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Lesson Note
Subject: Biology
Topic: Irritability / Cell Reactions To Its Environment

Lesson Objectives: By the end of the lesson, the learners should be able to:

  1. Explain Irritability and Type of Responses;
  2. Explain the concept of Movement in lower organism;
  3. Identify the Organelles for Movement.

Irritability is the ability of organisms to respond to stimuli. A stimulus is any change in external or internal environmental condition which can bring about a change in the activity of the whole or part of the organism.
Response is the term used for the change in activity of the organism. There are three major types of responses, these include tactic, nastic and tropic movements.

TAXIS OR TACTIC MOVEMENT: is a directional movement or response of a whole organism from one place to another in response to external stimuli such as light, temperature, water and certain chemicals. Examples of tactic movement include;
Euglena or chlamydomonas swimming away from high light intensity (Negative phototaxis).
In a moss plant, sperm swim towards the chemical produced by the egg cell (positive chemotaxis).
NASTISM OR NASTIC MOVEMENT: is a non-directional sleep movement or response of a part of a plant in response to non-directional stimuli such as light intensity, temperature and humidity. Example of nastic movement include;
The folding of the leaflets of mimosa plant when touched.
Closing of the morning glory flower when light intensity is low.
TROPISM OR TROPIC MOVEMENT: is a unilateral growth and directional movement of a part of a plant in response to directional stimuli. These responses are experienced in growth regions (root and shoot apices) and are controlled by certain plant hormones known as auxins. Tropic movement are named according to the stimuli e.g.
Shoots bend towards light (positive phototropism) while roots bend away from light (negative phototropism).
Shoots bend away from gravity (negative geotropism) while roots bend toward gravity (positive geotropism).
Tendrils of climbing plants twine around a support (positive thigmotropism) while root tips grow away from it (negative thigmotropism).

Organisms moves from one place to the other in search of food, water, mates and escaping predator or harsh weather conditions.

Cyclosis (cytoplasmic streaming) is the mass rotational movement of the cytoplasm and its contents in cells. Cyclosis brings about the transportation of substances from one part of the cell to the other and the exchange of materials between the cell organelles. Cyclosis occur in Protozoa like amoeba known as amoeboid movement.
Chloroplasts of some plants where they move independently to place their broad surface parallel to the surface of the leaf to receive sufficient sunlight for photosynthesis.

1. Flagella: they are long whip like projections usually one or two on the cell surface. Flagella are organelles for movement in Euglena, Trypanosome, Spermatozoa, Chlamydomonas etc.
2. Cilia: They are short hair like structures, numerous and closely packed together on the cell surface. Cilia can be found in paramecium and on cells lining the human wind pipe.
3. Pseudopodia: this is found in amoeba for locomotion.

Done studying? See all previous lessons in biology.
Take a quick test for this lesson.

What is irritability?
What do you understand by response?
Mention the structure for movement in paramecium, amoeba and euglena.
Describe briefly movement in amoeba.
Define cyclosis?
Mention two examples.
Outline two functions of cyclosis.
State two examples of nastic movement.
How is tropic movement related to growth?
Differentiate between tactic and nastic movement.
Describe an experiment to demonstrate geotropism in root and shoot.

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