With the help of ultra-fast imaging of moving energy in photosynthesis, scientists from Imperial College London (ICL) and the Johannes Kepler University (JKU) in Australia have determined the speed of its key stages, solving a decades-old debate.
This could lead to a better understanding of how plants, algae and certain bacteria have tweaked the process of photosynthesis to make it more efficient, and techniques for replicating it artificially to produce better fuels.
Even though photosynthesis is performed differently by different species (and there are two different types of photosynthesis), it is always triggered by sunlight. As photons are absorbed by proteins called “reaction centres”, water is oxidised (stripped of electrons), producing oxygen gas (released as a by-product of the reaction), hydrogen ions, and electrons.
Most of the removed electrons and hydrogen ions are transferred to carbon dioxide, which is t hen reduced to nicotine adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP) – the cells’ primary source of energy.
The oxidation of water is carried out by an enzyme called Photosystem II. Light energy is harvested by “antennae”, and transferred to the reaction centre of Photosystem II, which strips electrons from water. This conversion of excitation energy into chemical energy, known as “charge separation”, is the first step in splitting water.
Since the discovery of Photosystem II back in 2001, it was thought that the process of charge separation in the reaction centre was a “bottleneck” in photosynthesis – the slowest step in the process – rather than the transfer of energy along the antennae.
Now, however, thanks to ultra-fast imaging of electronic excitation that uses small crystals of Photosystem II, that hypothesis has been overruled.
“We can now see how nature has optimised the physics of converting light energy to fuel, and can probe this process using our new technique of ultra-fast crystal measurements,” said study co-author Dr. Jasper van Thor from the Department of Life Sciences at ICL.
“For example, is it important that the bottleneck occurs at this stage, in order to preserve overall efficiency? Can we mimic it or tune it to make artificial photosynthesis more efficient? These questions, and many others, can now be explored”.
Although the researchers could determine which step is faster, both steps occur incredibly quickly – the whole process takes a matter of nanoseconds (billionths of a second), with the individual steps of energy transfer and charge separation taking only picoseconds (trillionths of a second).
“There had been clues that the earlier models of the bottleneck of photosynthesis were incorrect, but until now we had no direct experimental proof. We can now show that what I was lectured as an undergraduate in the 1990s is no longer supported,” concludes Dr. van Thor.