Daily Newsletter

September 10, 2012 Cell Theory and Domains

The fundamental unit of life is the cell. This is one of characteristics of life, and is a foundational principle of biology. Based on the discussions from last week, you see that for life to exist, we must first create a barrier that seperates and inside fluid compartment from the outside (external) fluid compartment. This allows us to create electrochemical gradients that will be necessary for life functions. When that membrane seperating inside from outside is disrupted and broken, life functions cease. While the cell membrane may be the defining structure of the cell, the cell is more than just the cell membrane.
This week, we turn our attention to cellular structure, and begin to look at the life functions of a cell. The first step is to recognize that all life is made up of cells, and that all living cells come from pre-existing cells. The following phrase, highlighted in yellow, is the core of the modern cell theory. This theory, which we credit to the work of Matthais Schleiden (1838) and Theodor Schwann (1839), was based upon growing microscopic work in the early 1800's. Work, even into the modern day, has provided robust evidence to support this theory, and has helped in some refinements.
What is interesting about the cell theory are the implications, e.g., a complex organism, such as man, is made up of millions of individual, and seemingly independent, cells. The overall organism, is an expression of the total activity of each of these cells. This has led to people studying communication between cells, for how do you coordinate the action of millions of individual cells? Another study is how the individual metabolic operations of cells can sum to the overall metabolic operation of an organism. As you can see, the idea of the cell being the fundamental unit of life has rather important implications for biology.
When we look at life, we begin to notice that there are different types of cells. The most basic difference between cells is the presence or absence of a nucleus. Prokaryotic cells lack a nucleus, while eukaryotic cells contain a nucleus. The nucleus, which will will cover in more depth later this week, is an internal comparment that contains the cells DNA. An internal compartment means that there is a membrane barrier that seperates the contents of the compartment from the rest of the cell. Like the cell membrane, these internal compartments are surrounded by protein containing phospholipid bilayers (fluid mosaics). As the cell membrane seperates the extracellular fluid from the intracellular, so to do these internal membranes create new fluid compartments. Just like the cell membrane, these internal membranes are selectively permeable, and allow for internal spaces with different chemical concentrations.
The presence or absence of a nucleus is a major taxonomic feature for organisms, and the ability to form and maintain internal compartments has many implications for the organism (some of which we will discuss this week). One of the major differences is in size. Remember all the proteins found on the cell membrane? An important thing to remember is that there are a termendous number of metabolic reactions that occur at the membrane, as well as the import and export of ions. We have to have enough membrane to satisfy our metabolic needs. The larger the volume of the cell, the smaller the surface area, so cells have to remain small. That is, unless you have the ability to make internal membranes. Internal membranes increase the available surface area...so eukaryotic cells, which are able to make internal membranes, can be larger than prokaryotic cells.
Living organisms are divided into three domains: Eukarya, Archaea and Bacteria. Eukarya contains all cells that are eukaryotic (nucleus containing). The Archaea and Bacteria are prokaryotic. The division of organisms into three domains was based on genetic and biochemical analysis conducted over the previous 30 years. Each domain represents a unique cellular structure.

Daily Challenge

Members of all three domains are based on the fundamental unit of the cell. For today's challenge, you are to discuss the differences that led to the three domains being declared unique from one another.
Link to Forum

Reflection

Cells are the fundamental unit of life, but over the past four years, virologists have been saying that the definition of life should be expanded to include acellular structures such as viruses. Currently, viruses and other acellular structures are considered to be biological agents; i.e., structures that can affect the genetics and metabolism of a cell, but are not technically living. What would be the logic behind expanding the definition of life to include viruses? What problems would it create?