1.2 Carbohydrates

• What are monosaccharides? Give 3 common examples

  ● Monomers from which larger carbohydrates are made

  ● Glucose, fructose, galactose
• Describe the structure of α-glucose

  ● Left - full structure, carbon atoms labelled

  ● Right - simplified structure as in the specification to be memorised for exam

  
• 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 - what does this mean?


  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
• Draw a diagram to show how two monosaccharides are joined together

  
• 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

  Function = Energy store in plant cells

  
  

  Structure:

  ● 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

  Function = Energy store in animal cells

  Structure:

  ● Polysaccharide made of α-glucose

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

  ● Helical → compact for storage in cell

  ● Large, insoluble polysaccharide molecule → can’t leave cell / cross cell membrane

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

  ● Branched → compact / fit more molecules in small area

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

  ● Large, insoluble polysaccharide molecule → can’t leave cell / cross cell membrane

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


  Function

  ● Provides strength and structural support to plant / algal cell walls

  Structure

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

  ● Chains linked in parallel by hydrogen bonds forming microfibrils

  ● Polysaccharide of β-glucose
• 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

  1. Add Benedict’s solution (blue) to sample
  2. Heat in a boiling water bath
  3. Positive result = green / yellow / orange / red precipitate
• Which are the reducing sugars?

  monosaccharides, maltose, lactose
• Describe the test for non-reducing sugars

  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
• What is the non reducing sugar?

  sucrose
• 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 (colorimeter)

  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