Response to Stimuli

Any change in the internal or external environment is called a stimulus. There are three general responses to stimuli. These are:

1) Tactic responses. These are directional whole body movements in response to a stimulus where the direction of the stimulus affects the response. For example, a fly moving towards a flame. Tactic responses towards the stimuli are called positive tactic responses, while tactic responses away from the stimuli are called negative tactic responses.

2) Kinetic responses. These are non-directional whole body movements in response to a stimulus, where the intensity of the stimulus affects the response. For example, in response to low humidity woodlice will move faster and turn more often.

3) Tropic response. These are directional growth movements in response to a stimulus. For example, the stem of a plant growing towards the light. Animals also have tactic responses, like fingernails growing in a specific way to move around something.  

phototropism, beginner biology

Phototropism is the growth of a plant in response to light. Shoots are positively phototrophic, while roots are negatively phototropic. Gravitropism is the growth of a plant in response to gravity. Shoots are negatively gravitropic, while roots are positively gravitropic.

In phototropism, the hormone IAA, which is a type of auxin, moves to the shaded parts of roots and shoots thus increasing the concentration of IAA in these parts. In shoots, the IAA stimulates the cells in the shaded parts to elongate. As a result, the shoots bend towards the light. In roots, the IAA inhibits the elongation of root cells. As a result, the roots bend away from the light.

In gravitropism the IAA moves to the underside of shoots and roots, increasing the concentration of IAA in these parts. In shoots, the IAA stimulates the cells in the underside to elongate. As a result, the shoots bend upwards. In roots, the IAA inhibits the elongation of root cells. As a result, the roots bend downwards.

1 2 3 

Close Menu