Food Molecules AS Bilogy

Glucose, Disachhariedes and Polysacchariedes for unit 1.2 for edexcel AS bio

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  • Created on: 05-12-11 16:33
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Polysaccharides
Polysaccharides are long chains of many monosaccharides joined together by glycosidic
bonds.
There are three important polysaccharides:
Starch
Is the plant storage polysaccharide. It is insoluble and forms starch granules inside many
plant cells. Being insoluble means starch does not change the water potential of cells, so does
not cause the cells to take up water by osmosis. It is not a pure substance, but is a mixture
of amylose and amylopectin.
Amylose is composed of glucose molecules
with 1,4 glycosidic links, so is a straight
chain. the chain is floppy, and it tends to coil
up into a helix.
Amylopectin is also a polymer of glucose but
it has side brances. Alpha 1,6 glycosidic links
hold each side branch onto the main chain.
This gives it a more open molecular
structure than amylose. Because it has more
ends, it can be broken more quickly than
amylose by amylase enzymes.
Both amylose and amylopectin are broken down by the enzyme amylase into maltose, though
at different rates.
The compact spiral structure of starch and its insoluble nature makes it an excellent storage
molecule.
Glycogen
is similar in structure to amylopectin. It is poly
(1-4) glucose with (1-6) branches. It is made by
animals as their storage polysaccharide, and is
found mainly in muscle and liver. Because it is
so highly branched, it can broken down to
glucose for energy very quickly.
Cellulose

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Is only found in plants, where it is the main component of cell walls. It is poly (1-4) glucose,
but with a different isomer of glucose. Cellulose contains beta-glucose, in which the hydroxyl
group on carbon 1 sticks up. This means that in a chain alternate glucose molecules are
inverted.
This apparently tiny difference makes a huge difference in structure and properties.
While the a1-4 glucose polymer in starch coils up to form granules, the beta1-4 glucose
polymer in cellulose forms straight chains.…read more

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Polysaccharide Function: Structure: Relationship of
: structure to function:
Starch Main storage Made of 2 polymers amylose and Insoluble therefore
polysaccharide amylopectin. good for storage.
in plants.
Amylose: a polymer of glucoses Helix is compact.
joined by 1,4glycosidic bonds.
Forms a helix with 6 glucose The branches mean
molecules per turn and about 300 that the compound can
per helix. easily hydrolysed to
release the glucose
Amylopectin: a polymer of monomers.
glucoses joined by 1,4glycosidic
bonds but with branches of
1,6glycosidic bonds.…read more

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