1. Biological Molecules

• Monomer: smaller units that join together to form larger molecules
• Examples of monomers:

  • Monosaccharides (glucose, fructose, galactose)
  • Amino acids
  • Nucleotides
• Polymer: molecules formed when many monomers join together
• Examples of polymers:

  • Polysaccharides
  • Proteins
  • DNA / RNA
• Condensation reaction:

  • A chemical bond forms between 2 molecules
  • A molecule of water is produced
• Hydrolysis reaction:

  • A water molecule is used to break a chemical bond between 2 molecules
• Hexose monosaccharides:

  • Glucose
  • Fructose
  • Galactose
  • All have the molecular formula C6H12O6
• Type of bond formed when monosaccharides react:

  • (1,4 or 1,6) glycosidic bond
  • 2 monomers = 1 chemical bond = disaccharide
  • Multiple monomers = many chemical bonds = polysaccharide
• Disaccharides and how they form:

  • Maltose: glucose + glucose
  • Sucrose: glucose + fructose
  • Lactose: glucose + galactose
  • All have molecular formula C12H22O11
  • Formed through a condensation reaction that forms a glycosidic bond between 2 monosaccharides
• Structure of α-glucose:

  • OH H
• Structure and functions of starch:

  • Storage polymer of α-glucose in plant cells
  • Insoluble = no osmotic effect on cells
  • Large = does not diffuse out of cells
  • Amylopectin: 1,4 & 1,6 glycosidic bonds, branched
  • Amylose: 1,4 glycosidic bonds, helix with intermolecular H-bonds = compact
• Structure and functions of glycogen:

  • Main storage polymer of α-glucose in animal cells (also found in plant cells)
  • 1,4 & 1,6 glycosidic bonds, branched
  • Insoluble = no osmotic effect & does not diffuse out of cells
  • Compact
• Structure and functions of cellulose:

  • Polymer of β-glucose
  • Gives rigidity to plant cell walls
  • 1,4 glycosidic bonds
  • Straight-chain, unbranched molecule
  • Alternate glucose molecules are rotated 180°
  • H-bond crosslinks between parallel strands form microfibrils = high tensile strength
• Benedict’s test for reducing sugars:

  1. Add an equal volume of Benedict’s reagent to a sample

  2. Heat the mixture in an electric water bath at 100 ℃ for 5 mins

  3. Positive result: colour change from blue to orange & brick-red precipitate forms
• Benedict’s test for non-reducing sugars:

  1. Negative result: Benedict’s reagent remains blue

  2. Hydrolyse non-reducing sugars e.g. sucrose into their monomers by adding 1cm3 of HCl. Heat in a boiling water bath for 5 mins

  3. Neutralise the mixture using sodium carbonate solution

  4. Proceed with the Benedict’s test as usual
• Test for starch:

  1. Add iodine solution

  2. Positive result: colour change from orange to blue-black
• Colorimetry for qualitative results of sugars and starch:

  1. Make standard solutions with known concentrations. Record absorbance or % transmission values

  2. Plot calibration curve: absorbance or % transmission (y-axis), concentration (x-axis)

  3. Record absorbance or % transmission values of unknown samples. Use calibration curve to read off concentration
• To test for lipids in a sample:

  • Dissolve solid samples in ethanol
  • Add an equal volume of water and shake
  • Positive result: milky white emulsion forms
• Triglycerides form through a condensation reaction between 1 molecule of glycerol and 3 fatty acids forming ester bonds
• Contrast between saturated and unsaturated fatty acids:

  Unsaturated:

  • Contain C=C double bonds
  • 'Kinked' molecules have fewer contact points
  • Lower melting point = liquid at room temperature
  • Found in plant oils

  Saturated:

  • Contain only single bonds
  • Straight-chain molecules have many contact points
  • Higher melting point = solid at room temperature
  • Found in animal fats
• Structure of triglycerides related to functions:

  • High energy:mass ratio = high calorific value from oxidation (energy storage)
  • Insoluble hydrocarbon chain = no effect on water potential of cells & used for waterproofing
  • Slow conductor of heat = thermal insulation e.g. adipose tissue
  • Less dense than water = buoyancy of aquatic animals
• Structure and function of phospholipids:

  • Amphipathic molecule: glycerol backbone attached to 2 hydrophobic fatty acid tails & 1 hydrophilic polar phosphate head
  • Forms phospholipid bilayer in water = component of membranes
  • Tails can splay outwards = waterproofing
• Comparison between phospholipids and triglycerides:

  • Both have glycerol backbone
  • Both may be attached to a mixture of saturated, monounsaturated & polyunsaturated fatty acids
  • Both contain the elements C, H, O
  • Both formed by condensation reactions
• Contrast between phospholipids and triglycerides:

  Phospholipids:

  • 2 fatty acids & 1 phosphate group attached
  • Hydrophilic head & hydrophobic tail
  • Used primarily in membrane formation

  Triglycerides:

  • 3 fatty acids attached
  • Entire molecule is hydrophobic
  • Used primarily as a storage molecule (oxidation releases energy)
• Water and inorganic ions are not polymers; they are macromolecules
• Water is a polar molecule because O is more electronegative than H, forming O 𝛿 - (slight negative charge) & H 𝛿 + (slight positive charge)
• Biologically important properties of water:

  • Metabolite/solvent for chemical reactions in the body
  • High specific heat capacity
  • High latent heat of vaporization
  • Cohesion between molecules
• Water is significant to living organisms because it is a solvent for polar molecules during metabolic reactions, enables organisms to avoid fluctuations in core temperature, and provides cohesion-tension of water molecules in the transpiration stream
• Inorganic ions do not contain carbon atoms and are found in cytoplasm & extracellular fluid in high or very low concentrations
• Role of hydrogen ions in the body:

  • High concentration of H+ = low (acidic) pH
  • H+ ions interact with H-bonds & ionic bonds in tertiary structure of proteins, which can cause them to denature
• Role of iron ions in the body:

  • Fe2+ bonds to porphyrin ring to form haem group in haemoglobin
  • Haem group has binding site to transport 1 molecule of O2 around body in bloodstream
  • 4 haem groups per haemoglobin molecule
• Role of sodium ions in the body:

  • Involved in co-transport for absorption of glucose & amino acids in lumen of gut
  • Involved in propagation of action potentials in neurons
• Role of phosphate ions in the body:

  • Component of DNA, ATP, NADP, cAMP
• General structure of an amino acid:

  • COOH: carboxyl/carboxylic acid group
  • R: variable side group consists of carbon chain & may include other functional groups e.g. benzene ring or -OH (alcohol)
  • NH2: amine/amino group
• Test for proteins in a sample:

  • Biuret test confirms presence of peptide bond
  • Add equal volume of sodium hydroxide to sample at room temperature
  • Add drops of dilute copper (II) sulfate solution. Swirl to mix
  • Positive result: colour changes from blue to purple
  • Negative result: solution remains blue
• Number of amino acids: 20 differ only by side 'R' group
• Formation of dipeptides and polypeptides:

  • Condensation reaction forms peptide bond (-CONH-) & eliminates molecule of water
  • Dipeptide: 2 amino acids
  • Polypeptide: 3 or more amino acids
• Levels of protein structure: 4
• Primary structure of a protein:

  • Sequence, number & type of amino acids in the polypeptide
  • Determined by sequence of codons on mRNA
• Secondary structure of a protein:

  • Hydrogen bonds form between O 𝛿- (slightly negative) attached to ‒ C=O & H 𝛿+ (slightly positive) attached to ‒NH