The living beings, whether animals or plants, need certain essential conditions for their life and existence they are:
(i) Food and nutrients
i) Food and Nutrients :
Both animals and plants must be supplied with food. Animals are dependent upon the food prepared by green plants, and plants are capable of preparing their own food from sunlight, CO2 (Carbon dioxide), water and the mineral salts absorbed from the soil. The food supply is an important factor in determining the habits and distribution of organisms.
Oxygen is essential to bring about oxidation in organisms. They lake oxygen from the atmosphere and give out CO2 in return which is a by-product of oxidation the air contains 21% of oxygen (O2) which is being constantly used up in respiration, decomposition of organic food, burning of wood and in return, CO2 is added to the atmosphere.
Green parts of plants take up CO2 from the air and synthesis it into carbohydrates in the presence of sunlight and release O2 as a by-product. Thus, a cycle is established between the animals and to maintain percentage of oxygen in the air.
iii) Water :
Water is indispensable to living organisms. It is a universal solvent and C02 is dissolved in it to produce hydro carbonic acid. It is a medium of absorption and translocation of various mineral salts present in the soil.
Dry seeds do not germinate unless water is available to them. Plants in the presence of sunlight, manufacture carbohydrates from the CO, and water absorbed from the soil. The absence of life in deserts may be due to lack of water and excessive heat.
iv) Light :
Energy is required for all chemical reactions. This energy is obtained from the rays of the sun, i.e., solar energy. When there is low intensity of light, plants grow weak and sickly. Animals and children grow weak in dark dwellings.
v) Temperature :
The range of temperature to which many organisms are adapted is relatively small. In man, the normal average temperature of the body is 98.6°F, but this temperature varies with the age. The temperature of the human body remains constant regardless of the atmospheric temperature, and man, is, thus, said to be a warm blooded animal.
In frog, the body temperature varies with the atmospheric temperature, and it is, thus, said to be a cold blooded animal. Temperature has considerable importance on the vegetation of a region. High temperature leads to rich and luxuriant growth as we find in tropical regions. On the other hand, low temperature is the cause for scanty and poor vegetation as seen in the Arctic and Alpine regions.
vi) Pressure :
The normal life is in the soil, where life exists under ordinary atmospheric pressure. The organisms living in deep water are filled with liquids under the same pressure as the water in their surroundings.
Hence, the pressure within and without is the same upon all parts of the body. If the deep-sea fishes are brought up to the surface of the ocean, they explode. Certain parts of their bodies may burst as a result of sudden release from high pressure.
The type of vegetation found at higher altitudes, where the air is thin and the pressure is low, is quite different from those found at the sea-level.
Living, Non-living and Dead :
Whatever present in this universe is made of elements and are governed by the same physical laws i.e., gravitation, magnetism, action and reaction. Still, there are many differences between living and nonliving things. Now the question arises, what is life? The most difficult riddle before biologists is to discover the exact meaning of life.
A living thing needs energy for its activities. This energy comes from a series of chemical reactions inside its cells, known as internal respiration, tissue respiration or cellular respiration.
The cells contain various simple food substances, which are the results of digestive breakdown in animals and photosynthesis in plants. All these substances contain stored energy, which is released when internal respiration breaks them down.