10. OXYGENIC PHOTOSYNTHESIS & SEX

This article is based on a trend of research relating to a theory of ageing. It may be that this has now been developed or supplanted by more recent research. I have included it here just because of its perspective on such issues. This does not imply acceptance of its conclusions as established; doubtless research continues as ever….

AGEING, RESPIRATION, & SEX 

A beautiful hypothesis of the nature of sexuality relates also to the ageing process, and is offered by Professor John Alan of UCL. The theory starts from the premise that the sexual participants, the gametes, must first find each other, and are more likely to succeed if one searches while the other stays put. But movement takes energy, and all energy generation is dangerous. The DNA in the energy generators of cells, the mitochondria, suffer degradation throughout life, from the oxidative stress which is a by-product of respiration. 

For this reason the mitochondria are kept separate from the nucleus of the cell. They can continue to function up to a point even when collateral damage to their DNA produces faulty genes. But damaged mitochondrial DNA in a fertilised egg could result in a newborn organism already stricken with age. 

Nature has therefore selected in favour of male gametes which can ditch their faulty mitochondrial DNA just before uniting with the egg cell in conception. And, whereas a male produces abundant sperm throughout most of adult life, a human female (like other female organisms) is born with more than a lifetime’s supply of eggs already in her ovaries. These are held in a state of suspended animation, so that their own mitochondria are in pristine condition for the new embryo. 

One could see in this a metaphor for feminine youth, beauty and passivity, but it also endorses the strength of the maternal line, since mitochondrial DNA is always inherited from the mother, and this has been a factor in tracing the origin of early human evolution to Africa, back to the matrilineal most recent common ancestor, sometimes referred to as ‘Mitochondrial Eve’ (though any literal notion of a single individual ‘mother of all living’ – as in Genesis 4, 20 – would be scientifically untenable) 

John Alan https://www.ucl.ac.uk/biosciences/people/john-allen summarises his research thus:“Fertilisation of an egg by a sperm, like all life processes, requires energy, which is provided in the form of ATP [Adenosine triphosphate]. ATP is made in mitochondria. However, there is a price to pay – mitochondria also contain DNA, which becomes progressively degraded by the chemistry required for ATP synthesis.

We show that egg mitochondria are radically different from all others by being in a state of suspended animation – unable to use the information in their DNA to make the protein machinery of respiration. Sperm mitochondria wear out and are discarded when their job is done. But, in their germ-lines, females carry mitochondria that never grow old.”  In many ways, including the wider scientific context of the energy cycles on Earth as a whole, as well as civilised culture in general, this is a stunning thought.

PHOTOSYNTHESIS, RESPIRATION,  SEX 

Just to amplify, many of the reactions involved in cellular respiration happen in the mitochondria, which are the organelles that keep the cell full of energy, the powerhouses in each cell. Respiration, if viewed in the context of ecology, is the reverse process to photosynthesis. Photosynthesis takes place in the chloroplasts of plants and other organisms and converts light energy into chemical energy, and is the beginning of the energy cycle (or ‘food-chain’) of the whole biosphere. In respiration, an organism obtains energy (originating from light) by oxidising nutrients and releasing waste products. In both photosynthesis and respiration, the cellular processes have to cope with the potentially destructive effects of oxygen.

This arises from the fact that photosynthesis uses light energy to split water molecules (H2O) into hydrogen and oxygen. Respiration recombines them. As we draw breath we forget how dangerous oxygen is, and how life processes have only temporarily calmed its fiery energy. In atmospheric oxygen, each atom is paired up with another oxygen atom, which, more or less, keeps them both out of trouble. 

(In this form it is properly called ‘dioxygen’, O2, as opposed to O, ‘atomic oxygen’. Ozone, O3, is formed when ultraviolet radiation in the upper atmosphere splits O2, producing atomic oxygen which then reacts with dioxygen.) 

But each molecule of water (H2O) has a single oxygen atom combined with two of hydrogen, so splitting or recombining them inevitably involves the release of single oxygen atoms, each with unpaired electrons. These, called free radicals, can run amuck in search of other electrons to pair with, and damage cells by ripping to pieces proteins and DNA. This is called ‘oxidative stress’, and it is implicated in the ageing process and in many diseases. From the start, plants and animal cells have had to evolve ways of controlling this, for instance by manufacturing anti-oxidants such as ascorbic acid, known as vitamin C. But, whereas the complex bodies of organisms ultimately succumb to oxidative stress, their genes find a potential immortality through reproduction. Even in the simplest of unicellular organisms, this can involve a state of prolonged dormancy or encystment, or of vernalisation, as in many seeds, during which the DNA is not endangered by active energy processes within the cell. The evolution of life by union of two sorts of sex cell, one of which burns itself out while the other suspends animation, has become the golden road to Samarkand. 

“And though Death taps down every street

Familiar as the postman on his beat

Remember this, remember this

That life has trembled in a kiss

From Genesis to Genesis

And what’s transfigured will live on

Long after death has come and gone.”

(Alun Lewis)