The quimiotropismo is growth or movement of a plant or plant part in response to a chemical stimulus. In positive chemotropism, the movement is toward the chemical; in the negative chemotropism movement, it is far from the chemical.
An example of this can be seen during pollination: the ovary releases sugars in the flower and these act positively to cause pollen and produce a pollen tube.
In tropism, the response of the organism is often due to its growth rather than its movement. There are many forms of tropisms and one of them is called chemotropism.
Characteristics of chemotropism
As we already mentioned, chemotropism is the growth of the organism, and it is based on its response to a chemical stimulus. The growth response can involve the whole organism or parts of the organism.
The growth response can also be positive or negative. A positive chemotropism is one in which the growth response is toward the stimulus, while a negative chemotropism is when the growth response is away from the stimulus.
Another example of chemotropic movement is the growth of individual neuronal cell axons in response to extracellular signals, which guide the developing axon to innervate the correct tissue.
Evidence of chemotropism has also been observed in neuronal regeneration, where chemotropic substances guide ganglionic neurites into the degenerated neuronal stem. Also, the addition of atmospheric nitrogen, also called nitrogen fixation, is an example of chemotropism.
Chemotropism is different from chemotaxis, the main difference is that chemotropism is related to growth, while chemotaxis is related to locomotion.
What is chemotaxis?
The amoeba feeds on other protists, algae, and bacteria. It must be able to adapt to the temporary absence of suitable prey, for example entering resting stages. This ability is chemotaxis.
All amoebas are likely to have this ability, as it would give these organisms a great advantage. In fact chemotaxis has been demonstrated in amoeba proteus, acanthamoeba, naegleria and entamoeba. However, the most studied chemotactic amoeboid organism is dictyostelium discoideum.
The term "chemotaxis" was first coined by W. Pfeffer in 1884. He did so to describe the attraction of fern sperm to ovules, but since then the phenomenon has been described in bacteria and many eukaryotic cells in different situations.
Specialized cells within metazoans have retained the ability to crawl toward bacteria to eliminate them from the body, and their mechanism is very similar to that used by primitive eukaryotes to find bacteria for food.
Much of what we know about chemotaxis has been learned by studying dctyostelium discoideum, and comparing this to our own neutrophils, the white blood cells that detect and consume invading bacteria in our bodies.
Neutrophils are differentiated cells and for the most part non-biosynthetic, which means that the usual molecular biological tools cannot be used.
In many ways, complex bacterial chemotaxis receptors appear to function like rudimentary brains. Since they are only a few hundred nanometers in diameter, we have called them nanobrains.
This raises a question about what a brain is. If a brain is an organ that uses sensory information to control motor activity, then the bacterial nanobrain would fit the definition.
However, neurobiologists struggle with this concept. They argue that bacteria are too small and too primitive to have brains: brains are relatively large, complex, being multicellular assemblages with neurons.
On the other hand, neurobiologists have no problem with the concept of artificial intelligence and machines that function as brains.
Considering the evolution of computer intelligence, it is obvious that size and apparent complexity are a poor measure of processing power. After all, today's small computers are far more powerful than their larger and superficially more complex predecessors.
The idea that bacteria are primitive is also a false notion, perhaps derived from the same source that leads to the belief that big is better when it comes to brains.
Bacteria have been evolving for billions of years longer than animals, and with their short generation times and huge population sizes, bacterial systems are probably far more evolved than anything the animal kingdom can offer.
In trying to assess bacterial intelligence one stumbles upon the fundamental questions of individual behavior vis-a-vis the population. Usually only average behaviors are considered.
However, due to the immense variety of non-genetic individuality in bacterial populations, among hundreds of bacteria swimming in an attractive gradient, some continually swim in the preferred direction.
Are these guys doing all the right moves by accident? And what about the few who swim in the wrong direction, down the alluring gradient?
In addition to being attracted to nutrients in their environment, bacteria secrete signaling molecules in ways that tend to associate in multicellular assemblages where there are other social interactions that lead to processes such as biofilm formation and pathogenesis.
Although well characterized with respect to its individual components, the complexities of the interactions between the components of the chemotaxis system have only just begun to be considered and appreciated.
For now, science leaves open the question of what smart bacteria really are like until you have a more complete understanding of what they might be thinking, and how much they might be talking to each other.
References
- Daniel J Webre. Bacterial chemotaxis (sf). Currente biology. cell.com.
- What is Chemotaxis (sf).. igi-global.com.
- Chemotaxis (nd). bms.ed.ac.uk.
- Tropism (March 2003). Encyclopædia Britannica. britannica.com.