Carbohydrates

• What is a polymer?


  Many of the same monomers chemically bonded together in a repeating pattern.
• What is a monomer?


  One repeating unit of a polymer.
• What is a monosaccharide?


  One sugar unit.
• Give three examples of monosaccharides.


  Glucose (alpha/beta), fructose and galactose.
• What is a disaccharide?


  A molecule of two sugar units chemically bonded together.
• Give three examples of disaccharides.


  Maltose, sucrose and lactose.
• What monosaccharides make up maltose?


  glucose + glucose
• What monosaccharides make up sucrose?


  fructose + glucose
• What monosaccharides make up lactose?


  glucose + galactose
• What form of glucose is this?

  alpha

  
• What form of glucose is this?

  beta

  
• How are disaccharides formed?


  In a condensation reaction, two monosaccharides react to produce water and form a glycosidic link between C1 of the first monosaccharide and C4 of the second.
• What are isomers?


  Substances with the same chemical formula, but different structural formulae.
• What are polysaccharides?


  Polymers of sugar monomers, joined by glycosidic links.
• What is another name for poly-alpha-glucose?


  Starch/glycogen
• What is another name for poly-beta-glucose?


  Cellulose
• What is the structure of poly-alpha-glucose (starch/glycogen) and why is this useful?


  Forms a branched helix

  The helix structure makes the molecule more compact, meaning many alpha glucose molecules can be stored in one place, making starch a good store of energy.

  Branching increases the number of points where enzymes can act to release glucose.

  Hydrogen bonds between the same chain hold the helix structure in place.
• What is the difference in structure between starch and glycogen?


  Glycogen has more branching, so has more places for enzymes to act, therefore glucose can be released faster. This leads to a faster rate of respiration.
• What is the structure of poly-beta-glucose (cellulose) and why is this useful?


  Forms long, straight, unbranched chains

  This allows chains to be bundled together forming microfibrils, that give the molecule strength and rigidity.

  Hydrogen bonds form between the chains to form these bundles.
• Is poly-alpha-glucose soluble or insoluble? Why is this useful?

  Insoluble

  This means it has no osmotic effect (it does not affect water potential, posing a risk of water entering the cell and causing it to burst)

  It also means, along with its large size, it is retained in the cell as it cannot pass across the cell membrane.
• What is useful about microfibrils (poly-beta-glucose)?


  They can oppose the force of a fluid-filled cell to prevent further water entering, that would cause the cell to burst.
• Which form of glucose forms a complex with iodine and how does this occur?


  poly-alpha-glucose

  Forms a helix shape into which an iodine molecule can fit.
• What is the test for starch (and results)?

  . add iodine

  . positive = blue/black

  . negative = orange/brown
• What is the test for reducing sugars (and results)?


  . add Benedict's reagent to the sample

  . heat in a water bath for 5 mins (above 60º)

  . positive = (lowest conc.) green, yellow, orange, brick red (highest conc.)

  . negative = blue
• When should the test for non-reducing sugars be carried out?


  If the Benedict's test for reducing sugars comes back negative.
• What is the test for non-reducing sugars (and results)?


  . (to a fresh sample) add hydrochloric acid

  . boil for 1 min in a water bath

  . neutralise by adding sodium hydrogencarbonate

  . test pH to confirm neutralisation (paper or meter)

  . re-run Benedict's test (the results are the same)

  . positive means non-reducing sugars are present

  . negative means there are no sugars present at all