23.
Energy Transformations: Respiration and Photosynthesis
The Light Reactions: Trapping Solar
Energy
Photosynthesis is simplest in bacteria, and the process
in green and purple sulfur bacteria is diagrammed opposite. Light
energy is absorbed by various antenna molecules and is passed on
to a bacteriochlorophyll molecule (BChl right) in the form of electronic
excitation. The chlorophyll molecule uses these excited electrons
to reduce NAD+ to NADH, passing them first to a flavodoxin
(FD, a nonheme iron protein) and then to a flavoprotein (FP). (Bacteria
use NADH, even in photosynthesis.) The chlorophyll molecule then
is deficient in electrons, but the shortage is compensated for by
an external reducing agent such as H2S. The H2S
is oxidized first to elemental sulfur and ultimately to sulfate.
Protons are released into solution, and electrons are fed into an
electron-transport chain that leads to the bacteriochlorophyll molecule.
This chain contains cytochromes b and c, and other
components such as quinones.
It resembles the electron-transport chain of respiration
in this and in another key property: Some of the energy that is
released when electrons run down the free energy scale from H2S
to chlorophyll is captured and used to synthesize ATP. Thus photosynthetic
bacteria obtain two benefits: energy stored as ATP, and energy and
reducing power combined in NADH.