3.1.2 Carbohydrates

• Starch- a polysaccharide made of alpha glucose molecules, acts as storage for glucose in plants
• Amylose- a polysaccharide made of long, unbranched chains of alpha glucose, alpha 1,4 glycosidic bonds between molecules, has a helical structure
• Amylopectin- a polysaccharide made of branched chains of alpha glucose, held by alpha 1,4 and 1,6 glycosidic bonds, has more exposed ends
• Glycogen- a polysaccharide made of branches of alpha glucose, alpha 1,4 and 1,6 glycosidic bonds, more highly branched at intervals, exposed ends
• Why is sugar stored as starch:

  • starch is insoluble- no effect on water potential (by effecting solute concentration) so osmosis isn't affected
  • starch is a large molecule- cannot diffuse out of cells
  • helical structure- can be stored in small spaces
  • easily hydrolysed- can be broken down to alpha glucose for energy release
  • branched structure- more energy can be released from exposed ends
• Cellulose- a polysaccharide made of beta glucose molecules, held by beta 1,4 glycosidic bonds, straight chains, forming microfibrils, cell wall of plants
• Examples of monosaccharides:

  • Glucose
  • Fructose
  • Galactose
  • Ribose
  • Deoxyribose
• Monosaccharides- simple sugars which are the most basic form of carbohydrate, which can join to form disaccharides
• Properties of monosaccharides:

  • General formula- (CH2O)
  • Equal number of oxygen and carbon atoms
  • White crystalline solids
  • Dissolves in water
  • Sweet tasting solutions
• A condensation reaction between two monosaccharides forms a glycosidic bond, and a disaccharide.

  A hydrolysis reaction between a disaccharide forms two monosaccharide, and breaks a glycosidic bond.

  A) condensation

  B) covalent

  C) glycosidic

  D) released

  E) used

  F) hydrolysis

  G) covalent

  
• Lactose- a disaccharide formed by the condensation of an alpha glucose molecule and a galactose molecule
• Maltose- a disaccharide formed by the condensation of two alpha glucose molecules
• Sucrose- a disaccharide formed by the condensation of an alpha glucose molecule and a fructose molecule
• Cellobiose- a disaccharide formed by the condensation of two beta glucose molecules
• Alpha glucose- hydroxyl group is below

  A) alpha

  B) H

  C) HO

  D) OH

  E) H

  
• Beta glucose- hydroxyl group is above

  A) beta

  B) H

  C) OH

  D) H

  E) HO

  
• Test for reducing sugars:

  • Add Benedict's solution to sample
  • Heat mixture in water bath
  • Positive test= colour change from blue to red
• Test for non-reducing sugars:

  • Add dilute hydrochloric acid to sample
  • Heat in water bath
  • Add sodium hydroxide
  • Carry out normal Benedict's test
  • Positive result= colour change from blue to red
• Test for starch:

  • Add iodine solution to the sample
  • Positive test= colour change from brown to blue-black
• Glycogen releases energy through hydrolysis of it's exposed ends, where it is hydrolysed into glucose which is used in respiration.