By Robert Hazen, George Mason University
There’s a central paradigm in physics and chemistry called reductionism. Many physicists and chemists believe that, at root, to understand nature you have to understand the smallest building block, the smallest unit. If you understand that unit and how it interacts with itself, how it puts together larger structures, then you can build up understanding, from the smallest scale to the largest.
Reductionism
Seeing matter at the smallest scale is only half the story, because we live in a macroscopic world; we live in a world of great complexity. If we’re interested in whole living organisms, in ecosystems, in the field of consciousness, we can’t just resort to the study of atoms and quarks and leptons. For this reason, biologists for a long time resisted any sort of reductionist approach.
It seems almost inconceivable that the complex behavior and strategies of living things can be reduced just to a discussion of a few simple subunits. With the introduction of more and more powerful microscopes, however, reductionism has also contributed to biological thinking, and it has become a central aspect of the cell theory.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
The Discovery of Cells
The first step in this process was the discovery in the 19th century that all organisms are made of cells, which are the chemical factories of life. The discovery of cells themselves was made much earlier than that.
It was made by the British physicist Robert Hooke, who lived from 1635 to 1702. Hooke was renowned for his ingenious mechanical inventions and his demonstrations of various principles of mechanics.
In 1838, the German botanist Matthias Schleiden proposed that all plants are made of cells. In the following year, his countryman, zoologist Theodor Schwann, extended this idea to animals, that all animals were also made of cells.
Tenets of the Cell Theory
Schwann proposed three tenets of what is now known as the cell theory.
The first of these tenets is that all living things are composed of cells. That’s a profound statement: all living things are composed of cells. This leads to the second tenet, which is that the cell is the fundamental building block of life. In a sense, we’ve come to a kind of reductionism with respect to biology. The third idea is equally profound: all cells arise from previous cells.
These ideas initiated the field of cellular biology, and that remains one of the central fields of biology today.
Varying Sizes of the Cells
The cells are remarkably varied in their size, in their shape, and even in their function, in higher animals and in plants. Most cells, of course, are microscopic. They range in size from only a few ten-thousandths of an inch, and that’s much too small to see even in normal light microscopes. You need very sophisticated instruments to see these tiniest of cells.
Some cells, on the other hand, are many feet long. The nerve cells of a giraffe, extending from the leg up to the brain, can be more than 10 feet long. The largest cell alive today is the yoke of a fertilized ostrich egg. That’s a very massive cell, and there are ancient birds, extinct birds, that were even larger than that.
Characteristics Shared by All Cells
Cells share three characteristics; three characteristics that are common to all cells.
First of all, all cells are bounded by a cell membrane. All cells have to be bounded, so there’s an inside and an outside, a characteristic common to all cells.
A second characteristic is that all cells have the ability to survive as isolated units. Actually, there’s a proviso there—in some animals, there are some cells that have gotten so specialized they can no longer survive—but in general, cells are able to survive as isolated units. Many cells in your body, and many cells from plants, can be isolated and cultured, and they will survive alone. They’ll adopt an amoeba-like shape; they’ll actually become individual, isolated cells.
The third characteristic, which follows from this last idea, is that cells perform all the functions of a living organism. Individual cells have all those characteristics of reproduction and growth, of being able to obtain energy and atoms from their environment, and so forth.
Independent Growth and Existence of Cells
It’s fascinating that you can actually take individual human cells from various parts of the body and grow them, so that they form this amoeba-like organism. You can put them in a nutrient dish. They’ll adopt this blob-like shape. They’ll multiply and divide just like normal single-celled organisms, and yet they contain all the genetic information required to make a complete human being.
We haven’t learned how to clone humans yet, from that single cell, but the cells contain all that information—and yet they behave like a single-celled organism. So in a sense, we humans are collective animals. We’re collective organisms composed of perhaps 100 trillion cells that are all cooperating, and yet many of those cells are capable of independent existence.
Common Questions about the Cell Theory
The cells are varied in their size and shape. Most cells, of course, are microscopic. They range in size from only a few ten-thousandths of an inch, and that’s much too small to see even in normal light microscopes. Some cells, on the other hand, are many feet long. The nerve cells of a giraffe, extending from the leg up to the brain, can be more than 10 feet long.
The cell theory has three tenets. The first of these tenets is that all living things are composed of cells. This leads to the second tenet, which is that the cell is the fundamental building block of life. In a sense, we’ve come to a kind of reductionism with respect to biology. The third idea is equally profound: all cells arise from previous cells.
There’s a central paradigm in physics and chemistry called reductionism. Many physicists and chemists believe that, at root, to understand nature you have to understand the smallest building block, the smallest unit. If you understand that unit and how it interacts with itself, how it puts together larger structures, then you can build up understanding, from the smallest scale to the largest.
