Introduction to thermodynamics- Thermodynamics governing biological systems

1. Thermodynamics

2. Introduction
 Thermodynamics (in Greek: Thermo-heat, dynamics: power) is the study of the
relations between heat, work, temperature, and energy.
 Thermodynamics is the branch of science which deals with all type of energy
transformations that accompany physical and chemical processes.
 The laws of thermodynamics describe how the energy in a system changes and
whether the system can perform useful work on its surroundings.
 The key concept is that heat is a form of energy corresponding to a definite
amount of mechanical work.

3. System, boundary, surroundings

5. Types of systems
 A system which can exchange both
matter and energy with its
surroundings is called an open
system.
 A system which can exchange energy,
but not matter, with the surroundings
is called a closed system.
 A system which can exchange neither
matter nor energy with its
surroundings is called an isolated
system.

6. State of a system
 A system is said to be in a definite state when its macroscopic properties
(e.g., pressure, volume, temperature, composition, density, etc.) have
definite values.
 When ever thetre occurs a change in one of the macroscopic properties of
a system, it is said to undergo a change in state.

7. State and path functions
 A property of the system whose value depends only upon the state of the system
and does not upon the path by which the state has been attained is called a state
function.
 E.g., Pressure, volume, temperature, internal energy, enthalpy, entropy, etc.
 A property of the system which depends upon the path followed in attaining its
state is called a path function.
 E.g., Heat, work etc.

8. Thermodynamic property

9. Intensive and extensive properties

10. Control volume, Control surface, Universe,
Temperature

11. Work
 Work (w) refers to the exertion of force (f) in moving an object through a distance (d).
w = f . d
 Work is any quantity that flows across the boundary of a system during a change in its
state and manifests its effect on the surroundings which is equivalent to the lifting of a
weight in its surroundings.
 Work done on the system during a process is regarded as positive work while work
done by the system during a process is regarded as negative work.
 One important form of work is the pressure-volume work or expansion work
whereby a system alters its volume against an opposing force.
w = -P. ΔV (where P = Pressure & ΔV= change in volume)

12. Adiabatic and isothermal processes
 A process in which no heat enters or leaves the system is called an adiabatic
process.
 In such a process, the system is completely insulated from its surroundings.
 An isothermal process is one in which the temperature of the system
remains constant throughout.
 In such a process, heat can flow from the system to surroundings and vice-
versa.

13. Endothermic& exothermic processes
Reversible and irreversible processes
 A process which absorbs
energy as heat is called an
endothermic process.
 A process which liberates
energy as heat is called an
exothermic process

14. Internal energy
 Internal energy of a system is the
energy associated with different
molecular motions and
intermolecular interactions.
 It is the sum total of the energies
associated with the translational,
rotational, vibrational, electronic and
nuclear motions at the molecular
level as well as the potential energy
of interaction between the
constituent particles.
 It is denoted by E.

15. Enthalpy

16. The zeroth law of thermodynamics
 When two systems are both in thermal
equilibrium with a third system are in
thermal equilibrium with each other.
 When a body ‘A’ is in thermal
equilibrium with a body ‘B’ and ‘B’ is
also separately in thermal equilibrium
with a body ‘C’, then A and C will be in
thermal equilibrium with each other.
 It is the basis of temperature
measurement.
 Two systems in thermal equilibrium with
each other have the same temperature;
two systems not in thermal equilibrium
have different temperature.

17. First law of thermodynamics
 The first law of thermodynamics (also known as the law of conservation of
energy) states that energy can neither be created nor destroyed although it may be
converted from one form to another.
 The mathematical statement of the first law of thermodynamics is
ΔE = q + w
where w is the work done on a system, q is the energy transferred as heaty to the
system & ΔE is the resulting change in internal energy of the system.

18. Enthalpy change in a process
 Enthalpy of a reaction at a given temperature is defined as the enthalpy change
produced when the number of moles of the reactants as shown in the
stoichiometric equation for the reaction are completely converted into products.
ΔH = Hproducts – Hreactants = qp
 ΔH is referred to as enthalpy of reaction or heat of reaction at constant pressure.
 qp is the heat absorbed at constant pressure.
qp = ΔE = P ΔV

19. Entropy
 Entropy is a property which is a measure of disorder or randomness in a system.
 The greater the disorder, the greater is the entropy.
 The entropy change (ΔS) in a process is given by
ΔS = S2 – S1
where S1 and S2 are respectively the entropies of the system in the initial and final
states.

20. Second law of thermodynamics
 The second law of thermodynamics states that the entropy of the universe remains
constant in a reversible process whereas it increases in an irreversible process.
 The entropy change of the universe in a process is the sum total of the entropy
changes of the system and the surroundings in the process.
 According to the second law,
For a reversible process, ΔS = ΔSsystem + ΔSsurroundings = 0
For an irreversible process, ΔS = ΔSsystem + ΔSsurroundings > 0
 In other words, for a reversible process, the entropy change of the universe is zero
whereas for an irreversible process, it is positive.

21. Free energy
 Free energy or Gibbs free energy (G) is a function that can be used for processes taking place at
constant temperature and pressure.
 The free energy of a system is defined by:
G = H – T S
where H is the enthalpy of the system, S its entropy and T is the Kelvin temperature.
 For a process taking place at constant temperature T, if G1, H1 & S1 represent respectively the free
energy, enthalpy and entropy of the system in its initial state and G2, H2 & S2 the corresponding
values in its final state, then the free energy change is given by
Δ G = Δ H – T Δ S (Gibbs- Helmholtz equation)
where Δ H = H2 – H1; Δ S = S2 - S1.
 A process would be spontaneous only if the free energy change (Δ G) in the process is negative.

22. Laws of Thermodynamics as Related to
Biology
 The laws of thermodynamics are
important unifying principles
of biology.
 These principles govern the
chemical processes (metabolism) in
all biological organisms

23. First law of thermodynamics in
Biological systems
 All biological organisms require energy to survive.
 In a closed system, such as the universe, this energy is not consumed but transformed
from one form to another.
 Cells, for example, perform a number of important processes. These processes require
energy.
 In photosynthesis, the energy is supplied by the sun. Light energy is absorbed by cells
in plant leaves and converted to chemical energy.
 The chemical energy is stored in the form of glucose, which is used to form complex
carbohydrates necessary to build plant mass.
 The energy stored in glucose can also be released through cellular respiration.
 This process allows plant and animal organisms to access the energy stored in
carbohydrates, lipids, and other macromolecules through the production of ATP.
 This energy is needed to perform cell functions such as DNA replication, mitosis,
meiosis, cell movement, endocytosis, exocytosis, and apoptosis.

24. Second law of thermodynamics in
Biological systems
 As with other biological processes, the transfer of energy is not 100 percent
efficient.
 In photosynthesis, for example, not all of the light energy is absorbed by the plant.
Some energy is reflected and some is lost as heat.
 The loss of energy to the surrounding environment results in an increase of
disorder or entropy.
 Unlike plants and other photosynthetic organisms, animals cannot generate energy
directly from the sunlight. They must consume plants or other animal organisms for
energy.

25.  The higher up an organism is on the food chain, the less available energy it
receives from its food sources.
 Much of this energy is lost during metabolic processes performed by the producers
and primary consumers that are eaten. Therefore, much less energy is available for
organisms at higher trophic levels. (Trophic levels are groups that help ecologists
understand the specific role of all living things in the ecosystem.)
 The lower the available energy, the less number of organisms can be supported.
 This is why there are more producers than consumers in an ecosystem.

26.  Living systems require constant energy input to maintain their highly ordered state.
 Cells, for example, are highly ordered and have low entropy.
 In the process of maintaining this order, some energy is lost to the surroundings or
transformed.
 So while cells are ordered, the processes performed to maintain that order result in
an increase in entropy in the cell's/organism's surroundings.
 The transfer of energy causes entropy in the universe to increase.

27. Free energy changes in biological
processes
 Among the metabolic reactions, the catabolic reactions (larger complex organic molecules are
broken down to smaller ones) generally are energy- releasing and have ΔG negative. They are
referred to as exergonic reactions.
 Anabolic reactions ( large macromolecules of the cell are synthesized)are generally energy-
requiring and have Δ G positive. These are called endergonic reactions & are not spontaneous.
 A cellular reaction with positive Δ G may occur provided it is coupled with a reaction having a
negative and larger Δ G such that the net Δ G is negative.
i. Endergonic reaction: AB; ΔG1 > 0
ii. Exergonic reaction: SP; ΔG2 < 0
iii. (i) + (ii): Coupling; ΔG = (ΔG 1 + ΔG2) < 0
 All energy-requiring cellular reactions proceed by coupling with energy-releasing cellular reactions
in such a way that the net ΔG for a set of coupled reactions is always negative.
 The transfer of energy between the two is not done directly but indirectly and the actual
mechanism of coupling is rather complicated.