Biological Molecules

Testing for Biological Molecules

 Benedict’s Test for Reducing Sugars:

o An equal volume of sample and Benedict’s solution (alkaline copper sulfate) is mixed

o Heat in a water bath at a temperature above 80°C

 Positive: blue (after changing several colours) → brick red precipitate (copper oxide)

 Reducing sugar + Cu2+ → oxidized sugar + Cu + ( Cu irons are reduced/gains electrons)

 Negative: Benedict’s reagent remains blue

 Hydrolysis for non-reducing sugars (sucrose):

In case of a negative result with Benedict’s test, take a fresh solution sample

o Hydrochloric acid is added to hydrolyse the sugar (if non-reducing sugar is present, it will break down
into monosaccharides glucose and fructose)

o Heat in a water bath for approximately 2 minutes

o Sodium hydroxide is added to make the solution alkaline.

o Benedict’s reagent is added, and Benedict’s test is carried out

 Semi-quantitative Benedict’s test:

o Time taken for first colour change: measure the time taken for the colour to change from blue to green

 The faster the colour change occurs, the greater the concentration of reducing sugar in the
sample

o Comparison to colour standards: Allow the Benedict’s test to completely run through a sample with an
unknown reducing sugar concentration

 Compare the final colour of the solution with the colours obtained from carrying out Benedict’s
test on solutions with known reducing sugar concentrations to make an estimate.

 Biuret’s test for proteins:

o Equal amounts of the sample and Biuret’s solution are added together (no heating required)

 Positive: blue → purple

 Negative: Biuret’s reagent remains blue

O the colour develops slowly over a few minutes.

 Emulsion test for lipids:

o The sample is added to 2cm3 of ethanol and mixed well until it dissolves (lipids are soluble in ethanol)

o This mixture is then placed into a test tube containing the same volume of cold water

 Positive: milky emulsion

 Negative: remains clear

 Iodine test for Starch:

o Add 2-3 drops of iodine to the liquid that is to be tested

 Positive: orange-brown → blue-black

 Negative: The iodine solution remains orange-brown

Carbohydrates and Lipids

 Monomer: a simple molecule that is used as a basic building block for the synthesis of a polymer; many
monomers are joined together to make the polymer, usually by condensation/dehydration reactions, e.g.
monosaccharides, amino acids, nucleotides.

o Glucose is a monomer (monosaccharide) with the molecular formula C₆H₁₂O₆

o Two kinds of glucose, alpha-glucose and beta-glucose, depending on the position of the OH (hydroxyl)
group in its ring structure

 Polymer: is a giant molecule made from repeating subunits of monomers that are similar or identical to each
other, e.g. polysaccharides, proteins, nucleic acids

o Lipids are NOT polymers, as they are not made of repeating subunits of monomers. They are made of 3
molecules of fatty acid and one molecule of glycerol, lacking a monomer unit

 Macromolecules: These are large and complex molecules that are formed due to the polymerisation of smaller
monomers, e.g. polysaccharides, nucleic acids, proteins

 Monosaccharide: This molecule consists of a single sugar unit, the simplest form of carbohydrate and cannot be
hydrolysed further. It has a general formula of (CH₂O)n

o Examples: glucose, fructose, galactose (all are reducing sugars)

 Disaccharide: a sugar molecule consisting of two monosaccharides joined together by a glycosidic bond.

o Examples: sucrose (non-reducing sugar), maltose, lactose (reducing sugars)

 Polysaccharide: a polymer whose subunits are monosaccharides joined together by glycosidic bonds.

o Examples: starch, glycogen, cellulose ( most abundant molecule on Earth)

 Glycosidic bonds: covalent bonds that occur between constituent monomers and are formed due to a
condensation reaction (involving removing a water molecule) to form polysaccharides and disaccharides such as
sucrose.

o Hydrolysis (addition of water) can also separate these constituent molecules, which breaks the glycosidic
bond between monomers.

- Reverse of condensation
- This is the reaction that occurs to break non-reducing sugars into reducing sugars when
hydrochloric acid is added before Benedict’s test
Polysaccharides

Starch:

 A macromolecule that is found in plant cells

 Polymer made up of glucose (monomer) subunits

 Contain 1,4 glycosidic bonds

 Highly compact and stores energy

 Made of two components: amylose and amylopectin

 Amylose Amylopectin

Structure α 1,4 glycosidic bonds α 1,4 and α 1,6 glycosidic bonds, giving it its branched structure

Shape Helical and more compact Branched

Glycogen:

 A macromolecule that is used for the storage of energy in animal cells

 Polymer made from α glucose subunits

 The structure of glycogen is very similar to that of amylopectin; however, it contains more α 1,6 glycosidic bonds
and is, hence, more branched

Cellulose:

 Found in the cell wall of plant cells

 Polymer made from β glucose units

 β-1,4 glycosidic bonds

 Alternate β-glucose molecules are rotated 180 degrees to form these bonds

 Hydrogen bonds are also formed between parallel cellulose molecules

 60 and 70 cellulose molecules become tightly cross-linked to form bundles called microfibrils

 Microfibrils are, in turn, held together in bundles called fibres by hydrogen bonding

 Fibres increase tensile strength to withstand osmotic pressure, making the plant rigid and determining cell shape
  Freely permeable

Triglycerides and Phospholipids

 Triglycerides:

o Formed by condensing 3 fatty acid chains and a glycerol molecule

o Joined by an ester bond

o Fatty acid chains are long hydrocarbon chains with a carboxylic head; glycerol is an alcohol containing 3
OH groups.

o Non-polar molecules

 Unsaturated fatty acids: contain c=c bonds that are easier to break and melt easily. Vegetable
oils.

 Saturated fatty acids: contain c-c bonds that are solids at room temperature. Animal fats.

 Role of triglyceride:

o Better energy reserves than carbohydrates as more CH bonds

o Acts as an insulator and provides buoyancy

o A metabolic source of water gives CO2 and H20 to oxidation in respiration

  Phospholipid:

o The hydrophilic head contains a phosphate group and glycerol, while the hydrophobic tail contains 2
fatty acid chains.

o The partial negative charge on the phosphate group gets attracted to the partial positive charge on the
hydrogen atom of the water molecule and thus faces the aqueous environment.

Proteins

Proteins: Made of amino acids which only differ in the R- groups/ variable side chains and will always contain an amine
group (basic), a carboxyl group (acidic) and a hydrogen atom attached to the central carbon atom.

 A peptide bond is formed by condensation between 2 amino acids, forming a dipeptide.

 Many amino acids that join together by peptide bonds form a polypeptide.
  Peptide bonds are broken when hydrolysed into amino acids, often occurring in the small intestine and stomach.

Protein Structure

 Primary structure:

o The sequence of amino acids in a polypeptide/protein

o A slight change in the sequence of amino acids can affect the protein’s structure and function

o It has a unique sequence for each protein.

 Secondary structure:

o The structure of a protein molecule resulting from the regular coiling or folding of the chain of amino
acids

 α- helix: the polypeptide chain twists into a regular spiral and is maintained by hydrogen bonds
between the (-NH) group of one amino acid and the (CO-) group of another amino acid 4 spaces
later in the polypeptide chain.

 β- pleated sheet: the chain is not tightly coiled and lies in a looser, straighter shape.
  Tertiary structure:

o The compact structure of a protein molecule results from the three-dimensional coiling of the already-
folded chain of amino acids.

 Hydrogen bonds between wide varieties of R-groups (can be broken by PH and temperature
changes)

 Disulphide bridges between two cysteine molecules (can be broken by reducing agents).
Covalent bonds

 Ionic bonds between R groups containing amine and carboxyl groups. (Can be broken by PH
changes.)

 Hydrophobic interactions between non-polar R groups.

 In order of increasing strength:

hydrophobic interactions < hydrogen bonds < ionic bonds < disulphide bridges
  Quaternary structure:

o The three-dimensional arrangement of two or more polypeptides or a polypeptide and a non-protein

component, such as haem, in a protein molecule

o The polypeptide chains are held together by bonds in the tertiary structure.

Globular and Fibrous Proteins

 Globular proteins:

o Curl up into a spherical shape with their hydrophobic regions pointing into the centre of the molecule
and hydrophilic regions pointing outwards

o They are soluble in water, e.g. enzymes and haemoglobin.

 Fibrous proteins:

o Form long strands, are insoluble in water, and have structural roles, e.g. collagen, hair, and nails.

 Haemoglobin:

o A globular protein that has a quaternary structure with 4 polypeptide chains, 2 α-globin and 2 β-globin
chains

o Each chain has one prosthetic haem group containing an iron atom that reversibly binds to an oxygen
molecule.

o Oxyhaemoglobin is bright red when the haem group is combined with oxygen; otherwise, it’s purplish.

 Collagen:

o A fibrous protein that is present in the skin, bones, teeth, cartilage and walls of blood vessels

o It is an important structural protein.

o A collagen molecule has 3 polypeptide chains that are coiled in the shape of a stretched-out helix

o Compact structure and almost every 3rd amino acid is glycine, the smallest amino acid which can form
H-bonds

o 3 polypeptide strands are held together by hydrogen and covalent bonds

o Many of these collagen molecules lie side by side, linked to each other by covalent cross-links between
the side chains of amino acids, forming fibrils, and many fibrils make up a fibre
Water

 Hydrogen Bonding

o A water molecule contains two hydrogen atoms and one oxygen atom held together by hydrogen bonds

 Solvent

o Water is an effective solvent because of its polarity so that it can form electrostatic interactions with
other polar molecules and ions

o Thus, it’s a transport medium and reagent for metabolic and other reactions in the cells of plants and
animals

 High surface tension and cohesion

o Cohesion refers to the attraction of one water molecule to the other

o Water molecules have strong, cohesive forces due to hydrogen bonds, thus having high surface tension

 High specific heat capacity

o The amount of heat energy required to raise the temperature of 1 kg of water by 1 °C

o Water has high SPC due to its hydrogen bonds

o Temperature within organisms remains constant compared to external temperature, and water bodies
also have a slow change in temperature, providing stable aquatic habitats.

 High latent heat of vaporisation

o A measure of the heat energy needed to vaporise a liquid

o Water has a high LHV due to its high SPC, as H bonds need to be broken before water can be vaporised,
cooling the surrounding environment.

o Sweating is a good cooling mechanism

o A large amount of energy can be lost for a small amount of water

o Thus, dehydration is prevented e.g. in transpiration.

 Density and freezing properties

o Ice is less dense than water and floats on it, insulating water and preventing it from freezing, preserving
aquatic life underneath it

o Changes in the density of water with temperature cause currents, which help to maintain the circulation
of nutrients in the oceans.