What is the difference between a living organism and a dead or inanimate organism? For example, what characteristics distinguish a living creature like a plant or animal from an inanimate object like a rock?
Over time, scientists have developed the following list of general characteristics that are used to determine whether or not an organism is "alive,"
1. Living Things are Composed of Cells
As is defined in Cell Theory, the cell is the unit of structure, physiology, and organization in living things.Every life form begins with one cell.
2. Living Things Have Different Levels of Cellular Organization
Through the process of cell division within organisms, the process of "differentiation" occurs. This means that cells will begin to change, or "differ." from one another and each cell type will perform different "jobs" such as skin cells, brain cells, nerve cells, etc.
3. Living Things Use Energy
All organisms must have a metabolic system which provides them with the energy to perform different tasks. Plants generally obtain energy from the sun through the process of photosynthesis, while animals ingest food for energy production.
4. Living Things Are Homeostatic
Homeostasis is the stable regulation of an organism's internal state. A living organism controls things like temperature, heartbeat, and hydration. Some scientists consider homeostasis to be a sub-characteristic to energy use.
5. Living Things Respond to Their Environment
Living organisms exhibit "behavior," which is in its most basic form a set of responses to various external stimuli. For example, certain plants may turn their leaves towards the sun to better obtain energy. Likewise, animals will contract away from a source of pain..
6. Living Things Grow
All living organisms grow and change. They increase in size through one of two methods - cell division and cell enlargement.Cell division is when cells divide to form new, identical cells. As the number of cells increases, the organism's overall size increases. Cell enlargement occurs when the size of the individual cells themselves increase.
7. Living Things Reproduce or Have DNA
All living organisms must be able to reproduce in order to ensure the survival of their respective species. Reproduction may be asexual (from a single parent organism) or sexual (from two parent organisms).
This characteristic used to state only that "living things reproduce." It became apparant, though, that there are exceptions to every rule. The most often cited exception to this rule was the mule! Mules are the sterile offspring of a donkey and a horse. Two mules, therefore, cannot mate and reproduce themseleves. However, mules are indeed living creatures. This characteristic therefore has been adapted to include the presence of DNA.
8. Living Things Move
All living things are capable of some degree of locomotion. This may be walking and running for animals, or the simple movement of a plant's leaves throughout the day. On the simplest level, this characteristic applies even to the movement of cells within a living organism.
9. Living Things Adapt to Their Environment
The ability of a species to survive and thrive is dependent upon organisms being able to adapt to changes in their environment. The process of evolution, in fact, can be boiled down simply to the process of change through time in response to environmental change.
10. Living Things Die
This is pretty self-explanatory. There comes a time in the lifecycle of all organisms when they die cease to exhibit the characteristics of life noted in Items 1 - 9 above.
What About Viruses?
There has been much debate about whether or not the virus should be considered a living organism. Viruses possess genes, evolve by natural selection (adapt to their environment), and can replicate themselves. On the other hand, viruses are not composed of cells, do not metabolize, and are unable to replicate without a host. At present, therefore, while viruses are generally talked about and referred to as living beings, scientists generally considered them to be in an entirely different category of their own.
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