“Any sufficiently advanced technology is indistinguishable from magic.”
(Arthur C. Clarke, 1917–2008)1
A photon is spewed out from our sun, along with countless trillions of others, into the cold and dark of space. It just so happens that this photon and its companions have been pushed on a flight path towards the earth, ninety-three million miles distant. It travels at 186,000 miles per second, and in around eight minutes it reaches the earth. What happens next appears to be pure magic.
We think that technology is the exclusive preserve of humans, and perhaps other, if they exist, advanced intelligent extra-terrestrial beings. Think again. Whatever technology humans have invented you can soundly bet that nature thought of it first. And there is one such ‘technology’ so advanced, so exquisitely evolved, so gut wrenchingly beautiful in its complexity, and so fundamental to all life on this planet, from the smallest organisms to the largest ever to have lived, that it does indeed seem, to ordinary human understanding, like magic. That is, if we think about it at all. We take it for granted and just assume it is something that is, and always has been. It has not ‘always been’. The process evolved, just like everything else on this planet, and if it ever failed almost all life would be extinguished, save for a very few organisms that use a different system to survive and grow. It operates at the atomic level, and is nearly 100% efficient. Physicists and engineers have for centuries been striving to invent a usable system that efficient. If we could replicate it, we could solve our energy problems and fix climate change at a stroke. We have tried, but our efforts have failed miserably.
You are probably thinking about nuclear fusion right now, but try something more down to earth. Photosynthesis2. The process by which plants use sunlight to make their own food3 and power their growth. Please do not yawn at this point. Read on, and be amazed.
The photon strikes and enters one of the leaves of an oak tree. An average oak leaf is roughly 0.14mm thick. The average 80gsm paper is 0.67mm thick. Everything that follows goes on within a space less than a quarter of the thickness of the paper you use in your home printer.
The leaf contains many different kinds of cells, each type performing a different job, such as transporting water, nutrients or waste products, protecting the outside of the leaf, and carrying out photosynthesis. The outermost protective layer (upper epidermis) is made up of translucent cells that allow the sunlight through to the photosynthesising cells directly underneath. It is in these photosynthesising cells that the magic happens. Remember that we are now at the cellular level, very small, but things are about to get very much smaller.
Inside each of the photosynthesing cells (called mesophyll cells) there are around 30 to 40 spherical structures, called chloroplasts. The chloroplasts are around 4 to 7 thousandths of a millimetre long (4–7 microns). We are getting very small here, but they can just about be seen with an ordinary light microscope. These little balls are thought to have once been free-living single-celled photosynthesising microbes4, maybe similar to cyanobacteria5, in the primordial soup on the hot young earth long before true plants and animals evolved, some one to two billion years ago. It is thought that perhaps some larger single celled organism, much like an amoeba, engulfed some, and instead of absorbing their nutrients, allowed them to live within their bodies in symbiosis. The larger cells were able to use their prisoners’ photosynthesing abilities to power their own bodies. The ingested small cells eventually lost their cell wall, and, only encased in a membrane, could no longer survive outside of their host cell.
So, our photon has passed through the translucent outer layer of cells, entered a mesophyll cell, and then a chloroplast, and its journey is nearing its end. However, things are going to get way, way smaller before it is done. Inside the chloroplast there are 40 to 60 layered stacks of flat pouch-like structures called thylakoids. They look a little like stacks of tiny green pancakes. These are so tiny that you would now need an electron microscope to see them. The thylakoids contain pigments that can absorb light energy. The majority of these pigment molecules are of the two kinds of green chlorophyll (‘a’ or ‘b’)6, but there are also some molecules of two types of yellow/orange carotenoids.
It might now be a good point to pause, make a cup of tea, and reflect upon why plants need to photosynthesise at all, and it is all down to the energy needed in order to grow a body. The vast bulk of the energy that drives all life on this planet comes from our sun7. Plant bodies are basically built from various kinds of starch (sugars) that are made up from varying amounts of carbon, hydrogen and oxygen atoms8. These complex molecules have to be constructed from simpler ones, which, in the case of plants, are water (H2O) and carbon dioxide (CO2). The problem is that both water and carbon dioxide are stable molecules that are quite happy existing as they are. Their individual atoms firmly hold hands and don’t want to be separated. Energy is needed to break the bonds so that the atoms can be ‘recycled’ and made into the sugars that comprise the plant’s body9, and it is the sun’s energy that plants use to do this.
Let us re-join our photon containing its little precious packet of energy that has just entered a chloroplast. Now, after ninety three million miles, its journey is almost at the end. Photons can be any wavelength of light, but this one happens to be around 680 nanometres (nm). Each of the various pigments within the thylakoid are capable of absorbing a particular range of light wavelengths, and between them can absorb almost all of the visible light spectrum. It is this that makes the process almost 100% efficient. Very few photons escape the process. Our photon crosses into a thylakoid and is almost instantly absorbed by a molecule of Chlorophyll a, which then produces an unimaginably tiny amount of electricity. What happens next is fast. In less than a nanosecond (one billionth of a second) the electricity is passed from molecule to molecule fuelling the breaking of chemical bonds and the building of sugar molecules. How this happens is a fascinating story all of itself, which I won’t tell here (maybe another time). Suffice enough to say that the process happens in two stages. The first splits the bonds of the water molecules releasing two hydrogen atoms and one oxygen atom from each one. The second splits the bonds of the carbon dioxide molecules releasing one carbon atom and two oxygen atoms from each one. For each set of six water molecules and six carbon dioxide molecules one sugar molecule is made with six oxygen molecules released as a waste product10. Both of these stages happen in the chloroplasts side by side at the same time. The sugar molecule joins the multitude of others to build the plant’s body. The oxygen? Well, take a deep breath and thank photosynthesis.
The end of the story? Not quite. From thin air, sunlight and water (and a sprinkling of nutrients and trace elements), both the smallest and largest organisms on the planet make their livings through the process of photosynthesis within a perfect circular economy. We breathe in what they breathe out and vice versa. If photosynthesis had not evolved we would not have the vibrant, dynamic living planet we are privileged to inhabit. Yes, maybe there would still be life, in hot thermal pools, or around hydrothermal vents in the cold and dark on the deep sea bed, but a tropical rainforest, or an African savannah, or a coral reef, or a wildflower meadow, with all their diversity and abundance? Or would an organism have been able to evolve that could sit here and write about it?
A ‘sufficiently advanced technology’ it certainly is, and it looks like magic to me.
2024
1 This quotation is from Clarke’s 1962 book “Profiles of the Future: An Inquiry into the Limits of the Possible”.
2 Oxford Dictionary’s definition: ‘Process by which the energy of sunlight is trapped by the chlorophyll of green plants and is used to build up complex materials from carbon dioxide and water.’
3 Plants are called ‘autotrophs’, which means ‘self-feeder’. Humans and other animals are called ‘heterotrophs’, which means other (or different) feeder, cannot make their own food in their own bodies and have to eat the autotrophs to obtain it (or eat other heterotrophs).
4 Indeed they still carry their own DNA within their bodies, quite distinct from that of their plant host.
5 Blue-green algae.
6 The reason most plants appear green is because the chlorophylls cannot absorb the green part of the spectrum and so reflect it back out for our eyes to detect.
7 A very small number of organisms use heat energy from the Earth’s hot core around undersea volcanic vents, or use chemosynthesis, whereby energy is released by breaking chemical bonds in inorganic compounds.
8 The chemical formula for a simple sugar is CH2O (I atom of carbon, 2 atoms of hydrogen and one atom of oxygen).
9 It is an anabolic process, where large molecules are built from smaller ones.
10 For those interested, the basic chemical equation for photosynthesis is: 6CO2 + 6H2O → C6H12O6 + 6O2
Nature Matters 