# Glycolysis --- **Glycolysis** (Greek: *glyko*=sweet *lysis*=dissolution/breakdown) is a [[catabolism|catabolic]] pathway that *every [[biology|organism]] on earth* can use in order to break down one molecule of [[glucose]] into two molecules of [[pyruvate]], and in doing so generate [[adenosine triphosphate]] and [[NAD|NADH]]. It is among the most ancient, fundamental and widespread of all life processes. Glycolysis takes place in the [[cytoplasm#Cytosol|cytosol]], which makes sense because it predates the rise of eukaryotes. Along with glucose, glycolysis relies on several **intermediate** molecules that can come from the breakdown of other macromolecules, as well as a bunch of [[enzyme|enzymes]]. In total, one glucose molecule produces two NADHs, two (net) ATPs and two pyruvates. It's completely [[cellular respiration|anaerobic]], as it arose before [[bacteria|cyanobacteria]] could generate an oxygen rich atmosphere. Although ancient and reliable, it does not produce a great amount of ATP. In organisms that cannot tolerate oxygen, the products of glycolysis are used in [[fermentation]]. For aerobic respirators, the products of glycolysis can continue the next step of [[cellular respiration]], the [[citric acid cycle]]. Cells will chose to use glycolysis if there is a need for a lot of ATP, but if there isn't that need present, the cell can choose to make building blocks with the glucose instead, in the [[pentose phosphate pathway]]. ## Steps There are ten reactions that take place during glycolysis, stating with a single 6-carbon [[glucose]] and ends with two 3-carbon [[pyruvate]]. These 10 reactions can be divided into three phases: - the **energy-investment phase** (steps 1-5) of glycolysis is named because because it starts by using [[adenosine triphosphate]], not making it, which makes these reactions are [[endergonic and exergonic|endergonic]]. - the **cleavage phase** (steps 4 and 5) are where the one 6-carbon molecule into two 3-carbon molecules. - the **energy-payoff phase** (steps 6-10) are where the reactions now become (quite strongly) [[endergonic and exergonic|exergonic]]. ![[glycolysis.png]] step 1 - [[phosphorylation|phosphorylates]] the glucose into [[glucose 6-phosphate]] (costs 1 ATP) step 2 - [[isomer|isomerizes]] that special glucose into *fructose* 6-phosphate to generate symmetry step 3 - once more phosphorilates the fructose 6-phosphate to turn it into **fructose biphosphate** (costs 1 ATP) step 4 - cleaves the fructose biphosphate into two slightly different 3-carbon molecules; [[glyceraldehyde 3-phosphate]] step 5 - isomerizes the one of the 3-carbon molecules so they become identical. steps 6-10 - a lot of stuff happens, two [[NAD|NAD+]] are [[redox|reduced]] into two [[NAD|NADH]]s. Two [[adenosine triphosphate]]s are formed from substrate-level [[phosphorylation]] of ADP and a phosphate group. In these steps we systematically take back the phosphate groups for ATP. ## Glycolysis Regulation Glycolysis is [[enzyme regulation#Allosteric Regulation|regulated]] *allosterically* when high levels of ATP are present. Enzymes are very sensitive to how much substrate is present, and *phosphofructokinase*–the enzyme that catalyzes the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate–has two binding sites for ATP. When ATP levels are low, it only binds to the main active site, where catalyzes the reaction. But, if ATP levels are high, then it will also bind to a [[enzyme regulation|allosteric]] regulatory site, which inhibits the reaction. ___