Sensitivity

Sensitivity refers to the ability of living organisms to detect and respond to changes in their environment. A stimulus is any environmental change that triggers a response. The capability of organisms to respond to stimuli is called irritability.

Types of Responses

  1. Taxism: The movement of an entire organism in response to a stimulus. If the movement is toward the stimulus, it is called positive taxism; if away, it is negative taxism. Examples include:
    • Phototaxis: Movement in response to light, e.g., in organisms like Euglena and Chlamydomonas.
    • Chemotaxis: Movement in response to chemicals, e.g., Amoeba moving away from acidic environments.
    • Aerotaxis: Movement toward or away from a source of oxygen.
    • Osmotaxis: Movement in response to changes in osmotic pressure.
  2. Nastism: Movement in plants that is independent of the direction of the stimulus. Examples include:
    • Nyctinasty: Movement in response to changes between day and night, such as the opening and closing of flowers or leaves.
    • Haptonasty: Movement in response to touch, as seen in insectivorous plants like the Venus flytrap.
    • Hydronasty: Movement in response to changes in humidity.
    • Chemonasty: Movement triggered by chemical stimuli.
  3. Tropism: Directional growth or bending of plant parts in response to a stimulus. Examples include:
    • Phototropism: Growth in response to light; plant shoots exhibit positive phototropism, while roots show negative phototropism.
    • Geotropism: Growth in response to gravity; roots grow toward gravity (positive geotropism), while shoots grow against it (negative geotropism).
    • Hydrotropism: Growth toward water, as seen in roots.
    • Chemotropism: Growth influenced by chemicals, e.g., pollen tubes growing toward ovules.
    • Thigmotropism: Growth in response to touch, as seen in climbing plants.

Movement

Movement is the ability of living organisms to change their position or location. Various structures or mechanisms enable movement in different organisms.

Cyclosis

Cyclosis refers to the movement of protoplasm within cells. In protozoa like Amoeba, this is referred to as amoeboid movement.

Locomotion Organelles in Unicellular Organisms

  1. Pseudopodia: Amoeba moves using lobe-shaped extensions of the cell called pseudopodia. These extensions form as the internal protoplasm (plasmosol) flows outward and transforms into a gel-like state. Amoeba extends one pseudopodium while retracting another, enabling movement.
  2. Cilia: Short, hair-like structures that beat in a coordinated manner to facilitate movement. In Paramecium, cilia allow the organism to move and help direct food into its gullet. In humans, cilia in the windpipe move mucus toward the throat.
  3. Flagella: Long, whip-like structures used for movement. Flagella generate waves along their length, allowing organisms like Euglena and Chlamydomonas to swim. Flagella also aid in the movement of motile cells like human sperm.