Biological molecules
Benedicts test - reducing sugars
1. Add a few drops of Benedict’s reagent (copper II sulphate) into the sample
2. Heat mixture (boiling water bath) — copper II sulphate -> copper (I) oxide
3. Blue -> coloured precipitate (green -> yellow-> red -> brown)
- Semi-quantitative -> also shows the amount of of reducing sugars in the sample
- Greater concentration, greater colour change
- Time to rst colour change
- Or comparison to colour standards
Safety precautions
- handle test tube with tongs
- Wear safety goggles and heatproof gloves
! not when measuring enzyme activity!
Benedicts test - non reducing sugars
1. No colour change = no reducing sugars
2. Add dilute hydrochloric acid and heat, then neutralise
- acid will break down non-reducing sugars into monosaccharides
- After adding acid neutralise with sodium hydrogencarbonate (NaHCO3)
3. Heat and observe colour change - repeat Benedict’s test
Iodine test - starch
1. Add aqueous iodine solution (Lugol’s solution) to the sample
- Iodine from potassium iodide solution - iodine doesn’t dissolve in water
2. Brown orange -> dark blue-black
- starch iodine complex
Emulsion test - lipids
1. Add ethanol and shake
- lipid dissolves in ethanol
2. Pour the solution into water
- since lipids are insoluble in water they will not mix with water and will oat on top
3. Check for formation of cloudy (milky white) emulsion
- if negative no emulsion
Safety precaution: Do not use near open ame, ethanol is amable
Biuret test - proteins
1. Add sodium hydroxide - to make the solution alkaline
2. Add copper (II) sulphate solution
- Benedict’s solution contains alkali and copper (II), no need for step one
3. Blue -> purple
- for the test to work there must be at least two peptide bonds
fi fl fl fl
, De nitions:
Monomer - a relatively simple molecule which is used as a basic building block for the synthesis of a polymer
Polymer - a molecule made from many similar repeating subunits (monomers) joined together in a chain, through
covalent bonds
Macromolecule - a very large molecule such as a polysaccharide, protein, nucleic acid
Carbohydrates
Cx(H2O)y
Monosaccharide - single sugar monomer
Disaccharide - a sugar formed from two monosaccharides joined together by a glycosidic bond by condensation
polysaccharide - a polymer formed by many monosaccharides joined by glycosidic bonds in a condensation reaction
to form chains
Covalent bonds
- Bonds where atoms are joined by sharing electrons
- Strong, require large energy to break them
Glucose
- a monosaccharide containing six carbon molecules = hexose
Glucose isomers
- in aqueous solutions glucose forms a ring structure
Reducing sugars: glucose, fructose, maltose
Non-reducing sugars: sucrose
fi
Benedicts test - reducing sugars
1. Add a few drops of Benedict’s reagent (copper II sulphate) into the sample
2. Heat mixture (boiling water bath) — copper II sulphate -> copper (I) oxide
3. Blue -> coloured precipitate (green -> yellow-> red -> brown)
- Semi-quantitative -> also shows the amount of of reducing sugars in the sample
- Greater concentration, greater colour change
- Time to rst colour change
- Or comparison to colour standards
Safety precautions
- handle test tube with tongs
- Wear safety goggles and heatproof gloves
! not when measuring enzyme activity!
Benedicts test - non reducing sugars
1. No colour change = no reducing sugars
2. Add dilute hydrochloric acid and heat, then neutralise
- acid will break down non-reducing sugars into monosaccharides
- After adding acid neutralise with sodium hydrogencarbonate (NaHCO3)
3. Heat and observe colour change - repeat Benedict’s test
Iodine test - starch
1. Add aqueous iodine solution (Lugol’s solution) to the sample
- Iodine from potassium iodide solution - iodine doesn’t dissolve in water
2. Brown orange -> dark blue-black
- starch iodine complex
Emulsion test - lipids
1. Add ethanol and shake
- lipid dissolves in ethanol
2. Pour the solution into water
- since lipids are insoluble in water they will not mix with water and will oat on top
3. Check for formation of cloudy (milky white) emulsion
- if negative no emulsion
Safety precaution: Do not use near open ame, ethanol is amable
Biuret test - proteins
1. Add sodium hydroxide - to make the solution alkaline
2. Add copper (II) sulphate solution
- Benedict’s solution contains alkali and copper (II), no need for step one
3. Blue -> purple
- for the test to work there must be at least two peptide bonds
fi fl fl fl
, De nitions:
Monomer - a relatively simple molecule which is used as a basic building block for the synthesis of a polymer
Polymer - a molecule made from many similar repeating subunits (monomers) joined together in a chain, through
covalent bonds
Macromolecule - a very large molecule such as a polysaccharide, protein, nucleic acid
Carbohydrates
Cx(H2O)y
Monosaccharide - single sugar monomer
Disaccharide - a sugar formed from two monosaccharides joined together by a glycosidic bond by condensation
polysaccharide - a polymer formed by many monosaccharides joined by glycosidic bonds in a condensation reaction
to form chains
Covalent bonds
- Bonds where atoms are joined by sharing electrons
- Strong, require large energy to break them
Glucose
- a monosaccharide containing six carbon molecules = hexose
Glucose isomers
- in aqueous solutions glucose forms a ring structure
Reducing sugars: glucose, fructose, maltose
Non-reducing sugars: sucrose
fi
