Polysaccharides

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.

Fig. 1 - One part of the long chain in the polysaccharide structure

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.

Fig. 2 - 1,4- and 1,6-glycosidic bonds in polysaccharide glycogen

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.

Fig. 3 - Hydrolysis of amylose and maltose

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.

Fig. 4 - The highly branched structure of glycogen

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

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 5 shows the cellulose structure. Notice the position of the $C{H}_{2}OH$ group, and hydrogen and oxygen atoms.

Figure 5 - Structure of cellulose molecule

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.