Scientists shed light on the mystery of photosynthesis

Scientists at the University of Sheffield are part of an international team that has become the first to successfully discover how the component parts of photosynthesis fit together within the cell membrane. In a paper, The native architecture of a photosynthetic membrane, published in Nature on 26 August 2004, they describe how the configuration of the three structures that allow photosynthesis to occur fit together, and find that Mother Nature has developed a much more complex and effective system than was previously thought.

Photosynthesis is the reaction that allows plants and bacteria to take in sunlight and convert it into chemical energy, by reducing carbon dioxide and water into carbohydrates and oxygen. Photosynthesis is the backbone of life on Earth – all the food we eat, the oxygen we breathe and the fossil fuel we burn are products of this reaction.

Professor Neil Hunter from the University of Sheffield explains, “Photosynthesis is the single most important chemical reaction on Earth and it is fascinating to see for the first time how nature has overcome the problem of harvesting and utilising solar energy.

“Although scientists have known the structures of the individual components involved in photosynthesis for some time, this is the first time we have managed to see how they all fit together and work as a system. To achieve this we have used an Atomic Force Microscope, which ‘feels’ the shape of individual molecules and converts this into a picture, to see the system within an individual cell membrane. We have discovered Nature’s way of collecting light for photosynthesis.

“We already knew that during photosynthesis light is collected by an antenna made up of two light harvesting complexes – LH1 and LH2, and then passed to a reaction centre (RC) where it is converted into chemical energy. However, these were like individual jigsaw pieces and we had yet to see the full picture.

“The way photosynthesis works is that groups of LH2 complexes pick up the light, and pass them it around among themselves until the light comes across an LH2 complex which is touching one of the larger LH1 complexes. The energy then circulates around the LH1 complex, or passes to another LH1, until it moves on to the reaction centre.

“We found that the LH2 complexes are structured in an antenna-like shape and when light is scarce they co-operate by joining together to allow them to make the best possible use of the limited light available.

“The LH1 complexes are each attached to their own RC and from looking at the images we believe that if an LH1 takes in light whilst its reaction centre is ‘busy’ then it will keep passing the energy on to neighbouring LH1 complexes, until an unoccupied reaction centre is found.

“We hope to test this particular theory further but the purpose of both of these systems would be to maximise the efficiency of photosynthesis. The process of harvesting light energy is over 95% efficient, which is an incredible figure.

“This work doesn’t only have implications for our understanding of photosynthesis, but also for the future of molecular science. By looking at the world on an individual molecular level scientists have the opportunity to learn more about an incredible number of biological systems and processes.”