• To perform various task cell require energy to grow, move, synthesis & transportation. In scientific concept energy means capacity to perform work or ability to cause some kind of change to occur. 
  • For cell to form function energy must flow into cell from its surrounding. 
  • Energy occurs in different form, cells obtain energy either in the form of organic molecules that contain chemical energy or energy from photosynthetic organisms as photons of light. 
  • This energy is of two types.
  • Kinetic energy deals with motion of molecules, thermal radiation etc. Potential energy is the energy stored in bonds connecting atoms.


  • Thermodynamics principle helps us to understand about these energy law & their transformation:

First Law – 

  • Energy can never be created nor be destroyed however it can be transform from one form to another .
  • Energy of universe is constant, either change occurs in energy form or it’s physical location. 
  • Transformation takes place between various forms of energy such as heat, light, electricity, mechanical energy & chemical energy. 
  • In thermodynamics particular location in which energy changes occurs is reffered as system & rest of the universe is surrounding. 
  • Energy content of an individual system can be changed, but total energy content of system & surrounding remains constant. In cell 1st law is applicable in the sense that energy is transformed from surroundings & is transformed into those forms which is used by cell, for example in autotrophs, light energy is converted into chemical energy which is essentially used by heterotrophs.

Second Law- 

  • 1st law does not tells us about the probability that any such process is actually occuring. As a biologist one must like to know the direction of reaction & whether energy is released or absorbed.
  • This law states that energy event in the universe occurs in the direction that cause the system & surrounding to exhibit a net increase in randomness i.e Entropy. 
  • According to second law the Entropy of system & surrounding always increases but this is not true that entropy of individual system always increase.
  • It may increase or decrease or remain same .
  • Free energy or G (Gibbs). 
  • Describes the thermodynamic factors that allow us to apply the second law of thermodynamics to a individual chemical reaction without requiring us to measure the entropy change. 
  • It represents the energy that can be harnessed to do useful work. 
  • For living organisms where pressure and volume remains constant, the change in free energy that accompanies any biological process is determined by two parameters;

Δ E =total internal energy/ enthalpy

    S=change in entropy

   G= Δ E – TΔS

ΔS; where G = freeen ergy, T=absolute temperature

If Entropy increases then free energy decrease ;

ie, (ΔS= positive)     (ΔG=negative)  

  (ΔE<0; ΔG<0 ; ΔS>0)

& reactions proceeds in the directions that causes decrease in the free energy of the system i.e Exergenic reactions , i.e they realize free energy (products contains less bond energy ) .

  • It indicate the direction of the reactions i.e breakdown of complex organic molecule into simpler one is exergenic reactions .
  • If Entropy decreases  then free energy increases ;

i.e (ΔS= negative)    (ΔG=positive)     (ΔE>0);

  • Such type of chemical reactions are called as Endergenic reactions.
  • Photosynthetic organism are huge, complex, 
  • Endergenic reactions centers in which ΔG can be made favorable by coupling them to external energy supply i.e light .
  •  ΔG indicate the direction of reactions.
  • Most biological reaction differ from standard condition, particularly in the concentrations of the reactants. 
  • We can estimate free energy changes for different temperature by using the equation;
  • ΔG’=ΔG0′ +2.303RT log(product/reactant) where, 
  • R=gas constant, 
  • T= absolute temperature, 
  • ΔG0’=standard free energy, 
  • ΔG’= is measure of actual change in free energy, that occurs with a particular mixture of reactants & products at given concentration, the value of ΔG’ thus varies, depending on the conditions involved.
  • ΔG0′ is constant under standard conditions. 
  • It can be calculated under conditions of equilibrium.

If 0is substituted in eq (a)for ΔG’

0=ΔG0’+2.303 RT log(product(eq)) ………..     (a)

                                  ( reactant(eq))

    ΔG0’= -2.303 RT log(product(eq)……….      (b)


Equilibrium constant Keq= (product(eq)) ……….. (c)


Keq can be substituted in eq (b)

ΔG0’= -2.303 RT  log Keq ………..(d)

(Equilibrium constant at pH=7)

If ΔG0’= positive, direction of reaction , reactant —–>  product,

reaction is exergonic i.e Keq>1 & (product)> (reactant);

If ΔG0’= positive, direction of reaction , reactant <—–  product,

reaction is endergonic i.e Keq<1 & (product)< (reactant);


  • One widely occurring pattern involves the use of high energy phosphorylated compounds that release energy when their phosphate groups are removed. 
  • Such compounds play a key role in transferring energy from thermodynamically favorable to thermodynamically unfavorable processes. 
  • The most common example of such high-energy compound is adenosine triphosphate (ATP) .
  • The structure of ATP and the reactions involved in the removal of its phosphate groups is hydrolysis reaction i.e exergonic, giving adenosine diphosphate (ADP) & a free phosphate group. 
  • The terminal phosphate of ADP can also be removed in another 

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