Irritability – Concept, meanings, importance and examples

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

Irritability - sunflowers
Living beings react in a particular way depending on the nature of the stimulus

What is irritability?

In biology, irritability is understood to be one of the fundamental properties of living beings, which allows them to detect unfavorable changes in the environment in which they find themselves and react to them, thus preventing these changes from damaging their well-being or compromising their survival.

Thus, irritability is part of the homeostatic capacities of living beings, that is to say, of its mechanisms destined to perpetuate an internal balance and thus adapt and survive changes in the environment that threaten it.

Thus, when faced with a stimulus from the environment (external) or from within the body (internal), living things 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.

Cellular irritability

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

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 environment 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 cells of the body’s immune system react to the presence of foreign agents in it.

Irritability in plants

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

Plants lack a complex nervous system that allows planned reactions to internal and external stimuli, so their modes of irritability usually involve more or less slow movements, governed by phytohormones, and that can be classified into two types:

  • Tropisms. Sustained reactions of orientation or growth of plants, in the face of a sustained stimulus, and which can be positive (towards 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 out their bodies. Therefore, plants will grow in search of the sun (positive phototropism) when it is not enough, and they will grow away from the sun (negative phototropism) when it is excessive.
    • Geotropism. Plants require fixing their roots in the ground and lifting their stems in the opposite direction, no matter where they are. For this reason the roots will always seek the Earth’s center of 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 the water reservoirs, and not the other way around.
    • Thigmotropism. Have we ever noticed how plants adapt their growth to the obstacles around them, surrounding them, growing above them or crawling away from them. This is because they react to the obstacle, preventing it from hindering or impeding their growth.
  • Nastias. Changes in leaves and other parts of plants, in response to a specific 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 reduce the surface area exposed to light (in case of excess light), or by flowering once the sun has set, if it is during that moment when it is more convenient to do so, due to the presence of insects or pollinating winds, for example.
    • Sismonastia. It is a type of reaction of the leaves of certain plants to physical stimuli such as touch 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.

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 to another habitat or, conversely, approaching a heat source in cold weather, or covering up from the sun in hot weather, 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 beneficial conditions for the animal. This is what happens when reptiles sunbathe to warm their cold blood, since they are unable to regulate it otherwise.
  • Reflex acts. Basic reaction mechanisms as an immediate response to a situation of danger, pain or threat, which occurs without prior planning, but rather as an automatic mechanism. This is what happens when we close our eyelids to the possibility that something hits our eye.

Importance of irritability

Irritability involves a fundamental principle for the success of life: adaptation. A living being must be able to perceive 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 balance is maintained as much as possible. This ability plays an important role in evolution, since as adaptations become more radical and more persistent, new species can be given rise.

Examples of irritability

Irritability - roots
An example of irritability might be a tree lifting concrete off the sidewalk.

Some simple examples of irritability in living things are:

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