MODULE 10: METABOLISM

• Potato


  • 20% dry matter
  • 60-80% of this dry matter is STARCH
• Starch


  • A polysaccharide that consists of glucose monomers
  • Amylose - bonded in a linear or helical chain as it is connected only via α(1->4) glycosidic linkage
  • Amylopectin - bonded branched form as it is connected not only with α(1->4) glycosidic linkage but also with α(1->6) glycosidic linkage
• Lugol's iodine


  • Contains iodine (12) in potassium iodine (KI)
  • Iodine molecule slips inside the amylose coil forming a triiodide complex that causes an intense blue-black color
• Starch hydrolysis


  1. Addition of water to break glycosidic linkages between glucose molecules

  2. Large Polysaccharides → Smaller Polysaccharides → Disaccharides → Monosaccharides
• Starch is a non-reducing sugar. Despite having a free hemiacetal group, the ratio of this residue is very low relative to the whole molecule
• Completion of the hydrolysis of the whole will produce the disaccharide maltose and monosaccharide glucose (both are reducing sugars)
• Digestion


  Mouth → Stomach → Small Intestine → Intestinal Mucosal Cells → Intestinal Lining
• Mouth


  Has salivary α-amylase (Ptyalin). As food is swallowed quickly, large polysaccharide is only converted to smaller polysaccharides
• Stomach

  No digestion of starch occurs as amylases are denatured and inactivated due to the acidic environment
• Small Intestine


  Has pancreatic a-amylase. Primary site of carbohydrate digestion. Converts small polysaccharide into disaccharides (reducing). As food is swallowed quickly, large polysaccharide is only converted to smaller polysaccharides
• Intestinal Mucosal Cells


  Secretes disaccharide enzymes. Converts disaccharides into monosaccharides (also reducing)
• Intestinal Lining


  Absorbs monosaccharide into the bloodstream
• Saliva hydrolysis


  1. Saliva α-amylase will reduce the length of the starch

  2. As a result, intensity of Lugol's iodine blue color will reduce

  3. Benedict's test is a test for reducing sugars

  4. Amylase will hydrolyze the polysaccharides up until it becomes a reducing sugar

  5. In this experiment, we would at least observe a trace amount of reducing sugar (green)

  6. In the case of negative result, this indicates that hydrolysis has not yet taken to completion (to form disaccharides)
• Acid hydrolysis


  Addition of acid in the presence of water will catalyze hydrolysis. This will reduce the length of the starch, which will also reduce intensity of iodine's blue color
• Fermentation


  1. Sealing of the flask with balloon and rubber will put it in aerobic condition

  2. At anaerobic condition, in the presence of yeast, glucose is converted into ethanol via fermentation

  3. Amylase can also be found in yeast. This will help in hydrolysing starch

  4. Yeast also has ethanol dehydrogenase, which facilitates the conversion of sugar to ethanol and carbon dioxide

  5. Liberated CO2 will inflate the balloon

  6. C6H12O6→2C2H5OH+2CO2

  7. Fermentation is successful as observed by the inflated balloon

  8. Small balloon inflation is due to the slow hydrolysis of starch by the yeast. As this reaction is slow, only few glucose can be acquired

  9. Fermentation can be easily observed with simpler sugars in fruit samples (contains fructose, monosaccharide) and table sugar (sucrose, a disaccharide)