4.2 Link Reaction and Krebs Cycle RESPIRATION 4.2 Link Reaction and Krebs Cycle RESOURCES: MITOCHONDRIA DIAGRAM HANDOUT MOLYMODS

Learning Objectives Success Criteria Produce accurate notes on the link reaction Summarise in a diagram the key events in the Krebs cycle Carry out an experiment and analyse the results to demonstrate the role of coenzymes in respiration Describe what the link reaction is Describe what happens during the Krebs cycle Explain the role of hydrogen carrier molecules in the Krebs cycle

Starter Write down the word equation for Glycolysis What is used up in glycolysis? What else is produced in glycolysis?

Glycolysis You finish with Pyruvate You start with glycogen. NAD+ + 2H  NADH + H+ (oxidised form ) (reduced form) NB Rather then write NADH, examiners often simply refer to it as reduced NAD or reduced coenzyme Glycogen You start with glycogen. 2 ATP 4 ADP + 4Pi 4 ATP Glyceraldehyde 3-phosphate (3C x 2) Pyruvate (3C x2) Glucose (6C) 2H x 2 NAD Reduced NAD 2 ADP Nicotinamide adenine dinucleotide = So it gains electrons You finish with Pyruvate

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

What Happens Where? Glycolysis = Cytoplasm of the cell. Link reaction = Matrix of the mitochondria. Krebs cycle = Also in the matrix. Electron transfer chain Utilises proteins found in the membrane of the christa.

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

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.

The Link Reaction Links Glycolysis to the Krebs Cycle. 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.

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

Link Reaction Pyruvate(3C) CO2 (1C) NAD+ NADH + H+ Coenzyme (CoA) One carbon atom is removed from pyruvate in the form of CO2. The remaining 2-carbon molecule combines with coenzyme A to produce acetylcoenzyme A (acetyl CoA). Another oxidation reaction occurs when NAD+ collects more hydrogen ions. This forms reduced NAD (NADH + H+) No ATP is produced in this reaction. Pyruvate(3C) Decarboxylation CO2 (1C) NAD+ Oxidation Reduction NADH + H+ Coenzyme (CoA) Acetyl CoA (2C)

The Link Reaction occurs Twice for every Glucose Molecule For each glucose molecule used in glycolysis, two pyruvate molecules are made. But the link reaction uses only one pyruvate molecule, so the link reaction and the Krebs cycle happen twice for every glucose molecule which goes through glycolysis. Overall equation for one link reaction and Kreb cycle: Pyruvate + NAD + CoA acetyl CoA + reduced NAD + CO2

The Products of the Link Reaction go to the Krebs Cycle and the ETC 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)

Krebs Cycle Sir Hans Krebs in 1937 AKA Citric acid cycle or the Tricarboxylic acid cycle (TCA cycle) Takes place in the matrix.

Krebs Cycle From one glucose molecules you get 2 pyruvate molecules. So Krebs cycle go around twice per glucose molecule.

The Krebs Cycle Acetyl Coenzyme A (2C) NAD Coenzyme A 2C Reduced NAD Oxaloacetate (4C) Citrate (6C) (6C) (4C) NAD (4C) CO2 FAD Reduced NAD Reduced FAD CO2 (5C) (4C) NAD ADP + Pi ATP (4C) Reduced NAD

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. Remember this: 665 and five 4’s.

DENA DENA A FA... NA 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

The Krebs Cycle: DENA DENA A FA... NA Acetyl Coenzyme A (2C) NAD Coenzyme A 2C NA Reduced NAD Oxaloacetate (4C) Citrate (6C) (6C) (4C) NAD (4C) DENA CO2 FAD Reduced NAD FA Reduced FAD CO2 (5C) (4C) DENA NAD ADP + Pi ATP A (4C) Reduced NAD

Krebs Cycle Summary 2 CO2 molecules 1 ATP molecules (S.L.P) 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

Coenzymes in Respiration Coenzymes are complex organic molecules that are used by enzymes to accept or donate molecules involved in a reaction. They are often referred to as ‘Helper’ molecules as they carry chemical groups or ions about, e.g. NAD removes H+ and carries it to other molecules. Now carry out the practical about Coenzymes in respiration

Homework Complete the questions on the practical sheet Read ‘coenzymes in respiration’ on page 55 of the textbook then answer questions 1-5