Classwork Series and Exercises {Biology – SS2}: Cell Reactions to Its Environment (Irritability)

Biology SS 2

Topic: Cell Reactions to Its Environment (Irritability)

Irritability or sensitivity is the ability of a cell to respond to stimuli. All living things respond to stimuli. A stimulus is a change in the cell’s environment that can make it change its activity. The cell’s change in activity is known as its response to the stimulus.

The cell responds both externally and internally to changes. The external stimuli are due to non-living or abiotic factors of the environment such as change in light intensity, temperature, humidity etc. while an internal stimulus is a change within the cell itself.

Types of Response

•   Phototropism: This is the response of plants to light. The shoots are positively phototropic while the roots are negatively phototropic.
•   Hydrotropism: This is the response of plants to water. The roots are positively hydrotropic while the shoots are negatively hydrotropic.
• Geotropism: This is the response of plants to gravity. The roots are positively geotropic while the shoots are negatively geotropic.

How Environmental Factors Evoke Response

A plant that is starved of adequate light responds towards any source of light available. The shoot will respond positively to the source of light while the root will respond negatively.

Placing a plant in a horizontal position causes an unequal diffusion of hormones. This will make the shoot negatively geotropic by growing upwards.

In a shaded area, smaller plants in an attempt to get light from top grow taller. This is called aetiolation.

Movement

Movements that are due to external stimuli may be nastic, tactic or tropic movement.

• Nastic movements: These are responses shown by a part of a plant to non-directional stimuli such as changes in light intensity, temperature or humidity. Examples are closing of the morning glory flower when the light intensity is low.
• Tactic movements: This is when a whole organism moves directly towards or away from a stimulus. The stimulus comes from a particular direction. This type of response is said to be directional, and is positive if the organism moves towards the stimulus and negative if it moves away from it. Tactic movement are named according to the type of external stimuli:

1. Phototaxis—if the stimulus is light.
2.  Chemotaxis—if the stimulus is chemical
3.  Thermotaxis—if the stimulus is temperature

Tactic movements occur in motile organisms such as Amoeba, Chlamydomonas, Euglena and Earthworm.

Tropic movements: when a part of a plant moves in response to a directional stimulus. The direction of the response is related to that of the stimulus, and is positive if the plant parts grow towards the stimulus and negative if the part grows away from it. Tropisms are very slow growth movements. They are named according to the stimuli, e.g. phototropism is a response to light, while hydrotropism is a response to water.

Cyclosis: This is the streaming rotary motion of protoplasm within certain cells and one-celled organisms, i.e. circulation of cytoplasm or cell organelles, such as food vacuoles in some protozoan.

Movement in plants is not as conspicuous as found in animals. Their movement is by continuous and uniform flow of cytoplasm. In animals, cyclosis occurs in protozoa like amoeba (amoeboid movement).

Organs for Movement

1.     Flagella: They are long and are used in moving in liquid medium by flashing against water current. Euglena and Chlamydomonas have flagella.
2.     Cilia: They are usually numerous and short. It aids movement in water just like flagella. Paramecium has cilia.