1.2 Carbohydrates

• What are monosaccharides? Give 3 common examples ?

  Monomers from which larger carbohydrates are made

  ● Glucose, fructose, galactose
• Cellulose:

  • Function: provides strength and structural support to plant/algal cell walls
  • Structure: polysaccharide of β-glucose with 1-4 glycosidic bonds, straight, unbranched chains linked in parallel by hydrogen bonds forming microfibrils
• Describe the difference between the structure of α-glucose and β-glucose? AND DRAW THEM

  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

  
• DISSACHARIDES?

  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

  Disaccharide Monosaccharides

  Maltose Glucose + glucose

  Sucrose Glucose + fructose

  Lactose Glucose + galactose
• 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

  Disaccharide Monosaccharides:

  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 and glycogen?

  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

  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 and relate to their functions?

  Starch (amylose)

  ● 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 and relate to their functions?

  Glycogen (and starch amylopectin)

  ● 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)
• 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

  • Positive result = blue-black
• Exam insight: common mistakes ❌(1/6)

  ❌- *Referring to alpha glucose as ‘a glucose’.*

  ✅-This is not an equivalent of writing ‘alpha glucose’ or ‘α-glucose’ and will usually be rejected.
• Exam insight: common mistakes ❌(2/6)

  ❌- “Glycogen and starch are energy stores as hydrolysis of glycosidic bonds releases energy.”

  ✅-Glycogen and starch are energy stores as they are made of α-glucose, which is the substrate for respiration in cells. This produces ATP for energy release
• Exam insight: common mistakes ❌(3/6)

  ❌- *Comparing and contrasting polysaccharides by making a list about each.*

  ✅-This will achieve no marks. In these questions, each statement needs to make a clear comparison. Aim to use a word like ‘whereas’ to ensure you’re covering both sides.
• Exam insight: common mistakes ❌(4/6)

  ❌- When chains of beta glucose are linked by hydrogen bonds, myofibrils form.”

  ✅-Myofibrils are found in muscle fibres and are completely different. Microfibrils are found in cellulose cell walls
• Exam insight: common mistakes ❌(5/6)

  ❌- “Cellulose is strong because of hydrogen bonds.”

  ✅-Hydrogen bonds are weak individually, but strong in high numbers. You need to say that there are many hydrogen bonds.
• Exam insight: common mistakes ❌(6/6)

  ❌- “Use the same amount of Benedict’s solution on each sample when comparing the quantity of reducing sugar.

  ✅-Amount is too vague. You need to use the term volume to get the mark.