Biological molecules

• Monosaccharides


  Small, soluble carbohydrate monomers including fructose and galactose
• Key features of Glucose


  • A monosaccharide with the formula C6H12O6
  • A hexose monosaccharide (six carbon atoms) in a ring structure
  • Soluble in water → easily transported
  • Main energy source for animals and plants → chemical bonds store lots of energy
  • Two isomers: α-glucose and β-glucose → H and OH groups on carbon 1 inverted in β-glucose
• Glycosidic bonds and condensation/hydrolysis reactions

  1. Condensation reaction: two molecules join to form a new chemical bond and a water molecule is eliminated

  2. Condensation reactions form glycosidic bonds between monosaccharides to create disaccharides and polysaccharides

  3. Hydrolysis reaction: a water molecule is used and the chemical bond is broken (reverse of a condensation reaction → breaks glycosidic bonds)
• Benedict’s test for sugars


  1. Add an excess of blue Benedict’s reagent to liquid food sample in a test tube.

  2. Heat the tube in a water bath set to boil.

  3. If reducing sugars are present: coloured precipitate forms. End test here.

  4. If no reducing sugars are present: solution stays blue. Go to step 5.

  5. Break down non-reducing sugars to monosaccharides: add dilute HCl to new sample and heat in a water bath set to boil.

  6. Neutralise with sodium hydrogencarbonate, then repeat steps 1) and 2).

  7. If coloured precipitate now forms, non-reducing sugars are present in the sample.

  8. If the solution is still blue, neither type of sugars are present.
• Disaccharides


  • Maltose = glucose + glucose
  • Sucrose = glucose + fructose
  • Lactose = glucose + galactose
• Polysaccharides


  • Large polymers of monosaccharides joined with glycosidic bonds
  • Starch and glycogen are large energy storage molecules which cannot leave cells
• Starch is insoluble in water and does not affect the water potential of cells so water is not drawn in by osmosis
• Amylose is an unbranched α-glucose polysaccharide with 1,4 glycosidic bonds, having a helical structure for compactness
• Amylopectin is a branched α-glucose polysaccharide with 1,4 and 1,6 glycosidic bonds, allowing faster glucose release due to easy access by enzymes
• Glycogen is excess glucose storage in animals, easily hydrolysed when glucose is needed, and insoluble in water
• Carbohydrates


  • Excess glucose storage in animals
  • Easily hydrolysed when glucose is needed
  • Insoluble in water
  • Highly branched α-glucose polysaccharide (1,4 and 1,6 glycosidic bonds)
• Glycogen


  • Excess glucose storage in animals
  • Easily hydrolysed when glucose is needed
  • Insoluble in water
  • Highly branched α-glucose polysaccharide (1,4 and 1,6 glycosidic bonds)
• Cellulose


  • Found in plant cell walls to give strength
  • Unbranched long and straight β-glucose polymers (1,4 glycosidic bonds)
  • Chains linked with many hydrogen bonds to form strong rigid microfibrils
• Iodine test for starch

  1. Add iodine in potassium iodide solution to sample

  2. If starch is present, it goes from browny-orange to blue-black
• Carrying out the Benedict’s test on all tubes and a negative control (distilled water) will produce a calibration curve
• Using a colorimeter


  1. A more accurate and quantitative way to measure glucose concentration after the Benedict’s test

  2. When the precipitate is filtered out of the solution, the solution left is the Benedict’s reagent

  3. A colorimeter measures absorbance of light, lower absorbance = more blue colour lost = more glucose

  4. Zero the colorimeter to distilled water to make sure values are comparable

  5. Control the volume and concentration of Benedict’s solution used

  6. Control the duration of time in the boiling water bath

  7. Can use a serial dilution of a known concentration of glucose to produce a calibration curve
• Interpolation


  Measure the absorbance of a glucose solution with an unknown concentration and use the calibration curve to find the concentration
• If the precipitate was not filtered out of the solution, absorbance would increase with increasing glucose concentration
• Triglycerides


  • One molecule of glycerol bound to three fatty acids
  • Fatty acids join to glycerol in a condensation reaction forming an ester bond and releasing a water molecule
  • Energy store, insoluble in water, clump together in droplets with hydrophobic tails facing inwards
• Fatty acids


  • Variable R group, saturated fatty acids have no double C=C bonds in the hydrocarbon tail
• Lipids


  Contain carbon, hydrogen, and oxygen
• Fatty acids


  • Have a variable R group (hydrocarbon tail)
  • Saturated fatty acids have no double C=C bonds in the hydrocarbon tail
  • Unsaturated fatty acids have one or more double C=C bonds in the hydrocarbon tail so the chain kinks
  • Hydrocarbon tails are hydrophobic (insoluble in water)
• Phospholipids


  • One molecule of glycerol bound to two fatty acids and a phosphate group
  • Phosphate groups are hydrophilic, fatty acids are hydrophobic
  • Form a bilayer in cell membranes, water-soluble (polar) substances cannot pass through the hydrophobic centre of the bilayer
• Emulsion test for lipids

  1. Mix food sample with ethanol and shake until dissolved

  2. Pour mixture into water

  3. If lipid is present, milky emulsion forms (the more obvious it is, the more lipid there is)

  4. If no lipid is present, stays clear
• Proteins are made from amino acids which are the monomers of proteins
• Amino acids contain


  • Carbon, hydrogen, nitrogen, oxygen, and sometimes sulfur
  • Have a carboxyl group, an amine group, and a variable R group
  • There are 20 different amino acids, each with a different R group
• Peptide bonds and dipeptides

  1. A condensation reaction joins two amino acids with a peptide bond producing a dipeptide and a molecule of water

  2. A hydrolysis reaction breaks a peptide bond by adding a molecule of water

  3. A polypeptide is a polymer of amino acids (a long chain of amino acids joined with peptide bonds)
• Protein structure


  • Primary structure determines the overall protein structure
  • Secondary structure includes alpha helix (coiled) or beta pleated sheet (folded) with hydrogen bonds between N-H and C=O parts of amino acids
  • Tertiary structure involves further folding with bonds between R groups of amino acids, more hydrogen bonds, ionic bonds between positively and negatively charged R groups, and disulfide bridges between cysteine amino acids, forming the final structure for proteins made from one polypeptide
  • Quaternary structure includes proteins with more than one polypeptide chain held together with bonds like disulfide bridges
• Polymers


  Molecules made from a large number of monomers joined together in a chain
• Biuret test for proteins

  1. Add a few drops of sodium hydroxide solution to the sample to make it alkaline

  2. Add copper (II) sulfate solution

  3. If protein is present: solution turns purple

  4. If no protein is present: solution stays blue
• Functions of proteins


  • Enzymes: soluble and almost spherical, tightly folded polypeptides, catalyse metabolic reactions, examples: amylase digests starch, lipase digests lipids
  • Structural proteins: provide strength and support, long polypeptide chains parallel or twisted, held together with cross-links e.g. disulfide bonds, examples: collagen in connective tissue, keratin in hair and nails
  • Antibodies: made by plasma cells in the immune response, bind to one specific antigen
  • Transport proteins: found in cell membranes, channel proteins and carrier proteins, hydrophobic and hydrophilic regions help form shape
• Chromatography


  1. Separates particles based on their different affinities for the stationary phase vs the mobile phase

  2. Particles can adsorb to stationary phase

  3. Particles are soluble in the mobile phase
• Chromatography types


  • Paper chromatography: stationary phase is paper, mobile phase is normally water
  • Thin layer chromatography (TLC): stationary phase is silica gel, mobile phase is normally an organic solvent
• R groups in amino acids

  • Only R groups differ in amino acids, R group determines the interaction with the stationary and mobile phases
  • A more soluble amino acid spends more time in the mobile phase
• Chromatography method


  1. Draw a pencil line near the bottom of the paper or TLC plate

  2. Put concentrated spots of mixtures onto the pencil line

  3. Place the paper or TLC plate into the solvent

  4. Allow the solvent to move up until nearly at the top, then remove paper or TLC plate

  5. Stain the paper or TLC plate with ninhydrin to visualise the amino acids

  6. Calculate the Rf values

  7. Look up Rf values in standard reference tables to identify amino acids in the mixture
• Enzymes


  • Proteins with a specific tertiary structure
  • Active site is complementary to a specific substrate
  • Biological catalysts speed up a reaction by lowering the activation energy
  • Can be intracellular (act inside cells) or extracellular (act outside cells)
  • Determine structure and function of cells and whole organisms
• As temperature increases


  Enzyme and substrate have more kinetic energy, leading to more frequent successful collisions and more enzyme-substrate complexes form
• After the optimum temperature

  The enzyme becomes denatured due to too much kinetic energy breaking the hydrogen bonds and ionic bonds between amino acid R groups, changing the shape of the active site so it is no longer complementary to the substrate, and enzyme-substrate complexes cannot form
• Effects of low temperature

  Are reversible, effects of high temperature are not
• Enzymes are denatured above and below the optimum pH

  H+ (acidic) or OH- (alkali) ions interfere with the hydrogen bonds and ionic bonds between amino acid R groups, changing the shape of the active site