Polysaccharides

Polysaccharides

Polysaccharides are polymers made up of many monosaccharides. A polysaccharide is created in the same way as a disaccharide, via condensation reactions.

There are three key polysaccharides that you need to learn the structure and function of: starch, glycogen and cellulose. Starch and glycogen are both energy stores, whereas cellulose provides structural support.

Polysaccharides, figure 1

Starch

Starch is found in plants, not in animal cells, and it is the major carbohydrate store. Starch is made from the excess glucose created during photosynthesis. Glucose is used in respiration, but if more glucose is created in photosynthesis than is currently needed it is converted into the polymer starch for storage.

The presence of starch can be confirmed by using iodine. Iodine is orange/brown in colour when no starch is present, but it turns blue/black if starch is present.

Polysaccharides, figure 1

Structure of Starch

Starch is a polymer made up of α-glucose. These α-glucose monomers are joined together via condensation reactions and are held in place by 1-4 and 1-6, glycosidic bonds. (Remember, the numbers refer to which carbon atoms the bond forms between)

Amylose is the name of the structure in starch in which the glucose monomers are all joined together by 1,4 - glycosidic bonds. This results in a spiral shaped polymer, see diagram below.

Amylopectin is the name given to the other structure in starch I which the glucose monomers are joined by a combination of 1,4 and 1,6 - glycosidic bonds. The 1,6-glycosidic bonds result in branches, see diagram below.

Amylose

Polysaccharides, figure 2

Amylopectin

Polysaccharides, figure 3

Properties of Starch

Starch is insoluble due to the fact it is such a large molecule. This is an advantage to a storage molecule as it means it can be stored within cells and not dissolve. Therefore is will not change the water potential of a cell or affect osmosis.

The fact the amylose is spiral in shape means that is can be readily compacted. This is an advantage as a lot of the molecule can fit into small spaces and be stored.

The fact that amylopectin has branching strands provides a larger surface area for enzymes to attach to. This means that starch is readily hydrolysed back into glucose when plant cells are running low on glucose.

Glycogen

Glycogen is the major carbohydrate storage molecule found in animal cells. The main cells glycogen is stored in are liver and muscle cells. Glycogen is made from the excess glucose that has been eaten and absorbed into the bloodstream.

Glucose is used in respiration, but if more glucose is eaten than the cells currently need for respiration it is converted into the polymer glycogen for storage. As liver cells are responsible for removing toxins and muscles are responsible for movement, glycogen is mainly stored in these cells to ensure they always have a store of glucose to respire to release energy.

Structure of Glycogen

Glycogen is a polymer made up of α-glucose and is very similar in structure to amylopectin in starch. The α-glucose monomers are joined together via condensation reactions and are held in place by 1,4 and 1,6-glycosidic bonds. The key difference between the structure of glycogen and starch is that glycogen contains more 1,6-glycosidic bonds and is therefore a more branched structure.

Polysaccharides, figure 1

Properties of Glycogen

Glycogen is insoluble due to the fact it is such a large molecule. This is an advantage to a storage molecule as it means it can be stored within cells and not dissolve. Therefore is will not change the water potential of a cell or affect osmosis.

The fact that glycogen is a highly branched molecule means it has a larger surface area for enzymes to attach to. This means that it is readily hydrolysed back into glucose when cells are running low on glucose. Glycogen is even more branched than starch, therefore it is hydrolysed back into glucose more rapidly. This is essential for animals because they have a higher metabolic rate and therefore need more glucose. For example, they may need this glucose to provide energy to run from a predator.

Cellulose

Unlike starch and glycogen, the function of cellulose is to provide structural strength in plants. Cellulose is located in the cell wall of plants and therefore prevents cells from bursting if they take in excess water.

Structure of Cellulose

Cellulose is the only polysaccharide that is made up of β-glucose monomers. These monomers are joined by 1,4-glycosidic bonds only. For this reason, the cellulose polymer is unbranched.

These long, straight chains of β-glucose accumulate and lie parallel to each other. The parallel chains are then held together by many hydrogen bonds, and the sheer number of hydrogen bonds provides strength. This structure is called a fibril, or microfibril. Fibrils will then also align in parallel and are held in place by even more hydrogen bonds to form a cellulose fibre.

Polysaccharides, figure 1

Which polysaccharide contains 1-4 and 1-6, glycosidic bonds?
Your answer should include: Starch / Glycogen
Which polysaccharide is made up of the monomer β-glucose?
Cellulose
Which polysaccharide only forms unbranched structures?
Cellulose
Which is the only polysaccharide found in animal cells?
Glycogen
7. Which polysaccharide contains microfibrils?
Cellulose
8. Which polysaccharide contains amylose and amylopection?
Starch