The following journal article I've reviewed this past week may well be the longest I've ever read - and it certainly took the better part of a week for me to read it, as 40 pages of tiny Times New Roman is not easily done in one sitting, regardless of caffeine consumption.
Despite its length, I found "Biophysics of Photosynthesis" (Barber, 1978) to be a juicy, insightful and worthwhile read, with great relevance to topics to be covered in this course, and extrapolations on topics covered previously in BIPH2000.
The paper covers the studies done of the cardinal process of photosynthesis from the very beginnings of the understandings of O2 production via photolysis of water in 1937, to thecurrent understandings of the inner workings of the Thylakoid membranes and chemiosis-assisted ATP synthesis.
The paper begins by roughly going over the central dogma of photosynthesis from splitting water via light energy to make high energy chemical products (in the light reactions) which are subsequently used in dark reactions (ie the Calvin cycle) to synthesis carbohydrates.
The author then delves into the logistics of light harvesting and the various pigments involved. As seen on many spectra regarding photosynthesis, the complete range of photosynthetic accessory pigments cover nearly the entire visible spectrum regarding absorption. With chlorophyll a alone, the mechanics of photosynthesis would be no where near as high yielding in terms of energy, as it only absorbs in the red and blue portions of the spectrum.
Barber then continues into the quantum behaviour of the pigments, stating that highly organic molecules show broad bands of absorption on spectra because of the bonding nature in said molecule - the π and π* orbitals experience "changes in vibrational quantum states".
Barber also elaborates on the photosynthetic unit - a required number of pigment molecules for each reaction centre to achieve charge separation. In 1932 research was conducted by Emerson and Arnold using short flashes of light, concluding that approximately 2400 chlorophyll a molecules were needed for the oxidation of one water, and with four steps in the light reactions in oxygen liberation, suggesting that 600 chlorophyll a molecules are needed to bring about a charge seperation (Barber, 1978).
Barber also discusses different types of coupling between energy donor and acceptor possibly observed regarding electronic excitation energy - strong, weak and very weak. The energy transfer rates are dependent on the type of coupling.
The rest of the paper covers a huge array of different topics of photosynthesis, including differences between singles and triplet states in pigment excitation and how this determines fluorescence or phosphorescence, various models of the photosynthetic unit, the "Z" scheme and ATP production via a proton gradient (which provides the electric potential for ATP synthase).
In closing, I did find this read to be useful in broadening what has already been learnt regarding photosynthesis. There are also many useful resources in the appendix regarding energy transfer mechanisms that may be of use.
This journal article, again, certainly is not for the faint of heart, but a very worthwhile bit of bed time reading.
Also, there's an interesting bit in the article regarding "spillover energy" from photo system 2 to 1. Apparently, there's evidence which suggests that there is control over quantal allotment in the reaction centres of both these photo systems, to accomodate for various types of sun exposure. Barber suggests that this may be due to the need to optimise electron transfer in all parts of the plant, for example in the lower leaves, which could be in the shade of the sun-bleached canopy.
ReplyDeleteI'd actually pondered about this concept previously - it seemed to me that in order to operate optimally, the top leaves which have more sun exposure, should be able to distribute the light energy around somehow, right? Otherwise, the leaves that weren't at the canopy would be essentially redundant.
For the millionth time, I may just be nit-picking.
Hi Wendy.
ReplyDeleteCan you clarify what you mean by `sharing around the light energy' please? It almost sounds as if you'd like plants with internal fiber optics... coolest plant ever!
That's actually kind of what it sounded like. It was a very long-winded paper! But yes, Barber did mention something regarding energy distribution over the entirety of the plant in differently sun-exposed environments as mentioned previously. If you can bare a 40 page article, please do have a read of it. Particularly the "spillover energy" part.
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