Light harvesting system

As Alfred Holzwarth explains in detail in the next chapter, photosynthetic organisms have evolved light-harvesting (LH) antenna systems that service photosynthetic reaction centres so that they can operate efficiently under relatively low light intensities. The nature of these LH systems varies considerably according to the type of organism, but all function to intercept light and transfer the excitation energy rapidly to the reaction centre. The process is efficient, so the overall transfer rate must be faster than the singlet lifetimes of the pigments, which are typically in the nanosecond time domain. In fact, overall transfer times of energy migration from the

LH system to the RC are in the sub-nanosecond time domain, and in most cases transfer seems to occur by the Forster resonance mechanism. This requires good overlap between the absorption and emission spectra of the pigments, location of each pair of energy donor and acceptor pigment molecules to be close (typically within 10- 15 8, centre-to-centre) and with appropriate orientations. To achieve these properties, the pigment molecules are bound to a protein scaffold and these pigment-proteins associate with the RC. The number of light-harvesting pigmcnt molecules servicing an RC varies according to the type of organism and the growth conditions, from 50 (in some purple photosynthetic bacteria) to many thousands (as in the case of the chlorosome of green sulphur bacteria). In plants and algae, the number is around 250 pigment molecules per reaction centre.
 

The LH system and RC together comprise the photosynthetic unit. In the case of higher plants and green algae, the pigments bound to LH proteins are chlorophyll a, chlorophyll b and carotenoids. In addition to chlorophyll a and carotenoids, red algae contain the phycobilin pigments that covalently bind to protein to form the phycobilisomes. large macromolecular structures that attach to the outer (stromnl) surface of the photosynthetic membrane. Cryptomonads also contain phycobilins but they do not associate to form phycobilisomes and are located on the other sidc of the membrane.

Like red algae, brown algae, dinoflagellates and diatoms do not contain chlorophyll b, but differ again in that they contain chlorophyll c as an LH pigment as well as chlorophyll a and carotenoids. Cyanobacteria also do nut contain chlorophyll 6. but like red algae they contain phycobiliproteins that assemble into phycobilisomes. However, as mentioned in Section 1.2, there are related prokaryotic organisms (oxyphotobacteria) known as prochlorophytes that do not contain phycobilins but instead have an LH system composed of chlorophyll a and chlorophyll 6. In contrast, the purple and green sulphur bacteria contain different forms of bacteriuchlorophyll and carotenoids.
 

The photosynthetic unit is a marvellously tuned sunlight-gathering apparatus. The different spectral properties of the wide range of LH pigments. coupled with finetuning of the IR spectra by interactions with the proteins to which they bind, allow photosynthetic organisms to absorb at all the wavelengths available in the solar spectrum at the Earth’s surface (35C-I000 nm).