This should help scientists understand how nature has perfected the process
of photosynthesis, and how this might be copied to produce fuels by artificial
During photosynthesis, plants harvest light and, though a chemical process
involving water and carbon dioxide, convert this into fuel for life.
A vital part of this process is using the light energy to split water into
oxygen and hydrogen. This is done 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
It was previously 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.
Since the structure of Photosystem II was first determined 2001, there was
some suggestion that in fact it could be the energy transfer step that was
slowest, but it was not yet possible to prove experimentally.
Now, using ultrafast imaging of electronic excitations that uses small
crystals of Photosystem II, scientists from Imperial College London and Johannes
Kepler University (JKU) in Austria have shown that the slowest step is in fact
the process through which the plants harvest light and transfer its energy
through the antennae to the reaction centre.
The new insights into the precise mechanics of photosynthesis should help
researchers hoping to copy the efficiency of natural
photosynthesis to produce green fuels. Study author Dr Jasper van Thor, from
the Department of Life Sciences at Imperial, said: "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 ultrafast crystal
"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."
Co-author Dr Thomas Renger from the Department of Theoretical Biophysics at
JKU added: "When we predicted the present model of energy transfer eight years
ago, this prediction was based on a structure-based calculation. Since such
calculations are far from trivial for a system as complex as this, some doubts
remained. The technique invented by Jasper's group at Imperial has allowed us to
remove these doubts and has fully confirmed our predictions."
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).
The team used a sophisticated system of lasers to cause reactions in crystals
of Photosystem II, and then to measure in space and time the movement of
excitations of electrons - and hence the transfer of energy - across the
antennae and reaction centre.
The resulting movie of the movement of excited electrons across minute
sections of the system revealed where energy is held and when it is passed
along. This proved that the initial step of separating charges for the
water-splitting reaction takes place relatively quickly, but that the light
harvesting and transfer pr
ocess is slower.
Dr van Thor added: "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."