MadSci Network: Biochemistry

Re: Is it possible to harness ATP to create an electric potential?

Date: Sat Sep 16 13:07:16 2000
Posted By: Jim Caryl, Grad student, PhD Plasmid Molecular Biology, University of Leeds
Area of science: Biochemistry
ID: 968734156.Bc

Hi there…that is an interesting question, which I’ve never really considered before.

However, after thinking about it for a while I would have to say ‘No’.

Now the reason for this answer is based on my interpretation of your question, which is that you are asking whether these biochemical energy sources can be used akin to a chemical battery (Incidentally, if I have missed the point, then I hope at some point in my answer I address your question as you had intended!)

An analogy between ATP and batteries is appropriate, moreover – a better analogy would be a ‘rechargeable battery’. When batteries are used, their potential energy is ‘used’ until it has all been converted into kinetic energy and heat/unusable energy. Recharged batteries (in which an electrical current has been used to restore electrical potential) can be used only after the input of additional energy. Thus, ATP is the higher energy form (the recharged battery) while ADP – is the lower energy form (the used battery).

So when the terminal (third) phosphate is cut loose, ATP becomes ADP and the stored energy is released for some biological process to utilise. The input of additional energy (plus a phosphate group) "recharges" ADP into ATP – hence the analogy.

Now the question remains – just how feasible would it be to harness this energy? Well it isn’t really the same as harnessing the energy of say a Mercury-Cadmium electrochemical gradient as we are not really talking about that kind of electrical potential. ATP is chemical energy stored in the form of pyrophosphate bond of the second and third phosphate group. I couldn’t really suggest how one could harness ATP in a battery like this – and if I could I think I’d be a rich man.

The point is, ATP acts as a rechargeable battery in a very specific biological environment that is based on enzymes, co-enzymes, electron- transport chains and terminal electron acceptors. The microstructure of the cellular organelles (in the eukarya) , i.e. mitochondria, is of principle importance to the process.

Two processes convert ADP into ATP:

1) Substrate-level phosphorylation; and
2) Chemiosmosis

Substrate-level phosphorylation occurs in the cytoplasm when an enzyme attaches a third phosphate to the ADP (both ADP and the phosphates are the substrates on which the enzyme acts).

Chemiosmosis involves more than the single enzyme of substrate-level phosphorylation. Enzymes in chemiosmotic synthesis are arranged in an electron transport chain (A series of coupled oxidation/reduction reactions where electrons are passed like hot potatoes from one membrane- bound protein/enzyme to another before being finally attached to a terminal electron acceptor - usually oxygen or NADPH - that is embedded in a membrane. In eukaryotes this membrane is in either the chloroplast or mitochondrion.

According to the chemiosmosis hypothesis a special ATP-synthesising enzyme is also located in the membranes – ATP synthase. During the course of electron transport chain, hydrogen-ions are pumped against a concentration gradient into a ‘compartment’ from which the only exit is through this membrane bound ATP synthase. As the hydrogen passes through the enzyme, energy from the enzyme is used to attach a third phosphate to ADP, converting it to ATP.

So this is where we run into a problem – Enzymes/Proteins and finely evolved biological membranes. Most biological energy processes have membrane-protein interactions and simply put – these would never survive in a ‘battery’ in the true sense of the word. Proteins are subject to limited temperature ranges – outside of which they cease to function. As for biological membranes – well there would be no way to maintain one in a closed battery cell.

I’m sure there are plenty of theoretical ways of using artificial solid phase media to support enzymes etc, but then that’d be silly – as how would you recharge the said battery? We recharge our ATP through carbohydrates and fats – you can’t really run an electrical current through it, so the only choice left is to ‘feed it’ – which is why I don't think ATP is really industrially applicable. [But feel free to re-educate me on this point anyone!]

The point about hydrogen ions pumps brings up the topic of membrane potential, which is a manifestation of a small separation of charge across the lipid bilayer of biological membranes.

As D.F. Davey (1998) puts it – “The lipid is the dielectric of a parallel plate capacitor in which the salty (conductive) solutions separated by the membrane are the plates of the capacitor. The charges involved are all ions, and the charge separation comes about through ion movements resulting directly or indirectly from the action of ion pumps”.

The values observed vary a little from cell to cell and across different species, but for nerve and muscle cells that are very well studied, a typical value is -80mV. Although the membrane potentials commonly observed in cells are less than 1/10th of a volt, the electrical field is very large….and I suppose these could be harnessed, but then there is still the issue of protein pumps.

So before I turn this into a circular argument, I’ll thank you for your question and point out that if you have any further questions – ask us…..but for now – make like the Ever Ready Rabbit and use Duracells instead!

Jim Caryl
MAD Scientist


Recharging your batteries
D.F. Davey's Teaching Resources - Membrane Potential
Stryer, L. ‘Biochemistry’. 4th Edition. W H Freeman Press.

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