Structure of monomers in starch and cellulose:
- Monomer of starch is α-glucose, while that of cellulose is β-glucose.
- In α-glucose, the hydroxyl group (-OH) of carbon number 1 lies on the lower side of the ring, while that in β-glucose, it lies on the upper side.
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- The hydroxyl groups of carbon number 2 and carbon number 3 in both α-glucose and β-glucose are transpositioned.

Starch:
- It is a storage polysaccharide found in plants.
- It consists of monomers of α-glucose joined together by glycosidic bonds.
- It is digestible by human enzymes.

Molecular structure of starch:
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- It contains two types of large molecules namely amylose and amylopectin.
- Amylose:
  - It is a long, linear, unbranched chain made up of α-glucose molecules joined by α-1,4 glycosidic bonds.
- The α-1,4 glycosidic bonds in amylose cause it to coil into helical structures like springs, making the final molecule more compact.
- It has only two ‘ends’ to attach or detach glucose molecules.
- Amylopectin:
  - It has a branched chain structure and has branches arising from it due to the presence of α-1,6 glycosidic bonds.
  - It has shorter chains of α-glucose molecules joined by α-1,4 glycosidic bonds, however, it does not coil up into a helical structure like amylose.
  - It has shorter chains of α-glucose molecules joined by α-1,4 glycosidic bonds, however, it does not coil up into a helical structure like amylose.
Since it has a branch-like appearance compared to amylose, there are more than two ‘ends’ to attach or detach glucose molecules.

Functions of starch in relation to its structure:
- The mixture of amylose and amylopectin builds up into large starch grains, which are stored in chloroplasts and in storage organs like potato tubers, seeds of cereals and legumes.
- The coiling and branching structures of amylose and amylopectin are suitable for storing glucose in the form of storage polysaccharides in plants.

Glycogen:
- It isa storage polysaccharide found in animals.
- It consists of α-glucose molecules as its monomers that are joined together by glycosidic bonds.

Molecular structure of glycogen:
- It has both α-1,4 glycosidic bonds and α-1,6 glycosidic bonds, similar to amylopectin present in starch. So, it looks structurally similar to amylopectin.
- Unlike starch, glycogen does not have the unbranched component, amylose.
- The glycogen molecule tends to be even more branched than the amylopectin molecule since it has more α-1,6 glycosidic bonds.
- The highly branched glycogen molecules clump together to form granules, which are insoluble in water.
- The more ‘ends’ available in a polysaccharide, the faster is the rate of detaching glucose for use in respiration, and the faster is the rate of energy production.

Function of glycogen in relation to its structure:
- The highly branched structure of glycogen is particularly useful for meeting the energy demands of metabolically more active animal cells.

Cellulose:
- It is a structural polysaccharide found in the cell walls of plant cells.
- It consists of β-glucose as its monomers that are joined together by β-1,4 glycosidic bonds.
- Unlike starch, it is indigestible because humans can only digest α-1,4 glycosidic bonds of starch but not β-1,4 glycosidic bonds of cellulose.
- Molecular structure:
- For carbon number 1 to join with carbon number 4 of the successive glucose molecule, one glucose molecule must be rotated by 180⁰.
- This rotation in successive glucose molecules prevents the long chain of glucose from coiling up unlike in the amylose, and therefore, results in a long straight chain.
- The straight chains of β-glucose molecules in cellulose run parallel to each other and are cross-linked by hydrogen bonds.

Functions of cellulose in relation to its structure:
- The hydrogen bonds that cross-link the straight chains of glucose are individually weak, but collectively they provide huge strength as they are formed in large numbers.
- This cross-linking causes them to form bundles called microfibrils. Microfibrils in turn are held together in bundles, called fibres, by hydrogen bonds that provide strength to cellulose.
- The cross-linking of the parallel chains of glucose by hydrogen bonds makes cellulose insoluble in water.
- The cellulose in the cell walls of plant cells makes them strong enough to prevent cell bursting when excess water flows in and instead the cells swell up.

Chitin:
- It is a structural polysaccharide found in the cell walls of fungi.
- Molecular structure:
- It consists of N-acetyl glucosamine as its monomers that are joined together by β-1,4 glycosidic bonds.
- Similar to cellulose, one of its successive monomers is rotated by 180 degrees.

2.02 - Carbohydrates (II)

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