Free Energy in Living Things

Living things represent rich stores of free energy. You, for example, represent an extremely highly ordered collection of atoms in intricately organized bonds. Where does the free energy for all this organization come from?

One of the hallmarks of living things is their ability to extract energy from exergonic reactions and use it to drive endergonic reactions needed to maintain themselves. For example, proteins are long chains of amino acids linked in specific patterns (see the Biomolecules Tutorial for more information about proteins). Amino acids are added one at a time to a growing chain in endergonic reactions:

protein(aq) (n amino acids)

+

amino acid(aq)

protein(aq) (n+1 amino acids)

+

H2O()

DGº' ~= 30 kJ

This reaction is made product-favored by being coupled to another exergonic reaction, the hydrolysis of a molecule called adenosine triphosphate (ATP) to adenosine diphosphate (ADP):

ATP4-(aq) +	H2O()		ADP3-(aq) +	H2PO4-(aq)	DGº' = -30.5 kJ

The actual mechanism of the coupling is complicated, but the point is that the hydrolysis of ATP drives the reactant-favored addition of amino acids to proteins forward to the formation of products. Many other endergonic reaction in living things are forced toward products in the same way.

Where does the ATP used to couple these reactions come from? For animals, fungi, and many bacteria, the only source of ATP is from the exergonic breakdown of food taken in (eaten) from the environment. For example, the oxidation of glucose is very exergonic:

C₆H₁₂O₆(s) + 6O₂(g) 6CO₂(g) + 6H₂O()     DGº' = -2870 kJ

In cells, this reaction takes place in many small steps, several of which are coupled to the formation of ATP:

ADP^3-(aq) + H₂PO₄^-(aq) ATP^4-(aq) + H₂O()     DGº' = 30.5 kJ

In humans, the complete breakdown of one molecule of glucose causes the formation of about 32 molecules of ATP. Thus, about 34% of the energy is stored in a form that the body can use.

The ultimate source of nearly all the energy used by living things comes from the sun via the process of photosynthesis carried out by plants and some bacteria. This process reduces atmospheric carbon dioxide to organic molecules such as glucose:

6CO₂(g) + 6H₂O(g) C₆H₁₂O₆(s) + 6O₂(g)     DGº' = 2870 kJ

The mechanism again is complex, but it starts with the splitting of water into oxygen gas, protons, and electrons bound to an organic carrier:

2H₂O() O₂(g) + 4H⁺(aq) + 4e^-(bound)     DGº' > 500 kJ

The separation of charge represented by the H⁺ and e^- is a free energy reservoir that can be used to drive the reduction of carbon dioxide forward to completion. The energy needed to split water comes ultimately from the very exergonic nuclear reactions carried out in the interior of the sun. Thus, coupling of reactions can occur even over millions of miles!

Free Energy in Living Things