2 Biomolecules
2.1 Testing for biomolecules • equal volumes of sample and Biuret reagent are
mixed
Food tests • if proteins are present, the colour changes from blue
to lilac
1) Reducing sugars
• instead of biuret reagent, KOH and dil. CuSO4 can be
• reduce soluble blue copper sulphate containing copper used
(II) ions to insoluble brick-red copper oxide, containing
copper (I) For extra details on performing food tests, see paper 3
• the copper oxide is seen as a brick-red ppt notes.
2.2 Carbohydrates and lipids
All living organisms are made of C, H, O + N molecules.
• add equal volumes of Benedict’s reagent and the food
sample to a test tube Carbohydrates
• heat in a water bath at 80°C
• composed of C, H, O
• if reducing sugars are present, the following colour • divided into monosaccharides, disaccharides,
changes are observed: polysaccharides
• monomer – one of many small molecules that
combine to form a polymer, e.g. – monosaccharides,
2) Non-reducing sugars amino acids, nucleotides
• e.g., sucrose • polymer – large molecule made from many similar
• disaccharide is first broken down into its 2 repeating subunits, e.g. – polysaccharides, proteins,
monosaccharide constituents in a hydrolysis reaction nucleic acids
• this is done by adding HCl, and then neutralising the
acid with an alkali such as sodium bicarbonate • macromolecule – large molecule formed due to
• constituent monosaccharides will be reducing sugars polymerisation of monomers, e.g. – polysaccharides,
and their presence can be tested by Benedict’s test proteins (polypeptides), nucleic acids
(polynucleotides)
3) Starch Monosaccharides
• add drops of iodine solution to the sample Molecule consisting of a single sugar unit with the general
• if blue-black colour is quickly produced, starch is formula C(H2O)n
present • dissolves in water
• iodine solution is yellow brown • main types of monosaccharides – trioses (3C),
pentoses (5C), hexoses (6C)
4) Lipids (emulsion test) • glucose, fructose galactose, ribose, deoxyribose
• sample is shaken with ethanol HEXOSES PENTOSES
• any lipids present will dissolve
• mixture of ethanol and sample is poured into a
beaker containing water
• if lipids are present, a cloudy-white suspension is
formed due to the lipids being unable to remain
dissolved when mixed with water
• therefore, the lipid molecules form droplets
throughout the liquid, this kind of mixture is called the
emulsion Image: https://tlamjs.com/2017/01/13/biological-molecules-carbohydrates/
Roles of monosaccharides
5) Proteins (Biuret test) 1) source of energy in respiration – C-H bonds can be
• all proteins have peptide bonds containing nitrogen broken to release a lot of energy which is
atoms which form a purple complex with Cu2+ ions transferred to help make ATP from ADP
1 www.alevel-notes.weebly.com
, 2) building blocks for larger molecules – glucose is uses chains are shorter than
to make the polysaccharides starch, glycogen, and long, unbranching chain amylose and branch out to
cellulose; ribose is one of the molecules used to make sides
RNA and ATP, deoxyribose is one of the molecules
used to make DNA chains are curved and
coil into helical
branches are formed by 1-6
Disaccharides structures making the
linkages
final molecule more
Sugar molecule consisting of 2 monosaccharides joined
compact
by a glycosidic bond.
2) Glycogen
• made of chains of 1-4 linked ⍺-glucose molecules
with 1-6 linkages forming branches
• tend to be more branched than amylopectin
molecules
• many ends due to branching aids in easy addition
and removal of glucose
• compact and insoluble, doesn’t affect the water
potential (Ψ)
3) Cellulose → polymer of β-glucose
• in order to form a glycosidic bond with the 4th carbon
Image: https://dopeahmeanbio.wordpress.com/tag/glycosidic-bonds/ atom where the OH group is below the ring; every
• H2O molecule is removed; the bond formed by other glucose is rotated 180°
condensation is called a glycosidic bond • so successive glucose molecules are linked 180° to
each other
Polysaccharides • one oxygen is up and the other is down
A polymer whose subunits are monosaccharides joined • the molecules are still linked 1-4
by glycosidic bonds
• e.g., starch, glycogen, cellulose (all polymers of
glucose)
• not sugars
• if glucose itself accumulated in cells, it would
dissolve and make the contents of the cell too
concentrated which affects its osmotic properties
• cellulose molecules become tightly cross-linked to
• storage polysaccharides – convenient, compact,
form bundles called microfibrils
inert, insoluble
• microfibrils are held together in bundles called fibres
by hydrogen bonding
1) Starch à amylose + amylopectin • cellulose fibres have very high tensile strength – this
AMYLOSE AMYLOPECTIN makes it possible for a cell to withstand large
pressures as a result of osmosis
• cellulose fibres, despite their strength, are freely
permeable
Image: http://futurefoodchemist.weebly.com/
Image: https://www.e-education.psu.edu/
made by condensation
reactions between 1,4 also made of 1,4 linked ⍺-
linked ⍺-glucose glucose molecules
molecules
Image: unknown
2 www.alevel-notes.weebly.com
2.1 Testing for biomolecules • equal volumes of sample and Biuret reagent are
mixed
Food tests • if proteins are present, the colour changes from blue
to lilac
1) Reducing sugars
• instead of biuret reagent, KOH and dil. CuSO4 can be
• reduce soluble blue copper sulphate containing copper used
(II) ions to insoluble brick-red copper oxide, containing
copper (I) For extra details on performing food tests, see paper 3
• the copper oxide is seen as a brick-red ppt notes.
2.2 Carbohydrates and lipids
All living organisms are made of C, H, O + N molecules.
• add equal volumes of Benedict’s reagent and the food
sample to a test tube Carbohydrates
• heat in a water bath at 80°C
• composed of C, H, O
• if reducing sugars are present, the following colour • divided into monosaccharides, disaccharides,
changes are observed: polysaccharides
• monomer – one of many small molecules that
combine to form a polymer, e.g. – monosaccharides,
2) Non-reducing sugars amino acids, nucleotides
• e.g., sucrose • polymer – large molecule made from many similar
• disaccharide is first broken down into its 2 repeating subunits, e.g. – polysaccharides, proteins,
monosaccharide constituents in a hydrolysis reaction nucleic acids
• this is done by adding HCl, and then neutralising the
acid with an alkali such as sodium bicarbonate • macromolecule – large molecule formed due to
• constituent monosaccharides will be reducing sugars polymerisation of monomers, e.g. – polysaccharides,
and their presence can be tested by Benedict’s test proteins (polypeptides), nucleic acids
(polynucleotides)
3) Starch Monosaccharides
• add drops of iodine solution to the sample Molecule consisting of a single sugar unit with the general
• if blue-black colour is quickly produced, starch is formula C(H2O)n
present • dissolves in water
• iodine solution is yellow brown • main types of monosaccharides – trioses (3C),
pentoses (5C), hexoses (6C)
4) Lipids (emulsion test) • glucose, fructose galactose, ribose, deoxyribose
• sample is shaken with ethanol HEXOSES PENTOSES
• any lipids present will dissolve
• mixture of ethanol and sample is poured into a
beaker containing water
• if lipids are present, a cloudy-white suspension is
formed due to the lipids being unable to remain
dissolved when mixed with water
• therefore, the lipid molecules form droplets
throughout the liquid, this kind of mixture is called the
emulsion Image: https://tlamjs.com/2017/01/13/biological-molecules-carbohydrates/
Roles of monosaccharides
5) Proteins (Biuret test) 1) source of energy in respiration – C-H bonds can be
• all proteins have peptide bonds containing nitrogen broken to release a lot of energy which is
atoms which form a purple complex with Cu2+ ions transferred to help make ATP from ADP
1 www.alevel-notes.weebly.com
, 2) building blocks for larger molecules – glucose is uses chains are shorter than
to make the polysaccharides starch, glycogen, and long, unbranching chain amylose and branch out to
cellulose; ribose is one of the molecules used to make sides
RNA and ATP, deoxyribose is one of the molecules
used to make DNA chains are curved and
coil into helical
branches are formed by 1-6
Disaccharides structures making the
linkages
final molecule more
Sugar molecule consisting of 2 monosaccharides joined
compact
by a glycosidic bond.
2) Glycogen
• made of chains of 1-4 linked ⍺-glucose molecules
with 1-6 linkages forming branches
• tend to be more branched than amylopectin
molecules
• many ends due to branching aids in easy addition
and removal of glucose
• compact and insoluble, doesn’t affect the water
potential (Ψ)
3) Cellulose → polymer of β-glucose
• in order to form a glycosidic bond with the 4th carbon
Image: https://dopeahmeanbio.wordpress.com/tag/glycosidic-bonds/ atom where the OH group is below the ring; every
• H2O molecule is removed; the bond formed by other glucose is rotated 180°
condensation is called a glycosidic bond • so successive glucose molecules are linked 180° to
each other
Polysaccharides • one oxygen is up and the other is down
A polymer whose subunits are monosaccharides joined • the molecules are still linked 1-4
by glycosidic bonds
• e.g., starch, glycogen, cellulose (all polymers of
glucose)
• not sugars
• if glucose itself accumulated in cells, it would
dissolve and make the contents of the cell too
concentrated which affects its osmotic properties
• cellulose molecules become tightly cross-linked to
• storage polysaccharides – convenient, compact,
form bundles called microfibrils
inert, insoluble
• microfibrils are held together in bundles called fibres
by hydrogen bonding
1) Starch à amylose + amylopectin • cellulose fibres have very high tensile strength – this
AMYLOSE AMYLOPECTIN makes it possible for a cell to withstand large
pressures as a result of osmosis
• cellulose fibres, despite their strength, are freely
permeable
Image: http://futurefoodchemist.weebly.com/
Image: https://www.e-education.psu.edu/
made by condensation
reactions between 1,4 also made of 1,4 linked ⍺-
linked ⍺-glucose glucose molecules
molecules
Image: unknown
2 www.alevel-notes.weebly.com
