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Carbohydrates Revision
Carbohydrates:
Carbohydrates contain carbon, hydrogen and oxygen
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The general formula for carbohydrates is: (CH₂O)ₙ
Carbon, hydrogen and oxygen appear in the ratio 1:2:1
Carbohydrates also known as saccharides (sugars
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White crystalline solids
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Soluble in water
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Insoluble in non-polar substance
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Polysaccharide = many monosaccharides linked together, like
glycogen, cellulose, starch
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Sweet, reducing agent
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In diagrams, the carbons are numbered clockwise, beginning with the
carbon to the right of the oxygen atom within the ring
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This solubility means glucose is dissolved in cytosol of the cell
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Alpha glucose = hydroxyl group on carbon 1
sits below the plain of the ring
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Condensation reactions:
When 2 alpha glucose molecules are side by side, 2 hydroxyl groups react
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A bond forms between carbons 1 and 4 on the glucose molecules and they
join
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Water molecule formed as product of reaction
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Carbohydrates Revision
Forming Disaccharides:
Glucose, fructose and galactose are isomers of each other –
Fructose and galactose are also hexose
same chemical formula but structurally different
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Fructose naturally occurs in fruit
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Fructose:
Pentose shape but a hexose
monosaccharide because it contains 6
carbons
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Present in nectar to attract insects for
pollination
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Hydrolysed to monosaccharides to be
oxidised in respiration for energy
production
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C6H22O11
Non reducing sugar
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Starch:
Many alpha glucose molecules join to form 2 different
polysaccharides, known collectively as starch
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Found in amyloplasts of plants in cytoplasm or
chloroplasts
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Amylose:
Formed by alpha glucose molecules joined in alpha 1,4
glycosidic linkages
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Iodine used to test for starch; it sits in the core of the
helix
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Pentose monosaccharides:
Contain 5 carbon atoms
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- Also made by alpha 1,4 glycosidic bonds
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- Alpha 1,6 glycosidic bonds create branched structure
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- Alpha linkages can be broken down by amylase enzyme
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Carbohydrates Revision
Glycogen:
- Functionally equivalent to starch
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- Stored in muscle and liver cells
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- Forms more branches than amylopectin – 3x more
highly branched
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)
- Branches at alpha 1,6 glycosidic linkages every 8
glucose subunits
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- Compact and insoluble
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Chitin:
- long-chain polymer of Nacetylglucosamine, a derivative of glucose
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- Added N-acetyl group
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- Used to make strong, flexible surgical
thread
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Hydrolysis:
- Glucose stored as starch by plants or glycogen
by animals/fungi until it is needed for
respiration
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- To release glucose for respiration, starch or
glycogen undergo hydrolysis reactions, which
require addition of water molecules
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- Reverse reaction of condensation reaction that
forms glycosidic bonds
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- Structural molecule
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- Hydroxyl groups on carbon 1 and 4 are too far away from each other
to react
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- This polysaccharide is unable to coil/form branches, forms straight
chained cellulose molecule
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- Wood = 50% cellulose
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- Straight chained cellulose molecules lie adjacent to each other,
forming hydrogen bonds
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- Microfibrils join together to form macrofibrils
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- They are strong and insoluble; have a high tensile strength – can be
pulled and won’t tear
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- Cellulose isn’t nutritional to humans because we don’t have cellulase
enzymes to hydrolyse the beta 1,4 linkage
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- Cows/horses/sheep have cellulase enzyme so can digest cellulose
and get its nutrients
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- All monosaccharides and some disaccharides (maltose and lactose)
are reducing sugars
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- Benedict’s reagent is an alkaline solution of copper sulfate
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)
2) Add equal volume of Benedict’s reagent
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- Repeat the experiment 3x in order to gain reproducible results
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- Reducing sugars react with the blue Cu2+ ions in Benedict’s
reagent, reducing them to brick red Cu+ ions
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- The more reducing sugar present, the more precipitate formed and
the less blue Cu2+ ions are left in the solution
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Suggest how you could distinguish a sample of glucose from a
sample of maltose using Benedict’s reagent
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Perform the test for reducing sugars on both (using Benedict’s reagent
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Maltose will
also be twice as concentrated
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- Colour change from yellow/brown to purple/black means starch is present
Reagent strips:
- Used to test for presence of reducing sugars, commonly glucose
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)
- Sucrose = most common non-reducing sugar
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- Sucrose is hydrolysed by the acid to produce glucose and fructose, which are
reducing sugars
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- The more concentrated a solution is, the more light it will absorb and the less light it will transmit
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1) Filter placed in colorimeter
2) Colorimeter calibrated using distilled water
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4) Resulting solutions filtered to remove precipitate
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6) Calibration curve plotted
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cuvette

Explain why the transmission of red light increases as the glucose concentration in the sample rises
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These cu+ ions form a precipitate in the bottom of the centrifuge tubes
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The more concentrated the glucose solution, the less unreacted cu2+
ions remain – therefore the solution is a paler blue colour
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1
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The solution is then
added to a cuvette, leaving the precipitate behind in the bottom of the centrifuge tubes
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1
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e
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Or, the test could be repeated with a glucose solution that is more concentrated
than 1%
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Centrifuge
tube



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