Irritability – cyclosis and movement

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 factors that evoke response. Organisms respond to stimulus in three ways namely tactic, nastic and trophic response.

Tactic Response or taxis 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 directional and positive if the organism moves towards the stimulus or it is negative if it moves away from the stimulus.

– Phototaxis is the response to variation in light intensity

– Chemotaxis is the response to variation in concentration of chemical substance

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 responses are usually described according to the stimulus evoking them.

– 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

– 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.

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.

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.

Geotropism: This is a response to gravitational force the shoot of plant is negatively geotropic while the root is positively geotropic.

Chemotropism is the response of plant to concentration of chemical substance e.g. roots respond positively to the presence of salt, particularly salt of calcium but negatively to alkaline or acid.

Haptrotropism /(Thigmotropism) is a response of plant to contact and is characteristic of tendrils and other organs by which the plant secures support. Tendrils show positive response to touch by twinkling around a support while roots on the other hand show a negative response to touch by growing away from stones.


Movement is the ability of living organisms to move from one place to another.

Reasons for Movement

  1. To search for food

ii          To escape from danger

iii.        To respond to stimulus either positively or negatively

  1. For the sake of reproduction


Cyclosis or cytoplasmic streaming: This is the mass rotational streaming or movement of the cytoplasm and its contents in cells with the vacuole. In some cases, the streaming may be restricted to a particular region or the cell while at times the whole cytoplasm is subjected to cyclical movement of the cell.

OrganismsOrganelles for movementMechanism of movement
AmoebaPseudopodiaCytoplasmic streaming (cyclosis)
ParameciumCiliaBeating the cilia against water
EuglenaFlagellumLashing movement of flagellum
HydraTentaclesSwimming, swaying, hoping and Somersaulting
Toads and frogLimbsHopping



  1. Explain (i)   Cell division

(ii)  Cell enlargement

(iii) Cell differentiation

  1. State the organelles or locomotion in the following organisms

(i) Amoeba

(ii) Paramecium

  • Toad
  • (iv) Earthworm

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