Cell Theory and Brief History
The story of cell biology started to unfold when the European scientists began to focus their
crude microscopes on a variety of biological material ranging from tree bark to bacteria to human s
perm. One such scientist was Robert Hooke, in 1665, Hooke built a microscope and examined thin slices
of cork cut with a penknife. He saw a network of tiny boxlike compartments that reminded him of a
honeycomb and called these little compartments cells, from the Latin word cellula, meaning "little room
". We now know that what Hooke observed were not cells at all. Those boxlike compartments were
formed by the empty cell walls of dead plant tissue, in a cork. Although he noticed that cells in other
plant tissues were filled with what he called "Juices”. Hooke's observations were limited by the
magnification power of his microscope, which enlarged objects to only 30 times (30X) their normal size.
This made it difficult to learn much about the internal organization of cells.
A few years later , Anton van Leeuwenhoek, a Dutch textile merchant, produced small lenses
that could magnify objects to almost 300 times (300 X) their size. Using these superior lenses, van
Leeuwenhoek became the first to observe living cells, including blood cells, sperm cells, bacteria, and
single celled organisms (algae and protozoa) found in pond water.Two factors restricted further
understanding of the nature of cells. First, the microscopes of the day had limited resolution (resolving
power)-the ability to see fine details of structure. The second factor was the descriptive nature of
seventeenth-century biology. It was an age of observation, with little thought given to explaining the
intriguing architectural details being discovered in biological materials.
By the 1830s, important optical improvements were made in lens quality and in the compound
microscope, in which one lens (the eyepiece) magnifies the image created by a second lens (the
objective). This allowed both higher magnification and better resolution. Now, structures only 1
micrometer (μm) in size could be seen clearly.
Aided by such improved lenses, Robert Brown found that every plant cell he looked at contained
a rounded structure, which he called a nucleus, a term derived from the Latin word for "kernel:' In 1838,
his German colleague Matthias Schleiden came to the important conclusion that all plant tissues are
composed of cells and that an embryonic plant always arises from a single cell. A year later, German
cytologist Theodor Schwann reported similar conclusions concerning animal tissue, thereby discrediting
earlier speculations that plants and animals do not resemble each other structurally. Schwann examined
animal cartilage cells, he saw that they were unlike most other animal cell s because they have
boundaries that are well defined by thick deposits of collagen fibers. Based on his incisive observations,
Schwann developed a single unified theory of cellular organization, which has stood the test of time and
continues to be the basis for our own understanding of the importance of cells and cell biology. As
originally postulate d by Schwann in 1839, the cell theory had two basic principles:
1. All organism s consist of one or more cells.
2. The cell is the basic unit of structure for all organisms.
Less than 20 years later, a third principle was added. This grew out of Brown's original
description of nuclei, extended by Swiss botanist Karl Nageli to include observations on the nature of cell
division. By 1855 Rudolf Virchow, a German Physiologist, concluded that cells arose only by the division
, of other, preexisting cell s. Virchow encapsulated the conclusion in the now-famous Latin phrase omnis
cellula e cellula, which in translation becomes the third principle of the modern cell theory:
3. All cells arise only from preexisting cells.
Thus, the cell is not only the basic unit of structure for all organisms but also the basic unit of
reproduction.
Cells exist in a wide variety of shapes and sizes, from filamentous fungal cells to spiral-shaped
Treponema bacteria to the differently-shaped cells of the human blood system. Often, an appreciation
of a cell's shape and structure gives clues about its function. For example, the large surface area of the
microvilli on our intestinal cells aids in maximizing nutrient absorption, the spiral thickenings in the cell
walls of plant xylem tissue give strength to these water-conducting vessels in wood, and the highly
branched cells of a human neuron allow it to interact with numerous other neurons.
The challenge of understanding cellular structure and organization is complicated by the
problem of size. Most cells and their organelles are too small to be seen by the unaided eye. In addition,
the units used to measure them are unfamiliar and therefore often difficult to appreciate .The problem
can be approached in two ways: by realizing that only two units are really necessary to express the
dimensions of most structures of interest and by considering a variety of structures that can be
appropriately measured with each of these two units.
The micrometer (μm) is the most useful unit for expressing the size of cells and organelles. A
micrometer (sometimes also called a micron) is one-millionth of a meter (10 - 6 m). In general, bacterial
cells are a few micrometers in diameter, and the cells of plants and animals are 10 to 20 times larger in
any single dimension. The nanometer (nm), on the other hand, is the unit of choice for molecules and
subcellular structures that are too small to be seen with the light microscope. A nanometer is one-
billionth of a meter (10 - 9 m), so it takes 1,000 nanometers to equal 1micrometer. A ribosome has a
diameter of about 25 nm. Other structures that can be measured conveniently in nanometers are
microtubules, microfilaments, membranes, and DNA molecules. A slightly smaller unit, the angstrom
(A), is occasionally used in cell biology when measuring dimensions within proteins and DNA
molecules .An angstrom equals 0. 1 nm, which is about the size of a hydrogen atom.
The Emergence of Modern Cell Biology
Modern cell biology results from the weaving together of three different strands of biological
inquiry- cytology,
biochemistry, and genetics into a single cord . Each of the strands has had its own historical origins, and
each one makes unique and significant contributions to modern cell biology.
Historically, the first of these strands to emerge is cytology, which is concerned primarily with
cellular structure. In our studies, we will often encounter words containing the Greek prefix cyto- or the
suffix -cyte, both of which mean "hollow vessel" and refer to cells. Cytology had its origins more than
three centuries ago and depended heavily on the light microscope for its initial push. The advent of
electron microscopy and other advanced optical techniques has dramatically increased our
The story of cell biology started to unfold when the European scientists began to focus their
crude microscopes on a variety of biological material ranging from tree bark to bacteria to human s
perm. One such scientist was Robert Hooke, in 1665, Hooke built a microscope and examined thin slices
of cork cut with a penknife. He saw a network of tiny boxlike compartments that reminded him of a
honeycomb and called these little compartments cells, from the Latin word cellula, meaning "little room
". We now know that what Hooke observed were not cells at all. Those boxlike compartments were
formed by the empty cell walls of dead plant tissue, in a cork. Although he noticed that cells in other
plant tissues were filled with what he called "Juices”. Hooke's observations were limited by the
magnification power of his microscope, which enlarged objects to only 30 times (30X) their normal size.
This made it difficult to learn much about the internal organization of cells.
A few years later , Anton van Leeuwenhoek, a Dutch textile merchant, produced small lenses
that could magnify objects to almost 300 times (300 X) their size. Using these superior lenses, van
Leeuwenhoek became the first to observe living cells, including blood cells, sperm cells, bacteria, and
single celled organisms (algae and protozoa) found in pond water.Two factors restricted further
understanding of the nature of cells. First, the microscopes of the day had limited resolution (resolving
power)-the ability to see fine details of structure. The second factor was the descriptive nature of
seventeenth-century biology. It was an age of observation, with little thought given to explaining the
intriguing architectural details being discovered in biological materials.
By the 1830s, important optical improvements were made in lens quality and in the compound
microscope, in which one lens (the eyepiece) magnifies the image created by a second lens (the
objective). This allowed both higher magnification and better resolution. Now, structures only 1
micrometer (μm) in size could be seen clearly.
Aided by such improved lenses, Robert Brown found that every plant cell he looked at contained
a rounded structure, which he called a nucleus, a term derived from the Latin word for "kernel:' In 1838,
his German colleague Matthias Schleiden came to the important conclusion that all plant tissues are
composed of cells and that an embryonic plant always arises from a single cell. A year later, German
cytologist Theodor Schwann reported similar conclusions concerning animal tissue, thereby discrediting
earlier speculations that plants and animals do not resemble each other structurally. Schwann examined
animal cartilage cells, he saw that they were unlike most other animal cell s because they have
boundaries that are well defined by thick deposits of collagen fibers. Based on his incisive observations,
Schwann developed a single unified theory of cellular organization, which has stood the test of time and
continues to be the basis for our own understanding of the importance of cells and cell biology. As
originally postulate d by Schwann in 1839, the cell theory had two basic principles:
1. All organism s consist of one or more cells.
2. The cell is the basic unit of structure for all organisms.
Less than 20 years later, a third principle was added. This grew out of Brown's original
description of nuclei, extended by Swiss botanist Karl Nageli to include observations on the nature of cell
division. By 1855 Rudolf Virchow, a German Physiologist, concluded that cells arose only by the division
, of other, preexisting cell s. Virchow encapsulated the conclusion in the now-famous Latin phrase omnis
cellula e cellula, which in translation becomes the third principle of the modern cell theory:
3. All cells arise only from preexisting cells.
Thus, the cell is not only the basic unit of structure for all organisms but also the basic unit of
reproduction.
Cells exist in a wide variety of shapes and sizes, from filamentous fungal cells to spiral-shaped
Treponema bacteria to the differently-shaped cells of the human blood system. Often, an appreciation
of a cell's shape and structure gives clues about its function. For example, the large surface area of the
microvilli on our intestinal cells aids in maximizing nutrient absorption, the spiral thickenings in the cell
walls of plant xylem tissue give strength to these water-conducting vessels in wood, and the highly
branched cells of a human neuron allow it to interact with numerous other neurons.
The challenge of understanding cellular structure and organization is complicated by the
problem of size. Most cells and their organelles are too small to be seen by the unaided eye. In addition,
the units used to measure them are unfamiliar and therefore often difficult to appreciate .The problem
can be approached in two ways: by realizing that only two units are really necessary to express the
dimensions of most structures of interest and by considering a variety of structures that can be
appropriately measured with each of these two units.
The micrometer (μm) is the most useful unit for expressing the size of cells and organelles. A
micrometer (sometimes also called a micron) is one-millionth of a meter (10 - 6 m). In general, bacterial
cells are a few micrometers in diameter, and the cells of plants and animals are 10 to 20 times larger in
any single dimension. The nanometer (nm), on the other hand, is the unit of choice for molecules and
subcellular structures that are too small to be seen with the light microscope. A nanometer is one-
billionth of a meter (10 - 9 m), so it takes 1,000 nanometers to equal 1micrometer. A ribosome has a
diameter of about 25 nm. Other structures that can be measured conveniently in nanometers are
microtubules, microfilaments, membranes, and DNA molecules. A slightly smaller unit, the angstrom
(A), is occasionally used in cell biology when measuring dimensions within proteins and DNA
molecules .An angstrom equals 0. 1 nm, which is about the size of a hydrogen atom.
The Emergence of Modern Cell Biology
Modern cell biology results from the weaving together of three different strands of biological
inquiry- cytology,
biochemistry, and genetics into a single cord . Each of the strands has had its own historical origins, and
each one makes unique and significant contributions to modern cell biology.
Historically, the first of these strands to emerge is cytology, which is concerned primarily with
cellular structure. In our studies, we will often encounter words containing the Greek prefix cyto- or the
suffix -cyte, both of which mean "hollow vessel" and refer to cells. Cytology had its origins more than
three centuries ago and depended heavily on the light microscope for its initial push. The advent of
electron microscopy and other advanced optical techniques has dramatically increased our
