[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe the stages of glycolysis you should then be able to State the end product of glycolysis in the last video we saw that during respiration a glucose molecule is gradually broken down and if you haven't seen that video then you need to watch it now at different stages the energy contained within glucose can be transferred to other molecules now this energy transfer can take place in two different ways in some reactions the energy transferred can be used to produce a molecule of ATP directly I'm showing you an example here in this case molecule a is converted to molecule B and this reaction releases energy the energies used to form a molecule of ATP from ADP and pi reactions like this are called substrate level phosphorylation alternatively a hydrogen and two electrons can be removed from a molecule this is called a dehydrogenation or oxidation reaction the hydrogen on two electrons are transferred to a hydrogen carrier such as the coenzyme NAD forming reduced NAD the reduced NAD is then used later to produce ATP in a process called oxidative phosphorylation okay so in this video we're looking at the first stage of respiration which is called glycolysis there are two key facts about glycolysis that you need to learn firstly glycolysis takes place in the cytoplasm and secondly glycolysis does not require oxygen now glycolysis actually consists of 10 different reactions but for a level biology we learn a simplified version I'm showing you here a glucose molecule I'm representing glucose as a chain of six carbon atoms rather than a ring as it's easier to visualize in the first stage of glycolysis the glucose molecule reacts with two molecules of ATP these ATP molecules each transfer one phosphate onto the glucose molecule so this is an example of a phosphorylation reaction this reaction activates the glucose molecule making it more reactive the product of this reaction is called hexos-biz phosphate in the next stage the hexos biz phosphate splits into two molecules of triosphate this is a lysis reaction which means splitting in stage 3 an inorganic phosphate ion from the cytoplasm now react with each molecule of trios phosphate this is a phosphorylation reaction and we formed two molecules of trios bisphosphate in the final stage each trials bisphosphate molecule is converted to a molecule of pyruvate during this reaction hydrogen is removed from the trials bisphosphate molecules so this is a dehydrogenation or oxidation reaction this hydrogen is added onto NAD forming reduced NAD also each phosphate group on Trials bisphosphate is added to ADP forming ATP so for each trials bisphosphate molecule we make two molecules of ATP and this is an example of substrate level phosphorylation okay I'm showing you here all the stages of glycolysis we need to work out the net or overall products of glycolysis at the start we used two ATP molecules but later we produced four ATP molecules so the net yield is two ATP molecules we also produce two molecules of reduced NAD these reduced NAD molecules will be used in a later stage of respiration called oxidative phosphorylation and we'll see that in a later video and finally we've made two molecules of pyruvate now you'll notice our glycolysis has not released a great deal of energy that's because the pyruvate molecules that we formed still contain a great deal of stored energy this energy is gradually released during the later stages of respiration in the next video we look at the next stage of respiration which is called the link reaction [Music]