Place a straight seedling on a cork disc floating on the surface of a beaker half-filled with water.

LIVINGS

In

BIOLOGY

FOR

GENERAL SECONDARY CERTIFICATE

Unit (I)

Chapter (5)

Sensitivity ( Irritability ) in Livings

Sensitivity ( Irritability ) in Livings

Sensitivity is one of the functions done by a living organism to maintain its life. Sensitivity in animals is more obvious than that in plants.

Importance of sensitivity:

1. Sensitivity is essential for adjustment:

The surrounding environment is full of various natural factors and forces:

Sounds may be low or high.

Light differs in its intensity.

The atmospheric temperature may be very high or very low.

We receive such different stimuli through special sense organs such as ears, eyes, skin…. Then we perceive them, and respond to them in an appropriate way. Thus, in noisy conditions we move away, in bright light we close our eyes, at high temperatures we sweat to cool the body, and in low temperatures we shiver, which generates enough heat energy to keep our body temperature constant.

Sensitivity also includes the response to the internal stimuli. In the case of hunger, the response is to eat. We drink as a response to thirst.

Sensitivity starts with the reception of the external or the internal stimuli, the perception of them, and ends with the appropriate response in order to adjust the body to its environment. Sensitivity exists in all livings from unicellular to Man.

2. Sensitivity is essential for coordination:

Each of the various body systems is specialized for a certain function. In Man, the digestive system is specialized for digestion, the circulatory system for circulation, the respiratory system for respiration and so on. Each system does not perform its function separately. They are very closely linked and dependent of each other. Any change in the function of one system affects the others.

During muscular exercises:

- The muscles need more food and Oxygen.

- The breathing rate increases.

- The heart beats faster.

- A great volume of oxygenated blood is sent to the muscles.

- The skin is activated to excrete more sweat, to get rid of the excess heat and wastes.

There should be a type of coordination between the different body organs and systems, so each organ serves the conditions of the other organs, and adjust itself accordingly. Different body organs and systems work as one unit. This is achieved by a network of widely spread nerves in the body, and the different nerve centres.

So, its obvious that the importance of sensitivity is the production of coordination between the different body organs and systems, and between the body and its environment.

Sensitivity in plants

1) Response to touch and darkness:

If you touch a Mimosa leaflet, its petiole soon droops as if it has wilted. Other neighboring petioles soon follow, till the effect is seen in all leaflets.

In the day time, the leaflets are held in a horizontal position. At night, the leaflets hang downwards and fold their upper surfaces (undergoing sleep movement)

Explanation of the movement:

These movements can be explained according to cell turgidity. As Mimosa leaves are compound, and pinnate, each has a primary rachis which carries at its end four secondary rachises. Each secondary rachis carries two rows of leaflets. At the base of each primary and secondary rachis, and also at the base of each leaflets, there is a swollen structure called a pulvinus.

When the leaflet is touched or at night, the primary rachis hangs downwards, the secondary ones become depressed and so the upper surfaces of the opposing leaflets become folded together. The pulvini act as joints in these movements, where the lower surfaces shrink when being touched, as the cell walls of the lower half of each pulvinus are more sensitive. This leads to water diffusing to the neighboring tissues and hence the leaflet drop. When the stimulus is removed, the cells regain their turgidity and the leaflets open once more.

2) Tropism:

The growth of roots and stems depends on many factors such as light, humidity, and gravity, when these factors are exerted on the sides of the stem or roots in an unequal form, curvature results. This curvature is called tropism.

Types of tropism:

A. Positive and negative phototropism:

Experiment:

Place a straight seedling on a cork disc floating on the surface of a beaker half-filled with water.

Date: 2015-12-17; view: 621