Exam qs

• Explain 5 properties that make water important for organisms

  • A metabolite in metabolic reactions such as hydrolysis
  • A good solvent so allows transport of substances
  • High specific heat capacity so buffers changes in temperature
  • High latent heat of vaporisation so provides a cooling effect (through evaporation)
  • Cohesion (between water molecules) so supports columns of water e.g. transpiration stream
• Compare and contrast the structure and properties of triglycerides and phospholipids (5)

  • both contain ester bonds
  • both contain glycerol
  • both insoluble
  • both contain carbon, hydrogen and oxygen, but phospholipids also contains phosphorous
  • triglycerides have 3 fatty acids but phospholipids have 2 fatty acids and a phosphate group.
• Describe how lactose is formed and where in the cell it would be attached to a polypeptide to form a glycoprotein (4)

  • glucose and galactose
  • joined by condensation reaction
  • joined by glycosidic bond
  • added to polypeptide in Golgi.
• Compare and contrast the structure of starch and cellulose (6)

  • both polysaccharides
  • both contain glycosidic bonds
  • both contain carbon, hydrogen and oxygen
  • starch is made of alpha glucose while cellulose is made up of beta glucose
  • starch is branched while cellulose is unbranched
  • starch is helical while cellulose is a straight chain molecule
• DNA helicase is important in DNA replication. Explain why. (2)

  • separates strands
  • so nucleotides can attach
• Explain one way in which the structure of cellulose is linked to its function (2)

  • hydrogen bonds/ microfibrils
  • give strength/ rigidity
• A student carried out the Benedict’s test. Suggest a method, other than using a colorimeter, that this student could use to measure the quantity of reducing sugar in a solution. (2)

  • filter and dry the precipitate
  • find mass/weight
• Why is a colorimeter used to improve the repeatability of a investigation that involves determining the colour/concentration of a precipitate?

  • quantitative
  • standardises the method
• The structure of cellulose is related to its role in plant cell walls. Explain how.

  • straight chains of beta glucose joined by hydrogen bonds
  • forms microfibrils
  • provides rigidity/strength/support
• Describe how monomers join to form the primary structure of a protein

  • condensation reaction between amino acids
  • forming peptide bonds
  • creating specific sequence of amino acids
• Describe one similarity and one difference between the induced-fit model of enzyme action and the lock and key model of enzyme action (2)

  • similarity: substrate binds to active site
  • difference: in induced-fit model, active site not complementary to substrate, but it is complementary in lock and key
• Describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst (3)

  • substrate binds to active site of enzyme
  • active site changes shape slightly so it is complementary to substrate
  • reduces activation energy
• A dipeptide consists of two amino acids joined by a peptide bond. Dipeptides may differ in the type of amino acids they contain.

  Describe two other ways in which all dipeptides are similar and one way in which they might differ. (3)

  • similarities: amine group and carboxyl group
  • difference: different R groups
• Describe how a peptide bond is formed between two amino acids to form a dipeptide (2)

  • condensation reaction
  • between amine group of one amino acid and carboxyl group of another
• The secondary structure of a polypeptide is produced by bonds between amino acids.

  Describe how. (2)

  • hydrogen bonds
  • between NH group of one amino acid and C=O of another
• Two proteins have the same number and type of amino acids but different tertiary structures.

  Explain why. (2)

  • different primary structure
  • form ionic/disulfide/hydrogen bonds in different places
• Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule (2)

  • condensation reaction
  • between phosphate and deoxyribose
  • catalysed by DNA polymerase
• Describe the structure of DNA (5)

  • polymer of nucleotides
  • each nucleotide is formed from deoxyribose, a phosphate group and an nitrogenous base
  • phosphodiester bonds between nucleotides
  • double helix
  • hydrogen bonds between adenine, thymine and guanine, cytosine
• Describe how an ATP molecule is formed from its component molecules (4)

  • adenine, ribose sugar and three phosphate groups
  • condensation reaction
  • ATP synthase
• Describe how ATP is resynthesised in cells (2)

  • from ADP and phosphate
  • by ATP synthase
  • during respiration/photosynthesis
• Give two ways in which the hydrolysis of ATP is used in cells (2)

  1. to provide energy for other reactions e.g. active transport
  2. to add phosphate to other substances and make them more reactive
• State and explain the property of water that can help to buffer changes in temperature (2)

  • high specific heat capacity
  • takes a lot of heat to change temperature
• Describe the biochemical tests you would use to confirm the presence of lipid, non-reducing sugar and amylase in a sample (5)

  LIpid

  • add ethanol then add water and shake/mix
  • white/milky emulsion

  Non-reducing sugar

  • do Benedict’s test and stays blue
  • boil with acid then neutralise with alkali
  • heat with Benedict’s test and becomes red/orange precipitate

  Amylase

  • add biuret reagent and becomes purple
• Give two properties of water that are important in the cytoplasm of cells.

  For each property of water explain its importance in the cytoplasm. (4)

  Property 1:

  • polar molecule
  • acts as a universal solvent

  Property 2:

  • universal solvent
  • metabolic reactions occur faster in solution
• Describe the roles of iron ions, sodium ions and phosphate ions in cells (5)

  Iron ions

  • haemoglobin binds with oxygen

  Sodium ions

  • co-transport of glucose/amino acids into cells
  • because sodium moved out by active transport
  • creates a sodium concentration gradient

  Phosphate ions

  • used to produce ATP
  • phosphorylates other compounds making them more reactive
• How can a colorimeter used to identify the sugar concentration of a precipitate?

  • higher absorbance = higher sugar concentration
• Describe how the structure of a protein depends on the amino acids it contains (5)

  • structure determined by position of R groups
  • primary structure is sequence of amino acids
  • secondary structure formed by hydrogen bonding between amino acids
  • tertiary structure formed by interactions between R groups
  • creates active site in enzymes
  • quaternary structure contains more than one polypeptide chain
• Describe how the structure of starch and cellulose molecules are related to their functions (5)

  Starch:

  • Helical/spiral shape so compact
  • Large (molecule)/insoluble so osmotically inactive
  • Branched so glucose is (easily) released for respiration;
  • Large (molecule) so cannot leave cell/cross cell-surface membrane

  Cellulose:

  • Long, straight/unbranched chains of beta glucose
  • Joined by hydrogen bonding
  • To form (micro/macro)fibrils
  • Provides rigidity/strength