Plant Cuttings

Rocking the photosynthetic cradle…

Image: Dominique Toussaint/Wikimedia Commons.
Image: Dominique Toussaint/Wikimedia Commons.

I’m sure that many of us in our teaching have demonstrated the transfer of light energy between adjacent accessory pigment molecules during the light-dependent stages of photosynthesis using the so-called Newton’s cradle. Perhaps more famous as a stress-relieving ‘toy’ for over-worked ‘executives’, the apparatus visually – and usually quite dramatically and memorably – demonstrates the principle of resonance transfer of energy – or exciton transfer – between the various antenna pigments, thereby extending the range of wavelengths that can be used in photosynthesis. And, if you were lucky enough (in ‘the old days’, but probably not in these ‘health-and-safety-gone-mad’ times…), you could suspend several students from the ceiling in a human version of the cradle. That would certainly make for a most memorable graphical illustration of the principles involved – with their screams representing the dissipation of some of the light energy that is not transferred ultimately to a molecule of chlorophyll a. Well, if the work of Dong Guo et al. applies to the vibrant, verdant, living world that is plant photobiology, there is now hope for an ever more energetic classroom demo. The team use the ‘same principle that causes a figure skater to spin faster and faster as they draw their arms into their bodies … to understand how molecules move energy around following the absorption of light’. So far this ‘angular momentum conservation in dipolar energy transfer’ has only been demonstrated between a rhenium(I)-based charge-transfer state to a chromium(III) acceptor, but one can expect it to be extended to photosynthetic photosystems soon. To ensure that your students understand how this might work, unhook them from the Newton cradle and tell them to spin (and spin and spin…). In fact, encourage them to emulate the antics of the Whirling Dervishes of the Mevlevi Order. Now who’d have thought we could ‘top’ the cradle as a visual aid! For readers who may seek more enlightenment – in the physical chemistry sense – on Förster energy transfer theory as reflected in the structures of photosynthetic light-harvesting systems, I’m happy to direct them to the article of that name by Melih Şener et al.


[Lest there be any doubt, in no way does the mischievous N. Chaffey or anybody associated with him, or his column, or the journal in which it exists, sanction the use of students as aerial components of a cradle, Newtonian or otherwise, nor indeed their use in any other way, shape or form that may cause distress (or humour – however unintentional) to participants, on-lookers (witnesses?), YouTube viewers, or the fabric of the building in which they be – however temporarily – suspended, whether for ‘instructional’ purposes or otherwise! – Ed.]

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