by Carl Shuster, M.A./M.S.
In 1655, the English scientist Robert Hooke made an observation that would change basic biological theory and research forever. While examining a dried section of cork tree with a crude light microscope, he observed small chambers and named them cells. Within a decade, researchers had determined that cells were not empty but instead were filled with a watery substance called cytoplasm.
Over the next 175 years, research led to the formation of the cell theory, first proposed by the German botanist Matthias Jacob Schleiden and the German physiologist Theodore Schwann in 1838 and formalized by the German researcher Rudolf Virchow in 1858. In its modern form, this theorem has four basic parts:
The cell theory leads to two very important generalities about cells and life as a whole:
Most of the activities of a cell (repair, reproduction, etc.) are carried out via the production of proteins. Proteins are large molecules that are made by specific organelles within the cell using the instructions contained within its genetic material (see our Fats and Proteins module).
Cytology is the study of cells, and cytologists are scientists that study cells. Cytologists have discovered that all cells are similar. They are all composed chiefly of molecules containing carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Although many nonliving structures also contain these elements, cells are different in their organization and maintenance of a boundary, their ability to regulate their own activity, and their controlled metabolism.
Figure 1: The plasma membrane, shown above, forms the outer boundary and barrier of a cell. The membrane protects the contents of the cell. The bulk of the membrane is made of the phospholipid bilayer. Cholesterol is also found in animal cell membranes to increase the fluidity of the membrane and prevent freezing of cells at low temperatures. Transmembrane proteins are proteins that are embedded in the membrane and may also have carbohydrates attached. These transmembrane proteins perform many important cellular functions, such as communication between cells, and can be used to form channels in the membrane that allow certain molecules in and out of the cell.
All cells contain three basic features:
Apart from these three similarities, cell structure and form are very diverse and are therefore difficult to generalize. Some cells are single, independent units and spend their entire existence as individual cells (these are the single-celled organisms such as amoebas and bacteria). Other cells are part of multicellular organisms and cannot survive alone.
One major difference among cells is the presence or absence of a nucleus, which is a subcellular structure that contains the genetic material. Prokaryotic cells (which include bacteria) lack a nucleus, whereas eukaryotic cells (which include protozoans, animal and plant cells) contain a nucleus.
Figure 2: Bacteria (on the left) are an example of prokaryotic cell, which lack a nucleus. Protozoa (on the left) are eukaryotic, and have a nucleus.
There are other major differences in cell structure and function between different types of organisms. For example:
Figure 3: Animal cells (left) have a cytoskeleton made of long, fibrous strands that helps to maintain their shape. Plant cells (right) have a cell wall consisting mainly of cellulose, and are typically more rigid structure than do animal cells.
There are even major differences in cells within the same organism, reflecting the different functions the cells serve within the organism. For example, the human body consists of trillions of cells, including some 200 different cell types that vary greatly in size, shape, and function. The smallest human cells, sperm cells, are a few micrometers wide (1/12,000 of an inch) whereas the longest cells, the neurons that run from the tip of the big toe to the spinal cord, are over a meter long in an average adult! Human cells also vary significantly in structure and function. For example:
Carl Shuster, M.A./M.S. "Cells: Discovery and Basic Structure," Visionlearning Vol. BIO-1 (2), 2003.