Properties of Living Systems
1. What are the properties of life?
2. What properties are shared among living cells?
3. Can cells change and evolve?
4. Are there any living things without cell structure?
Advances in scientific knowledge are often proceeded by technological breakthroughs that allow researchers to ask questions that were previously beyond their ability to answer. So it is with our understanding of the cell. Advances in cell biology often follow the development of new techniques providing fundamental insights into how cells work.
However, certain characteristics are common to all cells, indeed, these characteristics define life as we know it. Cells have a complex and highly structured internal organization. The original concept of the cell, derived from looking at cells under the microscope, was that of an aqueous solution with dense inclusions, isolated from the extracellular environment by a surrounding membrane. The cell was thought of essentially as a bag of water in which the chemistry of life was carried out and where life-sustaining reactions were limited by diffusion. With the development of physical and biochemical techniques allowing researchers to probe cellular architecture, our understanding of the cell has advanced to provide a complex picture of an intracellular organization in which structure and function are directly related.
The most fundamental difference between types of cells is evident in comparing the cellular organization of Prokaryotes and Eukaryotes. This difference, the internal compartmentalization of subcellular functions into discrete, membrane-bound organelles, is what distinguishes the Monera (archaebacteria, eubacteria, cyanobacteria) from all other organisms. However, within these two groups, each level of structure in cells is consistent from cell to cell. In eukaryotes, organelles have consistent macromolecular composition and function. Cell structure is similar from organism to organism, despite differences in cell specialization and organism complexity. But for a few details, there are no fundamental differences at the molecular level among the cells of corals, algae, and human beings. This commonality of cell structure and function is evidence that all cells evolved from a common ancestor with which they share a universal blueprint. All cells possess a genetic program and the means to use it. There is, however, variation in the way the basic blueprint is executed in different types of cells.
Christian de Duve, renown electron microscopist, has defined life as the ability of a system to maintain itself in a state far from equilibrium and to grow and multiply with the help of a continual flux of energy and matter supplied by the environment. All living things interact with their environment to extract factors necessary for self-support and development. Without these interactions, which vary from one living system to another, there can be no life. Therefore, any living systems must be able to carry out the following:
1. manufacture its components from material available in the environment;
2. extract energy from the environment and convert it to do the work necessary to stay alive;
3. catalyze the metabolic different reactions required to support its activities;
4. encode information to direct biosynthetic and other metabolic processes involved in its own reproduction;
5. selectively insulate itself from the environment to maintain control over exchanges with the environment
6. regulate its activities to rapidly respond to changing conditions in the external environment by maintaining a dynamic internal environment;
9. die, that is, no longer display the properties noted above.
Most living organisms consist of a single cell. While cells can be components of larger organisms, nothing less that a cell really possesses all the criteria for life. Viruses exhibit some of the characteristics of cells, but they donít have the ability to reproduce themselves on their own. They must parasitize the reproductive machinery of other cells in order to make copies of themselves. It is considered unlikely that viruses arrived on Earth before their cellular hosts since they require hosts for reproduction. Since viruses have the same genetic language as their hosts, they most likely did not arise independently as primitive form after cells had evolved. They are probably a degenerate form derived from a more complex organism, maybe from small nucleic acid fragments that maintained some type of autonomous existence. Eventually these autonomous genetic elements attained a protein coat and became infective agents.