Archive for the ‘photosynthesis’ Category
the photosynthesis challenge
c’est bien simple…
So I wrote last time about my non-career in science, with a promise to myself that I might try to get my head around photosynthesis. So, years ago I read Oliver Morton’s fiendishly difficult book, Eating the Sun, and all I clearly remembered about it was that there were two types, simply named type 1 and type 2, and that the processes are very very very complex, and took decades to unravel. And a feeling that it was all over my head. Or that one had to be there, in those labs, or chatting at science conferences with the experts, as Morton no doubt was, to have a chance of getting my dull head around it all. And yet, never say never…
So I’m rereading Morton’s book (published in 2007 – has more been nailed down about the processes since?), having nothing better to do – and that isn’t meant as an insult – and I’m currently about a quarter of the way through. What I note is that Morton intermixes the undeniably complex science with interesting character sketches of the major researchers and theorists, and descriptions of the mostly mid-twentieth century background of war and conflict (at least in this early part of the book). My aim here though is self-education, so I’ll be focussing entirely on the science as far as I can grapple with it. So there will be no names, just a lot of ‘this breakthrough led to a new avenue of research’, etc.
So, photosynthesis is about using sunlight as sustenance. Of course, water is also required, and CO2. The devil’s in the detail, and there’s an awful lot of it..
So, where to begin. Chlorophyll, I suppose. That’s the green pigment in the chloroplasts of plants and algae. They absorb light, mostly from the sun. But what does this mean? And how did we come to know this?
Chloophyll is green because the green area of the light spectrum is reflected, while blue and red light is trapped, mostly. So, though plants ‘eat’ sunlight, they don’t eat the green part of that light, which is fascinatingly counter-intuitive.
As to these chloroplasts, these key sub-cellular elements in plants and algae, they’re a product of endosymbiosis, the colonisation of eukaryotic cells by earlier microbes – bacteria or their ancestors. Mitochondria, the engines of ATP through oxidative phosphorylation (wateva), are another such product.
Which brings me to photophosphorylation in plants, as useful and interesting a starting point as any. This process, originally called photosynthetic phosphorylation, might be expected to be carried out by mitochondria in plants, as it is in mammals, but plant biologists noted that ‘the leaf cells with the greatest number of chloroplasts – the cells that do the bulk of the photosynthesis – contain remarkably few mitochondria’ (Morton, p76).
Phosphorylating, as I understand it, involves adding a phosphate to ADP (adenosine diphosphate) to create adenosine triphosphate (ATP), the principal ‘energy molecule’. Another such molecule is NADPH (Nicotinamide Adenine Dinucleotide Phosphate), of which more later, perhaps. They are both important in plants, and are key to what became known as the Calvin-Benson cycle, a set of chemical reactions (independent of light) within chloroplasts as a part of photosynthesis. These reactions convert CO2 from the atmosphere into sugars for energy.
There are apparently two main stages to photosynthesis, the light-dependent stage and the light-independent stage (aka the Calvin-Benson cycle, or even the Calvin-Benson-Bassham cycle). The light-dependent stage occurs inside the thylakoid membranes of the chloroplasts. Here’s Wikipedia:
Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as grana (singular: granum). Grana are connected by intergranal or stromal thylakoids, which join granum stacks together as a single functional compartment.
In thylakoid membranes, chlorophyll pigments are found in packets called quantasomes. Each quantasome contains 230 to 250 chlorophyll molecules.
So that’s something about thylakoid structure. Now to function. Or maybe not – it’s all so complex. The thylakoid proteome (set of individual proteins) consists of at least 335 protein types, of which 89 are in the lumen or central cavity. At this stage it’s calculated that about 42% of these proteins are involved in photosynthesis.
Jumping away from all this, it’s important to note that for us mammals, the essential value of photosynthesis is that it creates, or releases, oxygen. Think of the great oxygenation event, mainly involving oceanic cyanobacteria, ‘about 2.4 billion years ago, shortly after the end of the Archaean’, according to Morton (p175). So how does that oxygenising aspect of photosynthesis work?
But I was going to write about the light-dependent part of photosynthesis, requiring sunlight and water. It’s called photosystem 1, and Wikipedia calls it ‘a vital membrane protein complex that uses light energy to transfer electrons from plastocyanin to ferredoxin, producing NADPH and helping generate ATP.’ So, digging slightly deeper, plastocyanin is ‘a small copper-containing protein that acts as a mobile electron carrier in the photosynthetic electron transport chain of plants, green algae and cyanobacteria’, and ferredoxins are ‘small iron-sulphur proteins that act as electron carriers in a wide variety of biological redox reactions’.
Now, NADPH is, of course, ‘the reduced form of nicotinamide adenine dinucleotide phosphate’ which acts as ‘a key electron donor and energy carrier’ in various processes, including photosynthesis. So, my guess is that these movements of electrons create chemical changes, and a moment’s research tells me that’s what redox reactions are all about. In oxidation, electrons are lost, which takes the oxidation number up (the substance is oxidised), and in reduction they’re gained, which takes it down, and the substance is reduced.
In oxidation, electrons are lost, which takes the oxidation number up (the substance is oxidised), and in reduction they’re gained, which takes it down, and the substance is reduced.
Ok so that’s enough for now, there’s so much more to explore – rubisco, the z scheme… I cannot help but feel that it’s not light yet, but it’s getting there, to non-paraphrase Dylan….
References
Oliver Morton, Eating the Sun, 2007