Friday, March 11, 2016

Early Origination of Life – Organic Oceans – Part 6

The life origination theory discussed here, Organic Oceans, has as a basis that on the very early Earth, and possibly other planets in the liquid water zone (LWZ) of their stars, had not one type of liquid pools, but two. One of course was water. The other was a combination of organic compounds, immiscible in water. The organic ocean was, in places, a layer on top of the water oceans. There was a meniscus between them, and organic molecules which were partially hydrophobic and partially hydrophilic would be stuck at that meniscus, with the appropriate ends of the molecule in the appropriate liquid. Amino acids, nucleotides, lipids are all like this. In short, many of the key molecules which are likely to have played a role in the origination of life would concentrate on the meniscus. The concentration of these ambiphilic molecules (both hydrophilic on one end and lipophilic on the other) on the surface would greatly increase the interaction rate. Membranes could be formed by polymerization of oriented lipids. Polymerization of amino acids might lead to some replication.

This post is about phosphorus. When people normally think about life, they think about hydrocarbons, as that is the largest component of an animal body, except for water. Maybe they know that nitrogen plays a giant role in most of the molecules involved with cellular life. But phosphorus might be not appreciated, unless you are a gardener and know you must add fertilizer containing it, along with nitrogen and potassium. Phosphorus makes up about 1% of our body weight. It is crucial to every cell membrane. It is crucial to the energy storage and transport within the body. It is crucial to DNA coding. It is crucial to signaling within the cells. It is a critical element in almost every function the body does.

In order to originate life, there has to be phosphorus. Nitrogen is in the atmosphere and dissolved in the oceans. Hydrogen and oxygen are water itself. Carbon exists in the form of carbon dioxide, a common molecule in the atmosphere. Phosphorus, like calcium, potassium, sodium and the trace elements needed in an animal body come from rocks. Somehow it has to get to the location of life origination.

Most phosphorus in rocks near the surface of the Earth is recycled. It is in sedimentary rock, deposited in the ocean and either still there or thrust up to the dry surface through tectonic motion. The sediment has apatite, which is one of the few minerals in the body, making up bone and teeth. Dead creatures accumulate on the seabed and after some huge length of time, accumulate into minable minerals. But before life existed, there was no sedimentary rock with phosphorus. It exists in trace quantities in granite, transported up from the mantle, and exists also as a separated form in apatite there as well. It also occurs in other minerals such as feldspar.

Sodium and potassium phosphate are soluble in water, as is almost sodium anything and potassium anything. But calcium phosphate is not. This does not mean that if you dissolve sodium phosphate in water it will stay there, if calcium is present. Calcium phosphate precipitates. It gets even worse in hot water, and precipitates more. So how do you get the phosphate from the rock to the meniscus where is is absolutely needed in life origination?

Combining the phosphate ion with one or two hydrogens from the water helps, as that is more soluble than phosphate alone. But calcium is more electropositive than hydrogen, so that will not solve the precipitation problem.

Recall that solubility is all about the attraction between solvent molecules and solute molecules. If there is no attractive force between a solvent molecule and a solute molecule, but there is between two solute molecules, the solute molecules will stick together, make a nano-crystal or a nano-droplet, depending on the melting point, and precipitate out. Since we do not yet have a clear idea of what might be in the organic ocean, figuring out solubility of calcium phosphate or other phosphorus-containing compounds there is a bit chancy, but there are two rules of thumb that will help.

One is that 'like dissolves like'. Polar molecules like water dissolve other polar molecules, and, to a lesser extent, non-polar molecules dissolve better in non-polar solvents. So there is a good chance that there will be a solubility pathway to transport the phosphorus to the meniscus in this early Earth scenario.

The second thing about solutions is that cosolvents usually increase solubility. It is unlikely that the organic ocean will be all benzene or all octane or all toluene or anything else, but a mixture of many mutually tolerant organics. Alcohols will dissolve in the water, and any other larger hydrocarbon molecule else with bonds similar to water will, and the rest will not. So, the phosphorus compounds would have an easier time in this potpourri of organics than in anything pure. It needs to find something to attract to, in order to displace some space between the solvent molecules. And since they are dissimilar, the forces between them will be less than in a pure solvent, especially a smaller or symmetric one.

Thus, it looks like phosphorus may have a pathway to the life origination site that is much easier, meaning much more can be transported per century, than just a water ocean alone. So far, we have not talked about other elements that might be a problem in transporting them, like sulfur. Sulfates are similar to phosphates in their water solubility characteristics. Halides would of course take the water route.

Something related may have an even bigger effect. If the phosphate compounds interact with something in the organic ocean, making some large molecule which is ambiphilic, it will gradually drift to the meniscus. If the transport time is significantly shorter than the lifetime of such a molecule, there would be an accumulation of phosphorus compounds at the meniscus, just waiting there for some random events to form them into the lipids needed to make a membrane, or the nucleosides needed to form some type of replicator.

On first hearing the term, organic ocean, some people might think of the methane lakes expected to be on Titan, the largest moon of Saturn. The biochemistry of life origination there would be completely different that that on early Earth.

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