Capacitance and the Origin of Life

 Capacitance and the Origin of Life

Over 65 years have elapsed since Peter Mitchell published his chemiosmotic hypothesis1  in which he described how chemical free-energy from the oxidation of substrates such as glucose could be coupled to biosynthetic chemistry using energy stored in an electrochemical potential created  across phospholipid bilayers. Most of this potential, electrochemical energy, was manifested or ‘contained’ in proton gradients, that is in a difference in positive charge form H+ ( hydrogen ions, protons) either side of a non-conductive lipid membrane. It was given the short-hand symbol ΔpH. Chemiosmotic theory is fully accepted today as the prime generator of free chemical energy usually in the form of ATP or  powerful reducing agents such as NADH2  used in cellular growth, maintenance and reproduction…all overwhelmingly endothermic, and entropically negative.

Unsurprisingly, given the above, speculations on the origins of life; the proto-cell, focuses around naturally occurring pH gradients, (either from photo-chemical reactions or from those found in alkaline sub-ocean vents) coupled with lipid bilayers. Bilayers, in turn formed  from simple lipids ,default into vesicles, simple spheres and are well known to exist in primordial ‘soups’2.  And here it rests: a pH gradient and a dielectric barrier in the form of a vesicle..but this is not a viable proto-cell however you argue it.

Modern bioenergetic organelles such as mitochondria or chloroplasts have vast surface areas of membranes and sophisticated regulation of electrical potentials through arrays of proton pumps and controlled depolarisation. 

A mere 40 years ago my PhD3 thesis contained a chapter which focussed on the capacitance of mitochondria as they aged and as their internal membrane area (cristae) decreased (as shown on electron micrographs). The assertion was that although membrane potential was being maintained at levels essential for normal cell function, the capacitance or reservoir of charge was diminishing thereby bringing the ageing organism closer to an energetic ‘cliff-edge’, which if crossed possibly initiating failure and apoptosis.

Capacitance, is a term so intrinsic to electronics that even the most elementary of courses would expect an understanding of the role and mechanism of capacitors in circuitry whether electrolytic or solid state capacitors. Capacitors store charge, they store energy, they are used as reservoirs to smoothing current flows and maintaining voltages during times of energy draw off through demand or natural leakage. In electronic circuits they are ubiquitous.

In my assertion here, the capacitance of electrochemical organelles matters and that it plays a similar buffering/storage role in organelles as it does in electronics. One has only to see an electron micrograph of so called reticulate mitochondria wrapping themselves around a nucleus during mitosis to appreciate they are acting as a huge ‘battery’ of stored charge for a temporarily biochemically semi-dormant cell but one with still with a lot of energetic  work to do to bring about cell division.

Thus, in the speculations about proto-life I think capacitance matters here too. Imperfect evolving systems invoking early proton-pumps, or any other charge separating mechanisms including natural pH gradients all need a reliable store of potential energy to keep the chemical wheels rolling in the same direction for decent periods of time.

Most proto-life theories focus on the formation of natural lipid vesicles. Lipid bilayers will always default to simple vesicles with single membranes as exemplified by the fate of complex membranous structures in cells following homogenisation! But, vesicles have very little capacitance, something with a greater surface area is needed.

Two recent articles have delighted me. The first4 reports that lipid sponge-droplets can be simply formed. The investigators are attempting to create artificial organelles  and have succeeded not in producing sophisticated folded mitochondria full of cristae folded like net curtains but spongy lipid droplets.

No matter, sponges have large surface area and so potentially large capacitance.

The second article5 is from the electronics world where the researchers are attempting to produce nano-super-capacitors and yes, you guessed it, they have produced devices and from their structure are called sponge capacitors.

Sponges like all foam structures have large surface areas compared to their volume. In fact the surface area can be incredibly larger than the volume. A sponge chemiosmotic capacitor would be a potentially very significant reservoir of free energy in the form of a charge distributed across a very large surface.

So, in short, my guess for the proto-cell will involve a sponge-lipid with alkaline and relatively acidic lacunae generating a mesh of charged membranes which contain enough capacitance energy, (charge), to drive endothermic synthetic processes for substantial, crucially continuous periods. To use an analogy from chemistry: driving an endothermic reaction with a heat source, say a Bunsen burner, will not be successful if the burner is turned off for a period every few seconds or so. Biosynthesis requires a consistent if not constant input of free energy and only charge storage provides the possibility of a gradually evolving molecular refinement of the process. 

Capacitance, charge storage, is in my opinion, crucial to  all proto-life theories as, ironically  much as it is crucial now for the electricity-driven green revolution. 

1. Mitchell, P. (1966). "Chemiosmotic Coupling in Oxidative and Photosynthetic Phosphorylation". Biological Reviews. 41 (3): 445–502. doi:10.1111/j.1469-185X.1966.tb01501.x. PMID 5329743. S2CID 2073366.

1a.^ Mitchell, P. (1972). "Chemiosmotic coupling in energy transduction: A logical development of biochemical knowledge". Journal of Bioenergetics. 3 (1): 5–24. doi:10.1007/BF01515993. PMID 4263930. S2CID 20251582

2.https://ecoevocommunity.nature.com/posts/55368-protocells-in-deep-sea-hydrothermal-vents-another-piece-of-the-origin-of-life-puzzle 

3. John A Spencer Biochemistry of Ageing; Birmingham University 1980 Ph.D

4.https://www.pnas.org/doi/10.1073/pnas.2004408117 lipid sponges

5.https://pubs.acs.org/doi/10.1021/nl2023433 sponge capacitors