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Tuesday, July 21, 2015 Unit 1: Respiration Title: Glycolysis Keywords: • glycolysis • link reaction • Krebs cycle • activation energy Learning Objectives: We are learning…. • Where does glycolysis fit into the overall process of respiration? • What are the main stages of glycolysis? • What are the products of glycolysis? Starter: Write down as much information as you can about respiration. Some key terms Pyruvate or pyruvic acid is an organic acid made from glucose during glycolysis. It can be used to make the amino acid alanine or converted into ethanol or lactic acid. Phosphorylation is the addition of a phosphate group to an organic (carbon containing) molecule. Oxidation is the loss of at least one electron when two or more substances interact. As NAD is reduced, one electron is added at the Nitrogen atom (removing the + charge), and two hydrogens (electron + proton = H atom) are added at the upper position of the nicotinamide ring = NADH2 Cellular respiration Glucose cannot be used directly by cells as a source of energy. Instead, cells use ATP as their immediate energy source. The conversion of glucose to ATP takes place during the process of cellular respiration. There are two different forms of cellular respiration; Aerobic respiration – requires oxygen and produces carbon dioxide, water and around 36-38 molecules of ATP per molecule of glucose used. Anaerobic respiration (fermentation) – without the presence of oxygen, glucose is broken down to produce lactate (in animals), ethanol (in plants), carbon dioxide and around 2 molecules of ATP. Aerobic respiration Aerobic respiration can be divided into four stages: 1. Glycolysis – splitting of 6-carbon glucose molecule into two 3-carbon pyruvate molecules. 2. Link reaction – conversion of 3-carbon pyruvate molecule into carbon dioxide and a 2-carbon molecule called acetylcoenzyme A. 3. Krebs cycle – the yielding of ATP and electrons from a series of oxidation-reduction reactions involving acetylcoenzyme A. 4. Electron transport chain – the use of electrons from the Krebs cycle to synthesise ATP with water produced as a by-product. 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. Glycolysis Glycolysis (glyco – ‘sugar’; lyso – ‘breakdown’) is the breakdown of a hexose sugar, usually glucose, into two molecules of the three-carbon compound pyruvate (pyruvic acid). Where does glycolysis take place? Glycolysis takes place in the cytoplasm of cells. The enzymes involved in the glycolytic pathway are found in the cytoplasm, so no organelle or membrane is required. Glycolysis does not require oxygen, so can take place with or without it. Glycolysis in the absence of oxygen produces pyruvate which can be converted to lactate or ethanol during anaerobic respiration. Why must red blood cells rely on ATP from glycolysis? Red blood cells do not have a mitochondria so cannot obtain ATP from the Krebs cycle and electron transport chain Stages of Glycolysis Stage 1 – the glucose molecule is insufficiently reactive, so it is phosphorylated to make it more reactive. The phosphate molecule comes from the conversion of ATP to ADP. Hexokinase enzyme At this point we are down 1 ATP ! Endergonic reaction – absorbs energy Stages of Glycolysis Stage 2 – the glucose molecule is reorganised into its isomer, fructose phosphate. Stages of Glycolysis Stage 3 – further phosphorylation takes place, by the donation of another phosphate molecule from ATP to make the sugar yet more reactive. At this point we are down 2 ATP ! Stages of Glycolysis Stage 4 – the six-carbon sugar is split into two threecarbon sugars. Stages of Glycolysis Stage 5 – more phosphorylation occurs but in this case the source of the phosphate is inorganic (not attached to carbon and hydrogen atoms) and not ATP. Two pairs of hydrogen atoms are removed, so 2 x NAD become 2 x NADH2 NAD NADH2 Stages of Glycolysis Stage 6 – a phosphate molecule is lost from both molecules of glycerate 1,3-bisphosphate, thus yielding two molecules of ATP from ADP. Exergonic reaction – yields energy Now we are at 0 net ATP. Stages of Glycolysis Stage 7 – a further pair of phosphates are removed forming two more ATPs. Each glycerate 3-phosphate molecule also has a water molecule removed. Now we are up by 2 ATP! Energy yields from glycolysis The overall energy yield from one glucose molecule undergoing glycolysis is: Two molecules of ATP (four molecules of ATP are produced, but two were used up in the initial phosphorylation of glucose) Two molecules of reduced NAD (these have the potential to produce more ATP through the electron transport chain) Two molecules of pyruvate. What happens to the pyruvate? With oxygen – Link reaction then Krebs cycle Without oxygen – lactate or ethanol during anaerobic respiration Energy yields from glycolysis The energy yield for each molecule of NADH + H (reduced NAD) is 2.5 ATP’s. For FADH2 (another hydrogen carrier found in the Krebs cycle) it is 1.5 ATP’s. So from glycolysis two NADH2 molecules x 2.5 = 5 ATP’s. There are 2 ATP molecules formed directly (net gain), so this makes 7 ATP in total. Main Activity: Write a short paragraph summarising the events of glycolysis. Share your paragraph with others in the group and provide feedback. Use diagrams to help support your understanding. Glycolysis Summary For each glucose molecule entering the reaction: • 2 molecules of ATP are used • 4 molecules of ATP are produced. • So there is a net gain of 2 ATP molecules • 2 molecules of reduced NAD are produced • Glucose is converted to 2 molecules of pyruvate. Plenary: Quick quiz 1 – 8 (15 marks) 1. Briefly describe the function of glycolysis during aerobic respiration and indicate the reactants and products. (2 marks) Glycolysis is a partial breakdown of a six-carbon glucose molecule (1 mark) into two, three-carbon molecules of pyruvate, 2NADH +2H+, and 2 net ATP. (1 mark) Plenary: Quick quiz 2. State the reactants in glycolysis. (4 marks) The reactants in glycolysis are glucose (a 6 carbon sugar) (1 mark), 2 NAD+ (1 mark), 2 ATP (1 mark), and 2 inorganic phosphates(Pi) (1 mark). Plenary: Quick quiz 3. State the products in glycolysis. (4 marks) The products in glycolysis are 2 pyruvate (a 3 carbon molecule) (1 mark), 2 NADH (1 mark), 2 H+ (1 mark), and 2 net ATP (1 mark). Plenary: Quick quiz 4. Does glycolysis require oxygen? (1 mark) No: (1 mark) Glycolysis is a partial breakdown of a six-carbon glucose molecule into two, three-carbon molecules of pyruvate, 2NADH +2H+, and 2 net ATP. It is an anaerobic pathway. Plenary: Quick quiz 5. Is the following statement true or false? In eukaryotic cells, glycolysis takes place in the mitochondria. (1 mark) False: Glycolysis takes place in the cytoplasm in both prokaryotic and eukaryotic cells. (1 mark) Plenary: Quick quiz 6. State why one molecule of glucose is able to produce two molecules of pyruvate during glycolysis. (1 mark) In step 4 of glycolysis, a 6-carbon molecule (fructose 1,6 biphosphate) is split to form two, 3-carbon molecules (glyceraldehyde 3-phosphate). (1 mark) Plenary: Quick quiz 7. State the total number and the net number of ATP produced by substrate-level phosphorylation during glycolysis. (1 mark) A total of 4 ATP are produced by substrate-level phosphorylation during glycolysis, but since 2 ATP were used in endergonic steps 1 and 3, the net total is 2 ATP. (1 mark) Plenary: Quick quiz 8. Steps 1 and 3 of glycolysis are: (1 mark) a. exergonic b. endergonic Endergonic – to absorb energy Exergonic – to yield energy A - endergonic (1 mark) - An exergonic reaction yields energy. Each of the first two steps of glycolysis require hydrolysis of a molecule of ATP. How successful were we this lesson? Learning Objective We were learning…..