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Can plants break down cellulose for energy?

Can plants break down cellulose for energy?


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I know humans and other animals start using their own proteins as food when starving. This made me wonder if a plant that is deprived of sunlight, after using up its sugar reserves and other carbs, could break down parts of its own woody structure in order to feed the rest of itself.

Googling yielded no useful results, which leads me to think the answer is no. I've never heard of plants themselves possessing cellulase, only certain animals.

Additionally, I would imagine that cellulose is not easy to transport, so it would have to be decomposed by the region of tissue surrounding it - which obviously couldn't last for long before that region died due to cannibalizing itself.


Plants do not produce extra cellulase to break down cellulose for an energy boost when they are grown in dark conditions (that I know of). But remember, cell walls are the structures maintaining turgor pressure in the cells, so breaking them down would be very costly to plant survival. A couple notes: plant cell walls have more components than just cellulose, and they ARE dynamic. In dark-grown conditions, plants elongate (etiolation) during which time, enzymes (expansions) break down cell wall bonds to allow the cells to elongate the stems of the plants. Elongation is an evolutionary advantage to find light when things get shady.

To read more about what the cell wall is made of, and the function it provides to the cell, try watching the Khan Acadamy video about cell walls. Light signaling is an entire field in Plant Biology (example). Read more, if you are interested in how immobile plants handle shading from other plants or periods of darkness. Plants do break down proteins for reuse! Read more about plant vacuoles if you're interested in how they recycle and break down some molecules and proteins when needed (related to your question's parallel to animal cells; although, not cellulose breakdown to replace photosynthetic sugar output).


Plants do not break down cellulose for energy, although it does store energy. Plants store their energy in the form of starch, which is broken down into glucose for the plant to use for energy. Most plants do not survive once the starch is utilized (but they do not breakdown cellulose).

Because cellulose molecules bond strongly to each other, breakdown of cellulose is relatively difficult compared to the breakdown of starch. High temperature is required for breakdown of cellulose, which plants cannot breakdown.


Breaking Down Cellulose

Biofuel is considered as one of the alternative energy sources in the near future when oil and coal are gone. Currently the major feedstock of biofuel is sugar from starch, while little comes from lignocellulosic biomass because of cost. However, the majority of the total carbohydrates in biomass is presented in forms of lignocelluloses like cellulose, hemicellulose or lignin. [1] A lot of biomass cannot be utilized if there is no cheap way to break down cellulose. In this article, we will focus on the structure of cellulose, cellulose hydrolysis techniques, current research of enzymatic hydrolysis and the cost of enzymatic hydrolysis.


Breaking down stubborn cellulose in time lapse

The plant component cellulose is an extremely resistant, water-insoluble polymer that is difficult to break down. This makes the efficient and sustainable use of plant biomass in biorefineries more difficult. "Only when there are sustainable and cost-efficient approaches for the degradation of cellulose will we start to produce fuels, chemicals and materials on a large scale from plant biomass," explains Bernd Nidetzky, biotechnologist and head of the Institute of Biotechnology and Biochemical Engineering at TU Graz.

Cellulose degradation in nature

In nature, the biological breakdown of cellulose occurs either through cellulases or through cellulosomes. Cellulases are enzymes that differ in their specificity and mode of action and are synergistically involved in the degradation of cellulose from woody plants such as trees or shrubs. Although individual cellulases may be located in close proximity to each other, they are individual, physically independent units. A cellulosome, on the other hand, is a protein complex, an ordered and physically interconnected collection of enzymes necessary for cellulose degradation.

Bernd Nidetzky and his team have set themselves the task of better understanding and visualising cellulosomes as essentially cellulose-degrading biological nanomachines. The researchers have now taken a decisive step towards this goal in an Austrian Science Fund (FWF) supported project. They were able to visualize a cellulosome at the single-molecule level during cellulose degradation by means of time-lapse atomic force microscopy and thus gain insights into its mode of operation. The results have been published in the journal ACS Central Science.

Nanomachines at work

In concrete terms, the researchers document the degradation of cellulose using a cellulosome from the bacterium Clostridium thermocellum. It is shown that the cellulosome dynamically adapts to the different surface conditions of the cellulose. "When binding to cellulose, the cellulosome switches to elongated, even thread-like forms and morphs them dynamically on a time scale of less than one minute according to the requirements of the attacked cellulose surface. Compared to cellulases, which detach material when sliding along crystalline cellulose surfaces, cellulosomes remain locally bound for minutes and remove the underlying material. The consequent roughening of the surface leads to efficient degradation of cellulose nanocrystals," explains Bernd Nidetzky.

Outlook for biorefineries

"Our analyses prove that cellulosomes are extremely efficient in breaking down cellulose. They could therefore play a central role in the development of new approaches for biorefineries," stressed Nidetzky. By exploiting the different mechanisms of action of enzyme complexes in the form of a cellulosome and free enzymes, cellulose degradation can be carried out faster, more completely and with less enzyme requirement. The synergies between the degradation mechanisms of cellulase and cellulosomes could thus help in the design of hybrid cellulase systems and provide new perspectives for applications in biorefineries.


Behind the Scenes: How Fungi Make Nutrients Available to the World

To prevent this gruesome fate, they developed extremely tough cell walls around 400 million years ago. For millions of years, nothing could break down lignin, the strongest substance in those cell walls. When a tree died, it just sank into the swamp where it grew. When the fossil record started showing trees breaking down around 300 million years ago, most scientists assumed it was because the ubiquitous swamps of the time were drying up.

But biologist David Hibbett at Clark University suspected that wasn't the whole story. An alternative theory from researcher Jennifer Robinson intrigued him. She theorized that instead of ecosystem change alone, something else played a major role – something evolving the ability to break down lignin. Through evolutionary biology research supported by the Department of Energy's (DOE) Office of Science, Hibbett and his team confirmed her theory. They found that, just as she predicted, a group of fungi known as "white rot fungi" evolved the ability to break down lignin approximately the same time that coal formation drastically decreased. His research illustrated just how essential white rot fungi were to Earth's evolution.

Fungi are still indispensable. The short-order cooks of the natural world, they have an unheralded job making nutrients accessible to the rest of us. Just like cooking spinach makes it easier to digest, some fungi can break down plant cell walls, including lignin. That makes it easier for other organisms to use the carbon that is in those cell walls.

"We all live in the digestive tract of fungi," said Scott Baker, a biologist at DOE's Pacific Northwest National Laboratory. If we weren't surrounded by fungi that decay dead plant material, it would be much harder for plants to obtain the nutrients they need.

To understand fungi's role in the ecosystem and support biofuels research, scientists supported by DOE's Office of Science are studying how fungi have evolved to decompose wood and other plants.

The Special Skills of Fungi

Fungi face a tough task. Trees' cell walls contain lignin, which holds up trees and helps them resist rotting. Without lignin, California redwoods and Amazonian kapoks wouldn't be able to soar hundreds of feet into the air. Trees' cell walls also include cellulose, a similar compound that is more easily digested but still difficult to break down into simple sugars.

By co-evolving with trees, fungi managed to get around those defenses. Fungi are the only major organism that can break down or significantly modify lignin. They're also much better at breaking down cellulose than most other organisms.

In fact, fungi are even better at it than people and the machines we've developed. The bioenergy industry can't yet efficiently and affordably break down lignin, which is needed to transform non-food plants such as poplar trees into biofuels. Most current industrial processes burn the lignin or treat it with expensive and inefficient chemicals. Learning how fungi break down lignin and cellulose could make these processes more affordable and sustainable.

Tracing the Fungal Family Tree

While fungi live almost everywhere on Earth, advances in genetic and protein analysis now allow us to see how these short-order cooks work in their kitchen. Scientists can sample a fungus in the wild and analyze its genetic makeup in the laboratory.

By comparing genes in different types of fungi and how those fungi are evolutionarily related to each other, scientists can trace which genes fungi have gained or lost over time. They can also examine which genes an individual fungus has turned "on" or "off" at any one time.

By identifying a fungus's genes and the proteins it produces, scientists can match up which genes code for which proteins. A number of projects seeking to do this tap the resources of the Joint Genome Institute (JGI) and the Environmental Molecular Sciences Laboratory (EMSL), both Office of Science user facilities.

Understanding the Rot

Just as different chefs use different techniques, fungi have a variety of ways to break down lignin, cellulose, and other parts of wood's cell walls.

Although fungi appeared millions of years earlier, the group of fungi known as white rot was the first type to break down lignin. That group is still a major player, leaving wood flaky and bleached-looking in the forest.

"White rot is amazing," said Hibbett.

To break down lignin, white rot fungi use strong enzymes, proteins that speed up chemical reactions. These enzymes split many of lignin's chemical bonds, turning it into simple sugars and releasing carbon dioxide into the air. White rot is still better at rending lignin than any other type of fungus.

Compared to white rot's powerful effects, the scientific community long thought the group known as brown rot fungi was weak. That's because brown rot fungi can't fully break down lignin.

Recalling his college classes in the 1980s, Barry Goodell, a professor at the University of Massachusetts Amherst said, "Teachers at the time considered them these poor little things that were primitive."

Never underestimate a fungus. Even though brown rot fungi make up only 6 percent of the species that break down wood, they decompose 80 percent of the world's pine and other conifers. As scientists working with JGI in 2009 discovered, brown rot wasn't primitive compared to white rot. In fact, brown rot actually evolved from early white rot fungi. As the brown rot species evolved, they actually lost genes that code for lignin-destroying enzymes.

Like good cooks adjusting to a new kitchen, evolution led brown rot fungi to find a better way. Instead of unleashing the brute force of energy-intensive enzymes alone, they supplemented that enzyme action with the more efficient "chelator-mediated Fenton reaction" (CMF) process. This process breaks down wood cell walls by producing hydrogen peroxide and other chemicals. These chemicals react with iron naturally in the environment to break down the wood. Instead of fully breaking down the lignin, this process modifies it just enough for the fungus to reach the other chemicals in the cell wall.

There was just one problem with this discovery. In theory, the CMF chemical reaction is so strong it should break down both the fungus and the enzymes it relies on. "It would end up obliterating itself," said Jonathan Schilling, an associate professor at the University of Minnesota.

Scientists' main theory was that the fungus created a physical barrier between the reaction and the enzymes. To test that idea, Schilling and his team grew a brown rot fungus on very thin pieces of wood. As they watched the fungus work its way through the wood, they saw that the fungus was breaking up the process not in space, but in time. First, it expressed genes to produce the corrosive reaction. Two days later, it expressed genes to create enzymes. Considering fungi can take years or even decades to break down a log, 48 hours is a blip in time.

Scientists are still trying to figure out how much of a role the CMF process plays. Schilling and like-minded researchers think enzymes are still a major part of the process, while Goodell's research suggests that CMF reactions do most of the work. Goodell's team reported that CMF reactions could liquefy as much as 75 percent of a piece of pine wood.

Either way, the CMF process offers a great deal of potential for biorefineries. Using brown rot fungi's pretreatment could allow industry to use fewer expensive, energy-intensive enzymes.

A Close Collaboration

Not all fungi stand alone. Many types live in symbiosis with animals, as the fungus and animal rely on each other for essential services.

Cows and other animals that eat grass depend on gut fungi and other microorganisms to help break down lignin, cellulose, and other materials in wood's cell walls. While fungi only make up 8 percent of the gut microbes, they break down 50 percent of the biomass.

To figure out which enzymes the gut fungi produce, Michelle O'Malley and her team at the University of California, Santa Barbara grew several species of gut fungi on lignocellulose. They then fed them simple sugars. As the fungi "ate" the simple sugars, they stopped the hard work of breaking down the cell walls, like opting for take-out rather than cooking at home.

Depending on the food source, fungi "turned off" certain genes and shifted which enzymes they were producing. Scientists found that these fungi produced hundreds more enzymes than fungi used in industry can. They also discovered that the enzymes worked together to be even more effective than industrial processes currently are.

"That was a huge diversity in enzymes that we had never seen," said O'Malley.

O'Malley's recent research shows that industry may be able to produce biofuels even more effectively by connecting groups of enzymes like those produced by gut fungi.

Termites as Fungus Farmers

Some fungi work outside the guts of animals, like those that partner with termites. Tropical termites are far more effective at breaking down wood than animals that eat grass or leaves, both of which are far easier to digest. Young termites first mix fungal spores with the wood in their own stomachs, then poop it out in a protected chamber. After 45 days of fungal decomposition, older termites eat this mix. By the end, the wood is almost completely digested.

"The cultivation of fungus for food [by termites] is one of the most remarkable forms of symbiosis on the planet," said Cameron Currie, a professor at the University of Wisconsin, Madison and researcher with the DOE's Great Lakes Bioenergy Research Center.

Scientists assumed that the majority of the decomposition occurred outside of the gut, discounting the work of the younger termites. But Hongjie Li, a biologist at the University of Wisconsin, Madison, wondered if younger insects deserved more credit. He found that young workers' guts break down much of the lignin. In addition, the fungi involved don't use any of the typical enzymes white or brown rot fungi produce. Because the fungi and gut microbiota associated with termites have evolved more recently, this discovery may open the door to new innovations.

From the Lab to the Manufacturing Floor

From the forest floor to termite mounds, fungal decomposition could provide new tools for biofuels production. One route is for industry to directly produce the fungal and associated microbiota's enzymes and other chemicals. When they analyzed termite-fungi systems, scientists found hundreds of unique enzymes.

"We're trying to dig into the genes to discover some super enzyme to move into the industry level," said Li.

A more promising route may be for companies to transfer the genes that code for these enzymes into organisms they can already cultivate, like yeast or E. coli. An even more radical but potentially fruitful route is for industry to mimic natural fungal communities.

For millions of years, fungi have toiled as short-order cooks to break down wood and other plants. With a new understanding of their abilities, scientists are helping us comprehend how essential they are to Earth's past and future.


AAAS Project 2061 Textbook Evaluations

Biology: Visualizing Life . Holt, Rinehart and Winston, 1998

Matter and Energy Transformations: Content Analysis

Map: What the Reviewers Found

This map displays the Content Analysis findings for this textbook in graphical form, showing what the reviewers found in terms of the book&rsquos content alignment and coherence for the set of key ideas on matter and energy transformations. You may find it helpful to print out this map and refer to it as you read the rest of the Content Analysis:

Also helpful for reference are the Matter and Energy Transformations topic maps, which contrast the coherent set of key ideas that the reviewers looked for with a composite of the treatment actually found in all nine evaluated textbooks:

Alignment

The topic of matter and energy transformations brings together a number of key ideas from both the biological and physical sciences. Biology: Visualizing Life treats most of these ideas and distributes them over several chapters: Chapter 2: Discovering Life, Chapter 5: Energy and Life, Chapter 14: Ecosystems, and Chapter 22: Plants in Our Lives. The ideas appear mostly as assertions in text, although occasionally illustrations and chapter review questions deal with the ideas. Matter and energy are usually discussed together. Little attention is given to the cycling of matter. The material does not address the important idea of the conservation of matter and energy: “However complex the workings of living organisms, they share with all other natural systems the same physical principles of the conservation and transformation of matter and energy. ” (Idea e). The following analysis provides details on how the textbook treats each of the specific key ideas.

Matter is transformed in living systems.

Idea a1: Plants make sugar molecules from carbon dioxide (in the air) and water.

There is a content match to this idea, which is treated in the text only. No discussions or investigations focus on this idea. The material states this idea in several places. The idea is introduced in the context of describing cellular organelles:

Plant cells contain chloroplasts, organelles that have the amazing ability to make chemical energy in the form of sugars, using air, water, and the energy from sunlight. This process is called photosynthesis.

p. 52s

Near the beginning of Chapter 5: Energy and Life, the text addresses the first part of this idea by stating, “In plant cells, chemical reactions that absorb energy make glucose and other organic molecules that plants use for energy and growth” (p. 77s). A few pages later, in the discussion of photosynthesis, the text states, “During the third stage, the ATP and NADPH are used to power the manufacture of energy-rich carbohydrates using CO2 from the air” (p. 85s). In the same section, the text goes on to say, “In the final stage of photosynthesis, carbon atoms are captured from carbon dioxide in the air and used to make organic molecules, which store energy” (p. 89s). On the same page, the overall reaction for photosynthesis is given in the form of symbols and words, showing that both carbon dioxide and water are needed for this process. The presentations of Idea a1 in Chapter 5 are set among biochemical details that go well beyond the key idea. The relevant chapter review questions (p. 96st, questions 12, 13, and 14) also focus on biochemical details rather than on Idea a1.

Idea b1: Plants break down the sugar molecules that they have synthesized into carbon dioxide and water, use them as building materials, or store them for later use.

There is a content match to parts of this idea, but the complete idea is never presented. The following representation of Idea b1 shows which parts of the idea are treated (in bold) in Biology: Visualizing Life : Plants break down the sugar molecules that they have synthesized into carbon dioxide and water, use them as building materials, or store them for later use.

In the context of describing organic molecules, plants’ synthesis and use of cellulose molecules for structural and storage purposes is noted, but the text is not explicit about plants making cellulose from sugar molecules they have synthesized:

Many organisms use polysaccharides as structural molecules as well as for energy storage. Plants manufacture a polysaccharide called cellulose. Cellulose forms the major part of the cell walls of plants.

p. 30s

After describing the cycling of carbon atoms in the Calvin cycle (p. 89s), the text gives a nice example to illustrate the fate of the glucose made in photosynthesis:

Plants use the organic molecules they produce during photosynthesis for their life processes. For example, sugar made in the leaves of a potato plant can be used to make cellulose for building new cell walls. Some of the sugar is stored as starch in the potato tuber. The plant may later break down the starch to make the ATP needed for energy, as you will see in Section 5-4.

p. 89s

However, there is no mention that plants’ carbohydrates are broken down into carbon dioxide and water. While the text mentions that “All living things use a process called cellular respiration” (p. 84s), gives the equation for glucose breakdown to carbon dioxide and water (p. 92s), and asks teachers to “Remind students that mitochondria are the organelles in eukaryotic cells with the special function of releasing energy stored in food” (p. 92t), the material does not mention that plants break down sugar molecules into carbon dioxide and water.

The idea that plants store sugars is presented again, much later, in the context of describing specialized functions of roots: “Roots often store nutrients. For example, carrots and sweet potatoes have roots that store large amounts of carbohydrates” (p. 391s). However, no mention is made of the fact that the carbohydrates stored are those originally made during photosynthesis.

Idea c1: Other organisms break down the stored sugars or the body structures of the plants they eat (or animals they eat) into simpler substances, reassemble them into their own body structures, including some energy stores.

There is an incomplete content match to this idea. Parts of the idea are mentioned on widely separated pages, but the complete idea is never stated. The following representation of Idea c1 shows which parts of the idea are treated (in bold) and what alternative vocabulary is used (in brackets) in Biology: Visualizing Life : Other organisms break down the stored sugars or the body structures of the plants [food] they eat (or animals they eat) into simpler substances, reassemble them into their own body structures, including some energy stores [as glycogen].

In the introduction to the human digestive system, the text notes that “Whatever we eat must be processed into smaller pieces before it can be used by the body. Food undergoes this transformation in the digestive system. ” (p. 706s). In describing the role of the liver, the text notes its role in storing excess glucose: “. when you eat a meal, the liver removes excess glucose from the blood and stores it as glycogen. When the level of glucose in the blood falls, glucagon causes the liver to release some of this glucose back into the blood” (p. 709s).

Much earlier in the text, in the context of describing chemical reactions in living things, the text mentions that atoms are rearranged: “In your cells, chemical reactions rearrange the atoms in glucose molecules, making new products and releasing energy” (p. 77s). The text does not mention body structures of plants. A figure caption indicates that “When you eat a potato, chemical reactions in your mouth convert starch into glucose” and the accompanying text states that a potato “is an excellent food because it is crammed with starch (long chains of glucose molecules)” (p. 77s).

In the context of describing the carbon cycle in ecosystems, the text mentions that other organisms break down carbon-containing molecules to carbon dioxide, but the focus is on the cycling of carbon rather than on the consumers’ use of the plant molecules:

Consumers obtain energy-rich molecules that contain carbon by eating plants or other animals. As these molecules are broken down, carbon dioxide is produced and released into the Earth’s atmosphere.

p. 262s

The idea that sugars are reassembled into body structures is not presented.

Idea d1: The chemical elements that make up the molecules of living things pass repeatedly through food webs and the environment, and are combined and recombined in different ways.

There is an incomplete content match to this idea. The following representation of Idea d1 shows which parts of the idea are treated (in bold) and what alternative vocabulary is used (in brackets) in Biology: Visualizing Life : The chemical elements [Nitrogen and carbon] that make up the molecules of living things pass repeatedly through food webs and the environment, and are combined and recombined in different ways.

The text describes the nitrogen cycle (p. 260s) and the carbon cycle (p. 262s) but does not state the generalization about the repeated passage through food webs of all elements that make up the molecules of living things or that in this cycling these elements are combined and recombined in different ways. Of the chapter review questions, only one relates at all to this key idea, but it, too, focuses on an example rather than the generalization and does not get at the idea of repeated passage:

Question: Imagine an atom of nitrogen that is in a protein in the leaf of a plant. Trace the steps of the nitrogen cycle this atom must pass through in order to reach the atmosphere.

Suggested Response: The leaf falls and decays by bacterial action, and the nitrogen in the leaf becomes ammonia. The ammonia is then further broken down by other bacteria to produce nitrogen.

p. 270st, question 10

The diagram of the nitrogen cycle (p. 260s) shows nitrogen in two different combinations, whereas the diagram of the carbon cycle (p. 262s) only shows carbon in one combination. The text assumes that students will recall the meaning of the phrase “organic molecules,” which was defined much earlier: “Molecules with carbon-carbon bonds are called organic molecules” (p. 29s).

Energy is transformed in living systems.

Idea a2: Plants transfer the energy from light into “energy-rich” sugar molecules.

There is a content match to this idea, which is treated in text only. In the context of describing cell parts and functions, the text states the idea:

Plant cells contain chloroplasts, organelles that have the amazing ability to make chemical energy in the form of sugars, using air, water, and the energy from sunlight. This process is called photosynthesis.

p. 52s

The idea is presented again in the context of describing energy flow in food chains:

Plants, algae, and some bacteria capture energy directly from sunlight and use that energy to produce ATP, energy-storing carbohydrates, and other types of organic molecules.

p. 84s

In introducing details of light and dark reactions, the text again states the idea, this time noting some chemical intermediates:

In the first stage of photosynthesis, energy is captured from light. During the second stage, the energy is used to make ATP and an energy-carrying compound called NADPH. During the third stage, the ATP and NADPH are used to power the manufacture of energy-rich carbohydrates using CO2 from the air.

p. 85s

One of the chapter review questions relates to the key idea:

Question: Describe the role of chlorophyll in photosynthesis.

Suggested Response: Chlorophyll absorbs light energy that will be converted to chemical energy.

p. 96st, question 11

The idea is presented once again in the context of describing energy flow in ecosystems: “Nearly all producers are photosynthetic they capture the sun’s energy to synthesize carbohydrates. Plants, some bacteria, and algae are producers” (p. 256s).

Idea b2: Plants get energy to grow and function by oxidizing the sugar molecules. Some of the energy is released as heat.

There is a content match to this idea. However the two parts of the idea are treated separately. In the context of describing how energy is involved in chemical reactions, the text notes that plants use the glucose they have made for energy and growth: “In plant cells, chemical reactions that absorb energy make glucose and other organic molecules that plants use for energy and growth” (p. 77s). Later in the same chapter, the text explicitly states that “Both plants and animals use the process of cellular respiration to release energy stored in organic molecules” (p. 90s) and then notes that “Glycolysis breaks down glucose into smaller molecules” (p. 91s), thus explicitly mentioning the breaking down (though not the oxidizing) of sugar molecules. Review questions are provided to check student knowledge of this idea (pp. 96s and 95󈟌t, questions 3, 4, and 15).

The second part of this idea, that some of the energy is released as heat, is presented nine chapters later, when energy flow in ecosystems is described:

A plant absorbs energy from the sun and uses it to make carbohydrates. Only about one-half of the energy captured by a plant becomes part of the plant body, however. Part of the remaining energy is stored in ATP made during cellular respiration. Most of the remaining energy escapes as heat.

p. 258s

Idea c2: Other organisms break down the consumed body structures to sugars and get energy to grow and function by oxidizing their food, releasing some of the energy as heat.

There is a content match to this idea. In the context of describing the role of energy in living things, the text notes that humans break down (rather than oxidize) glucose and release energy from it: “In your cells, chemical reactions rearrange the atoms in glucose molecules, making new products and releasing energy” (p. 77s). The text then points out that some of the energy is used to do work (“Other reactions break down glucose and release energy that your body uses to do work” [p. 77s]) but that some is released as heat (“When living cells break down molecules, some of the energy released from the molecules is in the form of heat” [p. 83s]).

Two figures and accompanying captions extend the idea to organisms other than humans:

  • Figure 5-10 shows a series of organisms in a food chain. In one picture, a deer is eating plants. The figure caption states, “Light energy. is converted into carbohydrates by plants. When an animal eats plants, it gets energy from the carbohydrates in the plants. ” (p. 84s).
  • Figure 5-16 shows a cow eating grass. The caption states, “. Both plants and animals use the process of cellular respiration to release energy stored in organic molecules” (p. 90s).

The reaction by which energy is released from starch is contrasted with burning but is not characterized as being an oxidation:

When logs burn, the energy stored in wood is released in a single reaction as heat and light. But this is not what happens in cells. Instead, energy stored in food molecules is released at each step in a series of enzyme-catalyzed chemical reactions, as shown in Figure 5-8.

p. 82s

The text subsequently refers to the process as “oxidative respiration” but does not indicate that sugars are oxidized in the process:

In most living things, a second stage of cellular respiration, called oxidative respiration, follows glycolysis. Oxidative respiration, which requires oxygen, happens within the mitochondria. It is far more effective than glycolysis at recovering energy from organic molecules. Oxidative respiration is the method by which most living cells get the majority of their energy.

p. 90s

Chapter review questions 1c, 3, and 4 relate to the idea (p. 96st).

Idea d2: At each link in a food web, some energy is stored in newly made structures but much is dissipated into the environment as heat. Continual input of energy from sunlight keeps the process going.

There is an incomplete content match to this idea. The following representation of Idea d2 shows which part of the idea is treated (in bold) in Biology: Visualizing Life : At each link in a food web, some energy is stored in newly made structures but much is dissipated into the environment as heat. Continual input of energy from sunlight keeps the process going.

In the section on energy flow in ecosystems, the text states the first part of the idea—that energy is lost at each link in a food web—and describes ecologist Howard Odum’s findings that the loss amounts to about 90%:

A plant absorbs energy from the sun and uses it to make carbohydrates. Only about one-half of the energy captured by a plant becomes part of the plant body, however. Part of the remaining energy is stored in ATP made during cellular respiration. Most of the remaining energy escapes as heat. Similar losses of energy occur at each trophic level of an ecosystem.

. Odum found that when a herbivore eats a plant, only about 10 percent of the energy present in the plant’s molecules ends up in the herbivore’s molecules. The other 90 percent of the energy is “lost,” some as the cost of doing work (breathing, moving, chewing) and much more as heat. Likewise, when a carnivore eats the herbivore, only 10 percent of the energy in the herbivore goes toward making carnivore molecules. At each trophic level, the energy stored in the organisms is about one-tenth that of the level below it.

p. 258s

However, the text does not present the second part of the idea that continual input of energy from sunlight keeps the process going. The idea that living organisms require a constant input of energy is presented early in the text, in the context of describing the six themes that unify biology:

Organisms use energy to grow and to carry out their activities. Without it, life soon stops. Almost all the energy that drives life on Earth is obtained from the sun. Plants capture the energy of sunlight and use it to make complex molecules in a process called photosynthesis. These molecules then serve as the source of fuel for animals that eat them. Maintaining the complexity of living organisms requires a constant input of energy. The availability of energy is a major factor in limiting the size and complexity of biological communities.

p. 18s

And in the context of describing energy flow in living food chains, the text notes that the sun is the ultimate source of most energy and that it shines continuously: “Almost all of the energy needed for life comes ultimately from the sun, which shines continuously on Earth” (p. 84s). However, neither of these two examples relate the second part of the key idea to the first part.

The total amount of matter and energy stays the same.

Idea e: However complex the workings of living organisms, they share with all other natural systems the same physical principles of the conservation and transformation of matter and energy. Over long spans of time, matter and energy are transformed among living things, and between them and the physical environment. In these grand-scale cycles, the total amount of matter and energy remains constant, even though their form and location undergo continual change.

There is not a content match to this idea.

Building a Case

The material asserts the key ideas it does not develop an evidence-based argument to support them. The one phenomenon that could support a key idea (the demonstration that saliva transforms starch in a cracker into sugar [p. 77t]) is not used to do so. And no other phenomena are provided or explained by the key ideas to lend support for them.

Connections

The set of key ideas on matter and energy transformations is highly complex, spanning four levels of biological organization (molecular, cellular, organism, and ecosystem) and depending heavily on knowledge in physical science (e.g., energy forms and transformations among them, and recombination of atoms in chemical reactions).

Biology: Visualizing Life treats most of the key ideas and presents most of them in their entirety. However, in some instances key ideas are presented in pieces and the pieces are not tied together. For example, the two parts of Idea d2—that energy is lost as heat at each trophic level and that continual input of energy keeps the process going—are treated more than 150 pages apart and are not tied together. Similarly, the key idea that “Plants break down the sugar molecules that they have synthesized into carbon dioxide and water, use them as building materials, or store them for later use” (Idea b1) is presented in separate parts that are not tied together: plants’ use of synthesized sugars as building material or storage of such sugars is presented in the context of describing the Calvin cycle (p. 89s) and the breakdown of sugar to carbon dioxide and water is presented in the context of describing oxidative respiration but is not linked to plants (p. 92s). Nor are the ideas put together in Chapter 21: Plant Form and Function, which mentions carbohydrate storage in roots (p. 391s) but does not relate it to photosynthesis in leaves (p. 393s). And while the same chapter describes the distinction between monocots and dicots (p. 395s), it does not point out that cotyledons store food made during photosynthesis in the parent plant for its offspring.

Biology: Visualizing Life focuses mainly on the energy side of the story, which is more abstract than the matter side. The energy ideas are introduced at the cell level (p. 52s), then presented in the context of the chemistry of photosynthesis (pp. 85󈟅s) and respiration (pp. 90󈟊s), then in terms of energy flow and nutrient cycles in ecosystems (pp. 256�s), and finally in terms of human nutrition and digestion (pp. 700�s). Teachers are not alerted to these different places where ideas about energy transformations are treated nor is a rationale for this sequence conveyed. Given that students are more likely to be familiar with energy phenomena at the level of the human organism, the sequence followed in the text makes little sense. If the implied sequence is followed, a course might end without ever treating energy ideas in human organisms, relating human energy needs to processes occurring at molecular and cellular levels, or using processes of energy transformation in an individual organism to shed light on the sequence of such transformations in ecosystems.

Energy. Connections among key ideas. The text makes several connections among key ideas about energy transformations. For example, the text relates the idea that “[Humans] break down. sugars [to] get energy to grow and function” (part of Idea c2) to the idea that “Plants get energy to grow and function by [breaking down] the sugar molecules” (part of Idea b2) and relates both to the idea that “Plants transfer the energy from light into ‘energy-rich’ sugar molecules” (Idea a2) in the context of describing the role of cell organelles:

The energy that drives a cell’s activities is converted within organelles called mitochondria (myt uh KAHN dree uh). These organelles are specialized to convert energy stored in food. The number of mitochondria in most cells varies. A muscle cell in your heart, which may pump more than 70 times per minute, can contain thousands of mitochondria. A mature red blood cell has none.

The significant differences between you and plants is the source of the food processed by mitochondria for your energy. How do plants provide their mitochondria with food molecules? Plant cells contain chloroplasts, organelles that have the amazing ability to make chemical energy in the form of sugars, using air, water, and the energy from sunlight.

p. 52s

In addition, the text again relates the idea that “[Humans] break down. sugars [to] get energy to grow and function” (part of Idea c2) to the idea that “Plants transfer the energy from light into ‘energy-rich’ sugar molecules” (Idea a2) in the context of introducing the energy transformations in photosynthesis and respiration:

Have you ever eaten beef enchiladas? The beef came from a cow that ate grass. Other parts of the enchiladas came directly from plants. With few exceptions, you end up with plants (or some other photosynthetic organism) if you trace your food back to its origin. Clearly, you depend on plants for food, as do all plant eaters and organisms that eat plant eaters. The energy in that food came from sunlight.

p. 85s

The text also relates the energy transformations (including heat “loss”) in individual organisms (Ideas a2, b2, and c2) to the loss of energy at each trophic level in an ecosystem (Idea d2):

A plant absorbs energy from the sun and uses it to make carbohydrates. Only about one-half of the energy captured by a plant becomes part of the plant body, however. Part of the remaining energy is stored in ATP made during cellular respiration. Most of the remaining energy escapes as heat. Similar losses of energy occur at each trophic level of an ecosystem.

. Odum found that when a herbivore eats a plant, only about 10 percent of the energy present in the plant’s molecules ends up in the herbivore’s molecules. The other 90 percent of the energy is “lost,” some as the cost of doing work (breathing, moving, chewing) and much more as heat. Likewise, when a carnivore eats the herbivore, only 10 percent of the energy in the herbivore goes toward making carnivore molecules. At each trophic level, the energy stored in the organisms is about one-tenth that of the level below it.

p. 258s

Connections between key ideas and their prerequisites. Biology: Visualizing Life makes only one connection between a key idea and its prerequisite and this connection is weak. In the context of describing human digestion, the text somewhat connects the idea that humans break down the food they eat into simpler substances (part of Idea c1) to the prerequisite that food provides the molecules that serve as building materials for all organisms: “Supplies of energy and building materials for the body exist only in potential forms in food. Whatever we eat must be processed into smaller pieces before it can be used by the body” (p. 706s). However, the term “smaller pieces” does not adequately convey that food provides molecular building blocks, which is needed for students to appreciate that these molecules can be assembled, restructured, or broken down during respiration.

The material makes no other connections between key ideas and prerequisites. For example, the text does not present the prerequisite idea that “. Some [reactions] require an input of energy whereas others release energy” or the prerequisite about energy conservation. And it does not use them to explain that since energy is lost as heat at each trophic level (first part of Idea d2), energy must be supplied from somewhere else. Thus it misses the opportunity to relate the first part of Idea d2 to the second part—that “Continual input of energy from sunlight keeps the process going.” The text does not treat the prerequisite idea that “An especially important kind of reaction between substances involves combination of oxygen with something else—as in burning or rusting” or relate it to the oxidation of glucose, even though it refers to the process as “oxidative respiration” (p. 92s). Furthermore, even though the text goes into detail about light “excit[ing] an electron” in chlorophyll (pp. 87󈟄s) and shows energy diagrams of exothermic and endothermic reactions (pp. 77󈞺s) and of the role of enzymes in lowering activation energy (p. 79s), it fails to point out or make use of the prerequisite idea that “Arrangements of atoms have chemical energy” or that “Different amounts of energy are associated with different configurations of atoms” to make sense of energy transformations in photosynthesis and respiration.

Connections between key ideas and related ideas. The text presents the related idea that “Within cells are specialized parts for the capture and release of energy” and makes a connection to energy release in other organisms (Idea c2) and the idea that plants transfer the energy from light into “energy-rich” sugar molecules” (Idea a2). First, the section title “Organelles: A Cell’s Laborers” captures the idea that cells have specialized parts that do the work of the cell and the introductory paragraph states that “cells perform basic functions of life. ” (p. 52s). Next, a paragraph heading calls attention to the work of capturing and releasing energy: “Cells manufacture and release energy” (p. 52s). Finally, the text presents parts of both key ideas and connects them to the related idea:

Cells manufacture and release energy
By now you are aware that the life of a cell is not a restful one. Your cells are always at work. Where do they get the energy to perform all of life’s tasks? The energy that drives a cell’s activities is converted within organelles called mitochondria (myt uh KAHN dree uh). These organelles are specialized to convert energy stored in food. The number of mitochondria in most cells varies. A muscle cell in your heart, which may pump more than 70 times per minute, can contain thousands of mitochondria. A mature red blood cell has none.

The significant differences [sic] between you and plants is the source of the food processed by mitochondria for your energy. How do plants provide their mitochondria with food molecules? Plant cells contain chloroplasts, organelles that have the amazing ability to make chemical energy in the form of sugars, using air, water, and the energy from sunlight.

p. 52s

Matter. Biology: Visualizing Life does a much less complete job of presenting the matter side of the story. As noted in the discussion of alignment, key Ideas b1 and c1 are each presented as separate parts that are not tied together and key Idea d1 is presented only through examples that are not tied to the general idea about the repeated cycling of elements through their combination and recombination into different molecules and organisms. The idea that atoms that make up molecules combine and recombine is explicit for nitrogen but not for carbon.

Connections among key ideas. The text makes only one connection among key ideas about matter transformation. The text relates the idea that “Plants make sugar molecules from carbon dioxide (in the air) and water” (Idea a1) to the idea that “Plants. use [sugars] as building materials, or store them. ” (part of Idea b1) and to the idea that “Plants get energy to grow and function by [breaking down] the sugar molecules” (part of Idea b2) through an example of a potato plant. After describing how plants make organic molecules from carbon dioxide and water, the text describes the fate of these molecules:

Plants use the organic molecules they produce during photosynthesis for their life processes. For example, sugar made in the leaves of a potato plant can be used to make cellulose for building new cell walls. Some of the sugar is stored as starch in the potato tuber. The plant may later break down the starch to make the ATP needed for energy, as you will see in Section 5-4.

p. 89s

The text makes only weak connections between the combination and recombination of atoms in photosynthesis (Idea a1) and respiration (Ideas b1 and c1) and the repeated combination and recombination of atoms in ecosystems (Idea d1). For example, the text description of the carbon cycle does not convey the idea that carbon atoms combine and recombine as they move through the organisms in ecosystems and are released into the environment. Nor does it indicate that the repeated combination and recombination results from the repeated occurrence of either photosynthesis and respiration or photosynthesis and the use of the sugar products to build other molecules:

Like water, carbon also cycles between the nonliving environment and organisms. The Earth’s atmosphere contains carbon in the form of carbon dioxide. Plants use carbon dioxide to build organic molecules during photosynthesis. Consumers obtain energy-rich molecules that contain carbon by eating plants or other animals. As these molecules are broken down, carbon dioxide is produced and released into the Earth’s atmosphere. Cellular respiration by decomposers and photosynthetic organisms also returns carbon dioxide to the atmosphere. Figure 14-11 shows how carbon cycles within an ecosystem.

p. 262s

Connections between key ideas and their prerequisites. Even though the text presents two prerequisite ideas, it makes only one weak connection between them and key ideas. The text attempts to connect the prerequisite idea that “Food provides the molecules that serve as fuel and building materials for [humans]” (but not for “all organisms”) and the idea that humans break down food into simpler substances (part of Idea c1) in the introduction to the human digestive system:

Supplies of energy and building materials for the body exist only in potential forms in food. Whatever we eat must be processed into smaller pieces before it can be used by the body. Food undergoes this transformation in the digestive system.

p. 706s

However, the connection is not made at the molecular level.

And while the text presents the prerequisite idea that “Carbon and hydrogen are common elements of living matter” (pp. 26s and 29󈞊s) and shows examples of the related idea that “Carbon atoms. bond to several other carbon atoms in chains and rings to form large and complex molecules” (pp. 29󈞋s), connections are not made between these ideas and the key idea that “Plants make sugar molecules from carbon dioxide (in the air) and water” (Idea a1).

Furthermore, the idea that matter is conserved is neither presented nor related to matter cycles between ecosystems and the physical environment.

Connections between key ideas and related ideas. While the text presents two ideas that are relevant to key ideas about matter transformation, it does not make the connections between them explicit. For example, the text presents the related idea that “The chief elements that make up the molecules of living things are carbon, oxygen, hydrogen, nitrogen. ” (p. 26s) but does not relate it to the combination and recombination of these elements in ecosystems. When the nitrogen cycle is presented, the text only notes that “Organisms must have nitrogen to produce proteins and nucleic acids” (p. 260s). It does not restate the idea that nitrogen is one of the elements that make up the molecules of living things or make explicit that nitrogen is incorporated into the molecules of living things.

Matter and Energy. The material makes a brief connection between two key ideas about matter and energy transformation. In describing the chemistry of living things, the text makes the connection between matter and energy transformation in humans (part of Ideas c1 and c2) by noting that “In your cells, chemical reactions rearrange the atoms in glucose molecules, making new products and releasing energy” (p. 77s). The text attempts to connect key ideas about matter and energy transformation in ecosystems, but the connection does not relate the combination and recombination of atoms to the energy “loss” at each trophic level (and, hence, relates less sophisticated ideas about matter and energy in ecosystems). After describing how energy is lost (as heat) at each trophic level, the text introduces the topic of nutrient cycling by noting, “Unlike energy, which flows through an ecosystem, nutrients such as calcium and nitrogen circulate within an ecosystem” (p. 259s).

The material does not present the key idea that “However complex the workings of living organisms, they share with all other natural systems the same physical principles of the conservation and transformation of matter and energy. ” (part of Idea e). No connections are made between ideas about matter transformations and those about energy transformations in individual organisms.

Beyond Literacy

In presenting key ideas about matter and energy transformation, the text often includes more sophisticated material that interrupts the central story. For example, in presenting the idea that cells break down sugars into carbon dioxide and water, releasing energy in the process, the text includes details of enzymes, activation energy, and active sites (pp. 78󈞼s), the structure, synthesis, and decomposition of ATP (p. 83s), details of chloroplast structure and steps in capturing light and pumping protons across the thylakoid membrane (pp. 87󈟄s), and details of cellular respiration (pp. 90󈟉s) that contribute little to developing the key idea. This excessive detail is illustrated in the following text, which describes the role of water molecules in photosynthesis:

Before excited electrons can leave their chlorophyll molecules, the electrons must be replaced by other electrons. Water supplies these electrons. Plants obtain electrons from water by splitting water molecules, H2O. As water molecules split, chlorophyll takes the electrons from the hydrogen atoms, leaving protons. The remaining oxygen atoms combine to form oxygen gas.

p. 87s

The sophistication of these ideas is well beyond benchmarks and interrupts the central story being told.

Copyright 2005 by the American Association for the Advancement of Science. All rights reserved.


Can renewable energy really replace fossil fuels?

A Purdue University scientist is studying the role of plants in renewable energy sources. Maureen McCann, a professor of biological sciences, is studying a wide range of plants from poplar trees to zinnias. Her lab has characterized hundreds of plant genes and their products in an effort to understand how they all interact and how they could be manipulated in advantageous ways. (Purdue University photo/Rebecca McElhoe) Download image

Scientist turns to zinnias, roadside weeds, other plants to create efficient biofuels

WEST LAFAYETTE, Ind. — As global temperatures and energy demand rise simultaneously, the search for sustainable fuel sources is more urgent than ever. But how can renewable energy possibly scale up to replace the vast quantities of oil and gas we consume?

Plant power is a significant piece of the answer, says Purdue scientist Maureen McCann.

“Plants are the basis of the future bioeconomy,” she says. “In my mind, building a sustainable economy means we stop digging carbon out of ground and begin to make use of one and a half billion tons of biomass available in the U.S. on an annual basis. That's the strategic carbon reserve that we need to exploit in order to displace oil.”

McCann is a professor of biological sciences, former director of the Energy Center at Purdue’s Discovery Park, and president-elect of the American Society of Plant Biologists. She has spent her academic career looking at plant cell walls, which contain some of the most complicated molecules in nature. By studying a wide range of plants — from poplar trees to zinnias — ­her lab has characterized hundreds of plant genes and their products in an effort to understand how they all interact and how they could be manipulated in advantageous ways.

The ethanol industry uses enzymes to break starchy corn kernels down into glucose molecules, which, in turn, are fermented by microorganisms to produce usable fuel. Many researchers have been working on the possibility of getting more glucose by breaking down cellulose — the primary fibrous component of all plant cell walls — which is much more abundant than starch. However, McCann says their methods might be ignoring a valuable resource.

In addition to cellulose, cell walls contain many complex, poly-aromatic molecules called lignins. These compounds can get in the way of enzymes and catalysts that are trying to access cellulose and break it down into useful glucose. As a result, many labs have previously attempted to create plants that have more cellulose and fewer lignins in their cell walls.

But it turns out lignins are important for plant development and can be a valuable source of chemicals. As director of Purdue’s Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio), McCann collaborated with chemists and chemical engineers in maximizing utilization of available biomass, including lignins. A nine-year grant from the U.S. Department of Energy funded C3Bio researchers’ work toward using chemical catalysts to transform both cellulose and lignins into liquid hydrocarbons, which are more energy-dense than ethanol and fully compatible with engines and existing fuel infrastructure.

In light of lignins’ usefulness, McCann and her colleagues are interested in alternative biofuel optimization strategies that don’t involve reducing plants’ lignin content. For example, if the researchers can modulate the strength of the “glue” between plant cells, they can make it easier for enzymes to access cellulose and also reduce the amount of energy needed for shredding the plant material. Another approach involves genetically engineering living, growing plants to incorporate chemical catalysts into their own cell walls, which will help eventual breakdown be faster and more complete.

“In both cases, this work is a reflection of synthetic biology thinking,” McCann says. “We don't simply take what nature gives us we think of ways to improve the performance of the biomass using the entirety of the genetics toolkit.”

McCann encourages others to think about “pathways of carbon.”

“If we think of how plants grow, they're marvelous chemists. They're taking in carbon dioxide from the atmosphere and water through their roots, and converting those simple molecules into highly complex cell wall structures,” she says. “When we think about making use of plant material in a biorefinery, a key goal is to make sure that every carbon atom that the plants so carefully trapped as part of their bodies ends up in a useful target molecule — whether that's a liquid hydrocarbon or a component of some material with advanced properties.”

As synthetic biologists, McCann and her lab members think holistically about optimizing crops for producing food, biofuel and useful materials such as specialized chemicals. Regardless of end purpose, she says, she keeps three dimensions in mind when thinking about optimization: increasing yield per acre, increasing the quality and value of each plant and increasing the area of land on which crops can be grown profitably. The holistic approach is particularly important for ensuring that scientists and agricultural producers achieve these goals without compromising the global environment or local ecosystems.

“As a new bioeconomy emerges powered by the life sciences, plants are at the root of it in so many ways — both in terms of the energy they can provide and also the kinds of molecules that they can produce,” McCann says.

For now, she acknowledges that ending economic dependence on fossil fuels is a work in progress. The transition to a renewable energy economy will require multiple levels of change over time. For example, even if we made the switch entirely to electric cars, we would likely still need hydrocarbon fuels to mine lithium for the batteries and to run machines with longer lifetimes than cars, such as airplanes and ocean-going vessels. Yet she maintains a positive outlook.

“Something that gives me great optimism is that we're going through a revolution in our ability to make new discoveries that lead to technologies that enable acceleration of the pace of discovery,” she says. “We’re going to find new ways of converting energy from one form to another that we haven’t even imagined. The capacity to make this substantial change from a fossil-based to a renewables-based economy is going to be there. We just need to drive it forward.”

About Purdue University

Purdue University is a top public research institution developing practical solutions to today’s toughest challenges. Ranked the No. 6 Most Innovative University in the United States by U.S. News & World Report, Purdue delivers world-changing research and out-of-this-world discovery. Committed to hands-on and online, real-world learning, Purdue offers a transformative education to all. Committed to affordability and accessibility, Purdue has frozen tuition and most fees at 2012-13 levels, enabling more students than ever to graduate debt-free. See how Purdue never stops in the persistent pursuit of the next giant leap at purdue.edu.

Writer: Grace Niewijk

Media contact: Amy Patterson Neubert, 765-412-0864, [email protected]

Source: Maureen McCann, [email protected]

Note to Journalists: A photograph of Maureen McCann is available for journalists to use via a Google Drive folder.


Algae can draw energy from other plants

Flowers need water and light to grow. Even children learn that plants use sunlight to gather energy from earth and water. Members of Professor Dr. Olaf Kruse’s biological research team at Bielefeld University have made a groundbreaking discovery that one plant has another way of doing this. They have confirmed for the first time that a plant, the green alga Chlamydomonas reinhardtii, not only engages in photosynthesis, but also has an alternative source of energy: it can draw it from other plants. This finding could also have a major impact on the future of bioenergy. The research findings have been released on Tuesday 20 November in the online journal Nature Communications published by the renowned journal Nature.

Until now, it was believed that only worms, bacteria, and fungi could digest vegetable cellulose and use it as a source of carbon for their growth and survival. Plants, in contrast, engage in the photosynthesis of carbon dioxide, water, and light. In a series of experiments, Professor Dr. Olaf Kruse and his team cultivated the microscopically small green alga species Chlamydomonas reinhardtii in a low carbon dioxide environment and observed that when faced with such a shortage, these single-cell plants can draw energy from neighbouring vegetable cellulose instead. The alga secretes enzymes (so-called cellulose enzymes) that ‘digest’ the cellulose, breaking it down into smaller sugar components. These are then transported into the cells and transformed into a source of energy: the alga can continue to grow. ‘This is the first time that such a behaviour has been confirmed in a vegetable organism’, says Professor Kruse. ‘That algae can digest cellulose contradicts every previous textbook. To a certain extent, what we are seeing is plants eating plants’. Currently, the scientists are studying whether this mechanism can also be found in other types of alga. Preliminary findings indicate that this is the case.

In the future, this ‘new’ property of algae could also be of interest for bioenergy production. Breaking down vegetable cellulose biologically is one of the most important tasks in this field. Although vast quantities of waste containing cellulose are available from, for example, field crops, it cannot be transformed into biofuels in this form. Cellulose enzymes first have to break down the material and process it. At present, the necessary cellulose enzymes are extracted from fungi that, in turn, require organic material in order to grow. If, in future, cellulose enzymes can be obtained from algae, there would be no more need for the organic material to feed the fungi. Then even when it is confirmed that algae can use alternative nutrients, water and light suffice for them to grow in normal conditions.


How Fungal Enzymes Break Down Plant Cell Walls

Image courtesy of the Environmental Molecular Sciences Laboratory (EMSL).

The Science

Inside cows, goats, and other ruminants, fungal enzymes break down plant matter and extract nutrients. These enzymes essentially overcome a major bottleneck in biofuel production: breaking down lignocellulose—the primary building block of plant cell walls. The parts, assembly, and diverse roles of fungal enzyme complexes, called cellulosomes, have not been clearly defined. Until now. This study offers insights into the structure and function of fungal cellulosomes. In doing so, it shows that cellulosomes are evolutionary chimaeras that co-opted parts of enzymes from bacteria that are also found in the gut.

The Impact

The findings highlight the power of fungal enzymes in breaking down lignocellulose. These enzymes could be harnessed to develop novel strategies for efficient biofuel production.

Summary

Gut microbes play a major role in helping ruminants such as cows, goats, and sheep break down lignocellulose-rich plant matter in their diet. Anaerobic bacteria and fungi inhabiting the ruminant gut have evolved a suite of lignocellulose-degrading enzymes, whose activity supports microbial metabolism while supplying nutrients to ruminants. These enzymes often assemble into large, multi-protein complexes called cellulosomes, which enhance the ability of gut microbes to degrade lignocellulose by confining all the enzymes in one place. Although bacterial cellulosomes now serve as a standard model for biomass conversion and synthetic biology applications, fungal cellulosomes have not been well characterized due to the lack of genomic and proteomic data, despite their potential value for biofuel and bio-based chemical production. To address this knowledge gap, researchers combined next-generation sequencing with functional proteomics to describe the comprehensive set of proteins that play a role in fungal cellulosome assembly. The collaborative team was from the University of California, Santa Barbara the Environmental Molecular Sciences Laboratory (EMSL) the U.S. Department of Energy Joint Genome Institute (DOE JGI) Pacific Northwest National Laboratory Centre National de la Recherche Scientifique French National Institute for Agricultural Research Radboud University King Abdulaziz University and the University of California, Berkeley. This research was performed under the Facilities Integrating Collaborations for User Science (FICUS) initiative. The team used resources at JGI and EMSL, which are DOE Office of Science user facilities. This analysis revealed a new family of genes that likely serve as scaffolding proteins critical for cellulosome assemblies across diverse species of anaerobic gut fungi. Unlike bacterial cellulosomes, which have high species specificity, fungal cellulosomes are likely a composite of enzymes from several species of gut fungi. Although many bacterial and fungal plant biomass-degrading enzymes have shared similarities, the fungal cellulosomes were found to contain additional lignocellulose-degrading enzymes not found in bacterial cellulosomes. These additional enzymes may not only confer a selective advantage of fungi over bacteria in the ruminant gut, but also impart fungal cellulosomes with great potential for biomass conversion. Taken together, the findings highlight key differences in bacterial and fungal cellulosomes and suggest enzyme connections (known as tethering) play an important role in plant cell wall degradation.

Contact

BER Program Manager
Paul Bayer, SC-23.1
U.S. Department of Energy
301-903-5324

Principal Investigator
Michelle A. O'Malley
University of California, Santa Barbara
[email protected]

EMSL
Sam Purvine
Environmental Molecular Sciences Laboratory
[email protected]

Funding

This work was supported by the U.S. Department of Energy's (DOE's) Office of Science, Office of Biological and Environmental Research, including support of the Environmental Molecular Sciences Laboratory (EMSL) and the DOE Joint Genome Institute (JGI), DOE Office of Science user facilities U.S. Department of Agriculture National Science Foundation U.S. Army University of California, Santa Barbara and Berkeley and California NanoSystems Institute.

Publications

C.H. Haitjema, S.P. Gilmore, J.K. Henske, K.V. Solomon, R. de Groot, A. Kuo, S.J. Mondo, A.A. Salamov, K. LaButti, Z. Zhao, J. Chiniquy, K. Barry, H.M. Brewer, S.O. Purvine, A.T. Wright, M. Hainaut, B. Boxma, T. van Alen, J.H.P. Hackstein, B. Henrissat, S.E. Baker, I.V. Grigoriev, and M.A. O'Malley, "A parts list for fungal cellulosomes revealed by comparative genomics." Nature Microbiology 2, 17087 (2017). [DOI: 10.1038/nmicrobiol.2017.87]

Related Links

Environmental Molecular Sciences Laboratory science highlight: Unlocking the Potential of Fungal Enzymes to Break Down Plant Cell Walls

Environmental Molecular Sciences Laboratory science highlight: Biofuel Breakdown


Can plants break down cellulose for energy? - Biology

ABOUT THE PHOTOSYNTHESIS LESSON

This lesson is appropriate for children ages 11 and up. Students model the photosynthesis reaction by building the products (glucose and oxygen) from the reactants (carbon dioxide and water). Students can model cellular respiration and build starch and cellulose to show how plants use glucose.

The topic of photosynthesis is a fundamental concept in biology, chemistry, and earth science. Educational studies have found that despite classroom presentations, most students retain their naïve idea that a plant’s mass is mostly derived from the soil, and not from the air. To call students’ attention to this misconception, at the beginning of this lesson we will provide a surprising experimental result so that students will confront their mental mistake.

Next, we will help students better envision photosynthesis by modeling where the atoms come from in this important process that produces food for the planet. Using models, students will utilize the atoms from carbon dioxide and water to build glucose and oxygen.

Additionally, students can model cellular respiration and build both cellulose and starch from the same glucose molecules to demonstrate how glucose becomes incorporated into the roots, shoots and wood—the structures of the plants we see around us!

TEACHING THE PHOTOSYNTHESIS LESSON

This interactive video can be used to co-teach the lesson with Dr. Kathleen Vandiver (inventor of this lesson). Alternatively, teachers can watch the entirety of the lesson in advance. BLOSSOMS (Blended Learning Open Source Science or Math Studies) is a collaborative initiative seeking to begin to develop a large, free repository of video modules for high school math and science classes in multiple languages.

Original Teacher Guides and Resources:

MIT Edgerton Center Molecule videos on our YouTube channel:

You can make your own Molecule Sets by visiting our webpage: Information for Edgerton Center Molecule Sets. We are no longer able to sell Molecule Sets, unfortunately.

The following LEGO bricks are the minimum required (per kit/2 students) for the Photosynthesis Lesson:

Photosynthesis Lesson Mats (per kit/2 students):

16. Recognize that producers (plants that contain chlorophyll) use the energy from sunlight to make sugars from carbon dioxide and water through a process called photosynthesis. This food can be used immediately, stored for later use, or used by other organisms.

AAAS benchmarks:

4c, grades 9-12, Processes That Shape the Earth: Plants alter the earth's atmosphere by removing carbon dioxide from it, using the carbon to make sugars, and releasing oxygen. This process is responsible for the oxygen content of the air.

5E, grades 6-8, Flow of Matter and Energy: Food provides the fuel and the building material for all organisms. Plants use the energy from light to make sugars from carbon dioxide and water. This food can be used immediately or stored for later use. Organisms that eat plants break down the plant structures to produce the materials and energy they need to survive. Energy can change from one form to another in living things. Animals get energy from oxidizing their food, releasing some of its energy as heat. Almost all food energy comes originally from sunlight.

National Science Foundation Content Standard:

Content Standard C, grades 5-8: For ecosystems, the major source of energy is sunlight. Energy entering ecosystems as sunlight is transferred by producers into chemical energy through photosynthesis. That energy then passes from organism to organism in food webs.

LEGO®, the LEGO logo, and the brick and knob configurations are trademarks of the LEGO group, used here with permission. ©The LEGO Group and MIT. All rights reserved.


Breakdown and Synthesis of Sucrose, Starch and Cellulose

Sucrose is broken down or hydrolysed to yield glucose and fructose in the presence of the enzyme invertase or sucrase. The reaction is irreversible.

Synthesis of Sucrose:

Synthesis of sucrose in plants may take place by 3 different ways:

(1) From Glucose-1-Phosphate and Fructose in the presence of the enzyme sucrose phosphorylase e.g., in bacteria.

(2) From UDPG (Uridine Di-Phosphate Glucose) and Fructose in the presence of the en­zyme sucrose synthetase e.g., in higher plants.

(3) From UDPG and Fructose-6-phosphate in the presence of the enzyme sucrose phos­phate synthetase e.g., in higher plants.

Sucrose-phosphate thus produced is hydrolysed in the presence of the enzyme phosphatase to yield sucrose.

Breakdown of Starch:

Breakdown or the hydrolysis of starch to yield its constituent a-D-Glucose units may take place in two ways:

(1) By the enzyme diastase:

In fact diastase is not a single enzyme but a complex of many enzymes which are as follows:

α-amylase and β-amylase attack 1 : 4 linkages of amylose and amylopectin (which constitute the starch) while R-Enzyme attacks 1 : 6 linkages of amylopectin, so that starch is hydrolysed to yield disaccharide units i.e., maltose. Finally, the enzyme maltase converts maltose into glucose molecules.

(2) By the enzyme starch phosphorylase.

Glucose-1-Phosphate may be converted into glucose by the enzyme phosphatase.

Synthesis of Starch:

Synthesis of starch involves the simultaneous synthesis of amylose (with α-(1: 4) glyco­sidic linkages) and amylopectin (with α-(1: 6) glycosidic linkages), the two important constitu­ents of starch.

(A) Synthesis of Amylose (Or α-(1: 4) Glycosidic Linkages):

Synthesis of amylose may take place by any of the following ways:-

(1) According to Hanes (1940) amylose can be synthesised in the presence of the en­zyme starch phosphorylase from glucose-1-phosphate and an acceptor molecule consisting of about 3 to 20 glucose units joined together by α-(1: 4) glycosidic linkages.

(2) Formation of α-(1 : 4) glycosidic linkages may also take place in the presence of the enzyme UDPG-transglycosylase (amylose synthetase) by the transfer of glucose from UDPG (Uridine Di Phosphate Glucose) to an acceptor molecule consisting of 2 to 4 or more glucose units joined together by α-(1 : 4) glycosidic linkages or even a starch molecule.

The structure of UDPG is given below:

UDPG (Uridine Diphosphate Glucose)

(3) According to Akazawa et al (1964) glucose molecule obtained as a result of the hydrolysis of sucrose in the presence of enzyme sucrase is transferred to UDP (Uridine Di Phos­phate) molecule to form UDPG. Form UDPG the glucose molecule is transferred to starch (Fig. 13.2)

(4) Formation of α-(1: 4) glycosidic linkages leading to the synthesis of amylose may also take place in the presence of D-Enzyme by the transfer of two or more glucose units from maltodextrins (consisting of more than two glucose units) to a variety of acceptors such as maltotroise, maltotetrose molecules.

(B) Synthesis of Amylopectin (Or α-(1: 6) Glycosidic Linkages):

It takes place in the presence of Q-Enzyme by the transfer of small chains of glucose units joined together by α-(1: 4) glycosidic linkages to an acceptor molecule consisting of at least four α (1:4) linked glucose units. The α-(1: 6) glycosidic bond is established between C-1 of the terminal glucose unit of donor molecule and C-6 of one of the glucose units of the acceptor molecule (Fig. 13.3).

Breakdown of Cellulose:

Cellulose is a straight chain polymeric carbohydrate molecule (a glucan), composed of a large number of D-glucopyranose units joined together by β(1 → 4) glycosidic linkages. In nature, cellulose is broken down by enzymatic hydrolysis through the enzymes called celluloses. These enzymes which are often grouped under generic name cellulase, randomly attack β(1 → 4) glycosidic linkages of the cellulose chain first forming cellodextrins and then disaccharides called as cellobiose. Cellobiose is then hydrolyzed to glucose by the enzyme cellobiose.

Cellulose degrading enzymes are not found in plants or humans. These are found only in certain organisms such as ruminants, termites, some bacteria and certain protozoa.

(Division Ruminantia of even-toed ungulates such as a deer, antelope, sheep, goat or cow).

Synthesis of Cellulose:

Long un-branched chains of cellulose (consisting of β(1→4) linked glucose residues) are synthesized in plants by the enzymes called cellulose synthases. The enzyme cellulose synthase is a multi-submit complex that is situated on plasma membrane and transfers a glucose residue from a sugar nucleotide donor called uridine diphosphate glucose (UDPG) to an acceptor molecule forming β (1 → 4) glucosyl acceptor.

UDPG + Acceptor → UDP + β (1→4) glucosyl-acceptor

It is believed that sterol-glycosides (i.e., sterols joined to a chain of one or more glucose units) such as β-sitosterol glucoside (Fig. 13.4), probably act as initial acceptors that start the elongation of cellulose chain. The process continues, and after the cellulose chain has attained desired length, the sterol is cut off from the glucan (Cellulose Chain) by the enzyme endoglucanase present in the plasma membrane. The separated cellulose chains are then extruded on the outer side of the plasma membrane (Fig. 13.5).

There are evidences to suggest that glucose in UDPG comes from sucrose, by the action of the reversible enzyme sucrose synthetase (Fig. 13.5). Alternatively, UDP-glucose may be directly obtained from cytoplasm.