MARKING SCHEME TRIAL PENGGAL 1 STPM 2016

SECTION A :

1 D 4 C 7 C 10 D 13 B 2 A 5 A 8 B 11 C 14 D 3 B 6 C 9 A 12 D 15 B

SECTION B [15 marks] Answer all questions in this section

16. Penicillinase which is found in many bacteria, breaks down penicillin. It gives bacteria resistance to penicillin. The figure below shows a ribbon model of the structure of the enzyme penicillinase. The arrow indicates the active site of the enzyme.

(a) Explain why the shape of the active site of the enzyme is important. [2 marks] - Active site of the enzyme has complementary shape for substrate / active site of enzyme is specific for its substrate. - Enables the substrate to bind to the active site like the lock and key /induced fit model. - Substrate binds with the active site to form enzyme substrate complex {Any two} (b) With reference to the figure, identify the (i) aspects of protein structure that are shown. [2 marks] - α-helix structure - Folding of the tertiary structure (ii) aspects of protein structure that are not shown. [2 marks] - Primary structure / sequence o f amino acids - R groups of amino acid - peptide bonds between amino acids

- Internal bonds {Any two}

(c) The figure below shows the changes in energy during the progress of an uncatalysed reaction.

(i) Draw a curve on the figure to show changes in energy during the progress of the same reaction

catalysed by an enzyme. [1 mark] - Lower peak (ii) State the term given to the energy level that must be overcome before a reaction can progress.

[1 mark] - Activation energy 17. The diagram below shows the process of ATP synthesis in aerobic respiration.

(a)

Name the multiprotein complex labelled I and IV. [2 marks]

I : NADH dehydrogenase / Flavoprotein / NADH-ubiquinone oxidoreductase IV : Cytochrome oxidase (aa3)/cytochrome c oxidase (b) Name the process labelled P in aerobic respiration. [1 mark] - Electron transport chain / electron transport system

P

Q

(c) State the role of oxygen in the process labelled P. [1 mark] - Oxygen is the final electron acceptor, it combines with hydrogen ions to form water (d) (i)

Name the process of ATP synthesis labelled Q. [1 mark]

- Chemiosmosis (ii) Explain how ATP is synthesis from the process you mention in (d) (i). [2 marks] - The electrochemical proton gradient forces hydrogen ions to diffuse through the ATP synthase complex - down its electrochemical gradient across the membrane // This potential energy is used to synthesis ATP.

Marks SECTION C [30 marks]

Answer any two questions in this section.

18. (a) With a suitable example, explain in detailed about three types of cofactors. [9 marks] Prosthetic Groups [3 m] • A prosthetic group is a non-protein organic molecule that binds tightly on a

permanent basis to the protein part of the enzyme (apoenzyme). 1

• The prosthetic group is involved in the catalytic function of the enzyme. 1 • Haem is present in the enzyme catalase which catalyses hydrogen peroxide into

oxygen and water. // Haem is found in the prosthetic group of cytochromes which are electron carriers. It takes part in oxidation-reduction reactions.

1

Coenzymes [3 m] • Coenzymes are small, non-protein organic molecules 1 • They bind loosely and temporarily to the active site of the enzyme 1 • The coenzymes readily detach and help to transfer chemical group, atoms or

electrons from one enzyme to another 1

• Many coenzymes are derivatives of vitamins especially group B vitamins, NAD (nicotinamide adenine dinucleotide) for example is formed from niacin // Other examples include NADP and CoA.

1

• It is a coenzyme for a number of dehydrogenase enzymes and acts as a hydrogen acceptor.

1

Succinate dehydrogenase Succinate + NAD+ -------------------------------> Fumarate + NADH + H+ (coenzyme)

Max 3m

Enzyme Activators / Metal ions[3 m] • Activators are inorganic ions such as Ca2+, Zn2+, Mg2+, Fe2+ and Cl-. 1 • They may attach temporarily to the enzyme and change its active site to make the

shape more suitable for a reaction to take place 1

• The ion may also bind the enzyme and substrate together. 1 • Calcium ions are needed to activate thrombokinase which converts prothrombin to

thrombin in blood clotting // Chloride ions increase salivary amylase activity. 1

Max 3 m

(b) Tabulate the differences between cellular respiration in animal cells under two

distinct conditions. [6 marks]

Condition A - sufficient oxygen supply

Condition B - sufficient oxygen supply but in presence of high partial pressure of carbon monoxide

Condition with sufficient oxygen With sufficient oxygen and high partial pressure of carbon monoxide

Marks

P1 Aerobic respiration occurs (in cytoplasm and mitochondria).

Anaerobic respiration occurs (only in cytoplasm).

1/0

P2 38 molecules of ATP are produced from one glucose molecule.

2 molecules of ATP are produced from one glucose molecule.

1/0

P3 Complete oxidation of glucose can occur. Incomplete oxidation of glucose occurs. 1/0 P4 The respiration involves glycolysis, link

reaction, Krebs cycle and electron transport chain.

The respiration only involves glycolysis. Link reaction, Krebs cycle and the electron transport chain are not involved.

1/0

P5 Carbon dioxide and water are produced. lactate are produced // no carbon dioxide and water are produced.

1/0

P6 Respiration in this condition is normal for the animal cells to carry out cell activities.

Prolonged respiration in this condition causes damage to the central nervous system and heart and even death.

1/0

Total(max): 6 m 19. (a) The final stage in cellular respiration is the electron transport chain and the

synthesis of ATP by chemiosmosis. Explain these reactions. [9 marks]

• The electrons supplied by NADH and FADH2 enter the electron transport system which consist of a series of electron carriers, namely flavoprotein (FAD), coenzyme Q (ubiquinone) and cytochromes.

1

• These electron carriers are located in the inner membrane of the mitochondrion. 1

• The electrons from NADH enter the electron transport system at the first carrier, flavoprotein.

1

• The two electrons are passed from one carrier to another. The carrier molecules that receive the electrons are reduced and the ones losing the electron are oxidized.

1

• Energy is released each time the electrons are passed from one carrier to the next. Sufficient energy is released for the phosphorylation of ADP to form ATP.

1

• FADH2 enters the electron transport system at ubiquinone molecule (coenzyme Q). 1

• The final carrier in the chain, cytochrome a3, transfers electrons and the two protons to oxygen to form water.

1

• When electrons are transferred along the electron transport system, hydrogen ions are pumped from the matrix into the intermembrane space.

1

• This creates electrochemical proton gradient across the inner membrane. 1

• Hydrogen ions diffuse down concentration gradient from intermembrane space back to matrix through ATP synthase.

1

• The flow of H+ through ATP synthase release energy. This energy provides the energy necessary for enzymes ATP synthases, to catalyze the synthesis of ATP from ADP and phosphate

1

• Each NADH generates 3 ATP and each FADH2 generates 2 ATP. 1

Max 9m (b) Describe the effect of cyanide to electron transport system in aerobic respiration.

[6 marks]

• Cyanide is a non-competitive inhibitor and irreversible. 1

• It binds to the iron component of cytochrome a3 in the cytochrome oxidase of the electron transport system.

1

• This inhibits the terminal transfer of electrons to the oxygen as cytochrome a3 is the final carrier of the electron transport system.

1

• The flow of electrons along the electron transport system is blocked. Hydrogen ions cannot be pumped across the inner membrane from the matrix into the intermembrane space of the mitochondria.

1

• Failure to form a hydrogen concentration gradient results in hydrogen ions not flowing through the ATP synthase. As a result, no ATP is formed through oxidative phosphorylation.

1

• The cells are unable to carry out cellular activities without ATP. Hence, cyanide causes death.

1

20. (a) Explain how the dark reaction occurs in the C4 plants.

[10 marks]

l In the anatomy of C4 plants, the concentric arrangement pattern of mesophyll cell

layer and inner bundle sheath cell layer around the vascular bundle is known as Krantz anatomy.

1

l In C4 plants, carbon dioxide is first fixed in the mesophyll cells by phosphoenolpyruvate (PEP) to form oxaloacetate (4C).

1

l This fixation is catalysed by phosphoenolpyruvate (PEP) carboxylase. 1 l Oxaloacetate is then reduced by NADPH to form malate. 1 l Malate is transported to the inner bundle sheath cells through plasmodesmata. 1 l Metabolism of malate to pyruvate regenerate carbon dioxide. 1 l Pyruvate formed enters the mesophyll cell and is reduced by NADPH to regenerate

PEP 1

l Carbon dioxide concentration will increase in bundle sheath cells and prevents photorespiration.

1

l CO2 combines with ribulose bisphosphate (RuBP) a 5C sugar in a carboxylation reaction, catalysed by RuBP carboxylase (rubisco) to produce an unstable 6C sugar

1

l The 6C sugar immediately splits into 2 molecules of 3C glycerate 3-phosphate or 3-phosphoglycerate

1

l Each 3-phosphoglycerate receives a phosphate group from ATP to become 1,3-biphosphoglycerate

1

l 1,3-biphosphoglycerate is then reduced to glyceraldehyde-3-phosphate (GALP) or triose phosphate (PGAL)

1

l 1/6 of all PGAL molecules are used to regenerate RuBP molecules 1 l 5/6 of PGAL molecules are used in the product synthesis phase, which includes

synthesis of carbohydrates,synthesis of lipids and synthesis of proteins 1

max 10 m

(b) With an aid of a diagram, explain the compensation point of a plant. [5 marks]

l Compensation point is the level of light intensity at which the rate of CO2 produced during respiration equals the rate of CO2 absorbed during photosynthesis

1

l There is no net gain or loss in the amount of sugar 1 l Only when the light intensity is higher than the compensation point, production of

sugar exceeds the use of sugar and the excess sugar is stored 1

l Shade plants can reach the compensation point faster at low light intensities than light plants

1

l Diagram with correct label 2

Max 5m