& Data Analysis
Protein gel analysis
Keeping a lab notebook
Writing research papers
Dimensions & units
Using figures (graphs)
Examples of graphs
Principles of microscopy
Solutions & dilutions
Fractionation & centrifugation
Radioisotopes and detection
Homeostasis, Steady States, and Equilibria
This web page is part of a very small collection of essays on concepts related to the scientific method and to specific laboratory studies.
Misuse of the term "equilibrium" is one of the most common mistakes in the biological sciences. Guilty parties include textbook authors, teachers, researchers, and even students. Here is an illustration of the difference between an equilibrium and other situations in which there is constancy.
A 1981 edition of Webster's dictionary provides a rather narrow definition of the term homeostasis, refering specifically to animals. Homeostasis was defined as the maintenance in an animal of a "constant internal milieu," that is, a relatively constant internal environment, despite changes to the external environment. More generally, homeostasis can refer to the maintenance of relatively constant conditions within any system. In fact the term is now used in reference to cells, animals, plants, and local or global ecosystems. The term could probably apply to a self-sufficient machine, for that matter. A key concept is that mechanisms must be in place to maintain constancy within a system, and that the system is itself dynamic. The latter quality is essential to the definition. The living tissue in a tree maintains homeostasis, but not so a block of wood after it is cut from the tree.
Steady state (dynamic equilibrum)
You may also have heard the phrase steady state. An organism an be said to be in a steady state, in which case we are using the phrase interchangeably with the term homeostasis. Why not take advantage of the opportunity to be more precise, though?. While homeostasis refers to the entire internal environment, the term steady state can be restricted to describing specific mechanisms. A cell is in homeostasis because every mechanism that keeps it alive is in a steady state. For example, an enzyme complex called sodium/potassium ATPase (also known as the sodium/potassium pump) uses energy from the hydrolysis of ATP to "trade" sodium ions for potassium ions, thus maintaining a constant internal concentration of potassium. Potassium concentration can be said to be in a steady state. The term dynamic equilibrium is also used synonymously with steady state, but the use of that term can be confusing. A dynamic equilibrium is not the same as a chemical equilibrium.