BioPHYSICAL chemistry

these molecules. In the photosynthetic membrane, the energy is trans-
ferred in 3 ps to a second protein called the light-harvesting complex I,
which also has rings of proteins and bacteriochlorophylls. Again, the
energy is effectively delocalized among the bacteriochlorophylls of that
complex until transfer to the central site, the bacterial reaction center,
occurs in 35 ps. The last step represents the slowest step of the entire
process, presumably due to the relatively large distance of 35 – 40 Å to
the bacteriochlorophylls of the reaction center.

Electron transfer, bacterial reaction centers, and photosystem I

The reaction center can be considered to be
an energy-transducing device that converts the
energy of sunlight into chemical energy.
Reaction centers are integral membrane
proteins that are largely buried in the cell
membrane. Each reaction center has two core
protein subunits that surround a number of
cofactors. For reaction centers from purple
bacteria, reaction centers have four bac-
teriochlorophylls, two bacteriopheophytins

CHAPTER 20 PHOTOSYNTHESIS 425

(a) Exciton transfer

Excited states

Ground states

Antenna pigment molecules Reaction

center

pigment

molecule

Energy hv

Light-harvesting complex I Light-harvesting complex II

35 ps from ring

to reaction center

Reaction

center

80 fs

within ring

B875 ring

(b)

3 ps between

rings

100 fs

within ring

B850 ring

0.7 ps

between rings

B800 ring

Figure 20.4Energy transfer in photosynthetic bacteria. (a) Light absorption by the
bacteriochlorophylls of the light-harvesting complexes results in the formation of an excited state
followed by energy transfer, or excition transfer, among the different bacteriochlorophylls until the
energy is transferred to the reaction center bacteriochlorophyll dimer. (b) A schematic view of the
energy transfer process in the cell showing the transfer times, which range from 80 fs to 35 ps,
between the different bacteriochlorophylls of the light-harvesting complexes I and II and the
reaction center. Modified from Fleming and van Grondelle (1997).