Chapter 3

• Hydrogen bonding


  • water molecules are polar - oxygen atom has partial negative charge & hydrogen atoms have partial positive charge - oxygen atom is attracted hydrogen atoms

  
• Characteristics of water 


  • good solvent
  • high specific heat capacity
  • high heat of vaporisation
  • cohesive
  • low density solid (ice)
• Why does ice float on water?

  • crystalline structure of ice is less dense than liquid water
  • as water cooled below 4°C, hydrogen bonds fix the positions of polar molecules further apart
  • produces giant, rigid but open structure w every oxygen atom at centre of a tetrahedral arrangement of H bonds
• Importance of ice for life

  • provides an insulating layer for aquatic habitats in cold climates
  • ice surface provides habitat for some organisms (e.g. polar bears)
• Cohesive nature of water

  • hydrogen bonds cause molecules to be attracted to each other - move as one mass
  • allow plants to draw water up roots
  • water molecules more strongly cohesive to each other than they are to air - result in water having 'skin' of surface tension - allows small organisms to move on water surface
• Adhesive nature of water

  • occurs between water molecules & other polar molecules & surfaces
  • water molecules attracted to other materials
  • e.g. wash hands - hands are wet, water doesn't run straight off
• Capillary action 


  • process by which water can rise up a narrow tube against the force of gravity
  • result of effects of adhesion & cohesion
• How does water act as a solvent?

  • because it's a polar molecule it can attract other polar molecules (solutes) & dissolve them
  • acts as a medium for chemical reactions & help transport dissolved compounds in & out of cells
• How does water act as a coolant?


  • due to large amount of energy required to overcome hydrogen bonding (high shc) it helps to buffer temp changes during chemical reactions
  • maintaining constant temps is important as enzymes only active in narrow temp range
  • high heat of vaporisation allows for thermoregulation - sweating & panting to cool organisms when water on body is evaporated
• Elements found in carbohydrates

  C, H, O
• Elements found in lipids 


  C, H, O
• Elements found in proteins

  C, H, O, N, S
• Elements found in nucleic acids 


  C, H, O, N, P
• Monosaccharide 


  • single sugar unit
  • e.g. glucose, fructose, ribose
• Disaccharide 


  • 2 monosaccharides linked together
  • e.g. lactose, sucrose
• Polysaccharide 


  • 2 or more monosaccharides - form a polymer
  • e.g. glycogen, cellulose, starch
• Difference between a hexose & pentose monosaccharide

  • pentose - 5 carbon atoms
  • hexose - 6 carbon atoms
• Structure of a-glucose & B-glucose 


  • hexose monosaccharides w ring structure
  • position of OH group: ABBA - alpha below beta above

  
• Properties of glucose 


  • polar - due to H bonds between hydroxyl groups & water molecules
  • water-soluble - important - means glucose is dissolved in cytosol of cell
• Structure of ribose 


  • pentose monosaccharide
  • ring structure

  
• Condensation reaction(carbohydrates)


  • 2 monosaccharides react to form a disaccharide
  • glycosidic bond is formed - bond forms between carbon 1 and carbon 4 - 1,4 glycosidic bond
  • water molecule produced
• Name 3 disaccharides & how they form

  • maltose - glucose + glucose
  • sucrose - glucose + fructose
  • lactose - glucose + galactose
• In what reaction are disaccharides broken down to reform original 2 monosaccharides?

  hydrolysis reaction
• As two OH are so close, they react, forming covalent bond called a glycosidic bond between 2 glucose molecules

  
• Function of starch 


  storage polysaccharide of a-glucose in plant cells
• 2 components of starch

  • amylose
  • amylopectin
• Amylose 


  • long chain of a-glucose monomers joined by 1,4 glycosidic bonds
  • twists to form helix which is further stabilised by hydrogen bonding in molecule - makes polysaccharide more compact & much less soluble than glucose

  
• Amylopectin 


  • chain of a-glucose monomers joined by 1,4 glycosidic bonds
  • branches created by 1,6 glycosidic bonds, occur every 25 glucose monomers - increases number of points at which glucose can be released through hydrolysis
• Function of glycogen 


  main storage polysaccharide of a-glucose in animals & fungi
• Structure of glycogen 


  • 1,4 & 1,6 glycosidic bonds
  • more branches than amylopectin - more compact
  • insoluble
  • branching allows for free ends where glucose molecules can be added or removed - speeds up process of storing or releasing glucose molecules required by cell
• Key properties of amylopectin & glycogen that make them ideally suited to storage roles

  • insoluble
  • branched
  • compact
• Function of cellulose 


  chain of B-glucose monomers which gives rigidity to plant cell walls
• Structure of cellulose 


  • chain of B-glucose monomers
  • 1,4 glycosidic bonds
  • alternate glucose molecules are rotated 180 degrees so OH groups are close enough to react - means its unable to coil or form branches - straight chain formed
  • hydrogen bonds form macrofibrils which join to produce strong & insoluble fibres - used to make cell walls

  
• How is cellulose important in humans?

  part of diet

  • forms fibre necessary for healthy digestive system
  • hard to break down into its monomers
• Hydrolysis


  addition of water molecules to break down branched molecules
• Test for reducing sugars

  Benedict's test
• How is Bendict's test carried out for reducing sugars? 


  • place sample in boiling tube
  • add equal volume of Benedict's reagent
  • heat mixture gently in boiling water bath for 5 mins
• What is the result of Benedict's test if no reducing sugars are present?

  solution remains blue
• What is the result of Benedict's test if reducing sugars are present?


  the more present, the more precipate formed & less blue copper ions left in solution

  • high concentration - brick-red
  • medium concentration - yellow/orange
  • low concentration - green
• Using Benedict's test for non-reducing sugars (e.g. sucrose)


  • non-reducing sugars don't react w Benedict's solution so
  • after negative result from Benedict's, boil with dilte hydrochloric acid & repeat Benedict's test to give a positive result
  • works because sucrose has been hydrolysed by acid to glucose & fructose (both reducing sugars)