SS1 Biology Lesson Note on Cell Reactions to its Environment

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TOPIC: PROPERTIES AND FUNCTIONS OF THE CELL – CONTINUED

CONTENT:

1. Cell Reactions to its Environment
2. Movement
3. Reproduction

SUB-TOPIC 1: CELL’S REACTIONS TO ITS ENVIRONMENT

The protoplasm of plant and animal cells is irritable. It can detect and respond to changes in their environment. Any change in conditions which is enough to produce a change in the activities of an organism or its part is called a stimulus.

Types of Responses

The three responses of the cell to its environment are tactic, nastic and tropic responses.

Tactic responses (Taxism): It is a response made by a whole organism or its freely locomotive part in response to an external stimulus. Types of tactic responses are:

• Phototactic (Phototaxis): response to variation and intensity of light e.g. in free swimming Euglena, Chlamydomonas etc.
• Chemotactic (Chemotaxis): response to the presence of chemical substances e.g. chemotactic movement of Amoeba away from an acidic environment.
• Aerotactic (Aerotaxis): response to the source of oxygen
• Osmotactic (Osmotaxis): response to variation in osmotic concentration

Nastic responses (Nastism): This is a response made by a fixed plant in response to a non-directional or diffuse stimulus. Various types of nastic responses include the following:

• Nyctinastic (Nyctinastism): response to changing day and night conditions as exhibited by some flowers and leaves due to changing conditions of temperature and light intensity. Nyctinastic movement is a combination of two factors- temperature and light i.e. thermonastic and photonastic respectively e.g. leaflets of leguminous plants such as acacia, groundnut and clover.
• Haptonastic (Haptonastism): response due to contact e.g. movement of leaves of insectivorous plants such as venus flytrap.
• Hydronastic (Hydronastism): response to humidity changes
• Chemonastic: response to presence of specific chemical substances.

Tropic Responses (Tropism): This is a response made by a fixed plant part to a stimulus. The direction of movement is determined by the direction from which the stimulus originates. Various types of tropic responses include the following:

1. Phototropism: growth movement in response to the stimulus of light e.g. growth of plant stems. Shoot is positively phototropic while the root is negatively phototropic.
2. Geotropism: a growth response to the stimulus of gravity e.g. geotropic response of root and shoot. Root is positively geotropic while shoot is negatively geotropic.
3. Hydrotropism: a growth response to the stimulus of water e.g. root of plants grow towards a water source.
4. Chemotropism: response to concentration of chemical substances.
5. Haptotropism (Thigmotropism): a response to the stimulus of touch e.g. tendrils of climbing plants.

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