Laws of thermodynamics

Laws of thermodynamics

     Introduction
Branch dealing with energy changes or energy transformations is called thermodynamics.

The branch of thermodynamics dealing with transformation and use of energy by living cells is called biochemical thermodynamics or bioenergetics.

Quantitative observations of different forms of energy led to formulation of two fundamental laws of thermodynamics:-

    1. First laws of thermodynamics (Laws of conservation of energy)
In thermodynamics, system (matter within a defined region) + surrounding (matter in rest of universe) constitute universe.

The first law of thermodynamics states that "The total amount of energy in the universe remains constant" or "Energy can neither be created not destroyed but whenever it is used to do work or is converted from one form to another, the total amount of energy remains unchanged," While doing work, energy is either transferred or transformed or both types of energy changes occur in a living cell.

The first law of thermodynamics can be rest understood in plants as follows:

   Radiant energy or light energy from sun

                                               ↓Photosynthesis in green cells

     Chemical energy is (food molecules) in form of potential energy

                     ↓ Respiration

      Chemical energy is released and stored in the form of ATP molecules

 ↓

Utilized in different anabolic like synthesis of macromolecules in cell and hence growth, development and repair.

Living world is dependent upon this transformation.

In a cell, chemical stored in the form of ATP is converted into different forms as shown below:

Mathematically,  first law of thermodynamics can be expressed as-

ΔE = EA - EB = Q-W

E = Change in internal energy
Q = heat absorbed by the system
W = work done by the system.

The change in energy of a system is dependent only on initial and final stages and not on the path of transformation.

   2. Second law of thermodynamics 

According to second law of thermodynamics, any system or universe has a tendency to increase 'Entropy' when left to itself because energy has a tendency of even distribution and it is transferred from high energy area to low energy area, (i.e., in energetically downhill direction). Energy transfer through movement and collision of particles results in disorder of system or increase in entropy (denoted by the symbol 'S'), i.e., entropy is a thermodynamic property which is used to measure the degree of randomness or disorder of a system. The term entropy was first used by R.Clasius (1851).

Further some energy is lost in the form of heat as a result of this disorder. This energy change and loss in the form of heat. Living system involves a huge number of chemical reactions and physical changes and all of these require energy changes (transfer or transformation of energy). 

As these energy changes are never cent per cent efficient so there is a continuous loss of energy from the living which leads to increase in entropy. The increase in entropy input of usuable energy or free energy (i.e., energy available to do work). 

This free energy decreases entropy. A living system or organisms obtains free energy from environment either in the form of sunlight (e.g., in autotrophs) or in time form of food. It is for this reason that a living system cannot be isolated from its environment. 

Such a system, which has a continuous influx of energy (directly or indirectly) is called open system. (On the other hand the system where there as no exchange with the surroundings is called closed system).

Further, open system is a system in a steady-state (homeostasis) as rate: of input of matter and energy is equal to that of their output, e.g., in a living organism (system), there should be continuous input of water, by energy and variety of other materials and simultaneously there should be output of CO₂ nitrogenous waste materials, heat, etc.

Thus second law of thermodynamics expresses the concept that in occurrence of any event or process in the universe, there is decrease in availability of free energy and increase in entropy. Further, a process can spontaneously only if the sum of entropies of the system and its surroundings increases, i.e.

ΔS system + ΔS surroundings >0

    3. Third law of thermodynamics
According to the Third Law of Thermodynamics-

"The entropy of an ideal or perfect crystalline may be taken as zero as absolute zero temperature".

Actually the absolute zero temperature is so low that the constituents in a crystalline solid are not expected to have any movement to entropy. Therefore, their entropy is expected to be zero. However, this is true only for an ideal crystal. In general, crystalline solids suffer from certain defects (for details, please refer to unit 2 on solid state) also called imperfections. This leads certain entropy even at the absolute temperature. Consequently the law is applicable only to the ideal or perfect crystals free from any defect.