Biological molecules

Describe how a molecule of beta glucose is different from a molecule of alpha glucose. (2 marks)

Describe how a molecule of beta glucose is different from a molecule of alpha glucose. (2 marks)

• Hydroxyl group on carbon 1 is in a different position

• In alpha glucose it is below the ring, in beta glucose it is above the ring

Glycogen and cellulose are both carbohydrates. Describe two differences between the structure of a cellulose molecule and a glycogen molecule. (2 marks)

• Cellulose is made up of β-glucose (monomers) and glycogen is made up of α-glucose

• Cellulose molecule has straight chain and glycogen is branched

• Glycogen has 1,4- and 1,6- glycosidic bonds and cellulose has only 1,4- glycosidic bonds

A covering of lignin protects cellulose from enzyme attack . Use your knowledge of the way in which enzymes work to explain why cellulose-digesting enzymes do not digest lignin. (2 marks)

• Enzymes are specific

• Shape of lignin molecules will not fit active site (of enzyme)

Describe the structure of a cellulose molecule and explain how cellulose is adapted for its function in cells. (6 marks)

• made from β-glucose;

• joined by condensation, glycosidic bond;

• hydrogen bonds linking long straight chains;

• cellulose fibres makes cell walls strong

• can resist osmotic pressure;

• bond difficult to break; resists digestion / enzymes

Explain how cellulose molecules are adapted for their function in plant cells (3 marks)

• Long and straight chains

• Become linked together by many hydrogen bonds to form fibrils

• Provide strength to cell wall

Use the information in the figure to explain two ways in which fatty acids are important in the formation of new cells. (4 marks)

1. Fatty acids used to make phospholipids; Phospholipids in membranes; More phospholipids more membranes made;
2. Fatty acids respired to release energy; More triglycerides more energy released; Energy used for cell production

Draw a diagram to show the structure of a triglyceride molecule. (2 marks)

Define the quaternary structure of a protein. (1 mark)

Multiple polypeptide chains held together by hydrogen bonds and ionic bonds

Explain how two enzymes with different amino acid sequences can catalyse the same reaction. (2 marks)

Both active sites have similar tertiary structures so can form enzyme-substrate complexes with the same substrates

Explain how a change in the primary structure of a globular protein may result in a different three-dimensional structure. (3 marks)

• Sequence of amino acids change

• Tertiary structure folds in a different way

• As bonds form in different places

Explain how the structure of fibrous proteins is related to their functions.

• Long chains of amino acids;

• Folding of chain into a coil /pleated sheet;

• Association of several polypeptide chains together;

• Formation of fibres / sheets explained;

• H bonds / Disulphide bonding (In context);

• Fibres provide strength (and flexibility);

• Sheets provide flexibility;

• Example e.g. keratin in hair, collagen in bone; (MUST be in context)

• Insoluble because external R-groups are non-polar

Explain why initial rate of this reaction was faster at 65 °C than it was at 55 °C (3 marks)

• Particles have more KE at higher temperatures; move faster

• Greater chance of collision

• More enzyme-substrate complexes form

Raffinose is an example of a trisaccharide made up of glucose, fructose and galactose. The chemical formulae of these monosaccharides are listed below: 

Glucose = C₆H₁₂O₆

Fructose = C₆H₁₂O₆

Galactose = C₆H₁₂O₆

Determine the number of carbon, hydrogen, and oxygen atoms in a single molecule of raffinose. (1 mark)

18 carbon atoms, 32 hydrogen atoms, 16 oxygen atoms (two molecules of H₂O are lost when forming the glycosidic bonds between the three monosaccharides)

Describe the test for starch.

• Add iodine (potassium iodide solution) to the sample.

• If the solution goes from brown-orange to blue-black starch is present.

Describe the test for lipids.

• Emulsion test - add ethanol to the sample and shake the test tube then add water to it.

• Lipids are present if a white emulsion appears.
  

Describe the test for reducing sugars.

• Add Benedict's reagent to the sample then heat the solution gently.

• If it changes from light blue to a brick-red precipitate, indicates that reducing sugars are present.

Describe the test for non-reducing sugars.

• Add HCl to the sample and heat gently, neutralise the sample with NaHCO₃ solution.

• Add Benedict's reagent to the sample and heat in water bath

• If change from light blue to brick-red precipitate, then result is positive.

Describe the test for proteins.

• Add Biuret solution

• If amylase present, solution turns from blue to purple

When the reaction with catalase is carried out in a test tube, the test tube feels warm at the end of the reaction. Explain why. (2 marks)

• Energy in products less than energy in substrate

• Energy is released as heat

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 marks)

• Filter and dry the sample

• Find the mass

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

• A = glucose and B = maltose;

• Because more sugar/precipitate after hydrolysis/maltase action;

Use of a colorimeter in this investigation would improve the repeatability of the student’s results. Give one reason why. (1 mark)

• Gives quantitative results

• So standardises the method

Name the type of peptidase which will hydrolyse the bond labelled G in the diagram. (1 mark)

Endo(peptidase)