A tropism (from Greek τροπή, trope, "a turning") is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the stimulus (as opposed to nastic movements which are non-directional responses). Viruses and other pathogens also affect what is called "host tropism" or "cell tropism" in which case tropism refers to the way in which different viruses/pathogens have evolved to preferentially target specific host species, or specific cell types within those species. Tropisms are usually named for the stimulus involved (for example, a phototropism is a reaction to light) and may be either positive (towards the stimulus) or negative (away from the stimulus).
Tropisms are typically associated with plants (although not necessarily restricted to them).[1] Where an organism is capable of directed physical movement (motility), movement or activity in response to a specific stimulus is more likely to be regarded by behaviorists as a taxis (directional response) or a kinesis (non-directional response).
In English, the word tropism is used in sometimes derisive way to indicate an action done without cognitive thought: However, "tropism" in this sense has a proper, although non-scientific, meaning as an innate tendency, natural inclination, or propensity to act in a certain manner.
[edit] Types of tropisms
- Chemotropism, movement or growth in response to chemicals
- Geotropism (or gravitropism), movement or growth in response to gravity
- Heliotropism, movement or growth in response to sunlight
- Hydrotropism, movement or growth in response to water
- Phototropism, movement or growth in response to lights or colors of light (e.g.- the sunflower)
- Thermotropism, movement or growth in response to temperature
- Electrotropism, movement or growth in response to an electric field
- Thigmotropism, movement or growth in response to touch or contact
- Host tropism or cell tropism, the host range of pathogens
- Ecotropism, limited host range (e.g. infects only one species or cell type)
- Amphotropism, wide host range (e.g. infects many of entry into cells used by a given strain of HIV)
- Neurotropism, a virus that preferentially infects the host's nervous system.
- Photoperiodism, is response to seasons
[edit] See also
[edit] References
- ^ For example, some cells may not be conducive for the growth of a virus, which determines its tropism. The stimulus of light on insects may also be seen as a type of ethological tropism
