2016 FRQ by topics Core topics Respiration and Photosynthesis ANS

Respiration 1. [Tutorial 9286/08/2/7a] Outline the main stages of glycolysis. [8] Glycolysis occurs in the cytoplasm and does not require the presence of oxygen. It involves the oxidation / breakdown of glucose to yield two molecules of pyruvate; Glycolysis can be divided into two phases: energy investment phase and energy pay-off phase; During the energy investment phase , energy in the form of 2 ATP is used per glucose molecule; Activation of glucose occurs to make it more chemically reactive with the phosphorylation of glucose using ATP to produce glucose-6-phosphate. This reaction is catalysed by hexokinase; Glucose-6-phosphate is then isomerised to fructose-6-phosphate by isomerase; Phosphorylation of fructose-6-phosphate using ATP produces fructose-1,6-bisphosphate. This reaction is catalysed by phosphofructokinase; Cleavage / Splitting of fructose-1,6-bisphosphate produces 2 molecules of glyceraldehyde- 3-phosphate (GALP); During the energy payoff phase, energy in the form of 4 ATP is produced via substrate level phosphorylation; Each GALP is subsequently converted to pyruvate via multiple steps where 2 ATP are generated via substrate level phosphorylation and protons and e lectrons released via dehydrogenation are transferred to 1 oxidised NAD to form 1 reduced NAD; Since 2 molecules of GALP is formed from 1 glucose molecule, therefore 2 pyruvate , 4 ATP and 2 reduced NAD are produced per glucose molecule; Or Breakdown down 1 molecule of glucose to 2 pyruvate, 2 reduced NAD, with net gain of 2 ATP; [AJC 2013] Explain the significance of the steps in glycolysis. [5]  Glycolysis is a common step in both anaerobic and aerobic respiration;  Phosphorylation of glucose (by 2 ATP) is to activate it;  Phosphorylation of glucose (by 2 ATP) / glucose -6-phosphate results in glucose being trapped in the cytosol/ unable to leave the cell through the same glucose carrier protein/ committed to the end of glycolysis;  PFK which catalyse the phosphorylation of fructose phosphate also control rate of glycolysis/ high rate of ATP act as allosteric inhibitor to PFK;  ATP synthesis by substrate level phosphorylation;  Forms two glyceraldehyde-3-phosphate/ triose phosphate from one glucose;  Forms two reduced NAD (NADH) (by dehydrogenation);  Which later give 6 ATP by oxidative phosphorylation;  Pyruvate can enter into link reaction/ mitochondria or be converted to lactate (in mammals) or ethanol and carbon dioxide (in yeast);  Pyruvate is small enough to enter mitochondrion for aerobic respiration; [ACJC 2010 H1 P2] Describe the role of glycolysis in respiration. [7] 1. glycolysis - oxidation of glucose to pyruvate; 2. phosphorylation of glucose; 3. add (2) phosphate groups to glucose; 4. from (2 molecules of) ATP;

5. to produce (1 molecule of) fructose-1,6-bisphosphate; 6. to raise the energy level of glucose; 7. so that useful energy can be harvested in later steps in the pathway; 8. oxidation / conversion of (2 molecules of) glyceraldehydes; 9. into (2 molecules of) pyruvate; 10. removal of H from glyceraldehyde; 11. to hydrogen carriers NAD+; 12. produce (2 molecules of) reduced NAD / NADH + H+; 13. form (4 molecules of) ATP; 14. by substrate-level phosphorylation; 15. using organic phosphates; 16. reduced NAD allows for ATP production in the mitochondria; 17. by oxidative phosphorylation; 18. pyruvate to enter mitochondria for further oxidation; 19. or converted to lactate / ethanol in anaerobic respiration; 20. to regenerate NAD+ to allow glycolysis to continue; ;@1/2m, max 7 2. Outline the link reaction. [2] Takes place in mitochondrial matrix; 1) Decarboxylation  Carboxyl group of pyruvate (3C) is removed and carbon dioxide (CO2) is released 2) Oxidation via dehydrogenation  Remaining 2C molecule undergoes oxidation via dehydrogenation by transferring protons and electrons to oxidised NAD, therefore converting it to reduced NAD  Acetate is produced 3) Addition of Coenzyme A (CoA)  Coenzyme A is attached to acetate to form acetyl-CoA (2C) ;; Substrate to product (1M) By-products & processes(1M) 3. [Tutorial 02/7a] Outline the main features of the Krebs cycle. [8] [BT 2016] Outline the main stages of the Krebs cycle. [8] Krebs cycle occurs in mitochondrial matrix; During Krebs cycle, acetyl-CoA (2 -carbon) is attached to a 4-carbon compound called oxaloacetate. The resulting 6-carbon compound, citrate is then gradually re-converted to oxaloacetate, making it a cycle; At 2 stages in the Krebs cycle, carbon is removed from the intermediate compounds via oxidative decarboxylation, forming intermediate 5C and 4C compounds respectively . 2 molecules of carbon dioxide are produced per cycle and carbon dioxide diffuses out of the mitochondrion, and out of the cell; 1 molecule of ATP is produced per cycle via substrate level phosphorylation where the phosphate group was derived from Guanosine triphosphate (GTP); Intermediate compounds undergo oxidation via dehydrogenation whereby protons and electrons are transferred to oxidised NAD (nicotinamide adenine dinucleotide) and oxidised FAD (flavin adenine dinucleotide) and reduced to reduced NAD and reduced FAD respectively;

These reduced coenzymes subsequently transfer these high energy protons and electrons to the electron transport chain for the synthesis of ATP; Since 2 molecules of acetyl CoA are formed per glucose molecule during the link reaction, the Krebs cycle runs twice to completely utilise them; Therefore for each glucose molecule, the products of Krebs cycle are 4 molecules of CO2, 6 molecules of reduced NAD, 2 molecules of reduced FAD and 2 molecules of ATP; Examiner’s comments: The term ‘inorganic phosphate’ is only applicable for Oxidative phosphorylation. Substrate- level phosphorylation involves an enzyme transfers a phosphate group from a substrate molecule to ADP, rather than adding an inorganic phosphate (free lying ions) to ADP as in oxidative phosphorylation. 4. [BT 2016] Describe the role of NAD and FAD in cellular respiration. [6] NAD and FAD are co-enzymes which function as protons and electrons carrier; ® acceptor

Oxidized NAD reduced to reduced NAD formed during glycolysis (cytosol), link reaction and Krebs cycle (both in mitochondria matrix) while oxidized FAD reduced to reduced FAD during Krebs cycle only; During the oxidation of respiratory substrate via dehydrogenation reactions, protons and electrons are released and transferred to oxidized FAD to form reduced FAD.; (Learning point) Reduced NAD and FAD transfer high energy protons and electrons to electron carriers on electron transport chain embedded in the inner mitochondrial membrane , where oxidative phosphorylation occurs; Leading to the regeneration of oxidised NAD and FAD to allow glycolysis, link reaction and Krebs cycle to continue; As the donated electrons travel down a series of electron carriers that are progressively electronegative, the energy released is coupled to the pumping to these H+ ions from the mitochondrial matrix into the intermembrane space; Leading to a build-up of a proton-motive force/ electrochemical proton gradient/ H+ ion gradient across the inner mitochondrial membrane which is essential for the formation of ATP from ADP + Pi via chemiosmosis using ATP synthase; [TPJC 2010] Outline the role of NAD in aerobic respiration. [6] 1. NAD acts as co-enzymes of the NADH dehyrogenase 2. removes H atoms from their active sites 3. NAD acts as electron acceptors at glycolysis, link reaction and Krebs cycle, receiving H+ and electrons from the organic intermediates via dehydrogenases 4. they act as carriers of H+ and electrons to the ETC 5. NADH is then oxidized at the ETC when it transfers H+ and electrons to the electron carriers of the ETC to produce ATP at the inner membrane of the mitochondria 6. helps to generate proton gradient for