Saturday, November 7, 2009

more on photosynthesis

a simplified version of glyceraldehide [C3H6O3], a triose three carbon sugar used in photosynthesis

Canto: So is it to be photosynthesis today?
Jacinta: Maybe. Not all life depends on photosynthesis, you realize.
Canto: Uhh, yes of course, but tell me what photosynthesis is, then I can be clear about what it is that life doesn't entirely depend on.
Jacinta: Well, most people know that the oxygen around us, the oxygen we breathe, that we depend upon, has been produced by microbial and planktonic life, in the oceans mainly. Now this oxygen is a kind of waste product of a process that transforms oxides into sugars by means of sunlight. What's really important about photosynthesis, apart from its obvious importance for us, is that it's a process that creates 'food' from the most ordinary, inanimate chemicals around - carbon dioxide and water. Did you know that water was an oxide?
Canto: No. I knew that it contained oxygen, though.
Jacinta: Well there you go. Would you like a glass of dihydrogen monoxide?
Canto: You can't get rid of me that easily. Okay, so your plankton or whatever takes a ray of sunlight, or a photon or whatever, and synthesizes sugars out of carbon dioxide or water. That's as clear as mud.
Jacinta: Well, I seem to remember that the exact mechanisms were worked out only quite recently. Let's take plant photosynthesis. Light energy is absorbed by proteins containing chlorophyll...
Canto: Hang on, hang on - absorbed? What's this absorbed? Is that where a miracle occurs?
Jacinta: A miracle is just something we haven't examined sufficiently.
Canto: Oh how prosaic, how materialist.
Jacinta: Okay I'm not sure if absorption is a technical term but sunlight excites molecules, and I presume that's the key. The excitation results in electron transfer reactions. Chlorophyll is involved, absorbing the light due to its pigmentation, and promoting electrons to higher energy levels, creating free energy.
Canto: The electrons then release energy in returning to their stable or ground state.
Jacinta: Too right. When an electron goes into a higher energy state, the molecule it inhabits is said to have a higher reduction potential, so that it tends to donate electrons. That's how light energy becomes chemical energy, apparently.
Canto: How?
Jacinta: Get an education Canto.
Canto: No I think I understand.
Jacinta: Well what follows is very complex and I can't say I fully understand it myself. Electrons are donated to electron acceptors in an electron transfer chain. There's a whole heap of them, and what results at the end of this chain is a reduced molecule called NADPH. This molecule, along with ATP [which is also produced through this process], is involved in the Calvin cycle which fixes carbon dioxide into triose sugars.
Canto: What? Do you really know what you're talking about Jass? What's this thing called 'fixing' I've heard so much about? What are these molecules? What's a triose sugar?
Jacinta: Okay, you’re right, I’ve not got my head around those details, but there’s so much to explore in this universe, mate. I’ll answer your questions, but I’m also interested in pre-photosynthetic life, and much else besides.
Canto: I can’t wait.

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