Video Transcript
In this video, we will learn how to
describe the principles of cell theory. We will dive into scientific
history and explore the contributions of the scientists Robert Hooke, Antonie van
Leeuwenhoek, Matthias Schleiden, Theodor Schwann, and Rudolf Virchow who are all
critical in the development of cell theory.
Humans are always looking for
answers. Thanks to this questioning
behavior, Newton created the theory of gravity after an apple fell on his head and
Darwin postulated the idea that organisms can change and evolve over time. One question that has been on the
minds of scientists for years is what the smallest building block of an organism may
be. If we were to take this apple, for
example, which just a moment ago was part of the tree, and then break it down into
its building blocks, we would see that it’s made up of cells. We take this idea for granted
today, but the truth is it took hundreds of years to realize this to eventually form
what we call cell theory.
Cell theory has three principles or
postulates which are as follows. All living things are made of at
least one cell. Cells are the basic unit of
life. And all cells come from preexisting
cells. These principles are universally
accepted today. But before the development of the
microscope in the 1600s, scientists had no way of knowing what living things were
made of. Let’s now take a look at some of
the scientists who contributed to cell theory and their work.
The beginning of the historical
development of cell theory starts with Robert Hooke. Hooke was an English scientist and
architect who coined the term cell in 1665. He built his own primitive compound
microscope much like the one shown here, which is a microscope with three sequential
lenses. And using this microscope, he made
several observations that he published in a book called Micrographia. In his book, he presented a drawing
of what he observed through the microscope when examining a very thin slice from a
cork such as the one used in wine bottles.
In this material, he noted that it
was filled with regular open spaces that he named cells. This word comes from the Latin word
cellula, which means small room. Hooke came up with the term cell,
and he got famous for it. But he didn’t know that cells were
alive. It’s understandable that he thought
that cells were dead because what he observed were the cell walls that remained in
the piece of dried cork after the plant cells within them had died. In fact, Hooke was a supporter of
the theory of spontaneous generation in which life simply arises when certain
conditions are met, for example, when a piece of meat begins to rot and maggots
seemingly appear spontaneously out of nowhere.
Spontaneous generation was a
popular belief at the time. He observed some mold under his
rudimentary microscope. And when he couldn’t identify a
reproductive apparatus, he concluded that the mold was born of warmth and
moisture. We have learned a lot since then,
but this kind of thinking was standard at the time. The next step in the development of
cell theory was to discover the fundamental living nature of cells. This was achieved by Antonie van
Leeuwenhoek who is often called the father of microbiology. Van Leeuwenhoek was a Dutch
businessman, politician, and microscope enthusiast. He learned to make his own glass
lenses and built a simple microscope. This was essentially a single
powerful magnifying glass lens that was able to magnify objects to over 200 times
their original size.
This microscope allowed him to
observe a wide variety in microscopic life. This allowed him to make many
important observations including the discovery of freshwater protozoa, which he
called animalcules. He also produced the first written
description of bacteria by observing plaques scraped from his own teeth. He published most of his findings
in letters to the Royal Society of London. The society actually turned to the
English scientist Robert Hooke to confirm these findings since van Leeuwenhoek had
no formal training in the sciences. Robert Hooke had trouble using the
simple microscopes, but he managed to reproduce van Leeuwenhoek’s work, which gave
it credibility among the scientific community.
The next major advancement in cell
theory is attributed to Matthias Schleiden whose work is often referenced to in
conjunction with Theodore Schwann. In fact, Schleiden and his close
friend Schwann are often called the founders of cell theory who worked with compound
microscopes again. Schleiden was a German botanist who
was famous for studying plant structures under a microscope. In a book published in 1838, he
stated that all plants are composed of cells and the products of cells. He also noted the importance of the
nucleus, which seemed important in the production of new cells, and observed that
new cells seem to arise from the nucleus of old cells. Schleiden shared his observations
about plants with his friend Theodor Schwann. You may recognize his name from the
Schwann cells in the nervous system, which are named after him.
Schwann was a German physiologist
particularly interested in the microscopic study of animal tissues. With the same type of microscope,
Schwann reproduced Schleiden’s observations about plants in the tissues of
animals. He found that all animal tissues
were made of cells that possess a nucleus. Schwann, backed by Schleiden, was
then able to state that all living things are composed of cells and cell products,
which is the foundation of modern cell theory. Schwann then made three more
concluding statements. The cell is the unit of structure,
function, and organization in living things. The cell is both a distinct entity
and the building block in the construction of organisms and that living cells form
in a way similar to the formation of crystals. However, this statement has since
been disproven.
The last discovery in our timeline
of cell theory, which is that all cells arise from preexisting cells, is credited to
a German pathologist and politician named Rudolf Virchow. The Latin statement “omnis cellula
e cellula,” meaning all cells come from cells, was part of the rise by Virchow but
was actually first coined by a French scientist named Francois Vincent Raspail. In fact, there is evidence that the
first scientist to discover that cells are produced by preexisting cells is another
German scientist whose publications went largely unnoticed named Robert Remak. This idea of all cells come from
cells is a rejection of the concept of spontaneous generation, which was a commonly
held belief at the time.
The discovery that all cells come
from cells gave rise to germ theory. This states that specific diseases
are caused by certain microorganisms invading the body and that infections in
hospitals can be significantly lowered by hand washing between patients. It’s interesting that Virchow, a
scientist who specialized in the study of diseases, disagreed with the increasingly
popular germ theory. He instead believed that diseases
were caused by some imbalances and abnormal activities inside of cells, not by
outside pathogens. The contributions of these
scientists led to cell theory, which is really the foundation of modern biology. Today, the three postulates of cell
theory are that all living things are made of at least one cell, that cells are the
basic unit of life, and that all cells come from preexisting cells.
As a review, let’s look at the
contributions that each of these scientists made to cell theory as a timeline. It begins in 1590 when the first
microscope was made. Then in 1655, Robert Hooke sees the
first cells in a cork. He describes them as looking like
little rectangular rooms. Then in 1675, Antonie van
Leeuwenhoek views tiny objects in pond water. In 1838, Matthias Schleiden
concludes that all plants are made of cells. In 1839, Theodor Schwann proposes
that all animals, and therefore all living things, are made of cells. And finally, in 1855, Rudolf
Virchow concludes that all cells are formed from preexisting cells. All these contributions have been
critical in forming cell theory as we know it today. Now let’s take a moment to apply
what we’ve learned with a practice question.
Which of the following is not a
principle that cell theory is based on? (A) Cells are the smallest
structures found within an organism. (B) All living organisms are made
up of one or more cells. (C) All cells are made from
preexisting cells. Or (D) the basic functional unit of
all living organisms is the cell.
This question is asking us about
cell theory and which of the provided choices is not a principle that it’s based
on. So what is cell theory exactly? Cell theory is the universally
accepted theory that all living things are made up of cells. There are actually three principles
or postulates of cell theory that were developed by numerous scientists from the
early 17th century through to the mid-19th century. A major technological contributor
to the development of cell theory was the invention and production of simple and
compound microscopes. This allowed scientists to view the
microscopic world clearly for the first time.
The three principles or postulates
of cell theory are as follows. All living things are made of at
least one cell, cells are the basic unit of life, and all cells come from
preexisting cells. While the cell is the basic unit of
life and the smallest object within an organism to be considered independently
alive, cells themselves are composed of smaller structures such as the endoplasmic
reticulum, the nucleus, and the mitochondrion. All of these smaller structures are
called organelles, so cells are not the smallest structures found within the
organisms. So therefore, the statement that is
not a principle of cell theory is that cells are the smallest structures found
within organisms.
Now let’s go over the key points
that we covered in this video. Cell theory is a universally
accepted theory that is made up of three principles or postulates. All living things are made of at
least one cell, cells are the basic unit of life, and all cells come from
preexisting cells. These three principles are based
off the work of Robert Hooke, Antonie van Leeuwenhoek, Matthias Schleiden, Theodor
Schwann, and Rudolf Virchow who contributed to cell theory between the early 17th
century through to the mid-19th century.
