A
tropism (from Greek,
tropos, to turn) is a biological
phenomenonA phenomenon is any observable occurrence. In popular usage, a phenomenon often refers to an extraordinary event. In scientific usage, a phenomenon is any event that is observable, however commonplace it might be, even if it requires the use of instrumentation to observe it...
, indicating growth or turning movement of a biological
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
, usually a plant, in response to an environmental
stimulusIn physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it elicits or influences a reflex via stimulus transduction...
. 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 tropismHost tropism is the name given a process of tropism that determines which cells can become infected by a given pathogen. Various factors determine the ability of a pathogen to infect a particular cell. Viruses, for example, must bind to specific cell surface receptors to enter a cell...
" 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.
A
tropism (from Greek,
tropos, to turn) is a biological
phenomenonA phenomenon is any observable occurrence. In popular usage, a phenomenon often refers to an extraordinary event. In scientific usage, a phenomenon is any event that is observable, however commonplace it might be, even if it requires the use of instrumentation to observe it...
, indicating growth or turning movement of a biological
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
, usually a plant, in response to an environmental
stimulusIn physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it elicits or influences a reflex via stimulus transduction...
. 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 tropismHost tropism is the name given a process of tropism that determines which cells can become infected by a given pathogen. Various factors determine the ability of a pathogen to infect a particular cell. Viruses, for example, must bind to specific cell surface receptors to enter a cell...
" 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. The word tropism comes from the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
trope ("to turn" or "to change"). Tropisms are usually named for the stimulus involved (for example, a
phototropismPhototropism is directional growth in which the direction of growth is determined by the direction of the light source. In other words, it is the growth and response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. 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
plantPlants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants,...
s (although not necessarily restricted to them). Where an organism is capable of directed physical movement (
motilityMotility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. It can apply to either single-celled or multicellular organisms. Motile marine animals are commonly called free-swimming...
), movement or activity in response to a specific stimulus is more likely to be regarded by behaviorists as a
taxisA taxis is an innate behavioral response by an organism to a directional stimulus or gradient of stimulus intensity. A taxis differs from a tropism in that the organism has motility and demonstrates guided movement towards or away from the stimulus source ...
(directional response) or a
kinesisKinesis, like a taxis, is a movement or activity of a cell or an organism in response to a stimulus. However, unlike taxis, the response to the stimulus provided is non-directional....
(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.
Ecotropism or ecotropic from Eco, meaning the hearth (from which words like ecology and enonomy are derived, and "Tropic," meaning to turn towards. The basic philosophy of ecotropism and ecotropics argues that to be healthy human culture itself must inhabit an ecological niche, and thereby, relate appropriately with all the co-evolving forces, organic and inorganic. In short, human culture must turn towards the environment for a sustainable and meaningful future.
Curiously, the scientific community has latched onto this word to formulate the exact inverse. Here Ecotropics indicates that a pathogen like a virus or a bacterium has a narrow host range and can infect only one or a small group of species or cell culture lines.
Types of tropisms
- Chemotropism
Chemotropism is movement caused by chemical stimulus in organisms such as bacteria and plants. An example of chemotropic movement can be seen during the growth of the pollen tube. This growth of the pollen tube is always towards the ovules....
, movement or growth in response to chemicals
- Gravitropism
Gravitropism is a turning or growth movement by a plant or fungus in response to gravity. Charles Darwin was one of the first Europeans to document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull and stems...
(or geotropism), movement or growth in response to gravity
- Heliotropism
Heliotropism is the diurnal motion of plant parts in response to the direction of the sun.Heliotropism was first described by Leonardo da Vinci in his botanical studies. The term "heliotropism," though, was introduced in the early 1800s by A. P. de Candolle, for the growth of the stem tip towards...
, movement or growth in response to sunlightSunlight, in the broad sense, is the total spectrum of the electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon. Near the poles in summer, the days are longer and the...
- Phototropism
Phototropism is directional growth in which the direction of growth is determined by the direction of the light source. In other words, it is the growth and response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. Phototropism...
, movement or growth in response to lights or colors of light
- Thermotropism
Thermotropism or thermotropic movement is the movement of a plant or plant part in response to changes in temperature. A common example is the curling of Rhododendron leaves in response to cold temperatures.-External links:*...
, movement or growth in response to temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
- Thigmotropism
Thigmotropism is a movement in which an organism moves or grows in response to touch or contact stimuli. The prefix thigmo- comes from the Greek for "touch". Usually thigmotropism occurs when plants grow around a surface, such as a wall, pot, or trellis. Climbing plants, such as vines, develop...
, movement or growth in response to touch or contact
- Host tropism
Host tropism is the name given a process of tropism that determines which cells can become infected by a given pathogen. Various factors determine the ability of a pathogen to infect a particular cell. Viruses, for example, must bind to specific cell surface receptors to enter a cell...
or cell tropism, the host range of pathogenA pathogen , infectious agent, or germ, is a biological agent that causes disease or illness to its host....
s
- Ecotropism
Ecotropism or ecotropic from Eco, meaning the hearth Ecotropism or ecotropic from Eco, meaning the hearth Ecotropism or ecotropic from Eco, meaning the hearth (from which words like ecology and economy are derived, and "Tropic," meaning to turn towards. The basic philosophy of ecotropism and...
, limited host range (e.g. infects only one species or cell type)
- Amphotropism
Amphotropism or amphotropic indicates that a pathogen like a virus or a bacterium has a wide host range and can infect more than one species or cell culture line.- See also :* Tropism, a list of tropisms* Ecotropism, indicating a narrow host range...
, wide host range (e.g. infects many species or cell types)
- HIV tropism
HIV tropism refers to the cell type that the human immunodeficiency virus infects and replicates in. HIV tropism of a patient's virus is measured by the Trofile assay....
, the means of entry into cells used by a given strain of HIV
- Neurotropism
A neurotropic virus is a virus which is capable of infecting nerve cells, or which does so preferentially.Related terms include neuroinvasive , and neurovirulent...
, a virusA virus is an infectious agent too small to be seen directly with a light microscope. They are not made of cells and can only replicate inside the cells of another organism . Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
that preferentially infects the host's nervous system.