The below mentioned article provides an explanatory note on bioenergetics and thermodynamics.
Energy is defined as a measure of a system’s capacity to do work. The various forms of energy inter convertible by suitable means, include potential, kinetic, electrical, heat, chemical, nuclear and radiant energy. Inter-conversions or transformations between these forms of energy can only occur in presence of matter. Energy can exist in the absence of matter only in the form of radiant energy. “The study of energy transformations in living organisms is called as bioenergetics”.
The matter has mass and occupies space, while energy has neither mass nor it occupies space. However, energy can transform or act on matter. We can observe energy only by observing its effect on matter. The derived SI unit of energy is joule. Symbol of energy is E.
The energetic of cellular processes may be related to chemical equilibrium and oxidation- reduction potential (redox potential) of chemical reactions. Whether at the level of molecules, cells, tissues, organs, whole organisms or ecosystems, the flow of energy is essential for maintenance of life. Therefore, bioenergetics is sometimes also defined as “field of study concerned with flow of energy (or energy transductions) through living organisms and the nature and function of the chemical processes underlying these transductions.”
Nearly all living organisms derive their energy, directly or indirectly from the radiant energy of sunlight. The latter arises from thermonuclear fusion reactions carried out in the sun. The thermonuclear fusion reactions carried out in the sun convert four protons (4H+) with atomic mass of 1.0079 × 4 = 4.0316 into one helium (He) atom with atomic mass of 4.0026. The remaining mass of 4.0316 – 4.0026 i.e., 0.0290 gm atoms is simultaneously transformed into energy in the form of electromagnetic radiations.
A small part of this radiant energy is in the form of visible light that reaches earth after travelling a long distance of about 160 million km. This conversion of mass into energy is in conformity with Einstein’s equation: E = mc2 (where E is energy; m = mass; c = speed of light).
In recent past, the main purpose of bioenergetics has been to unfold or explain intricacies of energy transductions in photosynthesis and respiration and to understand how this energy is used to carry out energy requiring reactions such as ATP synthesis and accumulation of ions across membranes against electrochemical gradients.
The photosynthesizing cells absorb light energy and convert it into chemical energy by driving electrons from water to carbon dioxide and forming energy-rich products such as glucose, starch and sucrose and releasing oxygen into the atmosphere during the process.
The energy stored in photosynthetic products such as glucose (a carbohydrate) is released during cellular oxidation of these products by passing electrons to atmospheric oxygen into H2O and CO2 in respiration. Part of this released energy is conserved in the form of ATP molecules; the rest is lost as heat.
Energy rich ATP molecules are now utilized to drive off various metabolic processes of the cells, thus fulfilling their energy need. In-fact, ATP is the major carrier of chemical energy in all the cells. Almost all energy transductions in the cells can be linked or traced to flow of electrons from one molecule to another in a ‘down hill’ manner from higher to lower electrochemical potential. As will be discussed later, energy transformations in living cells/organisms, like all other natural processes, are governed by laws of thermodynamics.
Thermodynamics is the science that deals with flow of heat and other forms of energy into or out of a system. The science of thermodynamics arose during 19th century out of efforts to understand working of steam engines and why heat is evolved when boring cannon barrels.
However, the principles of thermodynamics are now universal and apply to all forms of energy and are widely used in every field of science including biology. Thermodynamics, provides an indispensable quantitative framework for understanding energy transformations in/living organisms i.e., bioenergetics.
In thermodynamics, the term system means that region of space or quantity of matter on which one has focused his interest and attention, or in other words “everything within a defined region of space”. A system may be a chlorophyll molecule, a cell, a photosynthesizing leaf, a beaker of sugar or salt solution or the Milky Way Galaxy. The system is separated from its surroundings by a boundary which may be real or definite but very often imaginary. The system and its surroundings constitute the universe (Fig. 26.1).
Thermodynamics is often concerned with the energy transfer and/or interactions which take place across the boundary.
Depending upon the nature of boundary, there are three types of thermodynamic systems (Fig. 26.2):
(i) Isolated System:
If the boundary is sealed, no matter will pass through it. And, if the boundary is insulated, no energy (e.g., heat) can pass through it. When the boundary is both sealed and insulated, neither matter nor energy will pass through it or in other words there will be no interaction between system and its surroundings. Such a system where neither matter nor energy can pass through it is called as an isolated system (Fig. 26.2A). Example of such a system is hot water contained in a thermos flask.
(ii) Closed System:
In such a system (Fig. 26.2 B), the boundary is sealed but not insulated so that only energy (in the form of heat, work and radiation) and not matter can be transferred to and from its surroundings. A specific quantity of hot water contained in a sealed vessel is an example of closed system where no water vapours (matter) can escape from the system. It can only transfer heat (energy) through the walls of the vessel into the surroundings.
(iii) Open System:
In such a system (Fig. 26.2 C), the boundary is neither sealed nor insulated, so that both energy and matter can be transferred across it. For example, hot water contained in a beaker is an open system where both energy (heat) and matter (water vapours) are transferred across the imaginary boundary into the surrounding. A living organism is also an open system which exchanges both matter and energy with its surroundings.
The living organisms exist in dynamic steady state and never at equilibrium with their surroundings. To maintain this steady state, they require continuous input of energy. When cells can no longer generate energy, the organism dies and begins to decay towards equilibrium with its surroundings.