Science [4 marks Questions]

Hormones are secreted in appropriate quantities. Imbalance in the concentration of hormone causes disorders. Such hormonal disorders are as follows:
(1) Giants: Due to excess secretion of growth hormone, individual becomes extremely tall.
(2) Dwarfism: Deficiency of growth hormone in childhood leads to dwarfism (very short).
(3) Goitre: Iodine deficiency in our diet affect the synthesis of thyroxin and cause goitre. Due to enlarged thyroid gland, swelling in neck region seen.
(4) Diabetes: If insulin is not secreted in appropriate amounts, the sugar level rises in the blood. It causes diabetes.

Plant hormones: They help to coordinate growth, development and response to environment.
They are synthesised at places away from where they act and simply diffuse to the area of action.
(i) Growth promoting hormones:
(1) Auxin: It is synthesised at the shoot tip. It helps cells to grow longer. When light is coming from one side of the plant, auxin diffuses towards the side of the shoot which is in shade. Specific concentration of auxin stimulates the cells to grow longer on the side of the shoot which is away from light. Thus plant appears to bend towards light. The auxin is responsible for phototropism.
(2) Gibberellin: It helps in the growth of the stem.
(3) Cytokinin: It is naturally present in higher concentration in areas where rapid cell division occurs. In fruits and seeds, it is found in more concentration.
It promotes cell division.
(ii) Growth inhibiting hormone
Abscisic acid: It inhibits plant growth. Its effects include wilting of leaves.

As nerve impulses does not reach each and every cells of animal body, there is another means of communication between cells.
(i) Hormonal coordination achieved by special chemical compounds, called hormones.
(ii) Hormones are known as chemical coordinators and diffused by those cells which have specific molecules on their surface.
(iii) Hormonal coordination is slower but it can potentially reach all cells of the body.
(iv) Hormonal coordination is effective in both plants and animals.
(v) Compared to electrical impulse it is steady and persistent.

If the movement in a plant organ is induced by an external and directional stimulus, then it is called tropism or tropic movement.
The process induced by a stimulus is called a response or reaction. If the growth of the plant organ is towards the direction of the stimulus, it is called positive tropism and if the growth of the plant organ is in the opposite direction of the stimulus, it is called negative tropism.
Factors inducing stimulus: Light, gravitational force, water, chemical and physical touch are the common factors that induce stimuli.
Types of tropisms based on response shown by the plant organs:
(1) Phototropism: The reaction of plant organ in response to stimulus in the form of light is called phototropism or phototropic movement.
e.g., Root system of plant shows negative phototropism and the shoot system shows positive phototropism.
(2) Geotropism : The reaction of plant organ in response to the direction of the gravitational force is called geotropism.
e.g., Root system shows positive geotropism and the shoot system shows negative geotropism.
(3) Hydrotropism: The reaction or response of plant organ in response to the direction of water is called hydrotropism.
e.g., The root system shows positive hydrotropism and the stem and leaves show negative hydrotropism.
(4) Chemotropism: The reaction of plant organ in response to the direction of chemical substance is called chemotropism.
e.g., The elongation of the pollen tube in the direction of the micropyle of the ovule during fertilisation is the example of positive chemotropism.
(5) Thigmotropism: The reaction of plant organ in response to the direction of touch is called thigmotropism.
e.g., The tendrils coil spirally along the supporting object that comes in contact with the tendril. It is an example of positive thigmotropism.

Coordination in plants is different from coordination in animals as follows:
(1) Animals have a nervous system for controlling and coordinating the activities of the body. But plants have neither a nervous system nor muscles.
(2) Unlike that in animals, the plants do not possess any muscular tissue or any specialised protein, even then the plant cells change their shape by changing the quantity of water contained therein. The cell expands or swells when the amount of water therein increases and the cells shrink or contract when the amount of water is reduced (decreased).
(3) Unlike that in animals, the plants have no tissue to pass any message from one tissue to the other. Yet the plant cells can pass electrical and chemical signals and thereby convey this information from one cell to another.

Change in position of an organism is called movement. Movement is one characteristic feature of living beings.
Movements are seen for following purposes in organisms:
(1) Some movements are growth related. e.g., a seed germinates and grows. Seedling develops and shoot system comes out from soil.
(2) Running, playing, chewing, etc. are the examples of movement which are not related to growth but for other functions.
(3) Some movements are seen as a response to change in the environment of the organism or for their advantage. e.g., amoeba expands pseudopodia to ingest food particles, buffaloes chew cud to break-up tough food for better digestion.
(4) Some movements are seen for protection. e.g., our pupils constricted when bright light falls on eyes, we pull our hand instantly while touching a hot object.