Biological Molecules

• Monomers


  repeating molecules from which larger polymers are made
• Polymer


  molecule made up of many monomers
• Condensation reaction


  ● 2 molecules join together

  ● Forming a chemical bond

  ● Releasing a water molecule
• Hydrolysis reaction


  ● 2 molecules separated

  ● Breaking a chemical bond

  ● Using a water molecule
• Examples of polymers and the monomers


  Monomer - Polymer

  -Condensation of nucleotide = polynucleotide (DNA or RNA)

  -Condensation of monosaccharide (e.g glucose) = polysaccharide (e.g starch)

  -Condensation of amino acid = polypeptide (protein)
• What are monosaccharides? Give 3 common examples


  ● Monomers from which larger carbohydrates are made

  ● Glucose, fructose, galactose
• Describe the difference between the structure of α-glucose and β-glucose


  OH group is below carbon 1 in α-glucose but above carbon 1 in β-glucose
• Alpha & beta glucose are isomers →


  same molecular formula, differently arranged atoms
• What are disaccharides and how are they formed?


  ● Two monosaccharides joined together with a glycosidic bond

  ● Formed by a condensation reaction, releasing a water molecule
• List 3 common disaccharides & monosaccharides from which they're made


  Maltose = glucose + glucose

  Sucrose = glucose + fructose

  Lactose = glucose + galactose
• What are polysaccharides and how are they formed?


  ● Many monosaccharides joined together with glycosidic bonds

  ● Formed by many condensation reactions, releasing water molecules
• Describe the basic function and structure of starch


  Energy store in plant cells

  ● Polysaccharide of α-glucose

  ● Amylose - 1,4-glycosidic bonds → unbranched

  ● Amylopectin - 1,4- and 1,6-glycosidic bonds → branched
• Describe the basic function and structure of glycogen


  Energy store in animal cells

  ● Polysaccharide made of α-glucose

  ● 1,4- and 1,6-glycosidic bonds → branched
• Explain how the structures of starch (amylose) relate to their functions


  ● Helical → compact for storage in cell

  ● Large, insoluble polysaccharide molecule → can't leave cell membrane

  ● Insoluble in water → water potential of cell not affected (no osmotic effect)
• Explain how the structures of glycogen relate to their functions


  ● Branched → compact, more ends for faster hydrolysis → release glucose for respiration to make ATP for energy release

  ● Large, insoluble polysaccharide molecule → can't leave cell membrane

  ● Insoluble in water → water potential of cell not affected (no osmotic effect)
• Describe the basic function and structure of cellulose


  Provides strength and structural support to plant cell walls

  ● Polysaccharide of β-glucose

  ● 1,4-glycosidic bond → straight, unbranched chains

  ● Chains linked in parallel by hydrogen bonds forming microfibrils
• Explain how the structure of cellulose relates to its function


  ● Every other β-glucose molecule is inverted in a long, straight, unbranched chain

  ● Many hydrogen bonds link parallel strands (crosslinks) to form microfibrils (strong fibres)

  ● Hydrogen bonds are strong in high numbers - so provides strength to plant cell walls
• Describe the test for reducing sugars


  Reducing sugars = monosaccharides, maltose, lactose

  1. Add Benedict's solution (blue) to sample

  2. Heat in a boiling water bath

  3. Positive result = green / yellow / orange / red precipitate
• Describe the test for non-reducing sugars


  Non-reducing sugars = sucrose

  1. Do Benedict's test and stays blue / negative

  2. Heat in a boiling water bath with acid (to hydrolyse into reducing sugars)

  3. Neutralise with alkali (eg. sodium bicarbonate

  4. Heat in a boiling water bath with Benedict's solution

  5. Positive result = green / yellow / orange / red precipitate
• Suggest a method to measure the quantity of sugar in a solution


  ● Carry out Benedict's test as above, then filter and dry precipitate

  ● Find mass / weight
• Suggest another method to measure the quantity of sugar in a solution


  1. Make sugar solutions of known concentrations (eg. dilution series)

  2. Heat a set volume of each sample with a set volume of Benedict's solution for same time

  3. Use colorimeter to measure absorbance (of light) of each known concentration

  4. Plot calibration curve - concentration on x axis, absorbance on y axis and draw line of best fit

  5. Repeat Benedict's test with unknown sample and measure absorbance

  6. Read off calibration curve to find concentration associated with unknown sample's absorbance
• Describe the biochemical test for starch


  1. Add iodine dissolved in potassium iodide (orange / brown) and shake / stir

  2. Positive result = blue-black
• Name two groups of lipid


  Triglycerides and phospholipids
• Describe the structure of a fatty acid (RCOOH)


  ● Variable R-group - hydrocarbon chain (saturated or unsaturated)

  ● -COOH = carboxyl group
• Describe the difference between saturated and unsaturated fatty acids


  ● Saturated: no C=C double bonds in hydrocarbon chain; all carbons fully saturated with hydrogen

  ● Unsaturated: one or more C=C double bond in hydrocarbon chain (creating bend / kink)
• Describe how triglycerides form


  ● 1 glycerol molecule and 3 fatty acids

  ● Condensation reaction

  ● Removing 3 water molecules

  ● Forming 3 ester bonds
• Explain how the properties of triglycerides are related to their structure


  Function: energy storage

  ● High ratio of C-H bonds to carbon atoms in hydrocarbon chain

  ○ So used in respiration to release more energy than same mass of carbohydrates

  ● Hydrophobic fatty acids so insoluble in water (clump together as droplets)

  ○ So no effect on water potential of cell (or can be used for waterproofing)
• Describe the difference between the structure of triglycerides and phospholipids


  One of the fatty acids of a triglyceride is substituted by a phosphate-containing group
• Describe how the properties of phospholipids relate to their structure


  Function: form a bilayer in cell membrane, allowing diffusion of lipid-soluble or very small substances and restricting movement of water-soluble (polar) or larger substances
• Describe how the properties of phospholipids relate to their structure 2


  ● Phosphate heads are hydrophilic

  ○ Attracted to water so point to water (aqueous environment) either side of membrane

  ● Fatty acid tails are hydrophobic

  ○ Repelled by water so point away from water / to interior of membrane
• Describe the test for lipids


  1. Add ethanol, shake (to dissolve lipids), then add water

  2. Positive = milky white emulsion
• How many amino acids are common in all organisms? How do they vary?


  The 20 amino acids that are common in all organisms differ only in their side group (R)
• Describe how amino acids join together


  ● Condensation reaction

  ● Removing a water molecule

  ● Between carboxyl group of one and amine group of another

  ● Forming a peptide bond
• What are dipeptides and polypeptides?

  ● Dipeptide - 2 amino acids joined together

  ● Polypeptide - many amino acids joined together

  A functional protein may contain one or more polypeptides
• Describe the primary structure of a protein


  Sequence of amino acids in a polypeptide chain, joined by peptide bonds
• Describe the secondary structure of a protein


  ● Folding (repeating patterns) of polypeptide chain eg. alpha helix / beta pleated sheets

  ● Due to hydrogen bonding between amino acids

  ● Between NH (group of one amino acid) and C=O (group)
• Describe the tertiary structure of a protein


  ● 3D folding of polypeptide chain

  ● Due to interactions between amino acid R groups (dependent on sequence of amino acids)

  ● Forming hydrogen bonds, ionic bonds and disulfide bridges
• Describe the quaternary structure of a protein


  ● More than one polypeptide chain

  ● Formed by interactions between polypeptides (hydrogen bonds, ionic bonds, disulfide bridges)
• Describe the test for proteins


  1. Add biuret reagent (sodium hydroxide + copper (II) sulphate)

  2. Positive result = purple / lilac colour (negative stays blue) → indicates presence of peptide bonds
• Describe / draw the general structure of an amino acid


  ● COOH = carboxyl group

  ● R = variable side chain / group

  ● H2N = amine group