Biological Responses - SS1 Biology Lesson Note

Every living organism possesses the ability of irritability or sensitivity, which is the ability to respond to both internal and external stimuli. There are three basic types of responses organisms can have towards their environment:

1.     Tactic responses

2.     Nastic responses

3.     Tropic responses

 

TACTIC RESPONSES

Tactic response refers to the directed movement of an organism or a cell in response to a specific stimulus or environmental condition. This type of movement allows organisms to navigate their surroundings, locate favorable conditions, and avoid unfavorable ones.

There are several types of tactic movements observed in biology, including:

1.     Phototaxis: Phototaxis is the movement of an organism or cell in response to light. Positive phototaxis occurs when an organism moves towards a light source, while negative phototaxis involves movement away from light. This type of movement is commonly observed in many organisms, such as plants, algae, and certain types of microorganisms.

2.     Chemotaxis: Chemotaxis is the movement of cells or organisms in response to chemical stimuli. Cells or organisms can exhibit positive chemotaxis by moving towards higher concentrations of a particular chemical (chemoattractant), or negative chemotaxis by moving away from higher concentrations of a chemical (chemorepellent). Chemotaxis plays a crucial role in processes such as finding nutrients, avoiding toxins, and locating potential mates.

3.     Thermotaxis: Thermotaxis is the movement of organisms or cells in response to temperature gradients. Organisms exhibiting positive thermotaxis move towards higher temperatures, while those showing negative thermotaxis move away from higher temperatures. Thermotaxis enables organisms to regulate their body temperature, seek optimal thermal conditions, or avoid extreme temperatures.

4.     Hydrotaxis: Hydrotaxis is the movement of organisms or cells in response to water or moisture gradients. It is often observed in certain organisms, such as bacteria or plant roots, as they move towards or away from regions with different water availability.

5.     Gravitaxis/Gravitropism: Gravitaxis or gravitropism is the response of organisms or cells to gravity. Positive gravitaxis involves movement towards the direction of gravity, while negative gravitaxis involves movement away from gravity. This mechanism allows organisms to orient themselves with respect to gravity, influencing processes such as plant growth (root gravitropism) and the behavior of certain aquatic organisms.

 

NASTIC RESPONSES

Nastic response refers to a non-directional, reversible movement exhibited by plants in response to external stimuli. Unlike tactic movements that involve directed movement towards or away from a stimulus, nastic movements are independent of the direction of the stimulus. Instead, they are triggered by specific environmental cues and typically occur in response to changes in factors such as light, temperature, touch, humidity, or gravity.

Nastic responses are typically observed in plant organs such as leaves, flowers, and stems. These movements are a result of changes in turgor pressure and the differential growth rates of cells on different sides of the organ, rather than the direct action of muscle-like tissues found in animals.

Examples of nastic movements include:

1.     Photonastic movement: Photonastic movements are triggered by light stimuli. For instance, the opening and closing of flowers in response to light levels, known as nyctinasty, or the folding of leaves in response to light intensity changes, known as photonasty.

2.     Thigmonastic movement: Thigmonastic movements are in response to touch or mechanical stimulation. For example, the rapid movement of Venus flytrap leaves in response to an insect's touch or the folding of Mimosa pudica (sensitive plant) leaves when they are touched.

3.     Thermonastic movement: Thermonastic movements are induced by temperature changes. Some flowers, such as the flowers of certain Araceae plants, exhibit thermogenesis, which generates heat to attract pollinators.

4.     Hydronastic movement: Hydronastic movements are influenced by humidity or water availability. For instance, the opening and closing of pine cones in response to humidity changes or the movement of fern fronds to adjust water uptake.

5.     Seismonastic movement: Seismonastic movements are triggered by mechanical disturbances, such as vibrations or wind. Examples include the folding of leaves in response to wind stimulation.

 

TROPIC RESPONSES

Tropic responses, also known as tropisms, refer to directional growth or movement of a plant or plant organ in response to an external stimulus. Unlike nastic movements, which are non-directional and reversible, tropic movements involve oriented growth or movement towards or away from a specific stimulus.

Tropisms are typically observed in response to stimuli such as light, gravity, touch, moisture, or chemicals. These movements are controlled by differential growth rates on different sides of the plant or organ, causing it to bend or curve in a particular direction. Tropic movements allow plants to optimize their growth and development, enabling them to respond to environmental cues and adapt to their surroundings.

Here are some common types of tropic movements:

1. Phototropism: Phototropism is the growth or movement of plants in response to light. Positive phototropism occurs when plants bend or grow towards a light source, while negative phototropism (or skototropism) occurs when plants grow away from light. Phototropism is primarily controlled by the hormone auxin, which accumulates on the shaded side of the plant, causing elongation and bending towards the light source.

2. Gravitropism: Gravitropism, also known as geotropism, is the growth or movement of plants in response to gravity. Positive gravitropism occurs when plant roots grow towards gravity (downwards), while stems and leaves exhibit negative gravitropism by growing against gravity (upwards). Gravitropism is influenced by the redistribution of auxin, with root tips having higher auxin levels on the lower side, promoting downward growth, and shoot tips having higher auxin levels on the upper side, promoting upward growth.

3. Hydrotropism: Hydrotropism is the growth or movement of plants in response to water or moisture. Roots often exhibit positive hydrotropism, growing towards sources of water, while shoots may show negative hydrotropism, growing away from excess moisture. Hydrotropism allows plants to seek water sources for survival and efficient nutrient uptake.

4. Thigmotropism: Thigmotropism is the growth or movement of plants in response to touch or mechanical stimuli. Examples include the coiling of tendrils around support structures, such as a vine wrapping around a trellis, or the growth of plant roots along a rough surface. Thigmotropism helps plants find support, secure themselves, or navigate their environment.

5. Chemotropism: Chemotropism is the growth or movement of plants in response to chemical stimuli. It can involve the growth of pollen tubes towards ovules during fertilization or the movement of plant roots towards or away from certain chemicals in the soil.