Biological Molecules

• Define monomer and give 3 examples 


  A monomer is a small unit that can join together to form larger molecules.

  • Monosaccharides (glucose, fructose, galactose)
  • Amino acids
  • Nucleotides
• Define polymer and give 3 examples


  Molecules formed when many monomers join together

  • Polysacchardies
  • Proteins
  • DNA / RNA
• What happens in a condensation reaction? 


  A chemical bond forms between 2 molecules and a molecule of water is produced
• What happens in a hydrolysis reaction? 


  A water molecule is used to break a chemical bond between 2 molecules
• Name the 3 hexose monosaccharides

  • Glucose
  • Fructose
  • Galactose

  All have the molecular formula C6H12O6
• Name the type of bond formed when monosaccharides react

  (1,4 or 1,6) Glycosidic bond
• Name 3 disaccharides and describe how they form

  Condensation reaction forms glycosidic bond between two monosaccharides

  • Maltose: glucose + glucose
  • Sucrose: glucose + fructose
  • Lactose: glucose + galactose

  All have molecular formula C12H22O11
• Describe the structure and functions of starch

  Storage polymer of a-glucose in plant cells

  • Insoluble so no osmotic effect on cells
  • Large so does not diffuse out of cells

  Made from amylose:

  • 1,4 glycosidic bonds
  • Helix with intermolecular H-bonds = compact

  AND amylopectin:

  • 1,4 and 1,6 glycosidic bonds
  • Branched, so many terminal ends for hydrolysis into glucose
• Describe the structure and functions of glycogen

  Main storage polymer of a-glucose in animal cells (but also found in plant cells)

  • 1,4 AND 1,6 glycosidic bonds
  • Branched, so many terminal ends for hydrolysis
  • Insoluble so no osmotic effect and does not diffuse out of cells
  • Compact
• Describe the structure and functions of cellulose

  Polymer of b-glucose, gives rigidity to plant cell walls (prevents bursting under turgor pressure, holds stems up)

  • 1, 4 glycosidic bonds
  • Straight chain, branched molecule
  • Alternate glucose molecules are rotated 180 degrees
  • H-bond cross links between parallel strands form microfibrils = high tensile strength
• Describe the Benedict’s test for reducing sugars


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

  Heat the mixture in an electric weather bath at 100 Celsius for 5 mins

  Positive result: colour change from blue to orange. Brick-red ppt forms
• Describe the Benedict’s test for non reducing sugars

  Negative result: reagent remains blue

  • Hydrolyse non-reducing sugars E.g. sucrose into their monomers by adding 1cm3 of HCL. Heat in a boiling water bath for 5 mins
  • Neutralise the mixture using sodium carbonate solution
  • Proceed with the Benedict’s test as usual
• Describe the test for starch

  Add iodine solution

  Positive result: colour change from orange to blue-black
• Describe how 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
• How do triglycerides form?

  Condensation reaction between 1 molecule of glycerol and 3 fatty acids. Forms ester bonds
• Contrast saturated and unsaturated fatty acids

  Saturated

  • Contain only single bonds
  • Straight-chain molecules have many contact points
  • Higher melting point = solid at room temperature
  • Found in animal fats

  Unsaturated

  • Contain C=C double bonds
  • ’Kinked’ molecules have fewer contact points
  • Lower melting point = liquid at room temperature
  • Found in plant oils
• Relate the structure of triglycerides to their functions

  .

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

  Amphipathic molecule: glycerol backbone attached to 2 hydrophobic fatty acid tails and 1 hydrophilic polar phosphate head.

  • Forms phospholipid bilayer in water = component of membranes
  • Tails can splay outwards = waterproofing
• Compare phospholipids and triglycerides

  .

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

  Phospholipids:

  • 2 fatty acids & 1 phosphate group attached
  • Hydrophilic head and 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)
• Are phospholipids and triglycerides polymers?

  No. They are not made from a small repeating unit. They are macromolecules
• Why is water a polar molecule?

  O is more electronegative than H, so attracts the electron density in. the covalent bond more strongly

  Forms O delta negative and H delta positive
• State 4 biologically important properties of water

  Due to polarity and intermolecular H-bonds:

  • Metabolite / solvent for chemical reactions in the body
  • High specific heat capacity
  • High latent heat of vaporisation
  • Cohesion between molecules
• Explain why water is significant to living organisms

  .

  • Solvent for polar molecules during metabolic reactions
  • Enables organisms to avoid fluctuations in core temperature
  • Cohesion-tension of water molecules in transpiration stream
• What are inorganic ions and where are they found in the body?

  .

  • Ions that do NOT contain carbon atoms
  • Found in cytoplasm & extracellular fluid
  • May be in high or very low concentrations
• Explain the role of H+ 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
• Explain the 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 ONE MOLECULE of O2 around body in bloodstream.

  4 Haem groups per haemoglobin molecule
• Explain the role of Na+ ions in the body

  Involved in co-transport for absorption of gLacoste and amino acids in lumen of gut

  Involved in propagation of action potentials in neurons
• Explain the role of phosphate ions in the body

  Component of:

  • DNA
  • ATP
  • NADP
  • cAMP
• What is the general structure of an amino acid?

  Amino group (NH2), Carboxyl group (COOH), and Variable side group (R)
• Describe how to test for proteins in a sample


  Biuret test confirms presence of peptide bond

  1. Add equal volume of sodium hydroxide to sample at room temp
  2. Add drops of dilute copper (II) sulfate solution. Swirl to mix.
  3. Positive result: blue —> purple

  Negative result: solution remains blue
• How many amino acids are there and how do they differ from one another?

  20

  Differ only by R side group
• How do dipeptides and polypeptides form?


  Condensation reaction forms peptide bond and eliminates molecule of water

  • Dipeptides: 2 amino acids
  • Polypeptide: 3 or more amino acids
• How many levels of protein structure are ther?

  4
• Define primary structure of a protein


  Sequence, number and type of amino acids in the polypeptide.

  Determined by sequence of codons on mRNA
• Define secondary structure of a protein


  Hydrogen bonds form between different amino acids in the chain, causing coiling (alpha helices) or folding (beta sheets)
• Describe the two types of secondary protein structure 


  A helix:

  • All N-H bonds on same side of protein chain
  • Spiral shape
  • H-bonds parallel to helical axis

  B-pleated sheet:

  • N-H and C=O groups alternate from one side to another
• Define tertiary structure of a protein. Name the bonds present


  3D structure formed by further folding of polypeptide

  • Disulfide bridges
  • Hydrogen bonds
  • Ionic bonds
• Describe each type of bond in the tertiary structuse of proteins


  .

  • Disulfide bridges: strong covalent bonds betweeen molecules of the amino acid cystine
  • Ionic bonds: relatively strong bonds between charged R groups (pH changes causes them to break)
  • Hydrogen bonds: numerous and easily broken
• Describe quaternary structure of a protein


  .

  • Functional proteins may consist of more than one polypeptide
  • Precise 3D structure held together by the same type of bonds as the tertiary structure
  • May involve addition of prosthetic group, E.g. metal ions or phosphate groups