The following points highlight the two laws of thermodynamics in relation to biological system.
The First Law of Thermodynamics:
The Principle of Conservation of Energy. According to this law, “in any physical or chemical change, the total amount of energy in the universe remains constant, although the form of the energy may change”. In simpler words, the energy is never lost in a reaction; a decrease in one form of energy will be compensated or balanced by an increase in some other form of energy elsewhere. Or, the energy can neither be created nor destroyed.
Living cells are perfect transducers of energy, capable or transforming or inter-converting chemical, electromagnetic, mechanical, and osmotic energy with remarkable efficiency. An example of application of first law of thermodynamics to living systems is energetic of a leaf.
Leaves absorb radiant energy directly from the sun and also from surroundings. Some of this energy is emitted back to the surroundings as radiant energy and also as heat, while the rest is stored in photosynthetic products as chemical bond energy.
Although, radiant energy absorbed by leaf has been transformed, but the total amount of energy will remain constant:
Total energy absorbed by leaf = Energy emitted back from leaf + Energy stored by the leaf
The Second Law of Thermodynamics:
The Concept of Entropy:
The second law of thermodynamics is a bit more difficult to comprehend (understand fully) because it is very abstract or theoretical and involves concept of entropy. What is entropy? Entropy is a thermodynamic state quantity that is a measure of the randomness, disorder or chaos of the universe (i.e., system plus its surroundings).
The second law of thermodynamics which can be stated in several forms says that the universe always tends towards increasing disorder: “In all natural processes, the entropy of the universe increases.” According to famous 19th century physicist R.J. Clausius (1879), the second law of thermodynamics states that “the entropy of the universe tends towards a maximum”
The symbol of entropy is S. Since, entropy is a thermodynamic quantity like any other, it can be measured (except its absolute value) by experiments and expressed in entropy units. In the SI system, these units are joules per mol per degree i.e., J moL-1 K-1 and represented by EU.
Any change in entropy or disorder accompanying a process from start to finish is represented by ∆S. The change in entropy for any process is given by the equation,
∆S = SFinal – SInitial,
If, SFinal > SInitial, ∆S is positive and vice versa.
A process accompanied by an increase in entropy tends to be spontaneous. In all spontaneous processes the ∆S is positive. The rate at which the process occurs is determined by kinetic factors separate from the entropy change. If a system is at equilibrium, the entropy of the system plus its surroundings is maximal and ∆S is zero.
Let us consider a very common process of melting of ice which is a spontaneous process. In ice, water molecules are present in highly ordered state with least freedom of molecular movements. In liquid water, the water molecules are less orderly arranged with greater freedom of their molecular movements.
While in water vapours, the water molecules are randomly dispersed and present in highly disordered state with highest freedom of their molecular movements. The entropy of ice is therefore lowest that of liquid water is higher, while that of water vapours is highest (Fig. 26.3). For the process of melting of ice, therefore, ∆S is positive.
Because entropy is a thermodynamic concept, it is more appropriate to discuss it in terms of thermal energy. Any system not at absolute zero (-273°C or OK) has an irreducible minimum amount of energy — energy in the form of thermal motions of the molecules and in the vibrations and oscillations of their constituent atoms. The quantity of this energy and temperature are directly proportional.
As the temperature increases or decreases, so does the quantity of this energy. Because temperature cannot be held constant when this energy is given up (assuming that there is no physical or chemical change), it has been called as isothermally unavailable energy. Quantitatively, the isothermally unavailable energy for a particular system is given by ST, where T is the absolute temperature and S is the entropy.
Since isothermally unavailable energy and entropy are related to the energy of molecular motion, it is implied that for any particular temperature, more atoms and molecules are free to move and to vibrate, that is, the more random or less ordered or more chaotic the system, the greater will be its entropy.
At absolute zero, when all molecular motion ceases, the entropy of a pure substance is also zero; this statement is called as third law of thermodynamics.
S = 0 at 0K
Entropy is a state not only of energy but of matter. To explain it further, let us consider a very common example of oxidation of glucose,
C6H12O6 + 6O2 → 6CO2 + 6H2O
Which is also illustrated schematically in Fig. 26.4.
The atoms contained in one molecule of glucose and 6 molecules of oxygen, a total of 7 molecules, are more randomly dispersed by this oxidation reaction and are now present in 12 molecules (6CO2 + 6H2O) with more freedom of molecular motion. Further, the six carbon atoms are far less constrained in 6CO2 molecules than in one glucose molecule, thus having increased entropy than the latter.
In simpler words, whenever a chemical reaction results in an increase in the number of molecules, or when a solid substance is converted into liquid or gaseous products that allow more freedom of molecular movement than solids – molecular disorder, and thus entropy increases.
It is noteworthy, that for any given reaction, the entropy of all reactants and products must be taken into consideration. For a system not at rest or absolute zero, the natural tendency is for entropy to increase and system to be more disordered or chaotic.
Since entropy represents energy that is not available to do work (isothermally unavailable energy), one of the implications of second law of thermodynamics is that “no process can proceed with 100% efficiency”. Or, “it is never possible to utilize all the energy of a system to do work”.
For example, photosynthesis will never be 100% efficient because some of the light energy driving this process will be converted into heat. Because some of the energy driving any process will be converted to or will remain as heat, “there will never be a perpetual motion machine.”
Living organisms do obey second law of thermodynamics too. The order within cells as they grow and divide is more than compensated for by the disorder they create in their surroundings in the course of growth and division through processes like oxidation of food materials (See Fig. 26.4). A comprehensive treatment of thermodynamics and bioenergetics is to be found in excellent monographs by F. Harold (1986), and D.G. Nicholls and S.J. Ferguson (1992).