Photosynthetic membranes are common to all green plants. They are configured as stacks of tiny sacs encasing an aqueous lumen. The membranes use light energy to split water molecules into electrons, protons and oxygen. Moving left to right, and across the membrane fragment shown here, electrons from split water molecules are carried ‘uphill’ to NADP+ thus forming NADPH. Protons accumulated within the lumen power ATP synthesis. These two energy-storage substances, NADPH and ATP, then drive food production. Our new insights on PSI help explain these events in stressful environments. Image: Sharyn Wragg

Sunlight absorbed by green plants powers our entire biosphere, and the evolution of that capacity two billion years ago has had profound implications for life on earth. Photosynthetic energy conversion might well be called the single most significant photochemical reaction on our planet. All human life depends on it.

Embedded within the chloroplasts of mesophyll cells, chlorophyll molecules absorb photons and use their energy to split water molecules, release oxygen and instigate an electron flow across a photosynthetic membrane. With the conceptualisation of a photosynthetic electron transport chain known as the ‘Z scheme' in the early 1960's, global research into energy capture burgeoned. Australian research was at the forefront of this emerging field. More recently, Professor Fred Chow and colleagues in the Photobioenergetics group from the School of Biology have been applying their extensive skills in biophysics and photochemistry to research the functional details of how light-energy conversion is achieved by these membranes in living tissue.

Fred Chow recognised that photosynthetic efficiency at low light gives plants a selective advantage, but those attributes also render plants vulnerable to strong sunlight, especially under environmental stress. To survive, plants require two separate photo systems working in tandem. An energy dissipating mechanism is required in strong light, and Photosystem I (PS I) appears pivotal in this regard. Unlike Photosystem II (PS II), PS I is able to sustain a cyclic light-driven electron flow that does not involve generation of highly reactive oxygen species that would otherwise call for additional biochemical machinery for their harmless dissipation.

To confirm the special role of PS I, the group devised a technique based on light-induced absorbance changes at a wavelength of 820 nanometres, to gauge photosystem activity and estimate electron flux around PS I vis a vis PS II.

Moreover, in an extension of this same methodology, where photosynthetic electron flow was manipulated, they have discovered that the relative abundance of PS II and PS I in living leaves is typically 1.6, rather than 0.9 as previously thought; thus resolving a long-running controversy as to the composition of major components of a typical photosynthetic membrane

While this new knowledge revises traditional paradigms, Fred Chow and his team are especially pleased to have demonstrated that PS I activity is sustained under environmental conditions that would render PS II dysfunctional. Plants are thus equipped with an energy dissipating mechanism that is free of adverse side reactions, and less vulnerable to photo-oxidative damage in strong sun.

Further Info Online:

• http://www.rsbs.anu.edu.au/ResearchGroups/PBE/profiles/Fred_Chow