Chlorophyll, The Heart of Natural Solar Power by Emily Lauterpacht

This is a slightly adapted version of an article that I wrote for the school science magazine last year. It is titled "Chlorophyll, The Heart of Natural Solar Power".

The word chlorophyll derives from Greek; chloros, meaning green and phyllon, meaning leaf. It is the pigment in leaves that makes them green. Its main purpose is to act as a catalyst in photosynthesis, the reaction that turns water and carbon dioxide into glucose and oxygen using energy from the sun. Chlorophyll was first isolated by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817. It is made up of carbon, hydrogen, oxygen, nitrogen and magnesium in different proportions, depending on the type of chlorophyll. There are two main types of chlorophyll, type a and type b. They are very similar, but a variation in a side chain of the molecule, normally represented by an R in any diagrams, allows them to “tune” into slightly different parts of the visible light spectrum (the R group of chlorophyll a is CH₃, and of chlorophyll b is CHO).  The main function of the chlorophyll found in leaves is to absorb sunlight, at an efficiency of about 1-2%. This energy is then transferred to a chlorophyll molecule. This absorbed energy excites an electron in the chlorophyll to a higher energy state, meaning it is more easily transferred to carbon dioxide. The electron that the chlorophyll loses is replaced by one from water. Over twenty steps then occur without chlorophyll, called dark reactions, before the glucose is produced. Interestingly, a type of green sea slug, Elysia chlorotica, has evolved so that it uses the chlorophyll it eats from algae to perform photosynthesis itself. This is the only animal known to perform this process, called kleptoplasty. The earth absorbs roughly 89 petawatts (1 petawatt = 1 x 1015watt) of energy from the Sun per year. Humans meanwhile only use about 0.016 petawatts a year, and so if we could find an economical way of harnessing just 0.02% of the energy from the sun, we would have a “free”, green energy source, easily able to cope with the growing energy demands of humans. Solar panels are often used now to generate electricity, but since they do not work at night, they prove problematic. Scientists are also working to increase their efficiency from the ~15% that it is now. Finding a way to mimic photosynthesis would mean that we could start to reduce the amount of greenhouse gases in the atmosphere, instead of adding to them, which is a concern for many scientists around the globe. In recent years, leading scientists have developed ways of making fuel directly from sunlight, but this has involved very expensive catalysts, such as platinum, and the cells used have been nowhere near robust enough to be used outside in unpredictable weather conditions all year round. Professor Daniel Nocera and his colleagues have been perfecting using other catalysts, such as cobalt based compounds and a nickel-molybdenum-zinc alloy, to use the sun’s energy to convert water into hydrogen and oxygen. The hydrogen can then be used as a fuel, and although this would not remove greenhouse gases from the atmosphere, it wouldn’t create more either. You can find a video Professor Nocera’s cells working here. These new cells are cheaper as the catalysts are very abundant on earth, and the cells can be formed by self-assembly, so are easy to manufacture. They are also more durable and the hope is that it will soon be possible to economically produce large quantities. 

The BBC program Horizons reported on Dr Nocera's artificial leaves in 2013, and can be watched here.  

Emily Lauterpacht