objects appeared to be blurry
• In 1595, Jansen and his father built the first
compound microscope
• While simple glasses were able to magnify
objects, they were not considered to be a
microscope
• A compound microscope was defined by having
two or more lenses in a hollow tube
• In 1665, Robert Hooke used a microscope
about six inches long with two convex lenses
inside and examined specimens under
reflected light for the observations in his book
Micrographia
• Hooke also used a simpler microscope with a
single lens for examining specimens with
direct transmitted light, because this allowed
for a clearer image
• With continual improvements made to microscopes
over time, magnification technology advanced enough
to discover cells in the 17th century
• The first cell "discovery" is credited to Robert Hooke in
1665
• Examining very thin slices of cork under a rudimentary
microscope, he noticed that the cork was composed of
hundreds of tiny attached structures that reminded
him of the "cells" that monks of the time lived in rather like tiny one-room apartments
• The name "cell" stuck and the term is still used today
• Drawing of the
structure of cork by
Robert Hooke that
appeared in
Micrographia
Robert Hooke's microscope
• The cells that Hooke observed were non-living
cells, and the microscope he used was not
advanced enough to allow him to see any cell
contents, such as a nucleus or other
organelles
• However, the first real invention and use of a
microscope was by Anton van Leeuwenhoek
• At some point in his life before 1668, he was
able to learn how to grind lenses
• This eventually led to Leeuwenhoek making his
own microscope
• His were instead simple powerful magnifying
glasses, rather than a compound microscope
• This was because he was able to use a single lens
that was a small glass sphere but allowed for a
magnification of 270x
• This was a large progression since the
magnification before was only a maximum of 50x
• Anton van
Leeuwenhoek's
Microscope from the
17th century with a
magnification of 270x
• The first person to study a live cell under a
microscope was Antony van Leeuwenhoek
• Under these microscopes, Leeuwenhoek found
motile objects
• In a letter to The Royal Society on October 9,
1676, he states that motility is a quality of life
therefore these were living organisms
• Over time, he wrote many more papers in which
described many specific forms of microorganisms
• Leeuwenhoek named these moving organisms
“animalcules,” or "little animal“ which included
protozoa and other unicellular organisms, like
bacteria
• He was able to identify the first accurate
description of red blood cells
• He also found for the first time the sperm cells of
animals and humans
• Once discovering these types of cells,
Leeuwenhoek saw that the fertilization process
requires the sperm cell to enter the egg cell
• Building on the work of Hooke and Van
Leeuwenhoek, a German botanist Matthias
Jakob Schleiden examined many plant samples
under the microscope and came to recognize
that all plants and parts of plants were
composed of cells
• Later on, while dining with his colleague
Theodor Schwann, a zoologist, Schleiden
discussed his research
Matthias Jakob Schleiden (1804-1881)
Theodor Schwann (1810-1882)
• Schwann had made similar findings in his
research of animal cells, and in 1839 he
published the work "Microscopic
Investigations on the Accordance in the
Structure and Growth of Plants and Animals“
• In this work, the first statements of cell
theory were put forth, as follows:
• 1. The cell is the unit of structure, physiology, and
organization in living things
2. The cell retains a dual existence as a distinct entity
and a building block in the construction of organisms
3. Cells form by free-cell formation, similar to the
formation of crystals (spontaneous generation)
Classical Cell Theory
• In 1858, Rudolf Virchow researched and built
upon the theories of Schleiden and Schwann
• It was he who proposed the theory that all living
cells must rise from pre-existing cells
• While this may seem obvious to students of
science today, at the time it was quite a radical
idea and was in direct opposition to the third
tenet of the cell theory proposed by Schleiden, as
noted above
• Until this time, most scientists believed in the theory of
"spontaneous generation," which proposed that nonliving material could spontaneously generate into living
matter
• One of the examples of spontaneous generation often
presented was the appearance of maggots on a piece
of rotting meat
• They were not there, and then they were there, with
no discernible method of locomotion to the meat
• Hence, the living maggots were believed to have
spontaneously generated from the non-living meat
• It was the work of Louis Pasteur that provided the data
necessary to disprove the theory of spontaneous
generation
• Pasteur performed experiments under controlled
environments that showed how substances like broth
and milk became curdled or spoiled due to exposure to
airborne particles, not by spontaneous generation
• As a result of this new information, in 1858 Virchow
proposed a revised cell theory, now known as the
"classical cell theory," as follows:
• 1. All living organisms are made up of one or
more cells
2. Cells are the basic unit of life
3. All cells arise from pre-existing cells
4. The cell is the unit of structure, physiology,
and organization in living things
5. The cell retains a dual existence as a distinct
entity and a building block in the construction
of organisms
Modern Cell Theory
• Over the past 150 years, as a result of more
research and improved scientific equipment, the
generally accepted "modern cell theory" is now
as follows:
• 1. The cell is the fundamental unit of structure
and function in living organisms
2. All cells arise from pre-existing cells by division
3. Energy flow (metabolism and biochemistry)
occurs within cells
• 4. Cells contain hereditary information (DNA)
which is passed from cell to cell during cell
division
5. All cells are basically the same in chemical
composition in organisms of similar species
6. All known living things are made up of one or
more cells
• 7. Some organisms are made up of only one cell
and are known as unicellular organisms
• 8. Others are multi-cellular, composed of a
number of cells
9. The activity of an organism depends on the
total activity of independent cells
• Exceptions to the theory
• Viruses are considered by some to be alive,
yet they are not made up of cells
• The first cell did not originate from a preexisting cell
• Later in the 1920s, the electron microscope
was developed, making it possible to view
objects that are smaller than a wavelength,
once again, changing the possibilities in
science
• Electron microscopes can magnify an image
up to 200,000x, and they can be used to study
very small structures inside cells or on cell
surfaces
• A scanning electron micrograph (SEM) shows
three-dimensional of cell surfaces
Field Emission Scanning Electron
Microscope
SEM of Sperm production (left) and
sperm fertilization (right)