Page 154 - Algae Anatomy, Biochemistry, and Biotechnology

Photosynthesis 137

potential indicates a reducing capability of the system (the system possesses available electrons),
while a positive redox potential indicates an oxidizing capability of the system (the system lacks
available electrons).
Photosynthetic activity of algae, which roughly accounts for more than 50% of global photo-
synthesis, make it possible to convert the energy of PAR into biologically usable energy, by
means of reduction and oxidation reactions; hence, photosynthesis and respiration must be regarded
as complex redox processes.
As shown in Equation (3.2), during photosynthesis, carbon is converted from its maximally oxi-
dized state (þ4inCO 2 ) to strongly reduced compounds (0 in carbohydrates, [CH 2 O] n ) using the
light energy.

Chlorophyll a
nCO 2 þ nH 2 O þ light ! (CH 2 O) n þ nO 2 (3:2)
In this equation, light is specified as a substrate, chlorophyll a is a requisite catalytic agent, and
(CH 2 O) n represents organic matter reduced to the level of carbohydrate. These reduced compounds
may be reoxidized to CO 2 during respiration, liberating energy. The process of photosynthetic
electron transport takes place between þ0.82 eV (redox potential of the H 2 O/O 2 couple) and
20.42 eV (redox potential of the CO 2 /CH 2 O couple).
Approximately half of the incident light intensity impinging on the Earth’s surface
(0.42 kW m 22 ) belongs to PAR. In the water, as explained earlier, the useful energy for photo-
biochemical processes is even lower and distributed within a narrower wavelength range. About
95% of the PAR impinging on algal cell is mainly lost due to the absorption by components
other than chloroplasts and the ineffectiveness of the transduction of light energy into chemical
energy. Only 5% of the PAR is used by photosynthetic processes. Despite this high energy
waste, photosynthetic energy transformation is the basic energy-supplying process for algae.

PHOTOSYNTHESIS

Photosynthesis encompasses two major groups of reactions. Those in the first group, the “light-
dependent reactions,” involve the capture of the light energy and its conversion to energy currency
as NADPH and ATP. These reactions are absorption and transfer of photon energy, trapping of this
energy, and generation of a chemical potential. The latter reaction follows two routes: the first one
generates NADPH due to the falling of the high energy excited electron along an electron transport
system; the second one generates ATP by means of a proton gradient across the thylakoid mem-
brane. Water splitting is the source of both electrons and protons. Oxygen is released as a
by-product of the water splitting. The reactions of the second group are the “light-independent reac-
tions,” and involve the sequence of reactions by which this chemical potential is used to fix and
reduce inorganic carbon in triose phosphates (Figure 3.1).

LIGHT DEPENDENT REACTIONS
Photosynthetic light reactions take place in thylakoid membranes where chromophore–protein
complexes and membrane-bound enzymes are situated. The thylakoid membrane cannot be con-
sidered as a rigid, immutable structure. It is rather a highly dynamic system, the molecular compo-
sitions and conformation of which, including the spatial pattern of its components, can change very
rapidly. This flexibility, is, however, combined with a high degree of order necessary for the
energy-transforming processes.
Quantitative analysis established that the 7 nm thick thylakoid membrane consists of approxi-
mately 50% lipids and 50% proteins. Galactolipids, a constituent that is specific of thylakoid
membranes, make up approximately 40% of the lipid fraction. Chlorophylls a, b, c 1 and c 2 , phos-
pholipids, sulfolipids, carotenoids, xanthophylls, quinones, and sterols, all components occurring in