Today’s topic is the light-dependent reactions of photosynthesis. Now, you may think that it’s all fairly straightforward, thinking back to your GCSE biology classes (or O-Level for you oldies).
A bit of light for the leaves provides energy to turn water and carbon dioxide into sugar and oxygen. Simples, I hear you say. That is what I thought too. Just a short chapter, I imagined. How complicated can it be?
Well. Let me tell you that it’s very bloody complicated. I’ve drawn two diagrams, and I’m still not entirely sure I’ve understood it. And I’ve only done the light-dependent reactions! The dark reactions are yet to come. I’m expecting them to involve Voldemort in some way.
Here is a short account of the principle reactions involved in this stage of photosynthesis, which I wrote as part of an activity to help us to understand the processes. I would include my diagram, but I’m not drawing it on a crappy laptop. It’s not an Etch-a-sketch, you know. So I’ve pinched this one from my OU course book.
The thylakoids are part of the chloroplast in plants. I apologise for the word “thylakoid”. All its consonants are in the wrong place, making it a bit of an assault course for the tongue. It reminds me of trying to learn German at school – I never was very good at German, partly because I had trouble getting my tongue around their words. I do, however, love the phrase: “Schnell, schnell, kartoppelkopf!”
They have an outer membrane, and a really convoluted internal membrane which is stacked into grana – and each little disc (or sac) in an individual granum is a thylakoid. The space inside the thylakoid membrane is called the thylakoid lumen, while the space outside the membrane is called the stroma. As illustrated above.
My summary is as follows. It’s supposed to simplify the description of what’s going on, and complement the diagram above. I’m not sure I’ve achieved that; any and all feedback is welcome!
When light strikes a chlorophyll molecule, a photochemical reaction takes place in which the hydrogen atoms of water molecules are split into their constituent protons (H+ ions) and electrons. (Oxygen is released as a by-product.) As shown above, the electrons move from the thylakoid lumen through the membrane to the stroma, by means of protein carriers within an electron transport chain (ETC). The protons are left behind, increasing the concentration of protons in the lumen. With me?
In the stroma, coenzyme NADP collects a couple of electrons and combines them with a couple of protons, reducing to NADP.2H (see above). This lowers the concentration of protons in the stroma. This will be important later.
One of the electron carrier proteins in the ETC is a little shuttle that collects protons from the stroma, bimbles across the membrane, and deposits them in the lumen, further increasing the concentration of protons in the lumen.
As a result of these processes, a transmembrane (yes, it’s a word!) protein gradient is formed across the thylakoid membrane – this works much like a hydroelectric plant (think of the reservoir at the top, and all that potential energy waiting to be turned into electricity). Now there’s an imbalance of proton concentration, enabling the protons to flow down the concentration gradient back into the stroma through channel proteins called ATP synthase (shown on the right of the diagram above).
The flow of electrons through these proteins enables the manufacture of ATP from ADP (adenosine diphosphate) and Pi and their transfer provides the energy required.
The products of these light reactions, ATP and NADP.2H, are used in the dark reactions of photosynthesis by the Dark Lord to reduce carbon dioxide to glucose.
I do apologise for the extreme biology – but this is the third time I’ve written the process in my own words, and I do believe it’s finally beginning to sink in. In a manner that ensures I understand and remember it.
Stay tuned for the Dark Reactions – I suspect they may be sexier.