Polysaccharides - Online Tutor, Practice Problems & Exam Prep

Polysaccharides, or glycans, are large carbohydrates made of repeating monosaccharide units, serving structural or energy storage functions. Key examples include cellulose, a plant-based structural polysaccharide with unbranched β(1→4) linkages, and chitin, found in arthropods and fungi, which has similar linkages but features an amide group. For energy storage, amylose and amylopectin (plant starch) and glycogen (animal starch) utilize α(1→4) and α(1→6) linkages, with glycogen exhibiting more extensive branching to meet higher energy demands in animals.

Polysaccharides Concept 1

Now, polysaccharides, also called glycans, can be enormous, with over 1,000,000 units. While at the same time, their composition can stay relatively simple. We usually say they consist of one or two monosaccharides that are repetitively linked together. Their functions come in two types: we have those polysaccharides that are basically structural support, which we will mark in green, and then we have those that act as energy storage, which we mark in red. Let's take a look at our chart here; we have our most common polysaccharides, the sources where they come from—whether from animals, plants, or other sources. We talk about the different types of linkages they possess and then some descriptions concerning that particular polysaccharide.

First, we have cellulose, and we know that cellulose originates in plants. We say that, concerning cellulose, it's unbranched and possesses βN14 glycosidic linkages. So here are our linkages. They compose plant cell walls and typically have thousands of repeated units linked together in this βN14 linkage setup. Next, we have chitin, a structural support polysaccharide. Its source varies because it is connected to arthropods, like lobsters, to insects, and even to fungi. Like cellulose, it has unbranched and has βN14 linkages. But the difference is that in cellulose, in position 2, we have OH groups on each of these rings. The OH group is modified in chitin, where it will have an amide group instead. So, we have NH connected to a carbonyl, connected to a CH₃. This substance forms the exoskeletons of lobsters and different insects. Like cellulose, it's involved in cell walls but not of plants—rather of fungi—dealing with fungal cell walls.

For energy storage, we have amylose, amylopectin, and glycogen. The first two are connected to plants. The last one is connected to animals, to us. Amylose is unbranched but has αN14 glycosidic linkages. This one makes up about 20% of all plant starch. Starch, plant starch has these two as its primary components: 20% of it coming from amylose, the rest of it coming from amylopectin. Amylopectin is a bit different. It still has αN14 glycosidic linkages, but it also has αN16 glycosidic linkages. This one again makes up about 80% of all plant starch. It has some branching in the form of this αN16 glycosidic linkage.

Finally, we have glycogen related to animals, to us. We have the same types of connections: αN14 and αN16 linkages. This is known as animal starch because of the similar branching, but our branching occurs much more often. We have much more extensive branching, every certain number of monosaccharide units we have a 16 branching group. We move more, so we require more energy. These branching sites are ways of packing in more monosaccharides so that when we need that energy, we can break down these sugars to supply the animal with the much-needed energy they need to move. Remember, these last two may be similar, but glycogen has way more branching involved. So these are our most common types of polysaccharides, those that are in structural support and those that are in energy storage, and then they have different sources, whether they be from plants, various, or animal.

Polysaccharides Example 1

In this example, we need to match each of the following statements with the correct polysaccharides. So option 1, we have amylopectin and glycogen. Option 2, we have just glycogen. And option 3, we have cellulose.

Alright. So for the first statement, it says it's an unbranched polysaccharide composed of beta-1,4-glycosidic linkages. When it comes to amylopectin and glycogen, they have branching groups. They have alpha-1,6 branching groups, so they would be out. That means the answer is option 3. Cellulose is an unbranched polysaccharide, and also it's the only one with beta linkages.

For the next one, a polysaccharide that is stored in muscle tissues. Alright. So, where do these polysaccharides come from? And when we think about it, we'd say that glycogen has to be the answer here because glycogen's source is animals, and we're talking about muscle tissues. We wouldn't talk about muscle tissues with plants here, we talk about fibrous tissue. That's different. So here, this would have to be option 2.

And then finally, this is a polysaccharide containing alpha-1,4 and alpha-1,6 glycosidic linkages. This is true of both amylopectin and glycogen. So option 1 would be our final answer here. So this is how we would basically match each of the statements with the correct polysaccharide.

Which plant-based polysaccharide possesses only a-1,4 glycosidic linkages?

• A. Chitin
• B. Glycogen
• C. Glycan
• D. Amylose
• E. Cellulose

Here’s what students ask on this topic:

What are polysaccharides and what are their main functions?

Polysaccharides, also known as glycans, are large carbohydrates composed of repeating monosaccharide units. They serve two primary functions: structural support and energy storage. Structural polysaccharides, such as cellulose and chitin, provide rigidity and strength to plant cell walls and the exoskeletons of arthropods and fungi, respectively. Energy storage polysaccharides, like amylose, amylopectin, and glycogen, store glucose units for later use. Amylose and amylopectin are found in plants, while glycogen is found in animals. Glycogen has more extensive branching compared to plant starches, allowing for rapid mobilization of glucose to meet the higher energy demands of animals.

What is the difference between cellulose and chitin?

Both cellulose and chitin are structural polysaccharides with unbranched β(1→4) glycosidic linkages. However, cellulose is found in plant cell walls, while chitin is found in the exoskeletons of arthropods and the cell walls of fungi. The key difference lies in their chemical structure: cellulose has hydroxyl (OH) groups at the C2 position of each glucose unit, whereas chitin has an amide group (NHCOCH₃) at the same position. This modification in chitin provides additional strength and rigidity, making it suitable for forming hard exoskeletons.

How do amylose and amylopectin differ in structure and function?

Amylose and amylopectin are both components of plant starch, but they differ in structure and function. Amylose is unbranched and consists of α(1→4) glycosidic linkages, making up about 20% of plant starch. Amylopectin, on the other hand, is branched and contains both α(1→4) and α(1→6) glycosidic linkages, accounting for about 80% of plant starch. The branching in amylopectin allows for more rapid mobilization of glucose units when the plant needs energy. This structural difference also affects their solubility and digestibility, with amylopectin being more readily digestible due to its branched structure.

Why is glycogen more extensively branched than amylopectin?

Glycogen is more extensively branched than amylopectin to meet the higher energy demands of animals. Both glycogen and amylopectin have α(1→4) and α(1→6) glycosidic linkages, but glycogen has more frequent α(1→6) branching points. This extensive branching allows for rapid release of glucose units when energy is needed quickly, such as during physical activity. The increased branching also makes glycogen more compact, enabling animals to store large amounts of glucose in a small volume, primarily in the liver and muscle tissues.

What are the sources of the most common polysaccharides?

The most common polysaccharides come from various sources. Cellulose is found in plants and forms the structural component of plant cell walls. Chitin is found in the exoskeletons of arthropods (like lobsters and insects) and the cell walls of fungi. Amylose and amylopectin are components of plant starch, with amylose making up about 20% and amylopectin about 80%. Glycogen, often referred to as animal starch, is found in animals, particularly in the liver and muscle tissues, where it serves as a storage form of glucose for energy.