Any one give me an example of biological reaction relating 1st & 2nd laws of thermodynamics,free energy change

Question

i.e. relating enthalpy, entropy, free energy change of a biological reaction.

Answer 1

Hmm - a good one is:

Glutamic acid + Ammonia -->  Glutamine

This is a non-spontaneous, endothermic reaction, with a Gibbs free energy change of +3.4.

In order to get this reaction to occur, the body couples this reaction with the hydrolysis of ATP.  This reaction (ATP --> ADP + Pi) is exergonic and the free energy change is -7.3.

So, although the first reaction won't occur by itself, if it is coupled with the second reaction, then both reactions can take place.

An even better example for you is the reaction is ice --> water.

This reaction is favoured by entropy (i.e. when ice is converted to water, the entropy increases).

However, the enthalpy component of the free energy change is non-favourable (change in H is positive).  

How does this resolve?  Well, at high temperatures (> 0 celcius), the reaction ice --> water is favoured, but at low temperatures the reaction is non-spontaneous.