Cell reaction to its environment (IRRITABILITY)
Irritability is the ability of an organism to perceive and respond to changes in
internal and external environment or stimuli.
Stimulus is a change in condition which produces a change in the activities of the
organism or part of its body. A stimulus can either be external or internal.
External stimuli are those environmental factor that evoke response.
Organisms respond to stimulus in three ways namely
i. Tactic response
ii. Nastic response
iii. Tropic response.
1. Tactic Response or taxis: This is a directional type of response or movement in
which the whole organism moves from one place to another in response to external
stimulus such as light, temperature, water or certain chemical. The response is thus
said to be directional and positive if the organism moves towards the stimulus or it
is negative if it moves away from the stimulus.
- Photo taxis is the response to variation in light intensity
- Chemo taxis is the response to variation in concentration of chemical substance
- Aero taxis is the response to variation in concentration of oxygen
- Osmo-taxis is the response to variation in osmotic condition
- Rheo taxis is the response to variation in the direction of the flow of liquid.
2. Nastic Response or Nastism: This is a type of response in which a part of a
plant moves in response to non-directional stimulus such as changes in light
intensity, temperature and humidity.
Nastic movements are varied in nature and may be as a result of growth curvature
or of sudden changes in turgor. Nastic responses are usually described according to
the stimulus evoking them.
(i) Nyctinasty is a response to changing in day and night condition (temperature) or
light e.g.
- The opening of the petals of sun flower in the light and closes in the dark
, - The closing of the morning glory flower when light intensity is low
(ii)Haptonasty is the response due to contact or touch
- The infolding of the leaflet of mimosa plant when touched.
- The closing of leaflet of flamboyant tree.
- The nastic movement of floral parts during pollination mechanism and
movement of leaves of insectivorous plant
(iii) Hydronasty is the response to humidity changes
(iv) Chemonasty is the response to the presence of chemical
3. Tropic response or tropism: This is a type of response in which a part of a
plant moves in response to directional stimulus.
Tropism are growth movement named according to the stimulus e.g. phototropism,
hydrotropism, chemotropism, haptotropism or thigmotropism, geotropism.
a. Phototropism is the growth movement in response to light, thus the
direction of growth movement depends on the direction of light. The shoot
of plant is positively phototropic.
Experiment
Aim: To show that shoots are positively phototrophic
Material Required: Two boxes, seedling growing in pot, knife, aluminium foil
Procedure: The two boxes are arranged in a proper way. Some fermenting bean
seeds are placed in two boxes with a hole cut at one end. Box A contains normal
seedlings while B contains seedlings with a caps of aluminium. The inside of each
box is painted black to prevent light reflection. The entire experiment is put on the
window and observed for few days
Observation: The shoot of the seedling in box A will be observed to bend towards
the source of light while those in box B do not
Conclusion: Since the shoot of seedlings bend toward light, it shows that the shoot
is positively phototrophic
Irritability is the ability of an organism to perceive and respond to changes in
internal and external environment or stimuli.
Stimulus is a change in condition which produces a change in the activities of the
organism or part of its body. A stimulus can either be external or internal.
External stimuli are those environmental factor that evoke response.
Organisms respond to stimulus in three ways namely
i. Tactic response
ii. Nastic response
iii. Tropic response.
1. Tactic Response or taxis: This is a directional type of response or movement in
which the whole organism moves from one place to another in response to external
stimulus such as light, temperature, water or certain chemical. The response is thus
said to be directional and positive if the organism moves towards the stimulus or it
is negative if it moves away from the stimulus.
- Photo taxis is the response to variation in light intensity
- Chemo taxis is the response to variation in concentration of chemical substance
- Aero taxis is the response to variation in concentration of oxygen
- Osmo-taxis is the response to variation in osmotic condition
- Rheo taxis is the response to variation in the direction of the flow of liquid.
2. Nastic Response or Nastism: This is a type of response in which a part of a
plant moves in response to non-directional stimulus such as changes in light
intensity, temperature and humidity.
Nastic movements are varied in nature and may be as a result of growth curvature
or of sudden changes in turgor. Nastic responses are usually described according to
the stimulus evoking them.
(i) Nyctinasty is a response to changing in day and night condition (temperature) or
light e.g.
- The opening of the petals of sun flower in the light and closes in the dark
, - The closing of the morning glory flower when light intensity is low
(ii)Haptonasty is the response due to contact or touch
- The infolding of the leaflet of mimosa plant when touched.
- The closing of leaflet of flamboyant tree.
- The nastic movement of floral parts during pollination mechanism and
movement of leaves of insectivorous plant
(iii) Hydronasty is the response to humidity changes
(iv) Chemonasty is the response to the presence of chemical
3. Tropic response or tropism: This is a type of response in which a part of a
plant moves in response to directional stimulus.
Tropism are growth movement named according to the stimulus e.g. phototropism,
hydrotropism, chemotropism, haptotropism or thigmotropism, geotropism.
a. Phototropism is the growth movement in response to light, thus the
direction of growth movement depends on the direction of light. The shoot
of plant is positively phototropic.
Experiment
Aim: To show that shoots are positively phototrophic
Material Required: Two boxes, seedling growing in pot, knife, aluminium foil
Procedure: The two boxes are arranged in a proper way. Some fermenting bean
seeds are placed in two boxes with a hole cut at one end. Box A contains normal
seedlings while B contains seedlings with a caps of aluminium. The inside of each
box is painted black to prevent light reflection. The entire experiment is put on the
window and observed for few days
Observation: The shoot of the seedling in box A will be observed to bend towards
the source of light while those in box B do not
Conclusion: Since the shoot of seedlings bend toward light, it shows that the shoot
is positively phototrophic
