I’m fascinated by photosynthesis, but it wasn’t always this way. Like most people, I knew the headline – it’s how plants make food using light from the sun. But I hadn’t explored it any deeper. I’m not a botanist, a chemist, or an ecologist and it’s one of those tricky words. It’s an apt name (Photo = light; synthesis = to make, to combine), but it does the process no real favours. It doesn’t beckon you in, but once you’re there, it is tremendous company.
What follows then comes from enthusiasm, not expertise. There’s a list of excellent books and videos at the end which have really helped and inspired me to try and grasp photosynthesis. Where it seems appropriate I’ve included some particular references, and I’ve attempted to make my summary as accurate as my ability allows, but I have to say at the outset that this is really an ode to photosynthesis, not an academic text. Please do check out the writers, scientists and teachers I’ve listed to experience real knowledge!
For me, the thing that has felt urgent, is figuring out whether it is important to understand photosynthesis at all.
Without it, life as we know it, would not exist. You and I would not exist. But life is full of vital things that we don’t know or care about, and we still get by. Is photosynthesis any different? I’d argue it is. By learning about it and appreciating it, I believe it reveals something true about the world and our place in it. Something I feel it’s hard to get any other way.
One of the other great things about photosynthesis is that to talk about it we have to talk about the biggest things in the universe and the smallest. We’ll end up chasing atoms in plant cells, but we begin with the sun in our sky.
PART ONE: LIGHT FROM THE SUN
The sun, our star, burns out there in space, pulsating, exploding and raging. It scorches my skin easily from that vast distance, turning my pale skin red more than brown, turning me freckly. But it also radiates, bringing warmth, joy, optimism. The sun, whether in cosmic or mundane ways, orchestrates our lives; when we sleep, when we wake up, how we feel.
Our star is a throbbing mass of energy. The conversion of hydrogen into helium in the Sun creates enormous amounts of energy per second. This radiates out into the Universe and a tiny sliver of it hits the Earth, some 93 million miles (150,000,000 km) away. Plants – living things as much as we are – have evolved on Earth to capture that energy, use it to split water, and to take the electrons from that process to power the creation of chemicals that enable it to fix carbon dioxide into sugars. Interestingly, you cannot just expose water to sunlight to do this. Even though the light obviously has the energy, and water can be split, it takes the structure of the chloroplast in plant cells to do it. Chlorophyll, in the chloroplast, has the particular structure of single and double bonds which can trap or harness this energy. The process of photosynthesis is so unique and finely tuned to this role that it has barely evolved at all in about 2.5 billion years [1].
WHAT IS LIGHT?
The central nucleus of an atom holds vast amounts of ‘stored’ energy and nuclear fusion is a way that this energy can be released. It is a nuclear reaction (a reaction which brings about a change in the nucleus).
Nuclear fusion is the collision and combination of two ‘light’ (as in ‘not heavy’) nuclei to form a heavier, more stable nucleus, with the release of large amounts of energy. It requires temperatures of millions of degrees Celsius, to give the nuclei enough kinetic energy for them to fuse when they collide (the high temperatures mean it is called a ‘thermonuclear reaction’). It only occurs naturally in stars, including our own sun.
Within the sun, hydrogen ions (that is, lone protons without an electron) are being converted into helium, resulting in the release of huge amounts of energy in the form of electromagnetic waves (or light). Hydrogen is normally found as H2, which means with 1 proton and 1 neutron, in which case the mass is double and it is called ‘deuterium’ instead.
Hydrogen ions will normally avoid each other due to their positive charges, but in certain conditions (such as in the sun) they do collide and stick together to form helium. In this reaction, mass is lost and as Einstein stated with E = MC2, the energy is the mass x the constant (or speed of light, or more accurately the speed of mass-less things, which is 3,000,000 metres per second – there’s more here but that’s another project).
There are several ways in which hydrogen can become helium, but the most common journey is as follows: hydrogen atoms collide to form deuterium. After it is formed, deuterium fuses with another proton to produce the light isotope of helium, 3He (2 protons, 1 neutron). This happens extremely fast and it is predicted that deuterium nuclei last for about 4 seconds before being converted into 3He. There are four pathways from here, but the most common is called p-p l (a proton–proton chain reaction – see end note 2) in which two 3He fuse to form helium-4 (2 protons, 2 neutrons), resulting in the release of 2 protons.
The energy released by this reaction can be buried within the sun and take 100,000 years to reach the sun’s surface, where it is radiated out into space and is able to travel the 93 million miles to Earth in about 8 minutes. Isaac Asimov has written very clearly about how much energy actually reaches the Earth’s surface and is available for plants to use. But essentially some of this energy, in the form of photons of light, falls on plant leaves and when it reaches the chloroplast, the energy is used to split water – turning H2O into free oxygen, electrons, and two hydrogen ions (atoms electrically charged by the loss or gain of electrons).
It is worth remembering, there is a difference in mass between individual particles and the same amount of particles when fused together to form other atoms. So, 6 protons, 6 neutrons and 6 electrons have a certain mass. Combine them to form oxygen12 and they will be lighter. This sounds a little crazy, but the reason is that some mass has been converted into the energy necessary to hold the atom together. In an atom, there is the same number of protons and electrons. If an atom loses or gains an electron, it becomes ionised. With equal numbers of protons and electrons, the positive and negative charges are cancelled out. When an atom is ionised, the charge is changed.
A little bit of basic chemistry is useful for understanding photosynthesis – see end note 3 if, like me, you want a refresher.
THE MOST COMMON THING IN THE UNIVERSE
Hydrogen is an element and the most common chemical substance in the Universe, accounting for approx 75% of all the known mass. The second most abundant substance is helium, with approx 24%. All other substances fall within the remaining 1% of stuff.
The reason why hydrogen is so abundant is because it is so simple, just one proton and one electron. Helium is also very simple, and that simplicity makes them very common. This abundance (mind boggling abundance) is why amazing things can happen. The energy released by each individual chemical reaction might be tiny, but they add up to all the energy pouring out from the sun.
I have also read that although hydrogen does convert into helium in the sun, there is more to it than that. You might recall there are other steps involved; i) some hydrogen becomes hydrogen-2; ii) hydrogen-2 and a proton fuse to produce helium-3; iii) two helium-3 fuse together to create helium-4; and iv) helium3 fuses with helium4 producing beryllium7, which decays and fuses with another proton to yield two helium-4.
Only step ii actually involves hydrogen converting into helium, and this, according to Ethan Siegel, only accounts for 39.5% of the energy emitted by the sun [6]. If you want your brain scrambled, it’s worth reading up on ‘quantum tunneling’ in the sun too, but let’s not digress.
PART TWO: LIGHT REACHES EARTH
Find a tree, or almost any plant for that matter. I’ve found an oak tree. A 400 year-old oak tree can have over 700,000 leaves with a surface area the equivalent of three tennis courts. The leaves are like little chemical factories that make carbohydrates, or glucose, to feed the plant. It needs water, light and carbon dioxide to do this. But it doesn’t just mix them all together in a bowl like a cake and out comes food. Each is needed for a different reason.
A WAY IN
Imagine walking across the surface of the leaf or outer skin (usually called the epidermis). On the underside of that surface, there are tiny openings called stomata (or stoma if we’re just talking about one of them). The stomata allow an exchange of water and gases, which is very important as we’ll see. For our purposes as well, let’s use it as our gateway into the leaf itself. Each stoma has a pair of crescent shaped cells on either side, which change shape and enable the stoma to open and close (a bit like bouncers on our tiny imaginary journey). These ‘bouncers’ are cool because they are the only surface cells that have chloroplasts.
Beneath the epidermis we go and into the ‘palisade layer’. The décor here is lots of regular, oblong shaped cells. I reckon it looks quite funky, but there’s an ordered pattern. These cells do a serious amount of photosynthesis because they have a lot of chloroplasts. It’s one of these cells that we’re going to explore.
What a beautiful thing a plant cell is. There are hundreds of thousands of these in each leaf. It has a cell wall around it, made of cellulose. This is very different from animal cells which don’t have a cell wall, just a membrane. Plant cells have a membrane too, but the cell walls are important. They give the cells strength and ultimately make wood the strong material that it is.
Through the cell wall and here’s the membrane (or plasma membrane). I’m not going to list everything in the plant cell here, but we must get through these layers to get to the heart of photosynthesis. The plasma membrane is semi-permeable, allowing some molecules and substances to enter and others to exit. It’s a critical thing, protecting the internal components of the cell from the outside environment.
It’s quite crazy and weird on the other side of the membrane but don’t get distracted. We’re looking for chloroplasts and here’s one. It probably started life as an independent bacteria that could do photosynthesis, and found itself inside a larger ‘eukaryotic’ cell (I find that a scary word, hard to say and hard to spell, so I’m not worrying about it too much here. But it’s quite cool to realise that by starting life as a bacteria, the chloropast has some DNA of its own).
Chloroplasts can come in all shapes and sizes, but this one is sort of kidney shaped.
As soon as water – or H2O (two hydrogen atoms and one oxygen) – enters the chloroplast inside each leaf cell, it is split into separate hydrogen and oxygen atoms in a process called phytolosis. Splitting water might sound easy, but it is fantastically difficult.
Humans can just about do it, but it takes a huge amount of effort. At this moment, although a lot of research has been focused on it, nobody has been able to develop it enough to commercialise it [7]. Some of the best scientific minds in the world are working out how to do this properly, and here we are standing underneath a tree and above us in every leaf, that process is happening thousands of times a second – silently, seemingly effortlessly.
As far as photosynthesis is concerned, the main players are carbon dioxide (CO2) and water (H2O). As we know, these two molecules combined with light from the sun are the three ingredients of photosynthesis, but what exactly is happening?
Paradoxically, the answer reveals something both remarkable and beautiful, and something terribly inefficient and convoluted. That it happens at all is brilliant though and all life on Earth depends on it, but in case we start to think it is too amazing to be an accident, we just need to grasp its problems too (which I will come to later).
THE CHLOROPLAST
The chloroplast is an organelle within the plant cell. It has its own membrane and can move around inside a cell according to light intensity and direction. It has a liquid inside called the stroma. Within the stroma are tiny structures called granums which are actually stacks of pancake-looking things called thylakoids. These stacks are connected by ‘walkways’ called lamella. It all looks like a futuristic city of some kind.
Each of the thylakoids has its own membrane, called … drum-roll… the thylakoid membrane. Some of the main ‘machinery’ of photosynthesis is embedded within this membrane as we will see in a moment. The interior of the thylakoid is called the lumen. A nice word.
The chloroplast has the ability to split water as we mentioned, but how? This is where light comes in. Light, which has travelled 93 million miles from the sun, hits a leaf and photons enter the plant cells, reaching the chloroplast and then the thylakoid membrane, where it can be absorbed by chlorophyll.
Chlorophyll is a pigment that can absorb light from the red and blue parts of the electromagnetic spectrum (reflecting light from the green part, which famously gives leaves their colour). It is contained in Photosystem 1 and Photosystem 2 – the simply named parts within the thylakoid membrane where this absorption happens.
Photosystems are light harvesting complexes embedded in the thylakoid membrane with clusters of photopigments – chlorophyll a, chlorophyll b and carotenoids. The different chlorophylls (P680 and P700) are to do with wavelengths of light, and carotenoids are probably best known for giving trees their autumnal colours as the chlorophyll withdraws from the leaf (see more in Epilogue). Water is split using energy within light to power the process.
PART THREE: LIGHT DEPENDENT REACTIONS
Photosynthesis happens in two parts, the light reactions and the dark reactions. Both happen in the presence of sunlight so the name is a bit weird. Sometimes they are called light dependent and light independent reactions. The second part is also called ‘the Calvin Cycle’ or more accurately the Calvin-Benson Cycle. The first part happens in the thylakoid membrane. The second part happens in the stroma, the liquid in the chloroplast I introduced earlier.
A quick point about Photosystems. Photosystem 1 was discovered first so it is called 1, but it was later discovered that another Photosystem exists and actually occurs earlier in the process. This Photosystem was named ‘2’, but because it happens first we start there. A bit confusing, but there’s nothing we can do about it.
So, light reaches Photosystem 2. The energy in the light is used to split water, forcing apart the hydrogen and oxygen. Electrons are stripped from the hydrogen atom, turning it into a hydrogen ion or proton. These electrons move their way into Photosystem 2 where they are excited by light and bounce around the system until they reach a special area known as the Reaction Centre. When the electron reaches this point, it gets picked up by an electron acceptor (called plastoquinone), which takes it to an Electron Transport Chain (ETC), or cytochrome b6f.
It can help to picture this ETC as a staircase and as the electron goes down the staircase, it gives off energy. At the bottom of the ‘staircase’, it is now back in a low energy state, requiring another hit of light in order to be useful. The energy it has just given off is used to create ATP (adenosine triphosphate).
It’s worth noting here that there are two pathways for electrons in photosynthesis. The first is ‘cyclic’ and the electron goes round and round the process just described, creating ATP, which I will describe in a moment. The other pathway is ‘non-cyclic’ and the electron is passed from Photosystem 2 to Photosystem 1 where another hit of light enables it to create NADPH, which I will get to shortly.
Anyway, the creation of ATP is quite cool and a bit convoluted. It’s called photophosphorylation. Remember the hydrogen ion from a few paragraphs above, well there are now some of these ions within the thylakoid, in the lumen, and some outside the thylakoid, in the stroma. As the electron goes ‘down’ the ETC staircase mentioned earlier, it doesn’t just give off energy, it also grabs a hydrogen ion (which is a proton) from the stroma and pulls it through the thylakoid membrane and drops it into the lumen (I realise some pictures would help show this in all its Willy Wonka glory, so I’m working on some illustrations!).
The end result of this is that hydrogen ions are building up in the lumen, creating a disparity (or proton gradient) between ions in the stroma and in the lumen. Nature will want to equalise this gradient and ions will want to move back into the stroma. However, they are unable to just move back through the thylakoid membrane.
This means hydrogen ions are now building up in the lumen, like water behind a dam. The only way back to the stroma is through a mechanism called the ‘ATP synthase’. ATP synthase behaves like a hydroelectric dam. As the ions pass through it, it ‘turns’ and creates energy. This energy is used to turn ADP or adenosine diphosphate into ATP (the D means two phosphates and T means three phosphates – see end note 8). The ATP, thus created by this process, is available to be used by the Calvin-Benson Cycle in the light independent reactions.
Back to the hydrogen ions. The ones that pass through the dam (ATP synthase) are now back in the stroma and go through this cycle again where they are picked up by the electron at the ETC.
There is more going on here though. That’s just Photosystem 2. You might recall the low energy electron which had just been down the ETC. Well, in Photosystem 1 it now gets excited by another photon of light (in the ‘non-cyclic pathway). It goes on a similar journey to Photosystem 2, making its way to another electron acceptor, this time called ferredoxin. Ferredoxin passes the high-energy electron into another ETC (with a crazy name … Ferredoxin-NADP+ reductase), but this time rather than giving off its energy, it brings together the electron, NADP+ and a hydrogen ion to make NADPH.
NADPH now takes its electron and hydrogen ion on to the Calvin-Benson Cycle, in order to fix carbon.
PART FOUR: CARBON FIXATION (OR THE CALVIN-BENSON CYCLE)
The Calvin-Benson Cycle is quite something. I said before that leaves are effortlessly splitting water and creating glucose all around us, thousands of times a second. Well, here is where we realise it isn’t effortless at all – it’s very difficult and hugely inefficient, even for plants.
All the energy, water and effort required for photosynthesis is mostly used just to make sure the plant can photosynthesise again. It’s a bit like running to the shops to buy food and only ever eating enough so you can run back to the shops again. But every time the cycle revolves around, just enough glucose is created that can be used by the plant and this fraction of useful sugar is enough to create all the plants, trees, wood and fruit in the world.
Despite the fact that only about 1/6 of the process is actually useful for the plant, photosynthesis is still the most amazing process on the planet. Without it, life as we know it is impossible. Remarkable really.
Describing the Calvin-Benson Cycle pleasantly is quite hard. What we know is that the plant is now going to turn carbon into something useful – glucose. It needed light to power Photosystems 1 and 2 as we’ve just described. It needed water for its hydrogen and its electrons. It now has ATP and NADPH at its disposal and CO2 entering the leaf through the stomata.
I’ll quote Oliver Morton’s summary first: “The key reaction in this cycle is the reaction of carbon dioxide with ribulose biphosphate (RuBP), a sugar molecule containing five carbon atoms, to produce two molecules of phosphoglycerate (PGA); this is the reaction catalysed by the enzyme rubisco. Three such reactions produce six molecules of PGA. The other enzymes involved in the cycle can, if supplied with energy from ATP and NADPH, turn five of these three-carbon sugars into three molecules of the original five-carbon sugar ribulose biphosphate. The sixth three-carbon sugar is ‘profit’ that can be channelled off to the rest of the cell’s metabolism, or stored as starch.” [9].
I wish there was a nicer way to say it. Anyway, remember the main equation here: 6 CO2 + 6 H20 + light = C6H12O6 (a glucose molecule). The number six is important – I will come back to it.
Let’s unpick it a little bit. It takes place in the stroma, the liquid inside the chloroplast but outside the thylakoid, and is catalysed by enzymes found there too.
Carbon dioxide molecules enter the stroma in the chloroplast and react with the enzyme ‘Rubisco’ to covert ribulose biphosphate (a five carbon sugar molecule, RuBP) into an unstable intermediate six carbon molecule (six carbons with a phosphate at each end). This unstable molecule breaks into two phosphoglycerate (three carbons with a phosphate, known has PGA).
PGA now interacts with ATP and takes a phosphate. This does two things – it turns PGA into BPGA (i.e. there are now two phosphates so it becomes biphosphogylcerate) and it converts ATP back to ADP (because instead of three phosphates, it now has just two). ADP returns to the thylakoid.
BPGA then interacts with NADPH. Now if you recall, at the end of the light dependent reactions, NADPH had a high energy electron and a hydrogen ion. It now donates both of these to BPGA and creates phosphoglyceraldehyde (a three carbon molecule, called PGAL). The NADPH becomes NADP+ again and returns to the thylakoid.
Now two PGALs can join together to make a six carbon sugar that the tree can use. But there is a bit more to it than that. This is where the six becomes important.
It’s not just one CO2 that enters the stroma, but many more. Six is useful because six CO2 react with rubisco and RuBP to make six intermediates. These break in two to create 12 PGAs. 12 PGAs become 12 BPGAs and then 12 PGALs. We need two PGALs to make a six carbon sugar, and that leaves 10 PGALs left over.
Between them these 10 PGALs have 30 carbon atoms (each one is a three carbon molecule remember). By getting energy from more ATP, these 10 PGALs are turned back into five RuBPs. These RuBPs can now be used to keep the Calvin-Benson Cycle going.
So for every turn of the cycle that is useful to the tree, it gets one glucose molecule and five RuBPs. This is the 1/6 fraction I mentioned earlier.
Does this explain the C6H12O6 equation? It does if you remember that the 6 H20 (or H12O6) are from the light dependent reactions. You need six molecules of water to donate the hydrogen required for the Calvin-Benson Cycle to work.
PART FIVE: TREES BREATHE TOO (THE NEED FOR FREE OXYGEN)
It is worth remembering that trees, indeed all plants, also need ‘free oxygen’ in order to break down sugars. This is ‘aerobic respiration’ which living cells – including humans – carry out. So despite all this talk about plants using carbon dioxide and pumping out oxygen, plants do still need to do the opposite. When cells respire, they give off carbon dioxide. In humans, we think of this as breathing. Although plants do this too, the overall net output of plants is to use up carbon dioxide and give off oxygen.
This need for ‘free oxygen’, which means O2 rather than the oxygen molecules wrapped up in water, is why most trees don’t like water-logged soil. It can seem strange that trees which are absorbing thousands of litres of water from the ground, can also struggle if there is too much water. Basically, in most soil, there is a plenty of air between the soil but when that soil is submerged or waterlogged, the water fills those spaces. Plants have to adapt to survive this. Some have ‘stilt’ like roots which lift them up, or they send up ‘snorkel’ like breather roots which take oxygen from the air.
TWENTY ONE PER CENT
That, more or less, is what is happening all around us. I find it fascinating, but even if the chemistry isn’t your thing, it’s worth thinking about what it means for us in really basic terms. We’ve barely even talked about oxygen yet – the waste product of photosynthesis. Without oxygen the human brain dies in about six minutes. We breathe in air that is 21% oxygen and breathe out air that is 16% oxygen. With those numbers we will eventually use up all the oxygen in the atmosphere and die, but new oxygen is constantly appearing, replenishing the cupboards as it were. That’s because of plants. For me, when I first started to really think about it, it began a startling re-evaluation of something I had taken for granted.
In addition to producing all this oxygen and keeping our brains alive, plants are taking huge amounts of carbon dioxide out of the air and locking it up inside useful stuff like wood and food. As we know, if carbon dioxide levels rise too high and too quickly it can change the climate of the planet in ways that we and all other life will struggle to adapt to. Plants take it and store it away. In response, humans cut down and burn millions of plants – not only reducing the amount of plants in the world to do this vital job, but also releasing that stored up carbon back into the atmosphere.
It is an extraordinary response to the situation we find ourselves in. So I think the most basic thing to take away from all this is a simple appreciation and understanding of what trees do (and all other plants too of course – 50% of the world’s photosynthesis is carried out by cyanobacteria and phytoplankton in the oceans). You don’t have to be a botanist or a scientist, a forester or a farmer. If you are human, if you are alive, then photosynthesis matters to you, and future generations.
Trees are so useful to us that we can help in many ways; by thinking more about the supply chains of wooden furniture we purchase, the way food we buy is produced, the amount of meat we consume, the journeys we make and the mode of transport we use (and lots of other things we do), we can start to make little differences – little things matter in this context.
Perhaps, in the grand scheme of things, little things will matter much more than big things. I hope so. I hope the small differences add up, over time, to mean more than the large ones. If so, it means we all contribute, we all play a part, whether we realise it or not, whether it is apparent in our lifetime or not.
But what photosynthesis revealed to me – the thing that I mentioned right back at the beginning, the thing I feel it’s hard to appreciate any other way – is that the story of life as we know it is really the story of plants. Plants make up 98% of all the biomass on the planet (biomass is all the living material, not the number of individual species). Look around. This is their world. They were around millions and millions of years before us, and will no doubt be around millions more after us.
Anthropomorphism is that odd thing where people attribute human behaviour to animals, objects or plants. On one hand it seems to make ‘alien’ things make sense, but on the other it seems to bestow some quality or intelligence on things that don’t have it. But humans are late to the game, having just arrived on life’s timeline, and now claiming that everybody and everything else somehow has echoes of our own way of thinking and doing. I think the truth is we are the echo. If we see some connection between them and us then it is because we copy them, not the other way round. They figured out a way to live first, to grow, to learn, to fight, to reproduce, to cooperate, to die, to be. Humans, in these most recent moments of existence, have seemingly gathered up all the notions of living for ourselves, as if fighting wars, craving success, caring for our offspring and being creative are something we do, and then just project on others.
Plants do all that, and did it before us. But they do it another way.
Forgive me for straying slightly from the point, but even art, the thing we claim solely for humanity, seems to have existed somehow before us. The bowerbirds of New Guinea and Australia occupy a range of different habitats, including rainforest, eucalyptus and acacia forest, and shrublands. They are known for their courtship behaviour, building structures that are neither nest nor home. Their purpose is for something else, that we humans would perhaps call art. Many people have written books, songs or some other expression in order to find a mate or attract attention, and that’s what the birds do as well. Research has shown that they have an idea, correcting attempts by scientists to change or manipulate the structures. The birds are making creative decisions.
In this sense, I think anthropomorphism is just another humancentic definition. When we see links between our way of being and other living things, we figure it is just us endowing these other things we our characteristics. But really we have their characteristics, we echo what other species have been doing for millenia, some animal, some plant. Love, hate, anger, empathy, etc, may be peculiarly human strains of all these things, but they existed in some form before we came along.
For me, it is plants that show us this best, because of their ‘otherness’, because they do it a different way. And photosynthesis, despite its complexity or rather because of its complexity, reveals this in all its glory. Plants, quietly, humbly, but essentially, go about doing something that humans can’t, despite all our technology and all our intelligence; they turn light from the sun into life on earth, and bring life to everything else in the process.
It is their story. We have cameo roles. There’s no shame in that. We should celebrate it. We should embrace the sense of place it offers, the belonging. We should use it to get some perspective, and use that feeling to fuel what needs doing in the world.
EPILOGUE – A NOTE ABOUT SEASONS
The seasons are one of nature’s greatest transformations and we get a front row seat. Birds migrate, some animals hibernate, snow might fall, the days get short or long. But perhaps the real highlight is what happens to our deciduous trees. The loss of leaves in autumn and regrowth in spring provides us with a colourful spectacle unlike anything else.
In the UK, we live in a temperate zone. The temperate latitudes of the Earth lie between the subtropics and the polar circles., where average yearly temperatures are not that extreme. Although moderate, temperatures can vary greatly though, between summer and winter (unlike in the tropics), which means most places with a temperate climate have four seasons: summer, autumn, winter and spring.
The green of plants and especially their leaves is caused by the chlorophyll, but as the chlorophyll breaks down in autumn, other colours created by other pigments become visible. The two key pigments related to autumn colours are carotenoids and anthocyanins.
THE OTHER PIGMENTS
Carotenoids are responsible for the yellows, oranges and some reds in leaves. Plants appear to produce carotenoids to protect their stems and leaves from the energy of the sun. Ultraviolet (UV) wavelengths can generate molecules called free radicals that can damage living cells. Free radicals are produced as the result of a normal molecule losing or gaining an electron. Carotenoids as antioxidants limit free radical damage by donating electrons to quench, or neutralize, the oxidant radicals.
Anthocyanins are responsible for the reds and purples of flowers, fruits and leaves. The colours can help attract pollinating animals to flowers and animals that will help disperse seeds. Anthocyanins are also thought to help protect leaves from ultraviolet radiation but some botanists think that may not be true for all plant species. They may also deter herbivores in some species.
Being any plant, but perhaps trees in particular, is a careful balance. Leaves are the machine for carrying out photosynthesis, but they are also the main route for water to evaporate, in a process called transpiration. This is an important part of how trees get water up to the leaves, but it can be an issue if too much water is being lost and not enough light is being absorbed. The fall in autumn is the moment that trees ‘decide’ that having leaves is becoming a hindrance rather than a help in their survival.
It is understood that trees do not just rely on temperature in order to decide when to drop their leaves. Light levels are more consistent, as day lengths shorten as autumn arrives and winter approaches. A key player in this is a photoreceptor called ‘phytochrome’, which detects light in the red part of the spectrum.
Prof John Christie at the University of Glasgow has studied photoreceptor systems, in particular ‘phototropins’, which detect light in the blue part of the spectrum. His research has shown how two different phototropins are engaged in helping plants detect blue light and control their behaviour and growth in response [10]. Rather wonderfully, phototropins are also active in other aspects of plant behaviour, including stomata control and chloroplast movement. Chloroplast movement is much more interesting than I thought. I’ve seen films of chloroplasts moving towards light, but they actually react differently depending on light levels. In low light, they spread out in a ‘bunched’ up sort of way but in intense light they move to the edge of the cell walls, like dancers at the edge of the dance floor. By doing so, they protect each other from the harmful effects of strong UV light. Chloroplasts nearest the light are ‘sacrificing’ themselves in order to limit damage to other chloroplasts!
I realise by using metaphors like ‘dance floors’, I am comparing plant behaviour to human behaviour in order to make it comprehensible and engaging – an example of the easy anthropomorphism I mentioned earlier! Oh.
It may be obvious but still worth remembering that plant forms are incredibly varied because they have much more flexible systems than animals. Humans, dogs, horses, etc, always tend to have the same shape. It is very unusual if a human has three legs or one arm. Indeed, we would call it a disability. But plants do this all the time. They change how many branches, how many leaves, their overall shape and architecture, depending on the environment and circumstances. Unlike humans, they have no unique parts. Plants can lose any part of their structure and survive, often simply growing a new one and carrying on. Humans and other animals are of course vulnerable because some parts of their body are irreplaceable. The brain controls all the living systems in a person and is therefore utterly vital to living as we know it. A plant’s systems are dispersed. It is always complete and never complete, at the same time. In one location, a tree might be small and thin. Another, genetically identical tree, might grow to be huge and wide. It can make plants and trees hard to identify because they don’t always look the same and escape easy categorisation. There is something beautiful in that too.
P.S. There are lots of great photosynthesis resources out there and I highlight some below, but I found Craig Savage’s series of five YouTube videos particularly good at explaining it. Check them out here.
END NOTES:
[1] Three forms of photosynthesis have evolved over the last 2.5 billion years, enabling it to deal with extreme or less than ideal conditions / habitats. These are called C3, C4 and CAM (or Crassulacean-Acid metabolism) photosynthesis. When we talk about photosynthesis we are normally referring to C3, but even C4 and CAM are doing essentially the same thing. Part 5 of the Craig Savage videos I highlighted above explain the differences really well.
For context, an approximate timeline of plant (and other) life on earth is as follows:
4.6 billion years ago – Planet Earth forms
4.2 billion years ago – first possible appearance of life
3.5 billion years ago – emergence of bacteria
2.5 billion years ago – photosynthesis is performed by cyanobacteria
850 million years ago – primitive plants move onto land
420 million years ago – plants develop vessels
420 – 360 million years ago – wood evolves
360 million years ago – plants with seeds (gymnosperms) evolve
200 million years ago – warm-blooded mammals evolve
145 million years ago – plants with flowers (angiosperms) evolve
6 million years ago – humans diverge from closest relatives, chimpanzees and bonobos
200,000 years ago – modern humans, Homo sapiens, evolve
https://en.wikipedia.org/wiki/Evolutionary_history_of_plants
[2] https://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction
[3] Chemical reactions are what make photosynthesis occur so it is important to understand a few things. An atom is the smallest unit of matter that defines the chemical elements. They are very small: the size of atoms is measured in picometers – a unit of length equal to 1×10−12 m, or one trillionth of a metre (see end note 4 for more on power of ten numbers) An element is a material made of one kind of atom throughout, such as oxygen, which is two oxygen atoms or O2. A molecule is a material made of two or more kinds of atom, such as water, H2O, and carbon dioxide, CO2 Atoms have a nucleus which is made up of protons and neutrons. They also have electrons that orbit the nucleus in different energy states. Protons are positively charged. Neutrons are neutrally charged. Electrons are negatively charged. This ‘charge’ is the physical property of matter. Positively charged substances are repelled from other positively charged substances, but attracted to negatively charged substances; negatively charged substances are repelled from negative and attracted to positive Every solid, liquid, gas, and plasma is made up of neutral or ionized atoms. Most atoms have equal numbers of protons and electrons in which in which case their charges cancel out, yielding a net charge of zero, thus making the atom neutral. If an atom loses or gains an electron it becomes an ion. It becomes either positively or negatively charged. If an atom loses or gains a proton it changes its atomic number and becomes a different chemical element – see end note 5). There are 118 elements – 98 occur naturally and 20 have been synthesised in labs. Elements can have atoms with different numbers of neutrons and these are called isotopes. Atoms of different chemicals can join together to become chemical compounds.
[4]. A quick note about ‘power of ten numbers’. 10-12 is a quick way of writing 10 divided by 10 twelve times, or 10 / 10 / 10 /10 / 10 / 10 / 10 / 10 / 10 /10 / 10 / 10. Likewise, 1012 is a quick way of writing 10 x 10 twelve times, or 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10. It helps to represent very big or very small numbers simply. The main thing to remember is minus means divided by ten and just the number means multiply.
[5] For example: 1 proton = Hydrogen (H); 2 protons = Helium (He); 3 protons = Lithium (Li); 4 protons = Beryllium (Be); 5 protons = Boron (B); 6 protons = Carbon (C); 7 protons – Nitrogen (N); 8 protons = Oxygen (O); and so on.
[7] https://en.wikipedia.org/wiki/Photocatalytic_water_splitting
[8] It’s probably worth saying something about phosphates. Trees have trouble getting phosphates themselves, so they team up with fungi in a symbiotic relationship. The trees give the fungi carbohydrates and the fungi provide phosphates (and other minerals). They can do this because the fungi hyphae (tiny strands in the soil) are small enough to extract minerals from inside rocks and stones. Plants need phosphates to help break down sugars.
[9] Eating the Sun: The Every Day Process That Powers The Planet, by Oliver Morton
[10] https://www.sms.cam.ac.uk/media/2097658
SOURCES, INSPIRATION AND FURTHER INFORMATION:
If you find this interesting, I urge you to seek out the writers, scientists and teachers below. My project has attempted to distill what I found to be the most accessible and fascinating parts of the process, but these books and videos are where it’s really at.
Eating the sun by Oliver Morton
Trees by Roland Ennos
The Secret Life Of Trees by Colin Tudge
The Emerald Planet by David Beerling (and TV series ‘How to grow a planet’)
Photosynthesis by Isaac Asimov
Brilliant Green by Stefano Mancuso & Alessandro Viola
The Man Who Made Things Out Of Trees by Robert Penn
The Triumph of Seeds by Thor Hanson
A Cabaret Of Plants by Richard Mabey
Molecular Mechanics of Photosynthesis by Robert E Blankenship
Plants: From roots to riches by Caroline Fry and Kathy Willis
Where Our Food Comes From by Gary Paul Nabhan
Life on the Edge by Jim Al-Khalili and Johnjoe McFadden
The World Without Us by Alan Wisemann
Craig Savage photosynthesis videos on YouTube
Bozeman Science Biology videos by Paul Andersen on YouTube
Khan Academy
thesideshoots.com 