Biomedical Engineering Reference
In-Depth Information
1
Light Conversion in Photosynthetic Organisms
S. Frigerio, R. Bassi, and G.M. Giacometti
1.1 Introduction
The sun is the ultimate source of free energy that drives all of the processes
in living cells. The radiant energy of the sun is captured and converted to
chemical energy by photosynthesis. The flux of carbon through the biosphere
begins with photosynthesis. Photosynthetic organisms produce carbohydrates
and molecular oxygen from carbon dioxide and water:
6H
2
O+6CO
2
+ light
→
(CH
2
O)6 pedice + 6O
2
(1.1)
The carbohydrates produced by photosynthesis serve as the energy source for
other non-photosynthetic (heterotrophic) organisms. In this process, carbo-
hydrates are recycled to carbon dioxide and water by the combined action of
cellular catabolic processes.
The
fixation
of carbon dioxide into sugars requires free energy in the form
of ATP and reducing power in the form of NADPH. The light reactions of
photosynthesis respond to this need: the visible component of solar radiation is
captured and its energy is converted into ATP and NADPH through a complex
series of redox reactions and membrane-mediated energy conversions.
The following reactions, referred to as
dark phase
of photosynthesis, as,
in principle, they do not need solar radiation to be carried out if NADPH
and ATP are provided, drive the reduction of CO
2
to the carbohydrate GAP
(glyceraldehyde-3-phosphate). The series of reactions are altogether indicated
as the Calvin-Benson cycle [1].
The enzyme directly responsible for the addition of CO
2
to ribulose 1,5 -
bisphosphate is RubisCO [2]. The whole carboxylation cycle requires three
CO
2
molecules to generate a molecule of GAP, which is then used for the
synthesis of more complex sugars or other compounds. The energy required
for this cycle is nine molecules of ATP and six of NADPH, as summarized in
the following equation:
GAP + 9ADP + 8P
i
+ 6NADP
+
3CO
2
+ 9ATP + 6NADPH
→
(1.2)
