Understanding Tropism in Biology

The Meaning of Tropism

• The word "tropism" originates from the Greek word "tropos," meaning a turning.
• Tropism is the ability of an organism to respond to a stimulus by turning from one direction to another.
• It can be positive or negative, depending on the kind and source of stimulation.

Tropism in Plants

• Tropism in plants is a growth-mediated movement response to an external stimulus.
• It is different from taxis and kinesis, which involve movement responses to a stimulus.
• Tropism is directional specific and occurs due to the asymmetrical elongation of cells on either side of the responding organ.

Types of Tropism in Plants

• Phototropism: growth or movement of an organism toward light (positive) or away from light (negative/scototropism).
• Chemotropism: growth in response to chemicals, such as roots growing toward minerals or away from harmful acids.
• Geotropism (Gravitropism): growth in response to gravity, with roots growing toward the gravitational field and stems growing away from it.
• Thigmotropism: response to touch, with positive thigmotropism involving growth toward a stimulus and negative thigmotropism involving growth away from it.
• Hydrotropism: growth in response to water or moisture, with positive hydrotropism involving growth toward water and negative hydrotropism involving growth away from water.

Other Plant Tropisms

• Heliotropism: growth in response to sunlight, with positive heliotropism involving growth toward sunlight and negative heliotropism involving growth away from sunlight.
• Thermotropism: growth in response to temperature changes.
• Electrotropism: growth in response to an electric field.

Auxin and Directional Growth

• Auxin plays a major role in growth-mediated plant movements, promoting or inhibiting cell elongation.
• Auxin is produced at the apical region of the shoot and is carried to other plant parts through auxin carrier proteins.
• Auxin-binding protein and co-receptors initiate a signal transduction pathway that controls the expression of genes involved in cell wall loosening and extensibility.

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