BBC Radio 4 - In Our Time, Photosynthesis
"‘Oxygen is not just a waste product. It's a toxic waste product. We don't think of it as toxic, we may come back to that…
2.4 thousand million years ago, as you said in the introduction, the whole biosphere was working fine without free molecular oxygen. This was dangerous stuff to have around. It's chemically highly reactive. 2.4 thousand million equals billion years ago, this trick was discovered by accident, of taking electrons from water, producing this byproduct. This was a big shock to the system. There's never been an equivalent environmental catastrophe for life that existed before that time... as the production of oxygen, a poison gas actually, which, nevertheless, doing that trick had such immense value. Again, discovered by accident, it wasn't that they wanted to do this.
I know that you were speaking metaphorically there. This was, had such benefit that they had to learn to live with this poison gas and learning to live with it… The organism, everyone... that had to deal with oxygen as it came along. There are still environments today where oxygen doesn't permeate and there is still anoxic life, without oxygen, that was a relic of this former time-’
‘Which is down in the cleft of the ocean.’
‘Yeah. And under the, in the, in the rocks in the lithosphere’...
‘Why is chlorophyll green?’'
‘That's such a good question. And you know, I have to confess, I don't know the answer. And lots of people have asked that question. why is it green?’'
‘It’s not green, we see it as green because green is the only thing that isn't absorbed, or green is the only wavelength that isn't absorbed. So nothing really has color. We just perceive it because... of the wavelength of light which is reflected off it, so it's interesting. There's a concept called chlorophyllia, which is about the love of, the love of green things, which I think is a nice, a nice thing to be’...
‘It absorbs blue light and red light and doesn't absorb light in the middle of the visible spectrum, which is green. I mean, that's true. But that's just sort of stating, not starting an argument here. I mean, Melvin's question, if I may, could be rephrased, why aren't plants black? Right? If they were black, they would be absorbing all this’...
‘It's interesting that they use red light, mostly. So the spectrum of light that chlorophyll is absorbing which is taking the electrons from water and driving that whole process is red light, which is not energetically particularly strong. Blue light has far more energy in it than red light does. So you would have thought from first principles, and actually chlorophyll does absorb blue light as well as, as John said, but it's not using that wavelength. And the reason is not clear but it may simply be the destructiveness of UV and blue light. You know, it can damage our own retinas and so on as well, there are issues with higher energy wavelengths of light so I think it's it's, it's ended up as red partly because that's the the wavelengths that chlorophyll absorbs. And partly because selection has adapted the wavelengths that chlorophyll absorbs to being the, the gentlest on the plant itself, it’s less likely to do damage if you're absorbing light at that wavelength.’...
'Probably the very first forms very early on three and a half billion years ago. Those forms did not use water as the electron donor. They used things like hydrogen sulfide, and iron, and the products that they, they produce, they, so if you're using iron as an electron donor, what you're leaving behind is, is rusty iron, which precipitates out of the oceans and forms banded iron formations, which are the major sources of iron ore that we're using today. So, you know, some of the big mineral deposits derive from photosynthesis and our evidence that photosynthesis was happening at that time'... 'You think plants produce oxygen, animals consume oxygen, but that's a complete balance, and so nothing changes. That's why you have that boring billion. It’s only when, you know, if we die and we're buried and we are not then kind of broken back down to 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 oxidized you is left over in the air. And so the the dynamic over evolutionary time has 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'"
