Whether they are green algae (chlorella) or the higher plants, autotrophs in general are gifted in nature to fix carbon dioxide and produce biomass. In ecologic terms, these are producers. The dominant autotroph is pho­totrophic. Photosynthesis has two distinct aspects: the light dependent step, where photolysis of water takes place:

ADP + Pi ——^ ATP

During this reaction, oxygen is set free, Co II is reduced and phosphory­lation of ATP takes place. The photoenergy is chemically utilized twofold.

In the next step, through a very complex enzymic sequence, CO2 is incorporated into the existing metabolite pool and higher carbohydrates are biosynthesized. This step of the reaction finds variation in different species; carbohydrates, proteins, and lipids are biosynthesized. Then, the first part of the reaction makes the autotrophs unique. Light falling on chloroplasts develops an electrical field across the membrane.

In the presence of the pigments in chloroplasts, the light energy is trapped and activates water and lyses it. Ideally, water, if converted into its elemental components, requires (at 25°C) 68.3 kcal/mol (from liquid) or 57.8 kcal/mol (from vapor). Thermal energy is not sufficient to bring this change. During photolysis, the plant pigment augments electron flow, and the electron flow system culminates in the two energy-rich chemical prod­ucts (reduced Co II, ATP, and O2), as already mentioned (see Fig. 1.7).

Lester Packer’s group at the University of California at Berkeley has shown the steps of the pathway with chloroplasts from spinach leaves,

ferredoxin from Spirulina, and hydrogenase of Clostridium pasteuri — anum [3].

2H, O ^ 4H + 4e + O2
4 Ferredoxin+ + 4e~ ^ 4 Ferredoxin
4H+ + 4 Ferredoxin ^ 2H2 + 4 Ferredoxin+

O2 + Glucose ^ Gluconate + H2O2 H2O2 + Ethanol ^ 2H2O + Acetaldehyde The overall reaction is

Glucose + Ethanol ^ Gluconate + Acetaldehyde + H2

Two H2 are produced for each O2 produced (if not consumed by an oxidase-type reaction as shown previously). Dr John Benemann of the same university has also suggested that hydrogen and methane pro­duction is possible by designing a two-stage system separated from each other (see Fig. 1.8).

Dibromothymoquinone blocks the natural electron flow system at plastocyanin level (see Fig. 1.9). Thus, in the presence of an artificial donor or acceptor, the photo systems I and II can be separated at pre — and post-blocking points.