Monomers, Polymers and Carbohydrates

• Monomers:

  • monomers are the smaller units which polymers are made from. Monomers are usually based around carbon
  • monomers join by condensation reactions to form polymers. Polymers are broken down by hydrolysis reactions
  • monosaccharides, amino acid and nucleotides are all examples of monomers
• Condensation reactions:

  • a condensation reaction joins 2 molecules together with the formation of a chemical bond and releases 1 molecule of water for each bond formed between the monomers
• Hydrolysis reactions:

  • a hydrolysis reaction breaks a chemical bond between 2 molecules, and this uses up 1 water molecule for every bond broken
• Carbohydrates:

  • all carbohydrates are composed of carbon, hydrogen and oxygen
  • carbohydrates are divided into monosaccharides, disaccharides and polysaccharides
• Monosaccharides:

  • these are monomers of which other larger carbohydrates are composed of
  • monosaccharides are soluble and sweet and have the general formula (CH2O)n
  • glucose is a hexose because it has 6 carbon atoms (C6H12O6)
  • other monosaccharides are pentoses (ribose in RNA or deoxyribose in DNA) or trioses
• Glucose, Galactose and Fructose:

  • glucose is a product of photosynthesis and a major substrate of respiration
  • glucose has 2 isomers; alpha-glucose and beta-glucose. In a-glucose the OH is below the H and in b-glucose the OH is above the H
  • glucose, galactose (found in milk sugar) and fructose (found in fruits and most plants) are common hexose monosaccharides (6 carbons)
• Disaccharides:

  • two monosaccharides linked together forms a disaccharide
  • alpha glucose + alpha glucose = maltose
  • glucose + fructose = sucrose
  • glucose + galactose = lactose
  • when 2 monosaccharides join together, a water molecule is removed (condensation reaction). A glycosidic bond is formed
• Testing for sugars:

  • to test for reducing sugars (glucose, maltose, lactose):
  • add Benedict's reagent (blue)
  • heat to 90°C in a water bath
  • if a reducing sugar is present the sample will turn from blue to green to yellow to brick red (depending on the concentration)
• Testing for non-reducing sugars:

  • to test for non-reducing sugars (sucrose) heat the sample with Benedict's - if there's no change then there are no reducing sugars present
  • heat a fresh sample with acid (hydrochloric) to hydrolyse it into it's monosaccharides
  • then neutralise with an alkali (sodium hydroxide)
  • now redo the Benedict's test
• Problems with the Benedict's test:

  • it is non-specific - it doesn't tell you the type of sugar present
  • Benedict's is qualitative - it cannot tell you the exact value for the concentration of sugars present
  • highly subjective - colours may be perceived differently
• Polysaccharides:

  • polysaccharides are long chains (polymers) of monosaccharides joined with glycosidic bonds. They are formed by condensation reactions. The important polysaccharides are starch, cellulose and glycogen
  • because they are long molecules polysaccharides are often insoluble. This means that they are very suitable for storage
  • polysaccharides are broken down in hydrolysis reactions into monosaccharides or disaccharides which can then be used by the organism/cell
• Starch:

  • starch is a storage polysaccharide that is found as granules or grains in plant cells, usually concentrated in the seeds as an energy store or in storage organs, such as potato tubers. the main role of starch is energy storage
  • starch is made up of long chains of alpha-glucose - it has coiled sections and branched sections. Coiling occurs due to many alpha 1,4 glycosidic bonds. Branching occurs due to both alpha 1,4 and alpha 1,6 glycosidic bonds
• Features of starch:

  • polymer of a-glucose - provides glucose for respiration
  • helical/coiled - compact; lots of alpha-glucose can be tightly packed into a cell allowing more to be stored
  • branched - glycosidic bonds are easily hydrolysed by enzymes to release glucose for respiration
  • insoluble - osmotically inactive/ doesn't affect water potential
  • large - cannot diffuse out of cell membrane
  • chemically inert - doesn't become involved in chemical reactions
• To test for starch:

  • add iodine dissolved in potassium iodide solution
  • if starch is present, the iodine will turn from orange/brown to blue-black
• Glycogen:

  • glycogen is found in animal cells (mainly muscle and liver cells) . Glycogen is similar to starch; it is an energy storage molecule, it is made from long chains of alpha glucose molecules, joined together by glycosidic bonds
  • it has coiled sections and branched sections, however it is more highly branched than starch and is made up of shorter chains compared to amylopectin. Because it's made of shorter chains, it's more easily hydrolysed
• Features of glycogen:

  • polymer of a-glucose - provides glucose for respiration
  • helical/coiled - compact; lots of alpha-glucose can be tightly-packed into a cell allowing more to be stored
  • branched - glycosidic bonds are easily hydrolysed to release glucose. This happens even quicker in glycogen which is essential for the relatively higher metabolic rate of animals
  • insoluble - osmotically inactive / does not affect water potential
  • large - cannot diffuse across the cell membrane
  • chemically inert - doesn't become involved in chemical reactions
• Cellulose:

  • cellulose is a polymer of many beta glucoses joined together by glycosidic bonds. This provides differences in it's structure compared to starch and glycogen
  • cellulose is a major component of plant cell walls and provides strength and support
• Cellulose and hydrogen bonding:

  • cellulose forms straight, unbranched chains that run parallel to eachother, allowing hydrogen bonding to occur between the chains
  • cellulose molecules are grouped to form microfibrils
  • as there are so many hydrogen bonds, they are collectively strong, therefore the chains are held tightly providing high tensile strength. This makes the cell wall rigid, so prevents osmotic lysis
• Features of cellulose:

  • made of long straight unbranched chains of b-glucose / hydrogen bonding forms between parallel chains / forms microfibrils and macrofibrils - provides rigidity, strength and support to the plant cells, which prevents osmotic lysis (too much water moving in and the cell bursting)