Energy Metabolism and Energy Output (With Diagram) | Organisms | Biology

In this article we will discuss about the metabolism, body’s input and output of energy in organisms with the help of suitable diagram.

Metabolism of Energy:

There is a continual exchange of energy between a living organism and its environment as in accordance with the principles of thermodynamics applied to non-living physical and chemical systems. There is a fundamental difference between plants and animals, in the way of their utilization of energy for their life processes.

Plants unlike animals, can convert the energy of solar photons into the energy of chemical bonds (i.e. food, e.g., carbohydrate, containing energy which is released during its breakdown) and store them in their leaves, stems, fruits, etc., which serves as food for the animals.

Plants are, therefore, interposed between the sun and all animals in the energy-flow relationship in nature. It follows therefore that the energy required for our bodily activities is supplied by food which is stored with energy and the original source of energy is the sun.

The Body’s Input and Output of Energy:

The energy exchange is based on the first law of thermodynamics as derived by Mayer, Joule and Helmholtz and later proved applicable in case of the animal body also by Voit, Pettenkofer and Rubner, which states that energy is neither gained nor lost when it is converted from one form to the other mechanical, thermal, electrical, chemical, etc.

The unit of energy is mainly expressed as heat unit or energy equivalent of foodstuffs—the calorie (cal), also known as small calorie, standard calorie or gram-calorie. This is defined as the amount of heat energy which can raise the temperature of 1 gm of water from 15-16°C.

The measurement of heat is known as calorimetry the apparatus for measurement being known as the calorimeter. In the biological system, i.e., in physiology and medicine, the unit used is the large calorie or kilo-calorie (Cal or K cal) which is equal to 1000 small calories. It is defined as the amount of heat required to raise the temperature of 1Kgm of water from 15-16°C.

The total energy obtained from food sources are utilized as:

i. For the synthesis of different types of protoplasmic constituent’s characteristic of different cells and tissues, e.g., phospholipid, proteins, RNA, DNA, enzyme, coenzyme and other complex substances, etc.

ii. For the operation of different organs and as a whole organism, e.g., heart, lung, gland, kidney, etc.,

iii. For maintenance of body temperature,

iv. For generation of electrical potentials and current, viz., in central nervous system and heart, etc.,

v. For the transport of substances against the concentration gradient, and

vi. For growth and maintenance.

Joule Verses Calorie:

Amounts of heat may be expressed as equivalent amounts of energy, with the joule as the unit. According to the pamphlet circulated to the Nutrition Society by the Royal Society (1968) the amount of heat must be expressed in terms of energy and the relationship between the two units is 1 kilocalorie equal to 4184 or 4188.5 or 4186.8 joules or about 4.19 K joules.

Methods for Determination of Energy Output:

Energy output can be determined by measuring the heat production of an individual over a measured amount of time.

i. Direct Calorimetry:

This is done by measuring the heat output of the subject for a given period, by putting him inside a specially prepared heat proof chamber (Atwater—Benedict’s respiration calorimeter). Heat produced is measured by changes in temperature of circulating water. This method, although very accurate, requires much elaborate apparatus and can hardly be used for ready clinical purposes.

ii. Indirect Calorimetry:

Due to complications involved in direct calorimetry, heat output is calculated indi­rectly from O₂ consumption and CO₂ output.

a. Closed Circuit Method (Clinical Type):

Various apparatus may be used for this purpose, such as Benedict- Roth apparatus (Fig. 10.1) and other apparatus of similar type. Benedict-Roth apparatus is very useful for clinical purposes as the heat production can be calculated in this type of apparatus by the oxygen consumption only without determination of CO₂ elimination.

The subject is allowed to breathe from O₂ reservoir through a mouthpiece, the nose being clipped. The CO₂ eliminated in expiration is absorbed by soda lime to keep the O₂ reservoir pure. The fall in the level of O₂ during the experiment is recorded which gives the value of O₂ consumption at the specified time.

In this method, R.Q. of the subject is not determined and the average R.Q. is taken as 0.82. 4.825 Cal of heat is liberated at this R.Q. when one litre of O₂ consumed. The energy output during the experiment is calculated by multiplying litres of O₂ consumed at that time with 4.825.

Table 10.1 shows the amount of heat produced per litre of O₂ consumed, at different R.Q.

(b) Open Circuit Method (Haldane Type):

The subject inspires atmospheric air. The expired air is collected in a special air-tight bag known as Douglas bag. The total volume of expired air collected in the bag at the end of experiment is measured and the samples are analyzed for CO₂ and O₂ in Haldane gas-analysis apparatus or in Scholander’s micro-gas analysis apparatus.

The amount of oxygen consumed and carbon dioxide given off is calculated from the difference of percentage of O₂ and CO₂ between the atmospheric and expiratory air. The R.Q. and the calorific value of O₂ can then be determined. The Douglas bag method can conveniently be used to measure the energy output during different types of activities. A respirometer devised at the Max-Planck Institute at Dortmund is also recently used for this purpose, and is much less cumbersome and easier to manipulate than the Douglas bag.