Cells and Their Structures
Cells were first described by Robert Hooke in his book Micrographia, published in 1665. Using a microscope, he described the structure of cork as closely resembling prison chambers or monks’ quarters (there is some debate about this). He used the term “cell” to describe these hollow chambers. The Cell Theory was first described in 1839. While the Cell Theory has been altered and revised, most biologists today list three or four general characteristics shared by all cells:
1. The cell is the basic unit of life. Anything smaller than a cell is not alive by definition.
2. All organisms are composed of one or more cells.
3. Cells arise from pre-existing cells.
4. All cells, at some point in their life cycle, contain the genetic material for the entire organism.
The first two characteristics are definitions. The third characteristic was demonstrated in part by Louis Pasteur’s work through 1862. The fourth point is subject to controversy; advocates point to the fact that organisms begin as a single cell with all of the genetic information for the organism and most divide by mitosis. Opponents point to the loss of genetic material that occurs during meiosis, although the loss is of copies of genes. The cells still have all of the genetic information.
Different Types of Cells
While cells do share many traits in common, there are differences. The cells that make a tree are not the same that make a dog. Even within the same organism, there are different types of cells. Your skin cells are different than muscle cells, or bone cells, or blood cells. Like organisms, cells can be characterized by their traits. Two common methods of distinguishing cells are by feeding mechanisms and by internal structure.
Cell Feeding Mechanisms
Cells must acquire nutrients and eliminate waste products. This can be done in different ways. Some cells are capable of producing food from the raw materials in the cell. This type of cell is called an autotrophic cell. Autotroph literally translates as “self feeding”. Most autotrophic cells on earth are photosynthetic, although in areas where light is not available (ocean bottoms, deep caves underground, etc.) autotrophs carry out chemosynthesis. Some examples of autotrophic cells are plants, algae, and some bacteria. Other cells must acquire their nutrients from other cells. This type of cell is called a heterotrophic cell. Heterotroph literally means “other feeding”. These cells must be able to capture and take in other food stuffs. Some examples of heterotrophic cells include animals, fungi, and some bacteria. There are a few groups of organisms which are mixotrophs. These organisms, such as protists like Euglena, have the capability to photosynthesize when light is available and switch to predation when light is not available.
Cells also vary based on complexity and structure. The first cells were relatively simple in structure and complexity. They are still present and actually outnumber the more complex cells you may be more familiar with. The first cells are termed prokaryotic (literally “before kernel”, meaning before the nucleus). These cells are generally smaller and less active. Usually, prokaryotic cells utilize some form of anaerobic respiration. They have no nucleus or membrane-bound organelles. Their single loop of DNA is termed a nucleoid, but is not isolated from the cytoplasm by a membrane. Prokaryotic cells do have cytoplasm, ribosomes, cell walls, cell membranes and their associated materials. Today, two of the three domains of life are prokaryotic: Archaea and Bacteria (some scientists term this group Eubacter or Eubacteria). The second type of cell is termed eukaryotic (literally “true kernel” or having a true nucleus). These cells are larger and more complex. Membrane-bound organelles “compartmentalize” parts of the cell for specific functions. These cells can carry out anaerobic respiration, but most also carry out aerobic respiration due to the greater energy yield per molecule of glucose. Eukaryotic cells are found in the domain Eukarya. Remember though, while eukaryotic cells are larger and more complex, they are not “better” than prokaryotic, just different. Today, there is more prokaryotic biomass on earth than eukaryotic biomass.
These are two different ways to distinguish cell types. They are not related to each other. Do not fall into the trap some students succumb to by linking autotroph/heterotroph with prokaryotic/eukaryotic. There are cells that are prokaryotic and autotrophic (some primitive algae), prokaryotic and heterotrophic (bacteria), eukaryotic and autotrophic (most plants), eukaryotic and heterotrophic (animals). Additionally, there are other ways to distinguish cell types that your instructor may discuss.