Every living organism requires energy to perform biological processes such as movement, respiration, excretion, coordination, reproduction, active transport across membranes systems, etc. Such energy needs vary from one organism to another, depending however, on their nature. An active animal, e.g the bird will require more energy for flight than a snail will require for crawling.
Energy for biological function is in the form of chemical energy which is usually stored in food. Simple food nutrients derived from carbohydrate, protein and fats are fed into mitochondria units where respiration occurs with the aid of enzymes, resulting in the release of energy.
Mitochondria are tiny cellular organelles found in every cell of plants and animals.
They are the indices of energy and are commonly referred to as the powerhouse of the cell. They represent seats of metabolic functions.
Energy released in this way is in the form of adenosine tri phosphate (ATP), a high energy phosphate compound capable of releasing 7,500 calories/mole ( = 7,500 cal), as stored energy. For this energy to be released for use, it is converted to adenosine diphosphate (ADP) and phosphate.
The energy so released (after respiration) can be used for the following purposes:
(i) As chemical energy for the processes that occur inside the cell.
(ii) As light energy in phosphorescent (light producing) organisms such as fire flies.
(iii) As electrical energy as in certain cat fish (eel).
(iv) As mechanical energy during locomotion.
(v) As sound energy in the production of sound in toads, insects, etc.
(vi) As heat energy used in the maintenance of body temperature. During respiration, part of the energy produced is lost in the form of heat. The other by-products of cellular respiration are carbon dioxide and water, alcohol or lactic acid, depending on whether or not oxygen is involved.
Sun as the ultimate source of energy
The ultimate source of energy for carrying out biological activities is the sun. Radiant energy from the sun is trapped by chlorophyll-containing plants and used in the manufacture of carbohydrates in the process of photosynthesis. Food so manufactured is chemical energy which, when consumed, is converted into energy to do work through the high energy phosphate compound (ATP).
Since all other organisms depend on plants directly or indirectly on animals that feed on them for their energy needs, then the energy of the sun is the ultimate in performing biological activities. Energy derived from the sun is also used domestically and in industry. At home, the heat from the sun is used to dry wet clothes, dry fresh produce such as kernel, pepper, melon, beans, rice, cocoa and other cereals as a means of their preservation. Also, the energy for generating some industrial plants is derived from the sun. Similarly, the energy of the sun is used to power types of batteries (solar cells) for the generation of electricity. For instance, the Games Villages at the 2000 A.D. Olympic Games, Australia derived their electricity from turbines operated from solar energy. So also are solar lamps operated.
Moreover, solar energy stored in dead organic matter is used to power some engines, machines and several equipment when firewood, coal and other fossil fuels are burned.
The production and consumption of energy are important activities that occur daily in all living organisms, with one complementing the other. Metabolism is the sum total of all the complex physiochemical reactions which take place in living organisms, giving rise to energy changes in their bodies. It consists of two processes viz: anabolism and catabolism.
(i) Anabolism: Anabolism is the building up processes (synthesis), in which complex organic substances are produced from simpler substances. The primary anabolic process is the synthesis of carbohydrate by green plants, using simple inorganic substances during photosynthesis. Other anabolic processes include the synthesis of proteins, fat and oil, vitamins and other plant constituents. Growth is another important example of anabolism. In each case energy is absorbed.
(ii) Catabolism: This refers to all the breaking down processes that occur in living organisms. For instance the breaking down of food nutrients in the body for the release of energy is catabolic. Energy released is a major source of energy for biological activities in living organisms.
Forms of energy
The different forms of energy may be summarised as follows:
(i) Solar energy: This is energy from the sun. It is the ultimate and may be categorised as radiant or light energy. Radiant energy is absorbed by photosynthesising plants, and certain bacteria for the purpose of food manufacture.
(ii) Heat or thermal energy is energy from the sun that warms the earth. This form of energy provides optimum temperature within which life processes occur.
(iii) The other forms of energy include electrical, chemical, sound, kinetic and potential energy.
Laws of thermodynamics
In nature, energy is neither created nor destroyed. One form of energy may be changed or transformed to another. This principle is applied to the formulation of thermodynamic first and second laws.
The first law of thermodynamics states that energy is neither created nor destroyed, but can be changed from one form to another. In practice, we see how green plants absorb radiant energy and transform this into chemical energy that is stored in the form of carbohydrate food or even the conversion of the chemical energy in food to kinetic energy to accomplish biological processes. Such transformation may however, never be 100% complete as part of the energy transformed is usually lost to the environment in the form of heat.
The second law of thermodynamics states that energy transfer is from a higher to a lower level. This is seen when heat is being transferred. It moves from a hotter to a colder object, e.g. the transfer of (heat) energy from the sun (at a higher level), to.the earth (at a lower level), for biological functions.