Lecture 32 – Glycolysis, the Hexose Pathway and Endosymbiosis November 13, 2013
• Glucose enters the cell via diffusion
• Glucose is phosphorylated byATP to produce Glucose 6-phosphate
• Glucose 6-phosphate cannot exit through the glucose channel, so it is “trapped in the cell,
allowing the cell to build up this compound without needing to worry about the concentration
• The phosphorylation of Glucose also adds energy to the molecule, so Glucose 6-phosphate has
a higher energy than Glucose.
• An addition of a sugar on the Glucose 6-phosphate, results in Fructose 6-phosphate
• A second phosphorylation occurs via ATP results in Fructose 1,6-Biphosphate, which is a
• Fructose 1,6-Biphosphate is split into two different trioses.
• One is Dihydroxyacetone phosphate, the other is Glyceraldehyde 3-phosphate.
• Dihydroxyacetone phosphate is only found in low amounts in the cell, this is because
Glyceraldehyde 3-phosphate is quickly taken to the next step of glycolysis, so
Dihydroxyacetone phosphate is converted by an enzyme into Glyceraldehyde 3-phosphate, to
try to lessen the concentration gradient.
• Glyceraldehyde 3-phosphate loses a hydrogen to NAD
• This reduced molecule gains a phosphate which is then lost to generateATP
• The second phosphate on Glyceraldehyde 3-phosphate is also lost to generateATP, this forms
pyruvate, additionally 4 ATP and 2 NADH molecules are produced.
• Pyruvate undergoes both anaerobic and aerobic reactions.
• In an anaerobic reaction, pyruvate decarboxylase removes CO from p2ruvate, forming
acetaldehyde, which reacts with alcohol dehydrogenase and recieves two protons from two
NADH to form ethyl alcohol (this is the reaction for yeast fermentation). The NADH are
supplied from the products of glycolysis earlier.
• An alternative anaerobic reaction in mammals occurs when pyruvate reacts with lactate
dehydrogenase and recieves two protons from two NADH molecules to form lactate (the
burn!!) • For theAerobic reaction, pyruvate dehydrogenase and CoenzymeAto produce NADH and
Acetyl CoA. All of this occurs in a pyruvate dehydrogenase complex, which is a cube shaped
• This grouping of all the needed reactants results in a more efficient reaction.
Sum up - The Hexose Pathway
• The intitial phosphorylation of Glucose
• results in retainment of Glucose in the cell
• adds potential energy to the cell
• and reconfigures the glucose so it can be split (with the addition of two phosphates)
• This process consumes 2ATP
• Generates Glyceraldehyde 3-phosphate and Dihydroxyacetone phosphate.
• The rapid use of Glyceraldehyde 3-phosphate results in the conversion of Dihydroxyacetone