214 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY
Figure 10.15AFM images of membranes showing that they have large
structures in circular configurations. From Bahatyrova et al. (2004).
Resistance =(voltage/current)
The ability of AFM to provide detailed images of the organization of
biological cells has been demonstrated by its use to obtain images of
membranes from photosynthetic bacteria (Figure 10.15; Bahatyrova et al.
2004; Scheuring & Sturgis 2005; Muller et al. 2006). In photosynthesis,
the harvesting of light energy is performed by large arrays of chlorophyll–
protein complexes (Chapter 20). In purple bacteria, two types of light-
harvesting complex are present, identified as light-harvesting complexes
I and II. Both of these complexes contain bacteriochlorophylls and caro-
tenoids that absorb the light and transfer the energy to a central protein
complex, the reaction center, that performs the initial photochemistry.
All three of these protein complexes are embedded within the cell mem-
brane. The light-harvesting complexes I and II are oligomeric with eight
to nine and 16 pairs of subunits arranged in a ring, with a single reaction
center found within each light-harvesting I ring. The structures of the
individual, purified complexes have been determined using protein crys-
tallography but the cellular arrangements have been largely unknown.
