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  NICK LANE: They were simply engulfed by a larger cell and put to work, not quite as slaves, but they did what they always did, they continued to photosynthesise, they continued to take electrons from water, put them on to carbon dioxide and make sugars that way. And so the host cell, which had captured them, gained those benefits of getting a free lunch, you might say.

  Within the engine room of the cell, as Melvyn termed it, there is a series of membranes, with enormous complexes of proteins. Taking that analogy further, Nick Lane invited us to shrink ourselves down to the size of a small molecule, at which point those engines would seem like a huge industrial complex sitting there in the membranes. What they do, as Nick Lane mentioned, is extract electrons from water and pass them down a kind of a chain and eventually push them on to carbon dioxide to make the sugars. That may be simple at a chemical level, but not at a biochemical one.

  NICK LANE: It’s not easy to get electrons out of water in the first place. The largest storm, crashing water against a sea cliff, is not going to break water down into its component parts. But light can do that.

  Water is useful as it is everywhere. There are other materials that could be used, such as hydrogen sulphide gas or dissolved iron, but they are far less common.

  The chemical processes in photosynthesis are known as the light and the dark reactions.

  NICK LANE: At its simplest, the light reaction is driven by the absorption of photons of light and it’s simply dragging, stripping electrons from water. The waste of this is oxygen, which is just a waste product of photosynthesis, it’s just let go, it accumulates in the atmosphere.

  Then there is the dark reaction that takes those electrons and forces them on to carbon dioxide, which does not require light at all, hence the name.

  Picking up on Nick Lane’s comments about oxygen, John Allen observed that oxygen was not just a waste product but a toxic waste product. Before photosynthesis, the whole biosphere was working fine without free molecular oxygen.

  JOHN ALLEN: This was dangerous stuff to have around, it’s chemically highly reactive; 2.4 billion years ago, this trick was discovered, by accident, of taking electrons from water, producing this by-product. This was a big shock to the system. There’s never been an equivalent environmental catastrophe for life that existed before that time.

  This process of taking electrons from water had such benefits overall that the organisms on this planet had to learn to live with this poison gas. There are still environments today, such as in the rocks, the lithosphere, that oxygen does not permeate and there is still anoxic life there without oxygen, a relic of this former time.

  Chlorophyll, John Allen explained, is a word taken from Greek and simply means ‘green leaf’. Its structure, as mentioned above, is like a spider’s web with a magnesium atom at the middle and then four carbon atoms, linked in series with a fifth, nitrogen, and then folded around to make a circle or pentagon. There are four of these in the basic head group of chlorophyll and these are also arranged in a series, then folded into a circle with the nitrogen atoms pointing inwards towards the magnesium atom in the centre. There is, additionally, a hydrocarbon tail attached to one of the rings, which gives chlorophyll the property of being completely insoluble in water. The work it does is special and essential, but it is not magical.

  JOHN ALLEN: In the history of biology, people have always wanted to be focusing down on what’s the smallest thing that we can say is alive and people got very excited about chlorophyll. It seemed to be a very special molecule, sustaining life on earth. But it’s just a chemical; in fact, it’s been synthesised by synthetic organic chemists in Harvard University in the ’60s or ’70s from scratch.

  There are two sorts of chlorophyll, both doing essentially the same thing. In the 1960s, Robin Hill and Fay Bendall, noting that there is a chain of electron carriers, added that there are two points in that chain of electron carriers where the electrons would not go unless they were given a push by some energy input, which is provided by light energy acting on chlorophyll. The special chlorophyll that loses its electron to start the chain is just one of 300 chlorophyll molecules, approximately. All the others take the absorbed excitation energy and pass it among each other until it arrives at this special one, and that sets the whole process going and sustains life on earth.

  There is more than one type of photosynthesis. Most plants, Sandy Knapp said, are called C3 photosynthesisers as the first thing that happens to the carbon dioxide when split up is it turns into a three-carbon molecule. There is also a set of flowering plants that are called C4 plants, where the carbon dioxide is broken up into a four-carbon molecule and then the photosynthesis happens in two different types of cell. The carbon is stored in bundle sheath cells around the veins. These C4 plants are much more efficient at photosynthesis at high temperatures so, often, the plants that grow in deserts are C4 photosynthesisers.

  SANDY KNAPP: Corn, maize, for example, is a C4 plant, and this has been studied the most in the grasses. These C4 plants are highly photosynthetically efficient. One of the great holy grails in agriculture is to take a C3 plant, like rice or wheat, and turn it into a C4 plant, which would increase its efficiency and thereby perhaps increase its yield and its ability to grow in different parts of the world.

  There is a third kind, crassulacean acid metabolism or CAM, so named as it was first discovered in a sedum in a rock plant in the family Crassulaceae. In CAM plants, the stomates, the little holes that can open and close and through which CO2 enters, keep closed during the day and open at night, when it is not so hot. They can store up all the components for photosynthesis and do it later, which is useful if you live in a desert. Cacti are CAM plants.

  Melvyn returned to Nick Lane for what happens in the cells, and we heard how the flow of electrons is used to drive protons, the positive nuclei of hydrogen atoms, across a membrane. From this, there is a proton gradient across the membrane.

  MELVYN BRAGG: Gradient, like, you mean, slope?

  NICK LANE: Well, yes. Essentially, on one side of the membrane, you have a large number of protons; on the other side, you have relatively few. It’s essentially like a hydroelectric power scheme with a reservoir on one side and a turbine in the membrane itself. The turbine is an enzyme called the ATP synthase enzyme and that is powered by the flow of protons from the reservoir back to the side where it’s downhill, in effect, and that produces ATP.

  ATP, he said, is generally called the energy currency of life and is used by all living cells. We could think of it like a coin in a slot machine where all proteins, if they are to do any work at all, must change their conformation, and to do that requires splitting an ATP. The current of electrons that is flowing from water to CO2 is driving all of this.

  In addition, all those electrons end up on carbon dioxide, converting it into a sugar, and those sugars are then converted into the rest of the organic molecules that the plants need to live, that we need to eat.

  Besides photosynthesis in the plants, there is respiration. John Allen, for present purposes taking the definition of photosynthesis as release of oxygen and uptake of carbon dioxide, explained that aerobic respiration, which is the respiration that plants do and we do, is the uptake of oxygen, putting electrons on to it to make water, which is the reverse of taking electrons from water to liberate oxygen. However, he added, at the kind of level that Nick Lane had been describing, the power station was a good analogy.

  JOHN ALLEN: The way in which that is done is universal in biology, and photosynthesis and respiration are two ways of applying that same fundamental mechanism – electron transport, moving protons across a membrane to make a gradient, which is stored energy and used to make ATP. In that sense, they’re the same process, except that the chlorophyll in the photosynthetic reaction centre gives the electron that initial push that it needs; in respiration, the electrons just sort of flow where they want to go.

  Nick Lane agreed that the source of electrons was really the major difference between photosyn
thesis and respiration. In respiration, we need an easy source of electrons – in our case, food – that will react spontaneously with oxygen. The enzymes in the mitochondria allow that to happen.

  NICK LANE: What’s happening in photosynthesis is that light is providing that essential input of energy, which starts electrons flowing from far more difficult places, [from] water, in this case. Water really does not want to lose its electrons, but the input of light through chlorophyll extracts electrons from it, and sets them flowing in exactly the same way that they flow from food to oxygen in us. It’s exactly the same process; the source of electrons differs.

  There are limiting factors for photosynthesis. Sandy Knapp mentioned the need for water, carbon dioxide and light, and also nitrogen and phosphorous, which are part of the reason we fertilise crops, to increase photosynthetic efficiency and thereby the yield. Temperature is a very important limiting factor for photosynthesis as well, as it does not happen efficiently at very high or low temperatures. Plants regulate the degree to which they photosynthesise, depending on environmental conditions.

  SANDY KNAPP: One of the things that’s really interesting about plants is that people often think of plants as just sitting there. Plants behave – it’s just on a very different scale to our human behaviour. And so, if there’s not enough water, or if there’s too much light, the stomates will close and thereby no carbon dioxide is taken in and photosynthesis goes down.

  As for why chlorophyll is green, it appeared there was no simple answer. Sandy Knapp said that it is not green – we see it that way because green is the only wavelength that is not absorbed. Nothing really has colour, we just perceive it that way because of the wavelength of light reflected off it. That prompted John Allen to offer his perspective.

  JOHN ALLEN: Why is chlorophyll green? Because it absorbs blue light and red light and doesn’t absorb light in the middle of the visual spectrum that is green. That’s true. But Melvyn’s question could be rephrased: ‘Why aren’t plants black?’ If they were black, they would be absorbing all visible light.

  MELVYN BRAGG: We could start again.

  Colour transmission electron micrograph of two chloroplasts in the leaf of a pea plant.

  JOHN ALLEN: No, why are they green? Why are they not making use of green light? They should really – if they were interested in getting the most energy, they would use the whole of the visible spectrum and would be black, and they’re not.

  While he did not have an answer to that question, Nick Lane noted that plants use red light mostly, which is not energetically particularly strong. Blue light has far more energy in it than red light does. Chlorophyll does absorb blue light as well, but it does not use that wavelength, for reasons that are not clear.

  NICK LANE: It may simply be the destructiveness of UV and blue light; it can damage our own retinas as well. I think it’s ended up with red partly because that’s the wavelength that chlorophyll absorbs and partly because selection has adapted the wavelengths that chlorophyll absorbs to being the gentlest on the plant itself. It’s less likely to do damage if you’re absorbing light at that wavelength.

  The need for red light is such that many plants in the rainforest, nearer the ground and so shielded from the sun by the canopy, have a red layer on the underside of their leaves.

  SANDY KNAPP: It’s actually very striking in deep rainforest plants. And David W. Lee published a really nice paper where they showed that what happens in this anthocyanin layer is that it reflects light back into the chloroplast so that they get more of the little light that’s coming through. Anthocyanins are pigments that we see as red, so they’re absorbing everything except the red.

  There is quite a lot of controversy over when photosynthesis evolved, according to Nick Lane. The first forms, 3.5 billion years ago, did not use water as the electron donor but chemicals like hydrogen sulphide and iron. Where iron was being used as the electron donor, this left behind rusty iron.

  NICK ALLEN: [This] precipitates out of the oceans and forms banded iron formations that are the major sources of iron ore we are using today. So some of the big mineral deposits derive from photosynthesis and are evidence that photosynthesis was happening at that time.

  Oxygenic photosynthesis, he added, the splitting of water and release of toxic waste oxygen, probably arose around 2.5–3 billion years ago. We know for sure it happened by 2.4 billion years ago because there was a tremendous catastrophe, a snowball earth, at that time.

  NICK LANE: There have been several episodes of these snowball earths across earth history. Probably what happened was, as oxygen was being released, it oxidised the methane being produced by other bacteria, and methane is a greenhouse gas … as it stripped out of the atmosphere, the temperatures plummet. The other thing we see around that time is the oxidation of rocks and so on, on the continent. We see what are called red beds, basically rusty iron everywhere.

  Until ten or twenty years ago, it was thought that, after the great oxidation event, when oxygen levels started picking up in the atmosphere, everything changed. In fact, it did not. It was stuck in a rut for another billion years or so.

  MELVYN BRAGG: The billion boring years.

  NICK LANE: The boring billion, it’s called, yes. So nothing really happened. Actually, complex cells arose in that time, the acquisition of chloroplasts and so on happened in that time. But there’s very little in the fossil record. Then, at the end of this boring billion, we go into another global upheaval of more snowball earths, and right on the back of that is the Cambrian explosion and the appearance of animals, really for the first time, in the fossil record.

  While there are debates about what was happening at the Cambrian explosion, which was around half a billion years ago, it appears fairly certain that oxygen was produced at that time by plants and terrestrial algae rather than just cyanobacteria, and that gave the animals the energy they needed.

  Photosynthesis itself, Sandy Knapp said, drives life on earth, it drives ecosystems and it probably drives the composition of ecosystems as well – namely why organisms are in a particular place and the relationship between them.

  SANDY KNAPP: So many heterotrophs depend upon autotrophs, which are these photosynthesising organisms, for their daily lives and their food, and then other heterotrophs depend upon those. We eat beef, which is a heterotroph, which is eating grass, so we actually are eating sunlight via this complicated chain.

  MELVYN BRAGG: It’s a bit ‘Ilkla Moor Baht ’at’, really?

  SANDY KNAPP: Yes, basically. It’s a tangled web, as Darwin said.

  The programme closed with a reference back to the discovery of photosynthesis, which John Allen placed at 1772 when Joseph Priestley published an investigation of different kinds of air. He quoted this from Priestley’s paper:

  I flatter myself that I have accidentally hit upon a method of restoring air which has been injured by the burning of candles and that I have discovered at least one of the restoratives that nature employs for this purpose. It is vegetation. One might have imagined that, since common air is necessary to vegetable as well as animal life, both plants and animals had affected it in the same manner and I own that I had that expectation when I first put a sprig of mint into a glass jar standing inverted in a vessel of water, but when it had continued growing there for some months I found that the air would neither extinguish a candle nor was it at all inconvenient to a mouse, which I put into it.

  That, John Allen said, was the discovery of oxygen and the discovery of photosynthesis in one experiment.

  In the studio afterwards, John Allen picked up on the comment that all photosynthetic organisms are autotrophic, they build themselves up. He argued that this was not true.

  JOHN ALLEN: Just tell me if this gets boring.

  MELVYN BRAGG: We’ve been bored for a billion years … what’s a few minutes?

  There are two things you can ask about any living creature, he said. Firstly, where does it get its energy? Photosynthetic organisms are phototrophs but t
hey don’t have to be autotrophs because they can get the energy and still eat food. Secondly, where does it get its stuff that it makes itself from? If it apparently gets it from nowhere, from the air, it is then an autotroph. If it gets it from other living things, it is a heterotroph, meaning other feeding. And yet …

  JOHN ALLEN: There are photoheterotrophs that get their energy from light but they still eat food, they need other organic compounds to assimilate. The obverse of that is there are also chemoautotrophs, which are organisms that are not interested in light, they live deep in rocks …

  SANDY KNAPP: Those are the vents …

  JOHN ALLEN: The vents, they’re chemoautotrophs that get their energy from chemical reactions, but they fix CO2 and make carbohydrate.

  SANDY KNAPP: That’s what always amazes me, that life on earth is absolutely incredible, the number of different ways in which organisms make their living, grow, survive and reproduce …

  MELVYN BRAGG: When I walk through London, I feel the same, every time I look in a shop. Dr Johnson said that – the way people make a living.

  SANDY KNAPP: Nothing ever changes.

  Finally, Nick Lane recalled one of the points he had meant to get across, which was, while plants produce oxygen and animals consume oxygen, that is a complete balance. In the boring billion, nothing changed.

  NICK LANE: It’s only … if we die and we’re buried and we are not then broken back down into CO2 and oxygen again and water again, if we’re just buried intact as a fossil fuel, in effect, then the oxygen that would have oxidised us was left over in the air, and so the dynamic over evolutionary time is nothing to do with how much photosynthesis there is, it’s to do with how much carbon is buried, so it’s a geological process rather than really a biological process.