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Polysaccharides

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Biology

Polysaccharides are very large molecules composed of many monosaccharides (poly- stands for 'many').

Unlike monosaccharides and disaccharides, polysaccharides are hydrophobic, meaning insoluble in water. This is because they are much larger in size and more complex in structure.

The general structure of polysaccharides

Polysaccharides are organic compounds composed of multiple molecules of monosaccharides. They have a complex structure with hundreds or thousands of monosaccharides. That is why they are referred to as complex carbohydrates. Figure 1 shows only a fraction of the long polysaccharide chain. Notice the individual monosaccharides.

Polysaccharides One part of the long chain in polysaccharide structure StudySmarterFigure 1. One part of the long chain in the polysaccharide structure. Source: commons.wikimedia

How are polysaccharides formed and broken down?

Like all polymers, polysaccharides are formed during the reaction of condensation.

Polysaccharides are built of units of glucose. Therefore, during condensation, hundreds of glucose units bond together using covalent glycosidic bonds. Water is removed during the reaction as the bonds form. Glycosidic bonds between these glucose molecules can be 1,4- and 1,6-glycosidic bonds. Glycogen structure is an excellent example of the two bonds connecting individual monosaccharides into a complex structure. In figure 2, you can see the 1,4- and 1,6-glycosidic bonds.

Figure 2. 1,4- and 1,6-glycosidic bonds in polysaccharide glycogen. Source: commons.wikimedia

If you have already studied monosaccharides and glycosidic bonds that form during condensation, you will remember that 1,4-glycosidic and 1,6-glycosidic bonds form in different places in the structure. 1,4-glycosidic bonds form on the 1st carbon atom of one and the 4th carbon atom of the other monosaccharide (therefore, they are called 1,4). On the other hand, 1,6-glycosidic bonds form when the polysaccharide structure starts to branch. They form between the 1st carbon atom of one and the 6th carbon atom of another monosaccharide. In case you missed this, go back to figure 2 and observe the placement of the bonds.

Polysaccharides go through the reaction of hydrolysis to break down into their monomers. This happens when there is a need for energy. Since cells can absorb smaller units but not large molecules like complex macromolecules, polysaccharides need to be broken down.

Polysaccharides go through multiple hydrolysis reactions. In figure 3, you can see that the first hydrolysis of amylose (a polysaccharide in starch) doesn't produce monosaccharides (glucose) straight away.

Polysaccharides Hydrolysis of amylose and maltose StudySmarterFigure 3. Hydrolysis of amylose and maltose. Source: commons.wikimedia

Three types of polysaccharides

The three most important types of polysaccharides are starch, glycogen, and cellulose.

Starch

Starch is a polysaccharide that is built of α-glucose molecules.

The structure of starch

Starch is composed of two molecules: amylose and amylopectin. The two are classed as polysaccharides.

They are both composed of α-glucose units; however, they differ in structure. Amylose has a long unbranched chain that forms a helix, in which α-1,4-glycosidic bonds link glucose units. Amylopectin is a branched molecule, with α-1,4-glycosidic bonds between individual glucose units in the chain and α-1,6-glycosidic bonds where it branches.

The function of starch

You may already know that plants produce glucose and oxygen with photosynthesis. They use glucose for various important cellular processes, and all unused glucose molecules are stored in the form of starch. That is why we say that starch serves as energy storage in plants. It is stored as small grains in different parts of a plant.

Starch can serve as long-lasting energy storage too. For example, starch in roots and bulbs is a source of energy during the winter months. Animals, however, never store starch. Animals and humans consume plants and receive a large amount of energy for their own cellular activities.

Foods high in starch include potatoes, bread, pasta, rice, and grains. Some examples of grains are couscous, wheat, and oats.

The relationship between the structure and the function of starch

The structure of starch makes several functions possible:

  • Starch is compact because of the coiled and branched structures of amylose and amylopectin. This means that tiny plant cells can easily store it in great amounts.

  • Starch is large, complex, and insoluble. It, therefore, doesn't diffuse out of cells, and it doesn't affect osmosis. This, too, makes it an excellent storage compound, as it has no ill effect on the normal functioning of cells.

  • During hydrolysis, a branched structure like starch can readily give out small, easily transported glucose molecules from the ends of its branches.

Glycogen

Glycogen is a polysaccharide that is built of α-glucose molecules.

The structure of glycogen

Glycogen is similar to amylopectin in that it is a branched polysaccharide, with 1,4-glycosidic bonds between glucose units in a chain and 1,6‑glycosidic bonds where the branches link to the chain.

It is a highly branched polysaccharide, even more so than amylopectin. Have a look at the figure below. Notice the branches in the structure, as well as the position of the 1,4- and the 1,6-glycosidic bonds.

Polysaccharides highly branched structure of glycogen StudySmarterFigure 5. The highly branched structure of glycogen. Source: commons.wikimedia

The function of glycogen

Glycogen is energy storage in animals. It is usually stored in the liver and muscles. It is never stored in plants. Similar to starch, it is hydrolysed when there is a need for energy for various processes. Glycogen that is stored in the liver is used for the regulation of blood glucose concentration. In muscles, it is important for muscle contraction. It serves as a fast release energy source during physical activity.

Eating fruits, starchy vegetables, and whole grains can build up glycogen.

The relationship between the structure and the function of glycogen

Glycogen is compact, even more so than starch. This makes it a great storage compound because it can be stored in small spaces and in great amounts. The branched structure also means that hydrolysis is fast. The glucose molecules on the end of the branches can be released quickly during hydrolysis, which in turn means that the cells can absorb the much-needed energy faster. Like starch, it is large, complex, and insoluble in water, which means it does not diffuse out of cells and does not affect osmosis in cells.

Cellulose

Cellulose is a polysaccharide that is built of β-glucose molecules.

Remember: Starch and glycogen have α-glucose molecules!

The structure of cellulose

The β-glucose molecules form a long, straight chain. Therefore, cellulose is not branched or coiled. Every other β-glucose unit is inverted, or 'upside down'. These β-glucose molecules are linked by 1,4-glycosidic bonds. Long chains of β-glucose molecules are linked together by hydrogen bonds. Hydrogen bonds are weak on their own, but when there are many of them, such as in cellulose, they create a firm structure.

Figure 6 shows the cellulose structure. Notice the position of the group, and hydrogen and oxygen atoms.

Polysaccharides structure of cellulose StudySmarterFigure 6. Structure of cellulose molecules. Source: commons.wikimedia

Cellulose molecules can be stacked on top of each other (see in figure 6 as well) to form very strong but extremely small fibrils called microfibrils. Multiple microfibrils are then joined together to form fibres that build the cell walls in plant cells.

The function of cellulose

Cellulose is important in plant cells. It provides crucial structural support in cell walls in that it makes them rigid, not flexible. This means that cells are left structurally intact during osmosis, which is important as cells would burst if there was too much water inside without strong structural support. Cellulose also helps plants stay upright, and supports stems and leaves to stay firm and flat in order for photosynthesis to happen.

Some animals, for example, cattle, can digest cellulose and use its glucose units as an energy source. Humans cannot digest cellulose and do not use it as an energy source (instead we use glycogen). However, it is an important source of fibre, significant for digestion.

The relationship between the structure and the function of cellulose

Due to the long, strong chains, cellulose molecules can be “stacked” onto each other, linked by hydrogen bonds. This ensures that cellulose molecules are strong enough to support cell walls. The extra support allows for stems and leaves to stay firm and upright, which means plants can produce food (glucose) via photosynthesis.

Because cellulose is so strong, and it is insoluble in water, it helps cells preserve their shape and helps normal functioning by not allowing cells to burst during osmosis.

Polysaccharides
StarchGlycogenCellulose
AmyloseAmylopectin
Storage

plants

plantsanimalsplants
Glucose unitsα-glucoseα-glucoseα-glucoseβ-glucose
Glycosidic bonds1,41,4 and 1,61,4 and 1,61,4 and 1,6
Structurecoiledbranchedhighly branchedstraight
Functionenergy storageenergy storagestructural support
Shape

Polysaccharides branched structure of amylose and amylopectin starch StudySmarter

Polysaccharides highly branched shape of glycogen StudySmarter

Polysaccharides straight structure of cellulose StudySmarter

Polysaccharides - Key takeaways

  • Polysaccharides are very large molecules composed of many monosaccharides (poly- stands for ‘many’). They are referred to as complex carbohydrates.
  • Polysaccharides are formed during condensation when hundreds of glucose units bond with covalent glycosidic bonds. These glycosidic bonds can be 1,4- and 1,6-glycosidic.
  • Polysaccharides go through multiple hydrolysis reactions in order to break down into individual glucose units.
  • The three most important types of polysaccharides are starch, glycogen, and cellulose.
  • Starch and glycogen are built of α-glucose molecules, cellulose is built of β-glucose molecules.
  • Starch and glycogen have branched structures, while cellulose has long, straight chains.

Polysaccharides

Polysaccharides are complex carbohydrates composed of many glucose units.

Polysaccharide cellulose provides essential structural support to cell walls. Cellulose makes cell walls rigid, not flexible, which allows for cells to withstand osmotic pressure. It also means that plants are able to stand upright, and stems and leaves are adept to produce essential nutrients during photosynthesis.

Yes, starch is a polysaccharide. It is composed of repeating units of α-glucose molecules.

Yes, cellulose is a polysaccharide. It is built of repeating units of beta-glucose molecules.

Yes, glycogen is a polysaccharide. It consists of repeating units of α-glucose.

Final Polysaccharides Quiz

Question

What is the definition of polysaccharides?

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Answer

Polysaccharides are complex carbohydrates built of many glucose units.

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Question

Are polysaccharides soluble in water?

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Answer

No, polysaccharides are insoluble in water. Even though their monomer glucose units are soluble in water, polysaccharides are not due to their large and complex structures.

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Question

How are polysaccharides formed?


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Answer

Polysaccharides are formed during condensation when hundreds of glucose units bond with covalent glycosidic bonds.

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Question

What are two different glycosidic bonds between glucose molecules in polysaccharides?

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Answer

Two different glycosidic bonds are 1,4- and 1,6-glycosidic bonds.

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Question

How are polysaccharides broken down into glucose molecules?

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Answer

Polysaccharides go through multiple hydrolysis reactions in order to break down into individual glucose units.

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Question

What are the three types of polysaccharides?

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Answer

Starch, glycogen, and cellulose.

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Question

What kind of glucose molecules is starch made of?

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Answer

Starch is made of α-glucose molecules.

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Question

Which two polysaccharides does starch consist of?

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Answer

Amylose and amylopectin.

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What is the difference between amylose and amylopectin?

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Answer

Amylose is a long unbranched chain that forms a helix, in which glucose units are linked by α-1,4-glycosidic bonds. Amylopectin is a branched molecule, with α-1,4-glycosidic bonds between individual glucose units in a chain, and α-1,6-glycosidic bonds where it branches.

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Question

What is the function of starch and where is it stored?

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Answer

Starch serves as energy storage in plants. It is stored as small grains in different parts of a plant.

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Question

How does the structure of starch affect its function?

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Answer

 Starch’s coiled and branched structure allows for the molecule to be compact and easily stored in cells. Also, being large, complex, and insoluble, starch doesn’t diffuse out of cells or affect osmosis. Its structure allows for a fast release of glucose units and therefore energy for cellular processes.

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Question

What kind of glucose molecules is glycogen made of?

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Answer

Glycogen is made of β-glucose molecules.

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How is glycogen similar to amylopectin in starch?

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Answer

Glycogen is similar to amylopectin in that it is a branched polysaccharide, with 1,4-glycosidic bonds between glucose units in a chain and 1,6‑glycosidic bonds where the branches link to the chain. Glycogen is more branched than amylopectin.

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Question

What is the function of glycogen and where is it stored?

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Answer

Glycogen is energy storage in animals. It is usually stored in the liver and muscles. It is never stored in plants.

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Question

How does the structure of glycogen affect its function?

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Answer

Glycogen’s highly branched structure allows for the molecule to be compact, even more so than starch. This makes glycogen a great storage compound as it can be easily stored in cells and in great amounts. Like starch, it is large, complex, and insoluble in water, which means it does not diffuse out of cells nor does it affect osmosis in cells.

Its structure allows for a fast release of glucose units and therefore energy for cellular processes. 

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Question

What kind of glucose molecules is cellulose made of?

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Answer

Cellulose is made of β-glucose molecules.

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What are the two types of bonds that are important for cellulose?

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Answer

1,4-glycosidic bonds that form between glucose molecules (a covalent bond), and hydrogen bonds between the chains.

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What do cellulose molecules form when linked together?

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Answer

Cellulose molecules form microfibrils

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Question

What is the function of cellulose?

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Answer

Cellulose provides crucial structural support in cell walls. It makes them rigid and protects them from bursting during osmosis. Cellulose also helps plants stay upright, and stems and leaves strong and flat in order for photosynthesis to happen.

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Question

How does the structure of cellulose affect its function?

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Answer

Long, strong chains of cellulose molecules can be “stacked” onto each other, linked by hydrogen bonds. This ensures that cellulose molecules are strong enough to support cell walls and therefore protect cells during osmotic pressure and keep plants firm and upright.

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