Carbohydrates- polysaccharides

Polysaccharides are large complex polymers. They are formed from very large numbers of monosaccharide units, which are their monomers, linked by glycosidic bonds.

Starch:

This is the main store of glucose for plants. Grains are found in high concentrations in seeds and storage organs.

• Insoluble therefore good for storage as it has no osmatic effect.
• The helix is compact.
• The branches mean that the compound can easily hydrolysed to release the glucose monomers.

Starch is made of a-glucose molecules bonded together in 2 different ways, forming the 2 polymers, amylose and amylopectin:

Amylose

• a polymer of glucoses joined by α-1,4-glycosidic bonds.
• Forms a helix with 6 glucose molecules per turn and about 300 per helix. #

Amylopectin

• a polymer of glucoses joined by α-1,4-glycosidic bonds but with branches of α-1,6-glycosidic bonds.
• This causes the molecule to be branched rather than helical.

Biochemical testing for the presence of starch

Iodine solution or potassium iodide solution can be used to test for the presence of starch. A positive result changes the solution from an orange-brown to a blue-black colour.

Glycogen:

• Main storage polysaccharide in animals and fungi, it is our mammalian energy storage e.g. in the liver and muscles.

• Similar to amylopectin but with many more branches which are also shorter.

• The number and length of the branches means that it is extremely compact.

Cellulose:

• It is the main structural constituent of plant cell walls
• Structure made up of adjacent chains of long, unbranched polymers of glucose joined by β-1,4-glycosidic bonds.
• These straight chains laid next to one another form hydrogen bonds which strengthen them into larger sub-units called microfibrils.
• Microfibrils are what cellulose is made of, and what gives plant cell walls their great strength.
• Freely permeable, spaces between the fibres.

Chitin:

• Found in the exoskeleton of insects and in fungal cell walls.
• Resembles cellulose, with long chains of β-1,4-linked monomers, but has groups derived from amino acids added, to form a heteropolysaccharide.
• Strong, waterproof, leightweight.