Module 4 Respiration

1. Module 4Respiration 1.4.5. Link reaction and Krebs cycle

2. Starter • Complete Glycolysis summary sheet

3. Learning Objectives Success Criteria • Describe what the link reaction is • Describe what happens during the Krebs cycle • Explain the role of hydrogen carrier molecules in the Krebs cycle • Outline the link reaction, with reference to decarboxylation of pyruvate to acetate and the reduction of NAD (Grade D-E) • Explain that coenzyme A carries acetate from the link reaction to Krebs cycle (Grade C) • Outline the Krebs cycle, including the roles of NAD and FAD, and substrate-level phosphorylation (Grade A-B)

4. Respiration The process of respiration can be split into 4 stages. • Glycolysis • Link reaction • Krebs Cycle • Electron Transport Chain

5. The Fate of Pyruvate? Products of Glycolysis • 2 reduced NAD (NADH + H+) • 2 Pyruvate • 2 ATP THIS DEPENDS ON THE AVAILABILITY OF OXYGEN!!!

6. The fate of pyruvate

7. Aerobic Respiration • If O2 is present 3C pyruvate passes into mitochondria. • Here it is completely oxidised forming CO2 and H2O. • The second stage of aerobic respiration is the link reaction.

8. The Link Reaction Cytoplasm Mitochondria The Link reaction The Krebs Cycle Glycolysis • Links Glycolysis to the Krebs Cycle. • The end product of the link reaction can enter the Krebs Cycle.

9. NAD+ NADH + H+ Coenzyme A Acetyl coenzyme A (2C) Pyruvate (3C) Acetate (2C) CO2 Coenzyme A now combines with the Acetate to form Acetyl coenzyme A The Pyruvate is decarboxylated (CO2) It is also dehydrogenated (H+ removed) Start with Pyruvate produced in Glycolysis This produces Acetate The Acetyl coenzyme A now enters Krebs cycle

10. Link Reaction Pyruvate(3C) • Pyruvate decarboxylase removes a carboxyl group from pyruvate in the form of CO2. • Pyruvate dehydrogenase removes H atoms from pyruvate • NAD+ collects hydrogen ions. This forms reduced NAD (NADH + H+) • The remaining 2-carbon molecule combines with coenzyme A to produce acetylcoenzyme A (acetyl CoA). • No ATP is produced in this reaction. Decarboxylation CO2 (1C) NAD+ Oxidation Reduction NADH + H+ Coenzyme (CoA) Acetyl CoA (2C)

12. Task • Write down the equation for the link reaction Overall equation for one link reaction and Kreb cycle: Pyruvate + NAD + CoA acetyl CoA + reduced NAD + CO2

13. The Products of the Link Reaction go to the Krebs Cycle (takes place in matrix) So for each glucose molecule: • 2 acetylcoenzyme A (go into the Krebs cycle) • 2 Carbon dioxide (released as a waste products) • 2 Reduced NAD (go to the electron transport chain)

14. Krebs cycle – metabolic cycle Each step in the cycle requires enzymes to reduce the activation energy. The reactions all take place in the matrix of the mitochondria and are usually represented as a circular diagram.

15. Oxaloacetate citrate

16. DeNa DeNa A Fa... Na • This little tune will help you to remember the Krebs Cycle. • Remember, respiration is all about releasing energy from your food. • Oxidation releases energy. • When a carbon compound is oxidised, coenzymes are reduced. • The coenzymes involved are: NAD and FAD. • Carboxylation is the removal of CO2.

17. DENA DENA A FA... NA • DENA: Decarboxylation and production of reduced NAD • DENA: Decarboxylation and production of reduced NAD • A: Production of ATP • FA... : Production of reduced FAD (The ‘...’ means a gap) • NA: Production of reduced NAD

18. Oxaloacetate citrate DENA NA FA DENA A

19. Krebs cycle • Acetyl CoA joins with the C4(oxaloacetate) group • CoA is released to transport more pyruvate into the matrix • A C6 fragment is formed (citrate)   • C6 (citrate) is oxidativelydecarboxylated. • A C5 group is formed. • The carbon is given off as carbon dioxide • NAD+ is reduced to NADH + H+ DENA

20. Krebs cycle • The C5 fragment is oxidised and decarboxylated further to a C4 compound. • Again the carbon removed forms carbon dioxide. • NAD+ is further reduced to NADH + H+.   DENA

21. Krebs cycle • The final stage in the cycle has the C4 (oxaloacetate) acceptor regenerated. • ADP is reduced to ATP • FAD (Coenzyme)is reduced to FADH2 . • There is a reduction of NAD+ to NADH + H+. A FA NA

22. Oxaloacetate citrate DENA NA FA DENA A

23. Pyruvate (3C) • (b) Link reaction • (c) C4 + C2= C6 • (d) Recycling of CoA • (e) Decarboxylation C6 to C5 and the reduction of NAD • (f) Decarboxylation C5 to C4 and the reduction of NAD • (g) C4 to C4 with the reduction of coenzymes FAD and NAD. ATP is made directly. • (h) C4 to C4 acceptor This cycle follows one acetyl group. Each glucose that enters glycolysis will produce 2 acetyl groups.

24. Krebs Cycle Summary Each Acetyl CoA entering the cycle results in: • 2 CO2 molecules • 1 ATP molecules (S.L.P) • 8 pairs of H atoms (Later used in the E.T.C to produce ATP) Used to reduce NAD and FAD. Three reduced NAD are produced and 1 reduced FAD per cycle. NAD = Nicotinamide adenine dinucleotide FAD = Flavine adenine dinucleotide

25. Importance of the Krebs cycle • Breaks down macromolecules into smaller ones • Produces H atoms that are carried by NAD to ETC for oxidative phosphorylation, to produce ATP. • Regenerates 4 –carbon molecule that combines with acetylcoenzyme A, which would otherwise accumulate. • Source of intermediate compounds used by cells in the manufactures of other important substances such as fatty acids, amino acids and chlorophyll

26. Counting carbons - Plenary

27. Success Criteria Learning Objectives Describe what the link reaction is Describe what happens during the Krebs cycle Explain the role of hydrogen carrier molecules in the Krebs cycle Outline the link reaction, with reference to decarboxylation of pyruvate to acetate and the reduction of NAD (Grade D-E) Explain that coenzyme A carries acetate from the link reaction to Krebs cycle (Grade C) Outline the Krebs cycle, including the roles of NAD and FAD, and substrate-level phosphorylation (Grade A-B)