2.1.2 Biological Molecules

Question 1

(q) how to carry out and interpret the results of the following chemical tests:

• Biuret test for proteins
• Benedict’s test for reducing and non-reducing sugars
• Reagent test strips for reducing sugars
• Iodine test for starch
• Emulsion test for lipids

Answer 1

• Biuret test for proteins
  
  • Add Biuret reagents (NaOH + copper(II)sulfate)
  • tests for the presence of peptide bonds
  • ​if present: turns pale blue to lilac
  • coloured complex formed between nitrogen atoms and Cu²⁺
• Benedict’s test for reducing sugars
  
  • reducing sugars = all monosaccharides + disaccharides; maltose and lactose
  • Add equal volume of Benedicts reagent (alkaline copper(II)sulfate)
  • Boil for 2 minutes at 80^oC
  • if present: brick red precipitate forms

Question 2

(c) the chemical elements that make up biological molecules
To include:

• C, H and O for carbohydrates
• C, H and O for lipids
• C, H, O, N and S for proteins
• C, H, O, N and P for nucleic acids

Answer 2

• C, H and O for carbohydrates
• C, H and O for lipids
• C, H, O, N and S for proteins
• C, H, O, N and P for nucleic acids

Question 3

(d) the ring structure and properties of glucose as an example of a hexose monosaccharide and the structure of ribose as an example of a pentose monosaccharide
To include the structural difference between an α- and a β-glucose molecule AND the difference between a hexose and a pentose monosaccharide.

Question 4

(e) the synthesis and breakdown of a disaccharide and polysaccharide by the formation and breakage of glycosidic bonds
To include the disaccharides sucrose, lactose and maltose

Answer 4

Disaccharide/Polysaccharide formed by condensation reaction (water = coproduct), broken down by hydrolysis reaction (water = reactant)

• α-glucose + α-glucose –> maltose + H₂O
• α-glucose + β-glucose –> lactose +H₂O
• α-glucose + fructose (pentose sugar) –> sucrose + H₂O

all of the above form 1-4 glycosidic bonds (between carbon 1 and 4)

Question 5

(h) the structure of a triglyceride and a phospholipid as examples of macromolecules
To include an outline of saturated and unsaturated fatty acids

Answer 5

• Triglyceride:
  
  • Glycerol
  • 3 fatty acids
• Phospholipid:
  
  • Phosphate group
  • Glycerol
  • 2 fatty acids
    
    • (1 saturated + 1 unsaturated)

Saturated: No double carbon to carbon bonds (C=C)
Unsaturated:

• Monunsaturated: 1 C=C bond
• Polyunsaturated: Multiple C=C bonds

Question 6

(k) the general structure of an amino acid

Answer 6

• amino acid = monomer of a protein

Question 7

(l) the synthesis and breakdown of dipeptides and polypeptides, by the formation and breakage of peptide bonds

Answer 7

Dipeptide/Polypeptide synthesized by condensation reaction
Dipeptide/Polypeptide broken down by hydrolysis reaction

Peptide bond forms (amide link –> CONH)

Question 8

(b) the concept of monomers and polymers and the importance of condensation and hydrolysis reactions in a range of biological molecules

Answer 8

Monomer: A small molecule which binds onto many other identical molecules to form a polymer
Polymer: A large molecule made from many smaller molecules called monomers
Condensation reaction: Reaction that occurs when two molecules are joined together with the removal of water
Hydrolysis reaction: Reaction that occurs when a molecule is split into two smaller molecules with the addition of water

Question 9

(m) the levels of protein structure
To include primary, secondary, tertiary and quaternary structure AND hydrogen bonding, hydrophobic and hydrophilic interactions, disulfide bonds and ionic bonds.

Answer 9

Primary structure:

• The sequence of amino acids
  
  • Bonds: Peptide bonds (amide link CONH)

Secondary structure:

• The coiling/folding of parts of a polypeptide due to formation of H-bonds. Two forms are: α-helix and β-pleated sheet
  
  • Bonds: Hydrogen bonds + Peptide bonds

Tertiary structure:

• 3D shape of a protein molecule. Two forms are: supercoiled shape for fibrous proteins or spherical shape for globular proteins
  
  • Bonds: Hydrophobic + Hydrophilic interactions, Disulfide bridges, Ionic bonds, Hydrogen bonds + Peptide bonds

Quaternary structure:

• Protein structure where a protein consists of more than one polypeptide chains
  
  • Bonds: Hydrophobic + Hydrophilic interactions, Disulfide bridges, Ionic bonds, Hydrogen bonds + Peptide bonds

Question 10

(f) the structure of starch (amylose and amylopectin), glycogen and cellulose molecules

Answer 10

Question 11

(p) the key inorganic ions that are involved in biological processes
To include the correct chemical symbols for the following cations and anions: cations: calcium ions (Ca²⁺), sodium ions (Na⁺), potassium ions (K⁺), hydrogen ions (H⁺), ammonium ions (NH₄ ⁺) anions: nitrate (NO₃ ^–), hydrogencarbonate (HCO₃ ^–), chloride (Cl ^–), phosphate (PO₄ ^3–), hydroxide, (OH^–)

Answer 11

• Cations:
  
  • Ca²⁺
  • Na⁺
  • K⁺
  • H⁺
  • NH₄ ⁺
• Anions:
  
  • NO₃ ^–
  • HCO₃ ^–
  • Cl ^–
  • PO₄ ^3–
  • OH^–

Question 12

(o) the properties and functions of fibrous proteins
To include collagen, keratin and elastin (no details of structure are required)

Answer 12

• Fibrous protein:
  
  • Relatively long + thin structure
  • Insoluble in water
  • Metabolically inactive + often have structural functions in an organism

Question 13

(d) the ring structure and properties of glucose as an example of a hexose monosaccharide and the structure of ribose as an example of a pentose monosaccharide
To include the structural difference between an α- and a β-glucose molecule AND the difference between a hexose and a pentose monosaccharide

Answer 13

Question 14

(j) how the properties of triglyceride, phospholipid and cholesterol molecules relate to their functions in living organisms
To include hydrophobic and hydrophilic regions and energy content AND illustrated using examples of prokaryotes and eukaryotes