L9. Biomass Composition & Chemistry

In the last lecture we discussed how plants are able to convert carbon dioxide into sugar via photosynthesis. More specifically, 6 carbon dioxide molecules and 6 water molecules plus the energy from the sun are converted into 1 sugar molecule (glucose) and 6 oxygen molecules. The chemical equation for photosynthesis looks like this:

Every molecule in the equation above is made up of just three elements: C (Carbon), H (Hydrogen), and O (Oxygen). These are three of the most common elements that make up all living things, including humans, animals, and plants. The 4th most common element in living organisms is N (Nitrogen). The amazing thing about chemistry is that these 4 simple elements (plus a few others) can be rearranged and reconnected in many ways, giving rise to the vast number of molecules that make up everything from the human body to the trunk of a tree.

 

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Glucose is FUN(damental)

The sugar that’s created during photosynthesis is called glucose. Glucose is the most abundant simple sugar on Earth, and the very base of the food chain. ALL biological energy is originally captured from the sun by plants as glucose via photosynthesis, and every cell uses glucose to get energy via a process called respiration. Both animals and plants use respiration to generate the energy needed to grow. If you look at the image below you’ll notice that respiration is the reverse of photosynthesis: glucose and oxygen in → cell carbon dioxide, water, and energy out. Which means plants also use oxygen and create carbon dioxide. Until now, we’ve only discussed plants as a carbon sink taking in carbon dioxide, but they can also be a carbon source as they respire carbon dioxide. Good news is that they take in more more CO₂ than they release.

Glucose has 5 hydroxyl groups (OH combination also referred to as alcohol group) arranged in a specific way along what chemists call a 6-carbon backbone. Glucose molecules are usually shaped like a ring, as shown in the diagram below. If you’ll remember from your high school chemistry class, each of the letters represents an element (carbon, oxygen, or hydrogen) and the lines represent the covalent bond Links to an external site. between the elements.

Glucose is a monomer - which is a generic term for any simple molecule that can combine to form more complex molecules called polymers. Mono = 1, poly = many. The process by which monomers react to form chains is called polymerization. So let’s say you have a monomer called ethylene - what do you call it after it undergoes polymerization? Polyethylene. (Side note: polyethylene is the is the most popular plastic in the world. This is the polymer that makes grocery bags, shampoo bottles, children's toys, and even bullet proof vests.)

To be more specific, glucose is a monosaccharide - a simple sugar (saccharide means sugar). When a monosaccharide polymerizes, it becomes…. you guessed it! A polysaccharide. So monomer and polymer are generic terms, while monosaccharide and polysaccharide specifically refer to sugars. In this way, we can understand glucose as a building block for more complex molecules.

Glucose molecules can exist in one of two forms: alpha (α) or beta (β). The only difference between the two is the position of the hydroxyl group (OH) on the first carbon, which is circled in the image below:

This is a small difference that has BIG impacts. When alpha-glucose polymerizes (joins with others) it becomes a polysaccharide called starch, but when beta glucose polymerizes it becomes cellulose. You can see in the image below that both cellulose and starch are made up of glucose molecules linked together in a chain, but that the geometry of the molecules differs slightly. (It's like one of those picture puzzles: How many things are different in these two drawings?)

Starch and cellulose are just two of the many components that make up biomass. Let's take a look at some of the others.

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Biomass Composition

In the last lecture, you learned that the growth patterns of a tree are important because they ultimately determine what kind of products we can make. For example, we can make a table out of the heartwood of a tree, but not so much the bark.

The same idea applies on a microscopic scale - some components of biomass are easy to convert into bioproducts, and others are difficult and costly to extract. Learning about the composition of biomass sets us up to better understand the processes by which different bioproducts are made. The following molecules are all components of both woody and herbaceous biomass:

• Starch is the molecule plants use to store energy in the fruits, roots/tubbers (e.g. carrots and potatoes), and grains (e.g. wheat). The bonds that hold the glucoses together in a chain are bent, which prevents the starch molecules from forming sheets. As a result, starch is soluble in water and is easy to break down into usable glucose monomers, meaning that humans can digest starch. It also means that starch is easily recovered and converted from grains into products. Starch from corn grain is the primary feedstock (another word for raw material) for today’s sugar-based bioproducts such as ethanol fuel.
• Cellulose is the most common structural compound in plants, and the most abundant organic compound on the planet. Bonds form between the hydrogens on the chains of glucose, which leads to the formation of flat sheets. These sheets lay on top of each other in a staggered pattern, similar to the way we stagger bricks to build a wall. And just like bricks, cellulose molecules are what provide strength to the cell walls in plants (about half of the cell wall is made of cellulose). Since cellulose is so sturdy, it is insoluble in water and very difficult to break down. Cows, and other ruminant animals, have a special digestive system that can digest cellulose (this process generates methane - a greenhouse gas)  The human digestion system cannot break down cellulose which is why we eat bread, not grass. The strength of the cellulose fibers makes it useful for products like cotton and paper. However, converting cellulose into glucose molecules that can be used for bioproducts is difficult, so making a bioproduct, like ethanol, out of cellulose is much more expensive and time-intensive than with starch.
• Hemicellulose is also a polysaccharide like starch and cellulose. But instead of being made up of glucose monomers, it’s made of 5-carbon sugars such as xylose. Hemicellulose makes up about ¼ of the cell wall, and forms a matrix with cellulose. This helps connect and strengthen the cell wall. Hemicellulose is much easier to process than cellulose (but still more challenging than starch), and is found in many food products and cosmetics.

• Lignin is NOT a polysaccharide like the other components listed above. It is a plastic-like polymer that is more similar to polystyrene (what Styrofoam is made out of) or some polyesters (like the ones used to make plastic bottles). It makes up about ¼ of the cell wall, and helps stiffen and protect the cellulose and hemicellulose. It is not digestible, even by cows, and does not easily degrade. Because of the complex structure and characteristics  it is not really used in the bioproducts industry today. In the paper industry lignin is removed as it discolors the paper. It is then used as a bioenergy source back in the paper manufacturing process. Many are working hard at finding uses for this unique molecule.
• Oils are found in the seeds of certain plants like soybeans and rapeseed. Vegetable oils are composed primarily of triglycerides. Again a molecule with just carbon, hydrogen and oxygen but in a different formation (see below). Notice that triglycerides are not made of circular structures like cellulose and starch, but instead of long, straight carbon chains. A large portion of the oil recovered from oilseeds is processed for human or animal consumption, but it can also be used for things like lubricants and hydraulic fluids.

Just to show you that this chemistry is real but not all that complex, take a look at this short video that demonstrates one method of separating these different components from the wood. The system looks complicated, but basically it works just like a coffee maker. As water drips through the coffee, the water soluble components of the coffee are extracted from the ground coffee bean. If we used other chemicals (e.g. acetone) in our coffee machine we could remove (extract) other chemicals from the coffee beans. What he is doing here is using multiple chemicals in sequence to remove everything BUT the lignin. In this first step he is using water and extracting "extractives". These are some chemicals like waxes, fatty acids, resins and other compounds in the wood - also used for bioproducts. 

 

It is important to note that these biomass components exist in different percentages in different plants. For example, the trunks of softwood trees have more lignin (as a percentage) than hardwood trees. A coconut seed has a much higher oil content than a corn grain. The two tables below show the variation in the biomass composition of certain plant species.

Table 1. Composition of various biomass sources (Biswas et al. 2015 Links to an external site.)

Biomass source	% Cellulose	% Hemicellulose	% Lignin
Corn stover	37-42	20-28	18-22
Wheat straw	31-44	22-24	16-24
Rice straw	32-41	15-24	10-18
Hardwood	40-45	18-40	18-28
Softwood	34-50	21-35	28-35
Switchgrass	33-46	22-32	12-23

 

Table 2. Oil content in various oilseeds

Oilseed	% Oil
Soybean	17-21
Corn	3-6
Canola	43-47
Coconut	65-68
Cottonseed	15-24
Palm Kernel	49
Sunflower	40-50

 

These proportions of different moledules is important to the bioproducts industry because it determines which plants are best to use for which products. As discussed above, corn grains have been preferred to corn stover (the leaves and corn stalks) for ethanol fuel production because it is easier to obtain glucose from starch than cellulose. Similarly, seeds with higher oil content are preferred for the production of lubricants or biodiesel, because you can get more of the desired product out of them. The table below shows some examples of what bioproducts can be made from sugars versus oils.

Table 3. Some typical biproducts from sugars and oils.

Biomass component	Chemical	Applications
Sugars (from any polysaccharide: starch, cellulose, etc.)	Lactic acid	Food/drink flavoring, textile dyeing
	1,3-Propanediol	Apparel, upholstery, resins
	Succinic acid	Surfactants, detergents, food, pharmaceuticals, antibiotics, vitamins
	Polyactide	Packaging, fiber & fiberfill
Oils	Lubricants & hydraulic fluids	Lubricants & hydraulic fluids
	Polyurethanes	Foams, plastics, varnish

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This lesson has been only an introduction to chemistry, the composition of biomass, and some challenges we face when processing different components. How we actually convert biomass feedstocks into bioproducts and bioenergy will be discussed in much greater depth later in the semester. For now, it is important that you understand the basics of biomass. In the last four lectures you have been introduced to biomass resources and production systems, and you have learned about how biomass grows and what it is comprised of. This knowledge will serve as a toolkit for you as you navigate the rest of the course.

In the following unit, we will begin our discussion on forest resources and products.