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Making photosynthesis more energy efficient

Solar cells might be more efficient than plants at using the sun's energy. But researchers say there are ways scientists can give photosynthesis an energy-efficient boost.

Solar cells might be more efficient than plants at using the sun's energy. But researchers at Michigan State University say there are ways scientists can give photosynthesis an energy-efficient boost.

I spoke last week with David Kramer, an MSU researcher and an author of a recent paper that compares the two processes -- and suggests methods for improvement.

You found that solar cells are more efficient than plants at harvesting the sun's energy. Why is that?

The physics of energy capture for plants and photovoltaics is similar, but they have to do different things. The photovoltaic system is designed by engineers basically to make a profit. It just has to put electrons down a wire to power our instruments. In contrast, photosynthetic systems have to work within living things. There are things more important to living things than energy efficiency. It has to protect its DNA, for example. If you make a photosynthetic system that produces toxic by-products that destroy or damage the DNA, that wouldn't work. It has to be conservative to preserve the future.

Photosynthesis didn't evolve to make biofuels for us or even to be the most efficient system all the time in producing energy for life. But it evolved to allow these organisms to colonize new niches. This is where legacy biochemistry comes in. You had living things already established and photosynthesis came on and it had to mesh with this. The kind of light that can be used by photosynthesis is very energetic. Once you move electrons in photosynthesis, you can use that electricity to drive other reactions. You have a large amount of energy available to go into a system that evolved to use small energy gradients. The batteries of life are rather low energy. One of the main reasons why photosynthesis is less efficient is it has to work within an organism and this organism already evolved to handle lower energy inputs.

Why was this important to find out and understand?

One of the critical issues of our lifetime is: How are we going to power our machines and our lifestyle without disturbing the environment to the point where it severely affects us? Renewable energy sources are a big thing. The scale of the energy we use is amazing. The solutions we're going to come up with are massive. They're going to be very expensive. The question is: Where do we as a society put our resources? Do we put them into making plants better? Do we put them in photovoltaics? There's been an ongoing argument about which one is better. What are the potentials for improvements in each of these? It's probably the case that in the future we'll have lots of different energy sources for different purposes.

The other [reason] is understanding why living things do the things they do. We have a similar situation in our bodies, for example. In our case, we're using sugars or other energy sources. We're breaking them down. Some of the processes are similar. In our case, when the mitochondria don't do things properly, they also make reactive oxygen species. This is important for diseases of aging. From a basic point of view, both in plants and in us, the questions of how energy is transduced and why it's not as efficient are critical.

How can scientists improve the efficiency of photosynthesis?

There are lots of ways. There are large differences in the inherent efficiencies of plants or algae. A lot of these are more efficient producers of biomass than our crop plants. Even without radical change in the properties of photosynthesis, at least some gain is possible. Over the past millennium, humans have bred plants for certain purposes -- mostly for increasing yield. We haven't made a concerted effort to breed plants that produce huge amounts of biomass. In a lot of cases, we've actually bred for plants that weren't as big. At least some of these have lower photosynthetic efficiency. We're just starting at this as a process for trying to harness high bio-energy yields. There are some wild species that have a strategy of growing fast. These are a good starting place. These are where some of these biofuel crop ideas come from. What are the differences between these plants and crops which are less efficient? That's one area of research that could bring the most immediate gains.

Then you have more far-reaching, radical suggestions for where to go. There are a lot of different ideas. One of the slowest reactions in photosynthesis is RuBisCO. RuBisCO is slow and it needs a lot of CO2 to run efficiently. When it's presented with CO2 and oxygen, sometimes it can't tell between the two. This causes a problem. It costs it a lot of energy. Some plants deal with this by pumping CO2 into the compartment where this enzyme is to make it run faster. There are some algae that have different CO2 pumps in them. One thought is to take those pumps and put them into crop plants.

There are people who think we should replace this enzyme with a completely different one. Nature has generated some other ways of fixing CO2. The question is: Can you put those into plants and make that work? But you'd have to re-engineer all the biochemistry to fit that product.

Photo, top: Courtesy of U.S. Department of Energy National Laboratory

Photo, bottom: David Kramer

This post was originally published on Smartplanet.com