Center polarity

In biology, a axis of polarity is a lack or an absence of Symétrie in the Plan of organization of an organization. The axis indicates the 2 sides which are not symmetrical.

One can find at an organization at the same time symmetries and polarities. For example at the vertebrate ones, there is a symmetry imperfect right-hand side-left. But one also observes:

an axis of polarity back-belly,
an axis of polarity back-before,
an axis of polarity right-hand side-left, in the plan of organization intern (the lobes pulmon surfaces, the Intestin S are not symmetrical).

Determinism of the axes of polarity

Just like the other aspects of the Plane of organization, the axes of Polarité are hereditary thus given at least partly Génétique lies. Some of the Gène S persons in charge were identified at model organizations (Drosophile, Souris) then at the human ones. It acts inter alia homeotic genes.

At the plants, one generally finds an axis of polarity haut-bas (or root-foliage) and it can appear an additional axis of polarity, directed towards a source of light in the event of Phototropisme.

Therefore, in the animal kingdom, the polarities are relatively immutable of a generation with another, and one can thus deduce from it that the polarities are given in large part genetically. At the plants, it is a character which presents large a Plasticité since the polarities can adapt to a given environment.

In fact, like the animals are mobile, that mitigates this lack of plasticity in the development of the plan of organization. It is not the case at the plants.

Relation with the Environment

As all in biology is history of evolution, one can wonder why this evolution revealed and maintained these absences of Symétrie. In fact, any organization is in relation to its environment, and the external polarities are evolutionary answers to the Polarité S of the medium, or rather to the Anisotropie S of the medium. Indeed we do not live in a symmetrical and homogeneous world but in a polarized environment:

; The dorso-ventral axis There are bottom and the top, fields of gravity directed towards the center of the Earth. It results from this that the organizations have members directed to the bottom, to support them well-sure. The ventral side is thus differ on the dorsal side. When the effect of gravity is reduced as in an aquatic environment (the Poussée of Archimedes reduces the weight of the animal), the polarity dorso-ventral is marked, as at the Poisson S.

; The antéro-posterior axis The gradient of food in the environment supported a concentration of the bodies of ingestion (stops) and the bodies of perception (eye, nose, language…) on the same side. It is the front side. At certain species living in an environment where the gradient of food weak or is confused with the fields of gravity, the polarities are marked:

the Mould S.
the sea urchin S.
the Corals
certain animals parasite S.

; Polarity and symmetry right-hand side-left

Polarity D-G of the internal organization of the Vertébré S is little marked but is undeniable. Which factor of the environment could supported this lack of symmetry? In all Assumption, none. Because the internal bodies are little in relation to the environment. If the Cœur beats on the left, and, if the intestine is rolled up on a side and not in the other that is due….randomly. Indeed, if the Heart point towards the left at everyone, it is an effect of chance, genetic Dérive which fixed an allele given in the populations. The cases of Situs inversus show that the direction of internal polarity D-G is not fundamental for good food. The direction of polarity D-G would be a neutral Caractère, with respect to the Natural selection.

On the other hand, symmetry right-hand side-left was supported by the Natural selection, because there is no difference right-hand side-left in the environment. If a predator arrives by the left it is necessary to know to flee by the line and conversely…

; Polarity at the plants The Anisotropie of the medium touches also the plants for which there is a field of Gravité, and a gradient of resources: the Light, the rain or the air arrive of in top, the ions of the ground are lower. However, all these anisotropies are in the same direction! This is why, one finds one axis of polarity haut-bas at the plants. A second axis can appear if, for example, the light only comes on a side (Phototropisme).

One finds also a double polarity in the Fleur S Zygomorphe S. But, it is there still in relation to the environment and with the polarity of an aspect of the medium, and more exactly that of the animals Pollinisateur which must be attracted by them.

Installation of the axes of polarity

August 1st

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