Biological molecules

• polymers are large complex molecules composed of long chains of monomers joined together.
• monomers are small, basic molecular units.
• carbohydrates are made up of monosaccharides & disaccharides.

  These monomers include:

  • glucose
  • fructose
  • galactose
• Glucose is a hexose sugar. There are two types of glucose:

  • alpha glucose
  • beta glucose
• what is the structure of alpha glucose?


  right OH group on the lower side

  
• what is the structure of beta glucose?


  right OH on upper side

  
• a condensation reaction is when two molecules are joined together forming a new chemical bond and a water molecule is released.
• monosaccharides are joined together in a condensation reaction to form disaccharides.
• the bond between two monosaccharides is called a glycosidic bond
• maltose = alpha glucose + alpha glucose
• sucrose = glucose + fructose
• lactose = glucose + galactose
• polymers are broken apart by hydrolysis reactions by addition of water.
• what is the test for reducing sugars?

  1. add Benedicts reagent to a sample and heat in a water bath.
  2. if the test is positive it will go from a blue solution to a coloured precipitate.
  3. the more towards red a ppt is the higher the concentration of reducing sugar.
  4. to quantitively compare you can filter and weight the ppts.
• what is the test for non-reducing sugars?

  1. if the test for reducing sugars is negative there may still be non-reducing sugars present.
  2. add dilute HCl and heat gently.
  3. allow the solution to cool then neutralise it with sodium hydrogencarbonate. (Na2CO3)
  4. add benedicts reagent and if its positive a coloured precipitate will form.
  5. the more red the ppt is the more concentrated the sugar is.
• what is an example of reducing sugars?


  monosaccharides: glucose

  disaccharides: fructose & lactose
• polysaccharides are formed by the condensation of more than two monosaccharides.
• starch is the main energy storage in plants
• Starch is made up of alpha glucose.

  • It can for long chains with branched ends allowing many enzymes to bind when breakdown is needed for glucose.
  • Starch is compact so is easily stored in cells.
  • Starch is insoluble in water so does not affect water potential in cells.
• Test for starch:

  1. add iodine via potassium iodide.
  2. If starch is present the solution will change from orange/brown to blue/black.
• glycogen is the main energy store in animals.
• Glycogen is a polysaccharide of alpha glucose.

  • It has many branched chains allowing glucose to be released quickly by enzymes.
  • It is very compact so easy to store.
• Cellulose is a major component in plant cell walls.
• Cellulose is made up of beta glucose in long, unbranched chains.

  • each parallel chain is joined by hydrogen bonds to form microfibrils which form strong fibres.
  • These provide structural support in the cell wall e.g when the plant is turgid so it doesnt burst.
• triglycerides have one glycerol and three fatty acids joined by ester bonds in a condensation reaction.
• Fatty acids are made up of long hydrocarbons which are hydrophobic (repel water).

  These tails make lipids insoluble in water.
• Saturated fatty acids dont have double bonds between carbon atoms so is saturated with hydrogen atoms.
• unsaturated fatty acids have at least one double bond between carbon atoms so has less hydrogen atoms.
• what are the two types of fatty acids?

  saturated & unsaturated
• Phospholipids contain one glycerol, two fatty acids and a phosphate group.

  • The phosphate group is hydrophilic (attracts water)
  • The fatty acids are hydrophobic (repel water)
  • This is important for the cell membrane.
• Triglycerides are mainly used as energy stores.

  • long hydrocarbon tails release alot of energy when broken down.
  • They are insoluble so dont affect water potential of the cell by osmosis.
• Phospholipids make up the bilayer in cell membranes.

  • This allows them to control what enters and leaves the cell.
  • The hydrophilic heads face outwards and the hydrophobic tails face inwards.
  • This makes the centre of the bilayer hydrophobic so water soluble molecules cannot pass through by diffusion.
• Test for lipids:

  1. Grind up sample if needed.
  2. Add ethanol and shake.
  3. Pour the solution into water and shake gently.
  4. If lipids are present a cloudy white emulsion will form.
• Proteins are made up of long chains of amino acids joined by peptide bonds in a condensation reaction.
• a dipeptide is when two amino acids are joined together.
• a polypeptide is when more than two amino acids are joined together
• Amino acids have an R group which can be different depending on the type of amino acid. This gives proteins their unique properties.
• Primary structure is the specific number and sequence of amino acids in a polypeptide chain.
• Secondary structure is when hydrogen bonds form between amino acids forming either an alpha helix or beta pleated sheet.
• Tertiary structure is when the polypeptide chain folds into a 3D structure by interactions of R-groups forming more hydrogen bonds, ionic bonds and disulphide bonds.