The photosynthetic unit

Before delving into the details of specific light-harvesting systems in photosynthetic organisms, we will first consider the general concept of a photosynthetic unit (PSU). As Fig. 2.1 shows, a PSU consists of an array of light-absorbing chromophores

arranged around a reaction centre. The antenna chromophores may be arranged regularly or irregularly in space, as we shall see later when discussing specific systems. There exist two essential requirements for the functioning of an antenna system. These are: (1) the distances between the chromophores must be short enough in order to allow sufficient electronic interaction for fast, efficient transfer of energy to occur between individual chromophores; and (2) the ordering of their excited-state energies must be such that there is sufficient energetic overlap between adjacent chromophores for energy transfer to occur at physiological temperatures. While there exist additional limitations for an efficient antenna system, the presently known structures tell us that these essential requirements can be satisfied by a very wide range of spatial arrangements of pigments, pigment compositions erc. As we shall see when we discuss specific antennae systems, typical interchromophore distances are in the range 7-15 A, and the number of antenna chromophores in a photosynthetic antenna unit ranges from a minimum of 32 in the LHI antenna of purple photosynthetic bacteria to several hundred per reaction centre in higher plants and algae, and possibly even higher in green photosynthetic bacteria.