What is irritability?

We explain what irritability is, what is cellular irritability, irritability in plants and animals. Importance and examples.

  1. What is irritability?

In the field of biology , irritability is understood as one of the fundamental properties of living beings , which allows them to detect unfavorable changes in the environment in which they are found and react to them, thus preventing such changes from damaging their well-being. or compromise their survival.

In this way, irritability is part of the homeostatic abilities of living beings , that is, of their mechanisms intended to perpetuate an internal balance and thus adapt and survive the environmental changes that threaten it.

Thus, before a stimulus from the environment (external) or from the interior of the organism (internal), living beings react in a particular way , depending on the nature of the stimulus that triggers the reaction and the level of complexity of the living being.

  1. Cell irritability

Irritability - Cell - unicellular organism
The cells react to changes in pH, temperature, sunlight, among others.

The cells have a permeable plasma membrane , which allows the inner protoplasm to detect and react to chemical and physical changes in the environment, in order to provide a more suitable means for their development. Thus, cells react to changes in pH , temperature, sunlight , electricity or the presence of nutrients and organic matter .

This microscopic degree of irritability generally depends on biochemical reactions detectable by specialized organelles , or by the same plasma membrane. Thus, unicellular organisms adapt, for example, to environmental conditions, but also that the body’s immune system cells react to the presence of foreign agents in it.

  1. Irritability in plants

Irritability - mimosa plant
Certain leaves react to physical stimuli such as rubbing or touch.

The plants lack a nervous system complex reactions allowed planned to internal and external stimuli, so their modes of irritability often involve slow movements more or less governed by plant hormones, which can be classified into two types:

  • Tropicalisms Sustained reactions of orientation or growth of plants, in the face of a sustained stimulus, and which can be positive (in the direction of the stimulus) or negative (away from the stimulus). The cases of tropism are:
    • Phototropism. Plants use sunlight for their photosynthesis processes , but too much sun can burn their leaves or dry their bodies. Therefore, the plants will grow in search of the sun (positive phototropism) when they find it little, and will grow away from the sun (negative phototropism) when they find it excessive.
    • Geotropism Plants need to fix their roots in the ground and lift their stems in the opposite direction, regardless of where they are. For that reason the roots will always look for the center of Earth’s gravity , while the stems will grow in the opposite direction, never the other way around.
    • Hydrotropism Another component that plants require for their metabolism is water , and since their roots are the organs through which they can absorb it, it is common to see them grow and spread in the direction of water deposits, and not vice versa.
    • Tigmotropism We will have ever noticed how plants adapt their growth to the obstacles around them, surrounding them, growing above or crawling away from them. This is because they react to the obstacle, preventing it from hindering or impeding its growth.
  • Nastias Changes in the leaves and other parts of the plants, in response to a determined and momentary external stimulus. They can also be of different types, for example:
    • Photonastia Many plants respond to the presence or absence of sunlight in a certain way, either by wrinkling their leaves to decrease the surface exposed to light (in case of light excess), or blooming once the sun has fallen, if it is during that moment in which it is more convenient to do it, due to the presence of insects or pollinating winds, for example.
    • Sismonastia It is a type of reaction of the leaves of certain plants against physical stimuli such as rubbing or touch. In some cases they can close their leaves to protect or hide them, or they can secrete toxic substances in reaction to what is perceived as a threat.
  1. Irritability in animals

Irritability - insects - light
Some animals move in response to the appearance or disappearance of stimuli.

In the case of animals , the presence of a more or less complex nervous system largely determines their reactions to certain stimuli, based first of all on their behavior . Actively moving away from a source of discomfort, moving out of habitat or, on the contrary, approaching a source of heat when it is cold, or covering the sun when it is hot, are examples of this. These behaviors can be classified into:

  • Tactisms Displacements of the organism in response to the appearance or disappearance of certain stimuli, associated with conditions of benefit to the animal This is what happens when reptiles sunbathe to warm their cold blood, since they are unable to regulate it otherwise.
  • Acts reflexes. Basic reaction mechanisms as an immediate response to a situation of danger, pain or threat, and that occurs without prior planning, but as an automatic mechanism. This is what happens when we close our eyelids before the possibility of something hitting our eyes.
  1. Importance of irritability

Irritability implies a fundamental principle for the success of life: adaptation . A living being must be able to perceive the changes in its environment, especially those that threaten its well-being in one way or another, in order to react in such a way that its state of internal equilibrium is maintained as much as possible. This capacity plays an important role in evolution , since as the adaptations become more radical and more persistent, new species can be originated .

  1. Examples of irritability

Irritability - roots
An example of irritability can be a tree that lifts the concrete from the sidewalk.

Some simple examples of irritability in living things are:

  • The attraction of nocturnal moths to artificial light, which they associate with the moonlight (positive tactics) versus the flight of cockroaches when we turn on the kitchen light and run to hide (negative tactics).
  • The shrinkage of its leaves when we touch a “mimosa” or “roosting” plant, or the closing of the leaves of a carnivorous plant when it detects an insect between them.
  • The roots of trees of the genus ficus that lift the concrete from the sidewalks in their search for water from the pipes (positive hygrotropism).
  • The movement of the sunflower branches, following the path of the sun in the sky (positive phototropism).

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