Characteristics Of Living Organisms (DIFFERENCES BETWEEN ANIMALS AND PLANTS)



In most animals, the characteristics by which we know they are alive are self-evident: they move about, they feed, they have young, and they respond to changes in their surroundings.

These features are less obvious in plants and certain small animals; and when dealing with organisms like bacteria and viruses the distinctions between living and non-living can olten be drawn only by a trained scientist with the appropriate apparatus and techniques at his disposal.

  The main differences between living organisms and non-living objects can be sum- marized as follows:

1. Respiration. This is the process by which energy is made available as a result of chemical changes within the organism, the commonest of which is the chemical decomposition of food as a result of its combination with oxygen. This is not a particularly obvious occurrence in plants and animals; but it is fairly easy to demonstrate that living creatures take in air, remove some of the oxygen from it and increase the volume of carbon dioxide in it. More simply expressed it can be said that living organisms take in oxygen and give out carbon dioxide. Sometimes this takes place with obvious breathing movements. Respiration also results in a rise of temperature, which is more easily detectable in animals than in plants.

2. Feeding. This is an essential preliminary to respiration,since energy comes ultimately from food. The feeding of a tree by its leaves is less obvious than that of an animal, which moves actively in search of food. Feeding may also result in growth.

3. Excretion. Living involves a vast number of chemical processes, including respiration, many of which produce sub- stances that are poisonous when moderately concentrated. The elimination of these from the organism is called excretion.

4. Growth. Strictly, growth is simply an increase in size, but it usually implies also that the organism is belogos-know complicated and more efficient. An illustration of this is an animal which changes its form from larva to adult, for example, a frog or a butterfly.

5. Movement. An animal can generally move its whole body,whereas the movements of the higher plants are usually restricted to certain parts such as the opening and closing of petals, or to the movements of parts as a result of growth.

6. Reproduction. No organism has a limitless life, but although individuals must die sooner or later their life is handed on to new individuals by reproduction, resulting in the continued existence of the species.

7. Irritability (Sensitivity). Irritability is the ability to re- spond to a stimulus. Obvious signs of sensitivity are the move- ments made by animals as a result of noises, on being touched or on seeing an enemy. Fully grown plants do not show such responses under casual observation, but during growth they respond to the direction of light, gravity and moisture.

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Both plants and animals have in common, to a greater or lesser extent, all the features listed above, but there are some fundamental differences between them of which one of the most important is the method of feeding.

1. Method of feeding. Animals take in food that is chemically very complicated (i.e. composed of large molecules); it consists either of plant products or of other animals. This food is reduced to simpler material by the process of digestion, and in this form it can be taken up by the body. Plants, in general, take in very simple substances that are composed of small molecules, namely carbon dioxide from the air, and water and dissolved salts from the soil. 

In their leaves they combine this carbon dioxide and water into sugar, using sunlight as a source of energy. From the sugar so produced, and the salts taken in from the soil, green plants can make all of the substances needed for their existence. The feeding of animals thus involves a breaking-down process, while that of plants is a building-up, or synthesis.

2. Chlorophyll. The green colour found in most plants is important for the absorption of sunlight and is due to chlorophyll, which is not present in any animal. This difference is one indication of the fundamental difference in feeding. (However, many plants such as fungi do not possess chlorophyll.)

3. Cellulose. In their structures, notably their cell walls, plants have a large quantity of a substance called cellulose, which is never present in animal structures.

4. Movement. Unlike animals, most of the familiar plants do not move about as complete organisms, but certain micro- scopic plants move as actively as microscopic animals.

5. Sensitivity. Although both plants and animals respond to stimuli, the response of an animal usually follows almost immediately after the application of even a very brief stimulus. 

In plants, on the other hand, a response may take place over a matter of hours or days, and then only if the stimulus persists for a relatively long time.


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