biological molecules

• Carbohydrates are mostly polymers made of long chains of monosaccharide monomers
• Monosaccharide


  A molecule consisting of a single sugar unit, the simplest form of carbohydrate
• Disaccharide


  A sugar formed when two monosaccharides are joined together by glycosidic bonds
• Polysaccharide


  A carbohydrate which contains many monosaccharides bonded together by glycosidic bonds
• Glycosidic bonds


  Covalent bonds that form between monosaccharides in a condensation reaction
• Glycosidic bonds can be broken by hydrolysis, e.g. acid hydrolysis of non-reducing sugars to retrieve constituent monomers (sucrose)
• α-glucose


  Glucose monomers used in macromolecules that store energy, e.g. glycogen and starch
• β-glucose


  Glucose monomers used for structural purposes, e.g. cell walls
• Starch


  • A macromolecule found in plant cells, made up of amylose (helical) and amylopectin (branched) components
  • Highly compact and stores energy
• Glycogen


  • A macromolecule used for energy storage in animal cells, similar structure to amylopectin but more branched
• Cellulose


  • Found in plant cell walls, made from β-glucose units forming β-1,4 glycosidic bonds
  • Alternate β-glucose molecules are rotated 180 degrees
  • Hydrogen bonds form between parallel cellulose molecules
• Benedict's test for reducing sugars

  1. Add Benedict's reagent and heat in water bath

  2. Positive: forms coloured precipitate (bigger colour change indicates higher concentration)

  3. Negative: no colour change
• If Benedict's test is negative, there could still be non-reducing sugars present

  
• Hydrolyse glycosidic bonds


  1. Heat with dilute hydrochloric acid

  2. Run Benedict's test again

  3. Positive: non-reducing sugars present
• Iodine/potassium iodide test for starch

  1. Add iodine in potassium iodide

  2. Positive: iodine changes from red-brown to blue-black

  3. Negative: no change
• Monomer


  The simplest repeating unit of a polymer, e.g. glucose
• Polymer


  Made from monomers joined together by glycosidic bonds, e.g. starch
• Macromolecule


  Large and complex molecules formed by polymerisation of smaller subunits, e.g. starch
• Monosaccharides, nucleotides, and amino acids are examples of monomers
• Condensation reaction


  Joins two molecules together to form a chemical bond, involves elimination of water
• Hydrolysis reaction


  Breaks a chemical bond between two molecules, involves use of a water molecule
• Triglycerides


  • Made up of one glycerol molecule with three fatty acid chains attached
  • Formed by condensation reaction between glycerol and fatty acids
  • Can be saturated or unsaturated
• Phospholipids


  • Similar to triglycerides but one fatty acid is replaced by a hydrophilic phosphate group
  • Form the cell membrane bilayer with hydrophilic heads and hydrophobic tails

  
• Emulsion test for lipids

  1. Shake test substance with ethanol until dissolved, then add to water

  2. Positive: milky emulsion forms (more emulsion indicates more lipid)

  3. Negative: solution remains clear
• Amino acid


  The monomers that make up proteins, general formula NH2-CHR-COOH
• Peptide bond


  Forms between amino acids in a condensation reaction, joins them into a polypeptide chain
• Levels of protein structure

  • Primary
  • Secondary
  • Tertiary
  • Quaternary
• Primary structure


  The sequence of amino acids in the polypeptide chain
• Secondary structure


  Hydrogen bonds between -NH and -CO groups forming α-helix and β-pleated sheet structures
• Tertiary structure


  Further coiling and folding of the secondary structure, stabilised by hydrogen bonds, ionic bonds, and disulfide bridges
• Quaternary structure


  Several different polypeptide chains held together by bonds
• Proteins have many functions in living organisms e.g. enzymes, antibodies, transport proteins, and structural proteins
• Biuret test for proteins

  1. Add NaOH to make solution alkaline, then add copper(II) sulfate

  2. Positive: solution turns purple

  3. Negative: solution remains blue
• Enzyme


  A biological catalyst that lowers the activation energy of a metabolic reaction
• Overall energy released during a reaction is maintained, even though the enzyme lowers the activation energy
• Measuring enzyme effects


  1. Measure rates of product formation

  2. Measure rates of substrate decrease
• Enzyme-substrate complex


  The substrate is held in place at the enzyme's active site by weak bonds

  
• Lock-and-key theory


  The active site has a precise shape complementary to the substrate

  
• Induced-fit theory


  The enzyme's active site changes shape slightly as the substrate binds

  
• The induced-fit theory is a better model than the lock-and-key theory