Unlocking the biofuel energy stored in plant cell walls

Cross section of freshly cut sugar cane.

By virtue of their chloroplasts, plants are superb harvesters of solar energy. They use it to build leaves, flowers, fruits, stems, and roots. We harvest a small percentage of that energy in the form of food and a smaller amount in the form of wood for heating.

But the vast majority of plant biomass goes unused by us. In every agricultural region on Earth, huge amounts of the structural parts of crops — things like cornstalks, sugar canes, beanstalks, and wheat stems — are discarded because we haven’t figured out a way to convert them into fuel.

Plant scientist Daniel Cosgrove, who has devoted decades to studying the cell walls that make plant matter resistant to chemical conversion, thinks it doesn’t have to be that way.

Given the scale of our need for energy and the size of that untapped, renewable resource, he says, “plant cell walls are the obvious target.”

Unlocking plant energy

We are expert at using the structural parts of plants to make things — wood for buildings and furniture, flax and cotton for clothing — but when it comes to using them for energy, we haven’t progressed much beyond the Neanderthal stage: We burn them.

“I have a wood pellet stove at home, and with the cold spell we’ve got right now, we’re cranking through a lot of those wood pellets to keep warm,” says Cosgrove, holder of the Eberly Chair in Biology. “The problem is, unless you want to go to an old steam engine or something of that fashion, it’s hard to harvest the energy in wood pellets to power an automobile.”

Sugar Cane Research-22-DCosgrove
Sugar cane towers over Penn State biologist Dan Cosgrove in the greenhouse at Buckhout Lab. Cosgrove, a member of the National Academy of Sciences, studies how plant cells build, expand, and maintain their cell walls. Image: Patrick Mansell

What we need, he says, is a liquid fuel, preferably one that’s carbon-neutral, can be mass-produced, and is compatible with present-day internal-combustion engines. The closest we’ve come is ethanol, made mainly through fermentation of corn, which we then mix with petroleum gasoline. But corn ethanol is not a long-term solution. It diverts prime agricultural land away from food production, decreases ecosystem diversity, and, when production and transportation are considered, it is not carbon-neutral.

It also misses the point. We make ethanol from the starch inside corn kernels. The millions of tons of corn stalks are often used for erosion control or plowed under to return nutrients to the soil, but the energy stored in chemical bonds in their cell walls remains untapped, because we don’t know how to break down the walls and release it. Biofuel crops such as poplar and switchgrass are less valuable sources of energy than they could be, for the same reason.

“The problem is, we don’t understand cell wall structure well enough to approach its conversion scientifically,” says Cosgrove.

The U.S. Department of Energy (DOE) agrees with him. In 2009, the agency funded three Energy Frontier Research Centers geared toward finding a good way to turn wood and fibrous plant material into liquid fuel. Two of the programs focus on trying to break down cell walls. The third, the Center for Lignocellulose Structure and Formation headquartered at Penn State, looks at the problem from the opposite perspective: how cell walls are made in the first place. The idea is that understanding how cell walls are made will make it easier for us to take them apart.

“We have a unique angle and a unique group of investigators, mostly from Penn State but also from five other institutions,” says Cosgrove, the Center’s director. “We’re doing a variety of trans-disciplinary work that involves physicists and computational modelers and biologists and geneticists. We’re interested in the fundamental problems of how cell walls are put together, because it’s not just biochemistry that determines cell wall properties.”

How your garden grows

The mystery of plant structure starts with how plants grow. In specific zones at the tips of the stems, shoots, and buds, new cells are added through proliferation: The cells take in nutrients, approximately double in size, and then divide to form two daughter cells, which themselves grow and divide. When the structure or organism reaches full size, both cell expansion and cell division stop. That kind of growth is much like what happens in animals.

But other parts of a plant grow, sometimes massively, without cell division. The stem gets longer and thicker because the cells in it elongate and expand. This means of growth occurs in all plants, from petunias to redwoods — and it is essential for plants to attain large size. If plant cells didn’t get larger, if they stayed the same size they were in the seedling, the landscape would look much different.

“Someone has calculated that if the tallest tree in the world, a redwood tree, grew the way your liver cells grow, it would be about hip-high, waist-high. About three feet high,” says Cosgrove.

Let it flow

The puzzling thing is how, exactly, plant cells expand so much. Each cell is enclosed by a flexible cell membrane and, just outside that, by a cell wall, a box-like structure that is as strong and tough as it sounds.

Most major constituents of the cell wall have been known for almost 200 years. Perhaps the most familiar is cellulose, the indigestible-but-good-for-colon-health polymer we know as “dietary fiber.” In its most basic form, cellulose is a long chain of glucose (sugar) molecules. Cell walls contain dense layers of cellulose microfibrils, each one made of two dozen of these chains adhering to each other.

sugar cane cells stained blue
Freshly cut piece of sugar cane, stained with toluidine blue to make the cells easier to see. Each cell is surrounded by a thin, fibrous cell wall. The two large, dark areas are vascular bundles that transport water and sugars.