Carbohydrates
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Energy storage and supply (in respiration)
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Structure (e.g. cellulose in plant cell walls)
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Contain carbon and hydrogen
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General formula: Cx(H2O)y
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Polymer = polysaccharide
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Disaccharides and monosaccharide’s both dissolve in water to give a
sweet tasting solution
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Monomer = monosaccharide. Cn(H2O)n, 1 sugar unit.
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3 Carbon atoms = Triose, e.g. glyceraldehyde
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5 Carbon atoms = Pentose, e.g. ribose
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6 Carbon atoms = hexose, e.g. glucose
Glucose
Isomers are molecules with the same molecular formula, sometimes even
the same structural formula, but a different special arrangement.
Alpha glucose in the ring has the H atom Above the rest of the ring, with the
OH group pointing down.
Beta glucose in the ring has the H atom Below the ring, and the OH group
pointing up.
Alpha and Beta glucose are isomers because they have the same molecular
formula, but the special arrangement of the hydroxyl group when the ring is
formed is different.
Glucose is an energy source because it can be broken down into smaller
molecules of water and carbon dioxide in respiration, releasing energy for
ATP.
Glucose is a polar molecule so it can form hydrogen bonds and dissolves in
water.
Glucose condenses to form di and polysaccharides because of the hydroxyl
groups.
Other monosaccharides
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Ribose – present in RNA and ATP
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Deoxyribose – Part of DNA, information molecule
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Fructose – Fruit sugars, makes for a sweeter taste. Fructose + glucose
-> sucrose
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Galactose – Galactose +glucose -> lactose
Formation of a Disaccharide
Disaccharides are formed from two monosaccharides in a condensation
reaction. The reverse is a hydrolysis reaction. An enzyme is required to make
both of these reactions take place.
The free OH groups, both downward facing in Alpha molecules, join,
creating a spare water molecule, and creating a glycosidic bond.
A bond between the 1st carbon in one monosaccharide and the 4th carbon
in the second, both alpha monosaccharides, creates an alpha 1, 4 glycosidic
bond / linkage.
Polysaccharides
Polysaccharides are formed when many monosaccharides are joined
together in a condensation reaction to form a macromolecule (polymer).
Cells cannot store glucose because it is soluble and will dissolve, affecting
the water potential of the cell, so instead plants and animals store glucose
as starch (plants) and glycogen (animals). Both are polymers of alpha lucose
and act as an energy store.
Storage is an advantage because it is more compact than glucose, inert,
insoluble in water, can be quickly broken down into alpha glucose which
can be respired.
Starch
Starch is made of 80% amylopectin and 20% amylose. Amylose is a coiled,
unbranched, spiralling helical structure, because of its alpha 1, 4 glycosidic
bonds. Amylopectin has a branched shape because of its main chain of
alpha 1, 4 glycosidic bonds, and then at the branches alpha 1, 6 bonds.
These mean it has more ends for the enzymes to act on for rapid energy
release when necessary.
Glycogen
Glycogen also has a highly branched polymer of alpha glucose, with long
chains linked by alpha 1, 4 glycosidic bonds and with many, short branches
created by alpha 1, 6 glycosidic bonds. Because it has more and shorter
branches, it is even easier for the enzymes to act.
It is stored mainly in muscle and liver cells.
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Muscle – For easy release when muscle fatigue sets in, for more
movement.
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Liver – when blood glucose levels are too high, glucagon is released, so
that the excess glucose in the blood can be sieved out and stored in the liver
as glycogen.
Cellulose
Cellulose is a structural polysaccharide. It is a polymer of beta glucose.
Cellulose is a linear, unbranched chain, because the beta monosaccharides
need to be rotated 180 degrees before they can be glycosidically linked, so
the alternately rotated bonds mean it remains linear.
The chains form fibrils and hydrogen bonds form between adjacent fibrils,
forming bundles called fibres. Fibres have high tensile strength and they lay
in a mesh to create a strong cell wall.
Cellulose is bonded by beta 1, 4 glycosidic bonds.