BIOLOGICAL MOLECULES

• MONOMERS AND POLYMERS

  Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers.

  Give two named examples of polymers and their associated monomers to illustrate your answer. (5)

  1. A condensation reaction joins monomers together and forms a (chemical bond) and releases water

  2. A hydrolysis reaction breaks a (chemical) bond between monomers and uses water

  Examples:

  Amino acid-->polypeptide

  Cellulose--->B glucose
• What is a monomer? (1)


  A monomer is a small repeating unit from which larger molecules can be made.
• CARBOHYDRATES

  A biochemical test for reducing sugar provides a negative result with raffinose solution.

  Describe a biochemical test to show that raffinose contains a non-reducing sugar. (3)

  1. Heat with acid and neutralise

  2. Heat with Benedict's

  3. Red precipitate
• A precipitate is produced in a positive result for reducing sugar in a Benedict's test. A precipitate is solid matter suspended in a solution.

  
  

  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)

  1. Filter and dry

  2. Find mass
• In an investigation, a student wanted to identify the solutions in two beakers, A and B.

  She knew one beaker contained maltose solution and the other beaker contained glucose solution.

  Both solutions had the same concentration. She did two separate biochemical tests on a sample from each beaker.

  Test 1 - used Benedict's solution to test for reducing sugar.

  Test 2 - added the enzyme maltase, heated the mixture at 30 °C for 5 minutes, and then used Benedict's solution to test for reducing sugar.

  Maltose is hydrolysed by maltase. The student's results are shown in the table below.

  Colour of solution after testing with Benedict's solution Beaker

  Test 1 A red B red

  Test 2 A red B dark red

  
  

  (c) Explain the results for beakers A and B in the table. (2)

  1. A = glucose and B = maltose; 2. Because more sugar/precipitate after hydrolysis/maltase action; Accept 'higher concentration of sugar' for 'more sugar' Accept 'break down' for hydrolysis
• Use of a colorimeter in this investigation would improve the repeatability of the student's results. Give one reason why. (1)

  1. Quantitative OR (Colour change is) subjective; Accept: accurate/precise
• Lactulose is a disaccharide formed from one molecule of galactose and one molecule of fructose. Other than both being disaccharides, give one similarity and one difference between the structures of lactulose and lactose. (2)


  Similarity 1. Both contain galactose / a glycosidic bond; Ignore references to hydrolysis and / or condensation

  Difference 2. Lactulose contains fructose, whereas lactose contains glucose;
• Glycogen and cellulose are both carbohydrates. Describe two differences between the structure of a cellulose molecule and a glycogen molecule. (2)

  1. Cellulose is made up of β-glucose (monomers) and glycogen is made up of α-glucose (monomers);

  2. Cellulose molecule has straight chain and glycogen is branched;

  3. Cellulose molecule has straight chain and glycogen is coiled;

  4. glycogen has 1,4- and 1,6- glycosidic bonds and cellulose has only 1,4- glycosidic bonds; Ignore ref. to H bonds / microfibrils
• Starch is a carbohydrate often stored in plant cells. Describe and explain two features of starch that make it a good storage molecule. (2)

  ) Any two from:

  1. Insoluble (in water), so doesn't affect water potential;

  2. Branched / coiled / (α-)helix, so makes molecule compact; OR Branched / coiled / (α-)helix so can fit many (molecules) in small area;

  3. Polymer of (α-)glucose so provides glucose for respiration; 4. Branched / more ends for fast breakdown / enzyme action; 5. Large (molecule), so can't cross the cell membrane Require feature and explanation for 1 mark 1. Accept Ψ or WP 1. Accept Insoluble so doesn't affect osmosis 1. Do not allow ref to 'doesn't affect water leaving cells 4. Ignore 'surface area' 4. Accept 'branched so glucose readily released
• Which test identifies the presence of starch? (1)

  Iodine/potassium iodide
• Describe the structure of glycogen. (2)

  1. Polysaccharide of α-glucose; OR polymer of α-glucose;

  2. (Joined by) glycosidic bonds OR Branched structure;
• During early pregnancy, the glycogen in the cells lining the uterus is an important energy source for the embryo. Suggest how glycogen acts as a source of energy. Do not include transport across membranes in your answer. (2)

  ) 1. Hydrolysed (to glucose);

  2. Glucose used in respiration; 1. Ignore 'Broken down' 2. 'Energy produced' disqualifies mp2
• Name the monomers from which a maltose molecule is made. (1)

  Glucose (and glucose);
• Name the type of chemical bond that joins the two monomers to form maltose. (1)

  (α1,4) Glycosidic;
• Explain how you would use the graph to determine the maltose concentration with a light absorbance of 0.45 arbitrary units. (2)

  Line of best fit drawn; Read off value at 0.45.
• Explain the difference in the structure of the starch molecule and the cellulose molecule shown in the diagram above. (2)

  1. Starch formed from α-glucose but cellulose formed from β-glucose;

  2. Position of hydrogen and hydroxyl groups on carbon atom 1 inverted
• Starch molecules and cellulose molecules have different functions in plant cells. Each molecule is adapted for its function. Explain one way in which starch molecules are adapted for their function in plant cells. (2)

  1. Insoluble; 2. Don't affect water potential;

  OR 3. Helical; Accept form spirals 4. Compact;

  OR 5. Large molecule; 6. Cannot leave cell.
• Explain how cellulose molecules are adapted for their function in plant cells. (3)

  1. Long and straight chains;

  2. Become linked together by many hydrogen bonds to form fibrils;

  3. Provide strength (to cell wall).
• LIPIDS

  The general structure of a fatty acid is RCOOH. Name the group represented by COOH. (1)

  (a) Carboxyl; Accept carboxylic acid
• Describe how you would test for the presence of a lipid in a liquid sample of food. (2)

  1. Add ethanol/alcohol then add water and shake/mix

  OR

  Add ethanol/alcohol and shake/mix then pour into/add water;

  Reject heating the emulsion test

  Accept 'Add Sudan III and mix'

  Ignore a second shake

  2. White/milky (emulsion)

  OR

  (emulsion) test turns white/milky;

  Ignore cloudy

  Reject precipitate

  Accept (for Sudan III) top (layer) red
• Describe how a triglyceride molecule is formed. (3)

  1. One glycerol and three fatty acids;

  2. Condensation (reactions) and removal of three molecules of water;

  3. Ester bond(s) (formed);

  Accept all marks in suitably labelled diagram OR in

  a balanced equation
• Describe how an ester bond is formed in a phospholipid molecule. (2)

  1. Condensation (reaction) OR Loss of water; 2. Between of glycerol and fatty acid; Accept labelled diagram
• The scientists expressed their results as Percentage of lipid in plasma membrane by mass. Explain how they would find these values. (2)

  ) 1. Divide mass of each lipid by total mass of all lipids (in that type of cell); 2. Multiply answer by 100.
• PROTEINS AND ENZYMES

  Describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst. (3)


  1. Substrate binds to the active site/enzyme OR Enzyme-substrate complex forms; Accept for 'binds', fits

  2. Active site changes shape (slightly) so it is complementary to substrate OR Active site changes shape (slightly) so distorting/breaking/forming bonds in the substrate;

  3. Reduces activation energy;
• The scientists investigated the effect of concentration of inorganic phosphate (Pi) on ATP synthase activity. After 2 minutes, they stopped each reaction and then measured the concentration of ATP. The figure below shows the scientists' results.

  Suggest and explain a procedure the scientists could have used to stop each reaction. (2)

  1. Boil

  OR

  Add (strong) acid/alkali;

  Accept heat at > 50oC OR at very high

  temperatures

  2. Denatures the enzyme/ATP synthase;
• Explain the change in ATP concentration with increasing inorganic phosphate concentration. (2)

  1. (With) increasing Pi concentration, more enzymesubstrate complexes are formed;

  2. At or above 40 (mmol dm-3) all active sites occupied
• Describe the results you would expect the student to obtain. (1)

  As starch concentration increases, time to hydrolyse/digest starch

  increases;
• A competitive inhibitor decreases the rate of an enzyme-controlled

  reaction.

  Explain how. (3)

  1. Inhibitor similar shape to substrate;

  Reject same shape

  Accept 'complementary to active site'

  2. Fits/binds to active site;

  3. Prevents/reduces enzyme-substrate complex forming;
• When bread becomes stale, the structure of some of the starch is

  changed. This changed starch is called retrograded starch.

  Scientists have suggested retrograded starch is a competitive inhibitor of

  amylase in the small intestine.

  Assuming the scientists are correct, suggest how eating stale bread could

  help to reduce weight gain. (3)

  Less hydrolysis of starch;

  AQA Biology A-Level - Proteins and Enzymes MS PhysicsAndMathsTutor.com

  Accept no hydrolysis

  2. (To) maltose;

  3. (So) less absorption (of glucose)
• Describe how the structure of a protein depends on the amino acids it contains. (5)

  1. Structure is determined by (relative) position of amino acid/R

  group/interactions;

  Accept for 'interactions', hydrogen bonds / disulfide

  bridges / ionic bonds / hydrophobichydrophilic

  interactions

  2. Primary structure is sequence/order of amino acids;

  3. Secondary structure formed by hydrogen bonding (between amino

  acids);

  Accept alpha helix/β-pleated sheet for 'secondary

  structure'

  4. Tertiary structure formed by interactions (between R groups);

  Accept for 'interactions', hydrogen bonds / disulfide

  bridges / ionic bonds / hydrophobic hydrophilic

  interactions

  5. Creates active site in enzymes
• Describe how amino acids join to form a polypeptide so there is always NH2 at one end and COOH at the other end. (2)

  1. One amine/NH2 group joins to a carboxyl/COOH group to form a

  peptide bond;

  Accept on diagram, for example (at least) two

  amino acids joining by a correctly drawn peptide

  bond (MP1) with NH2 at one end and COOH at the

  other (MP2).

  Ignore incorrect names of NH2 and COOH groups.

  2. (So in chain) there is a free amine/NH2 group at one end

  and a free carboxyl/COOH group at the other
• Explain how the active site of an enzyme causes a high rate of reaction. (3)

  1. Lowers activation energy;

  2. Induced fit causes active site (of enzyme) to change shape;

  3. (So) enzyme-substrate complex causes bonds to form/break;

  Accept: description, of induced fit

  Accept: enzyme-substrate complex causes stress/strain

  on bonds
• Other than those stated, give one factor the student would have controlled in his investigation. (1)

  Size/dimensions /mass/variety of potato

  OR

  Temperature (of solution/flask)

  OR

  pH (of solution);

  Accept : weight of potato

  Ignore : amount of potato

  Ignore concentration/ volume of catalase
• Suggest a change the student could make to his procedure so that 10 cm3 of oxygen would be produced in less than 6 seconds. (1)

  Cut up/use discs/homogenise/increase surface area (of potato chips)

  OR

  Use bigger chips

  OR

  Increase temperature

  OR

  Change pH;
• Describe a biochemical test to confirm the presence of protein in a solution.(2)

  )1. Add biuret (reagent);

  Accept sodium hydroxide (solution) and copper sulphate

  (solution)

  Reject addition of other incorrect chemicals

  2. (Positive result) purple/lilac/violet /mauve;

  Reject other colours

  Ignore references to heating
• 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

  2 max for similarities

  Accept for three marks, a labelled diagram of a dipeptide

  showing NH2/NH3

  +, COOH/COO− and different R groups.

  1. Amine/NH2 (group at end);

  Accept amino/NH3

  +

  2. Carboxyl/COOH (group at end);

  Accept carboxylic / COO−

  3. Two R groups;

  4. All contain C and H and N and O;

  Accept examples of different R groups

  Difference

  5. Variable/different R group(s);
• Explain what the positions of the spots in the diagram show about these amino acids. (3)

  1. Moved to negative (electrode) because positive(ly charged);

  2. (Spots move) different distances/rates because (amino acids)

  different charge/mass;

  Accept size for mass.

  3. Two spots (not three) because (amino acids) same

  charge/mass

  OR

  One spot has 2 amino acids because (amino acids) same

  charge/mass;

  Accept size for mass.
• Describe how a non-competitive inhibitor can reduce the rate of an enzyme-controlled reaction. (3)

  1. Attaches to the enzyme at a site other than the active site;

  Accept 'attaches to allosteric/inhibitor site'

  2. Changes (shape of) the active site

  OR

  Changes tertiary structure (of enzyme);

  3. (So active site and substrate) no longer complementary so

  less/no substrate can fit/bind;

  Accept 'no longer complementary so less/no enzymesubstrate complexes form'

  Accept abbreviations of enzyme-substrate complex.
• The scientist concluded that pectin is a non-competitive inhibitor of the

  lipase enzyme.

  Use Figure 1 to explain why the scientist concluded that pectin is a non competitive inhibitor. (1)

  (With inhibitor) increase substrate/lipid (concentration) does not increase/affect/change rate of reactionOR

  (With inhibitor) increase substrate/lipid (concentration) does not

  increase/affect/change lipase activity

  OR

  High substrate (concentration) does not overcome inhibition

  OR

  High substrate (concentration) does not meet

  maximum rate of reaction/lipase activity;

  Ignore references to competitive inhibitors
• Describe how a peptide bond is formed between two amino acids to form a dipeptide (2)

  1. Condensation (reaction) / loss of water; Accept each marking point if shown clearly in diagram.

  2. Between amine / NH2 and carboxyl / COOH; Accept between amino (group) and carboxylic / acid (group)