2. carbohydrates
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CarbohydratesSaccharides
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The basic formula is:
Cx(H
20)
yor (CH
2O)
n
All carbohydrates are Aldehydes or ketones
Aldehydes oxidise easily and make good reducing agents.
They have many –OH groups
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MONOSACCHARIDESThese are simple sugars
They are used to release energy or as monomers in larger molecules.
They are sweet
They are soluble in water
e.g. glucose: C6H
12O
6
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TEST FOR REDUCING SUGARS
Reducing sugars are able to
Reduce Copper II Sulfate to Copper I oxide (Blue, soluble) (red, insoluble)
Benedict’s solution is made
using Copper II sulfate
which is a blue solution
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Benedict’s test
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This solution tested positive for reducing sugars
This solution is negative for reducing sugars
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Ring Structures
Glucose can exist as a chain or fold around to form a 6 sided ring structure
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Note the carbons are numbered 1-6
Carbon 6 is outside of the ring attached to carbon 5
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Note carbon 6 is above the ring Some OH and some H groups are below the ring
Carbon 1 is asymmetrical
It has 4 different groups attached
HOHO and C
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In nature glucose exists in two forms
In beta glucose the OH is above the ring structure
Alpha glucose Beta glucose
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ALPHA GLUCOSE BETA GLUCOSE
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DisaccharidesThese are formed by condensation – removing water
These are both molecules of -∝ glucoseA bond is formed by removing OH from carbon 1 on one moleculeAnd an H on carbon 4 of the adjacent molecule
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This is an ∝-1.4 glycosidic bond Water is made
Maltose is the disaccharide formed
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Lactose a disaccharide formed by joining Galactose and Glucose
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SucroseThis is a disaccharide that is a non-reducing
sugar
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Test for Non-Reducing Sugar1. Carry out Benedict's test: negative result so no reducing sugars
2. Hydrolyse the unknown to split the sugar into monosaccharides:
Add hydrochloric acid Heat in a water bath neutralise using hydrogen carbonate
3. Repeat Benedict's test
If it goes Brick red this time there is non-reducing sugar present
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POLYSACCHARIDES
e.g. starch, glycogen, cellulose
Polysaccharides are food and energy stores
They are large and insoluble so do not affect water potential or influence chemistry of cell
They can fold into compact shapes
Polysaccharides are easily converted by hydrolysis to sugars when required
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STARCHStarch is a polymer of α-glucose
This is the main storage compound in plants
Has two components:
Amylose and
amylopectin
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AMYLOSEStraight chain of -glucose molecules joined by 1,4 α αglycosidic bondsHelically coiled making it compact.200-1500 residues
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AMYLOPECTINMakes up about 70% starchMany more residues, 2000-200-000Some residues joined with 1,4 glycosidic bonds and αhelically coiled Some branched with 1,6 glycosidic bondsα
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Alpha 1,6, glycosidic bond
Alpha 1,4 glycosidic bond
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GLYCOGENGlycogen is the storage polysaccharide in animal
cells (particularly in liver and muscle cells) and many fungi
Similar in structure to amylopectin but has more branching and is larger
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CELLULOSE
50% of Carbon in a plant is cellulose
It is the most abundant organic molecule on earth
Cellulose is an unbranched polymer of 2000-3000 β-glucose molecules
Held by β 1,4-glycosidic bonds
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Beta glucoseCannot form glycosidic bonds like thisThe OH of the first molecule is above the ring structure
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If one molecule flips through 180 degrees then the OH groups align on carbon 1 and 4 and condensation can occur
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Beta 1,4,glycosidic bond
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The next glucose molecule will need to be the same as the first oneThis time the beta-1,4 glycosidic bond is formed below the ring structure
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And so on…Cellulose is a long, straight molecule
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Cellulose molecules lie next to each other and are held together by weak hydrogen bondsThere are many of these bonds which makes the overall structure strong
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These molecules are bundled into microfibrils which are in turn bundled into cellulose fibresThese fibres have a high tensile strength