Biological Thermodynamics

Biological Thermodynamics

All organisms require energy to stay alive. Organisms are energy transformers. Organisms take in energy and transduce it to new forms. All chemical reactions in cells involve energy transformations.

For example green plants transform radiant energy into chemical energy. Irrespective of form and complexity all organisms capture, transduce, store and use energy in order to live. Biological thermodynamics refers to bioenergetics, the study of energy transformation in the biological systems. The concept of thermodynamics is the basis of all sciences and engineering.

Bioenergetics

Bioenergetics is the quantitative study of energy transductions in living cells. The ‘energy industry’ (production, storage and use of energy) is central to the economy of the cell society. ATP plays the role of the main energy ‘currency’ of all biochemical processes in all organisms.

Definition of energy

·       Energy is defined as the ability to do work.

·       Organisms take in energy and transduce it to new forms.

·       The flow of energy maintains order and life.

What is thermodynamics?

Thermodynamics is simply the study of energy transformations. The science deals with energy in its various forms and the conversion of one form of energy into another. Thermodynamics is concerned with the storage, transformation and dissipation of energy.

 Objectives of thermodynamics:

All chemical, physical and biological processes are ultimately enabled and regulated by the laws of thermodynamics.

1.  Understand the relationship between quantities of   heat and work in biological systems.

2. Understand the influence of energy changes in biological phenomena.

3.  Predict the effect of temperature on a variety of physico-chemical and biological 
      phenomena in systems at equilibrium. e.g. bio-reactors.

4. Understand the biochemical processes.

Biological perspective of thermodynamics principles:

In living cells,  thermodynamic changes are essential for biological functions such as growth,
 reproduction photosynthesis and respiration.

Light à Chemical : photosynthesis.

Chemical à Chemical : cellular respiration.

Chemical à Electrical : Nervous system.

Chemical à Mechanical : Muscles.

Biological energy needs: In a living cell, thermodynamic changes are essential for biological functions such as growth, reproduction, photosynthesis and respiration.

1.    To generate and maintain its structure

2.    To generate all kinds of movements

3.    To generate concentration and electrical gradients across cell membranes

4.    To maintain body temperature

5.    To generate light in some animals

Basic types of energy

•         Kinetic energy - Energy in motion- is the net available energy which is utilized during actual work. e.g. synthesize chemical compounds, transfer ions or enable movement.  

•         Potential energy –is the energy stored in the chemical bonds of carbohydrates, lipids, proteins and ATP.

Energy can take many forms such as mechanical, electrical, thermal and chemical.

Animals are open thermodynamic systems

The matter flowing into the living system contains a high energy potential. The matter flowing out of the system is at a low energy potential. The energy changes that occur between these two mass flow events are used to perform chemical and physical work processes.

System, boundary and surroundings

An assemblage of matter, which can interact with energy is called a system. A system is separated from its surroundings by a boundary. E.g. an organism, a fermenter or a test tube.

Classes of thermodynamic systems

Based on the  differentiation between flows of energy and flow of matter across the system boundary, thermodynamics distinguishes 3 types of systems:

1.An open system exchanges matter and energy with its environment.

2.A closed system exchanges only energy with its environment.

3.An isolated system exchanges neither matter nor energy with its environment.

An isolated system has boundary which is impermeable to both matter and all forms of energy. It exchanges neither heat nor matter with its surroundings. A closed system may accept heat from the surroundings but there is no transfer of matter between the system and its environment. E.g. Universe. When heat flows out of the system, the energy of the system decreases. When heat flows in, the energy of the system increases. If the heat remains constant, it may be called an isothermal system. e.g. bomb calorimeter.

An open system is one which can exchange both energy and matter with its surroundings. Biological systems are open. E.g. living cells, living things. Earth is an open system.

The first law of thermodynamics- Law  of  conservation of energy – this law was put forward by Robert Mayer in 1941. The first law states that “ the total energy of a system plus its environment remains constant”. This law declares that “ energy is neither created nor destroyed in the universe and it allows to be exchanged between a system and its surroundings”.

·                   The sum of the energy before the conversion is equal to the sum of the energy after                             conversion. The total quantity of energy in the  universe remains constant.

·                   The energy conversion is never 100% efficient. Ecological efficiencies vary from 1% to 56%                depending on organisms. Some energy is wasted in increasing the disorder or entropy.

Explanation of first law - Light is a form of energy. It can be transformed in to work, heat or potential energy of food, depending on the situation, but none of it is destroyed.

Plants convert light energy from the sun into high energy compounds that help to build cell material.

When animals eat plants, their stomach and intestines  break down the compounds for further use.

Free energy refers to the amount of energy available during a chemical reaction to do cellular work.

The free energy concept was developed by Willard Gibbs in 1870s.

The Gibbs free energy is a thermodynamic quantity which can be used to determine, if a reaction is spontaneous or not.

Gibbs free energy equation = ∆G=∆H -T∆S

Where ∆G=Gibbs free energy in KJ

                                                      ∆H=enthalpy change

                                                        T = temperature in Kelvin K =273+^oC

                                                       ∆S=entropy change (in KJ K ^-1)

Gibbs free energy –

The driving force of a chemical as two components

∆H is the drive toward stability (enthalpy)

∆ S is the drive toward disorder (entropy)

∆ G is the net driving force of a chemical reaction.

∆ G values depend upon temperature, pressure and the concentration of the reactants and products.

If ∆ G<0  = the reaction is spontaneous.

If ∆ G>0 = the reaction is non-spontaneous.

If ∆ G=0 =the reaction is at equilibrium.

Significance of  ∆G –

·                      The sign ∆G is a predictive element.

·                             -∆G à reaction favorable (exergonic, spontaneous)

·                            + ∆G à reaction not favorable(endergonic, non-spontaneous).

·                              ∆G =0 à reaction at equilibrium (no change).

Second law of thermodynamics- also called law of the degradation of energy or law of entropy. This law was developed in 1850s by German Physicist Rudolf Clausius. This law states that “a system and its surroundings always proceed to a state of maximum disorder or maximum entropy”.

Explanation of second law - Living systems are ordered, while the natural tendency of the universe is to move toward systems of disorder with unavailable energy. The second law is an important indicator of the direction of the reaction. All reactions proceed in a direction with increase in entropy and decrease in free energy.

Concept of entropy (∆ S) - The word entropy (from the Greek entrope = change ) is a measure of the unavailable energy resulting from transformations. The  term is used as a general index of the molecular  disorder associated with energy degradation.

Second law implies that the entropy of the universe is increasing because energy conversions are not 100% efficient. i.e. some heat is always released. Second law also implies that if a particular system becomes more ordered, its surroundings become more disordered. Entropy is unavailable energy or molecular disorder.

Entropy is the capacity factor for thermal energy. It is a function of state. It is a function of the degree of disorder in the system. ‘Entropy tends to increase’ = a change to a more disordered state at a molecular level. ‘no process is 100% efficient.’ High S value refers to high degree of disorder in a system; Low S value refers to low degree of disorder in a system.

Concept of enthalpy (∆H) - Enthalpy is defined as a change in heat content or heat of formation of a system.

The change in enthalpy is given by ∆H= ∆U +P ∆V

Where ∆U= internal energy change

P=pressure

V=volume

∆U= the change in internal energy of a system is equal to the heat added to the system minus the work done by the system.

∆U=Q  - W

Where Q= heat added to the system

                                                     W=work done by the system.

Summary - Thermodynamic laws describe the flows and interchanges of heat, energy and matter. Almost all chemical and biochemical processes are as a result of  transformation of energy.

Laws can provide important insights into metabolism and bioenergetics. The energy exchanges between the system and the surroundings balance each other. There is a hierarchy of energetics among organisms: