Biochemistry Lecture No. 14: Biological Forms Of Energy & Reducing Power
Friday October 5 , 2012
Gibb’s Free Energy:
- Gibb’s free energy (ΔG) operates according to thermodynamics and determines whether a reaction is
energetically favourable (spontaneously proceeds) or unfavourable (doesn’t proceed spontaneously).
ΔG is affected by the properties of the molecules and also the reaction conditions.
-Energetically favourable reactions are where products are favoured over the reactants without energy
input and ΔG < 0. Equilibrium reactions are where neither the products nor the reactants are favoured
and ΔG = 0 (reaction is proceeding forwards and backwards at the same rate). Energetically
unfavourable reactions are where reactants are favoured over the products, ΔG > 0 and the conversion
of reactants to products requires energy input. In a cell, there are ways assisting the forward reaction to
proceed despite its non-spontaneity. E.g. The formation of glutamine.
Energy Coupling Of Reactions:
-In order to make an energetically unfavourable reaction proceed, it must be chemically “coupled”
together with an energetically favourable one. However, this may not always result in an efficient result.
For example, the coupling of cellular respiration (energetically favourable) with the formation of
glutamine (energetically unfavourable) may result in a great source of energy input, but most of the
energy is dissipated (wasted) as heat.
-The process of combusting glucose on its own is one that requires stepwise oxidation, or rather the
slow, controlled release of energy in order to conserve as much as possible for doing work in the body.
This is accomplished by cells storing energy from food in carrier molecules, to be spent a little at a time.
The Hydrolysis Of Adenosine Triphosphate (ATP):
-ATP is the main energy carrier in living systems. The hydrolysis (catabolic) of ATP is energetically
favourable (forward reaction proceeds spontaneously). It involves phosphate groups being hydrolyzed
by water and yielding ADP and inorganic phosphate molecules. This reaction is highly noted in its ability
to couple with and drive energetically unfavourable reactions. The phosphoryl groups, starting with the
group closest to the ribose, are referred to as the alpha (α), beta (β) and gamma (γ) phosphates.
-The hydrolysis of ATP is favourable because: the releas