[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe the link reaction you should then be able to explain how the link reaction is an example of oxidative decarboxylation in the last video we looked at glycolysis which is the first stage of respiration we saw that glycolysis takes place in the cytoplasm and does not require oxygen during glycolysis the six carbon glucose molecule is broken down in a series of reactions to form two molecules of pyruvate which has three carbon atoms during glycolysis we produce a net yield of two ATP molecules and two molecules of reduced NAD now the two pyruvate molecules produced in glycolysis still contain a great deal of stored energy so in the later stages of respiration this energy is released now what happens next depends on the level of oxygen in the absence of oxygen anaerobic respiration takes place in the cytoplasm and we'd be looking at anaerobic respiration in detail in later videos however if oxygen is present then the cell carries out aerobic respiration and in this case all of the remaining reactions of respiration take place in the mitochondria I'm showing you a mitochondrion here remember that mitochondria have a double membrane and the internal region of a mitochondrion is called the mitochondrial Matrix in the next stage of respiration the pyruvate molecules produced by glycolysis are actively transported from the cytoplasm into the mitochondrial Matrix at this point the pyruvate molecules take part in the link reaction I'm showing you the link reaction here now in order to understand this I need to show you the structures of the key molecules you don't need to learn these but it will help you to see what's taking place okay so we start with pyruvate which contains three carbon atoms the pyruvate now reacts with a molecule called coenzyme a now coenzyme a is a complex molecule so I'm showing this as a box now the pyruvate molecule splits a two carbon group from the pyruvate is added to the coenzyme a and I'm showing that as blue this forms the molecule acetyl coenzyme a the remaining one carbon part of the pyruvate leaves is a molecule of carbon dioxide and I'm showing that as yellow at the same time an oxidation reaction takes place forming a molecule of reduced NAD so at the end of the link reaction we've got three products we have one molecule of acetyl coenzyme a one molecule of carbon dioxide and one molecule of reduced NAD now there are several important facts about the link reaction that you need to remember firstly glycolysis produces two pyruvate molecules for each molecule of glucose so the link reaction takes place twice for each molecule of glucose entering respiration this means that per glucose the link reaction produces two molecules of acetyl coenzyme a two molecules of carbon dioxide and two molecules of reduced NAD secondly during the link reaction a molecule of carbon dioxide is released from the pyruvate when carbon dioxide is removed from a molecule scientists call this a decarboxylation reaction at the same time an oxidation reaction takes place producing reduced NAD so because we have an oxidation reaction plus a decarboxylation reaction scientists call this oxidative decarboxylation and lastly you'll notice that the link reaction does not require oxygen okay now at this point the acetyl coenzyme a produced in the link reaction enters the next stage of respiration this is called the Krebs cycle and we look at the Krebs cycle in the next video foreign [Music]