Photosynthesis

No other chemical process is as crucial for the existence of life as photosynthesis. Photosynthesis is the process of converting light energy to chemical energy. This Mini Lecture deals with the basic processes taking place in the chloroplasts of plants with lecture snippets of John Walker, Hartmut Michel, and Steven Chu.

Every mammal and thus every human needs oxygen to live. The gas makes up around 20 percent of the air around us. But where does it come from? It’s produced by photosynthesis a complex physical-chemical reaction that takes place in the leaves of plants. No other chemical process is as crucial for the existence of life on Earth. Plants don’t only generate the air we breathe, they are also so-called ‘primary producers’ that use inorganic materials to make the biomass that feeds other organisms. Without plants no food and no tertiary consumers like us humans. Photosynthesis captures the energy from sunlight to reduce carbon dioxide to sugars. This process involves splitting water. The photosynthetic process converts light energy into chemical energy. The all-important oxygen that results from the water splitting is merely a by-product. This process by which biology harvests energy is photosynthesis. And it’s an incredibly complex process but consists of 2 separate reactions, the so-called light reaction where solar energy is used to split water and generate oxygen and reducing equivalents. And of course without this reaction there would be no life on Earth. The reaction takes place in cell structures called chloroplasts. A single square millimeter on the surface of a leaf may have a half million of these kidney-shaped organelles. They contain the pigment chlorophyll which gives the leaf its green color. Two forms, chlorophyll a and b, along with other pigments called carotenoids, have a very special ability. Thanks to their chemical structure they are able to absorb sunlight and harness the captured energy to drive biochemical processes. Sunlight contains a broad spectrum of different wavelengths. When it strikes matter can be reflected, transmitted or absorbed. The wavelengths that are absorbed by a leaf are eliminated from the reflected or transmitted residual light. Chlorophyll absorbs light in the red and blue regions of the spectrum while green light in the wavelength range of around 500 to 570 nanometers is reflected. That’s why leaves appear green. Photosynthesis consists of two coupled reaction chains. In the first one, called the ‘light reaction’, the photons of the sunlight excite electrons in chlorophyll into to an energetically higher state. These electrons are captured by so-called ‘acceptors’ and passed on through a series of transport steps. The missing electrons in the chlorophyll molecules are replaced by electrons produced by the splitting of water molecules. This process releases the oxygen that we breathe in from the atmosphere. Two different complexes in the chloroplast membrane called ‘photosystems’ are excited by sunlight and the energy they capture is used to build up the energy-rich compounds adenosine triphosphate and NADPH along with a proton, H+ The electron flow in the photosynthetic membranes of chloroplasts and also cyanobacteria is seen here. You have in the plant and in cyanobacteria, you have first photo system 2. Photo system 2 is where the water splitting occurs with the release of the oxygen. You get a transfer of the electrons across the membrane. And there the electron moves on to another complex. Then you’re going to a PC1 complex where you transfer the electrons back across the membrane. You produce another molecule, called plastocyanin, this donates electrons to the photo system 1. Then the electrons are transferred here to another, to ferredoxins and at the end you reduce NADPH, which is a co-enzyme. So this is what happens in the light reaction. These molecules are used in the second set of photosynthetic reactions, the Calvin Cycle, to synthesize the sugar, glucose. The carbon dioxide needed for that is taken in through pores, or stomata, in the epidermis of the leaf. A certain protein, called the RuBisCo protein for short, is particularly important in the Calvin cycle. It catalyzes the first major step in the cascade of reactions that “fixates” the CO2 diffusing in through the stomata. RuBisCo is estimated to be the most abundant protein on earth. So, photosynthesis uses a highly elegant method of converting solar energy into chemical energy. We humans can indeed be jealous. If we were able to mimic the principles of photosynthesis on a large scale, it could have enormous potential for generating clean energy. Scientists around the world are working intensely on a way of imitating the process a collaboration of more than 160 researchers from various disciplines and universities in the United States. The project was founded in 2010. The researchers are focusing on developing high-performance photocatalysts based on the natural process of photosynthesis. Photocatalysts speed up light-triggered decomposition reactions that can be used to generate energy The idea is ultimately to produce a kind of “solar fuel”. Our goal is to produce an electric vehicle by 2022 that would cost... a 4 or 5 passenger car that would cost about $20,000/$25,000. And we call this 'EV Everywhere'. We got the president to announce this. So this was a good coup for us. You could say, well, you know electric cars, who wants to buy electric car, not a very exciting tiny thing. But let me tell you, I have a friend who owns one of these babies. This is a Tesla S, a very, very exciting car. You know, I don’t know why, who wants to go to 0-60 in 4.5 seconds but that’s what it does. And it goes 300 miles on a single charge. And if you look on the inside, that big thing in there is an LED display touch screen that is like a humongous iPhone. And you can touch it and it’s as intuitive as an iPhone. My friend... I’ve ridden in this a couple of times and he just raves about it. He loves this car. But this car costs nearly $80,000. And so you’ve got to get the price of the car down. The performance is there, it has more luggage space than a normal car. It has a front and a back trunk because the battery is underneath. One major problem is still how best to store the generated energy. Research is also focused on finding the appropriate materials for inexpensive and sustainable catalysts. A prototype of an artificial leaf already exists and in fact it has an energy yield ten times that of a normal leaf. Artificial photosynthesis could generate oxygen in extraterrestrial colonies or here on Earth provide entire towns in the developing world with electricity –at least that is the vision. But whether it will be realized remains to be seen. One thing is clear: the Age of Photosynthesis, a three billion year old natural processs, has really only just begun.