fixation, redox reactions, nucleotides and more

ATP - see the triphosphate bit?

Jacinta: Okay, so since we're both thoroughgoing obsessional types, we'll have to get to the bottom of photosynthesis, at least to clarify some of the cycles and molecules and such that we referred to last time.
Canto: Great, we're talking the same language. So tell me about this fixing, in relation to carbon dioxide. I'm sure I've also heard of nitrogen fixing...
Jacinta: Well let's not get too sidetracked, but fixation, in chemical terms, means transforming a substance, or molecule...
Canto: Or molecular substance?
Jacinta: Yeah, into a more usable form, like ammonia, in the case of atmospheric nitrogen.
Canto: N2 into NH3, I get it.
Jacinta: Right, though of course much more complicated. And carbon fixation is hideously complicated. The Calvin cycle, worked out many decades ago, basically traces the carbon fixation process. All I can competently say at this stage is that the key enzyme in the process has come to be known as rubisco. I won't say anything more about NADPH - it's essential in the photosynthetic process in chloroplasts. I could say more but it wouldn't make much sense to either of us.
Canto: It acts as a reducing agent, doesn't it?
Jacinta: Yes. You know about oxidation-reduction?
Canto: I know of it. If I was a hands-on biochemist or whatever I'm sure I'd know about it.
Jacinta: Actually redox reactions aren't too difficult to understand. They generally involve electron transfer. The reductant, or reducing agent transfers electrons to the oxidant, or oxidizing agent. So the reducing agent gets oxidized, and the oxidizing agent gets reduced.
Canto: Believe it or not, I follow you. So in the Calvin cycle, NADPH is the reduced form of NADP+, and therefore NADP+ is the oxidized form of NADPH.
Jacinta: You read that somewhere mate.
Canto: Yes but it makes sense all the same. So what's ATP? That's a biggie isn't it?
Jacinta: Adenosine triphosphate is indeed a very important wee nucleotide, the key molecule in cellular metabolism.
Canto: Let's do each other's heads in - what's a nucleotide, and what exactly is metabolism?
Jacinta: Come on Canto, metabolism's just what you think it is - it's the breaking down of food to construct proteins, nucleic acids and so forth. Without which not. A nucleotide is a molecule with a particular structure, found in all cells performing various metabolic functions. They also are the bases of polymeric nucleic acids, as well as being the structural units of DNA and RNA.
Canto: Well read Jacinta.
Jacinta: Well digested encore.
Canto: Whatever you say mate. So, getting back to photosynthesis, how did plants, or bacteria, start getting into this?
Jacinta: You could just as well ask how did more complex organisms start using oxygen and other nutrients to sustain themselves, or how did pre-photosynthesizing organisms, or non-photosynthesizing organisms start utilizing whatever they utilized...
Canto: I could just as well, but I asked about photosynthesis.
Jacinta: Okay, it's known that photosynthesis evolved in bacteria and that it's been going on on our planet for at least 2.5 billion years. As to the how, I'll try to answer that in the next post.