Cellular Respiration (Glycolysis and Transition reaction)
Cellular respiration is the process of breaking down glucose and turning it into ATP, the energy currency of the body.
Aerobic respiration, this topic focuses on eukaryotic cells.
Definition of terms:
Metabolism - overall processes that happens in our body
Catabolism - breakdowns larger molecules into smaller molecules
Anabolism - builds larger molecules from smaller molecules
Phosphorylation - process of transferring of phosphate from high energy molecule to low energy molecule.
Isomerization - process of rearranging atoms of a molecule
Oxidation - loss of electron of a molecule (either addition of oxygen or removal of hydrogen)
Enzyme/catalyst - substances that helps in the metabolism processes in the body
Isomer - molecules with the same molecular formula but different arrangements of atoms (e.g. glucose and fructose)
Isomerase/mutase - enzyme that helps in the isomerization process
Kinase - enzyme that helps in the phosphorylation process
Aldolase - enzyme that helps in the cleavage or splitting of molecule.
NAD+/NADH - NAD+ is an electron carrier, NADH has electrons or carries electron.
GADP/PGAL - 3-glyceraldehydephosphate / phosphoglyecraldehyde
1,3-BPG - 1,3-bisphosphoglycerate
3-PG/2-PG 3-phosphoglycerate and 2-phosphoglycerate are the same, just differ in the position of phosphate in the carbon.
PEP - Phosphoenolpyruvate
Glycolysis
Glycolysis is the initiator of the cell respiration by extracting energy in the form of ATP from the 6-carbon compound, glucose and splitting it into 2 pyruvate.
Glycolysis comprises 10 steps catalysed by 10 enzymes as well. Each of the steps plays a crucial role in producing the component necessary for the preceding processes in cell respiration. This takes place in the cytoplasm of the cell where it invests 2 ATP in the preparatory stage or the energy-consuming stage (first 5 steps) in order to produce 4 ATP in the payoff stage or the energy-producing stage (last 5 steps).
Preparatory stage:
Step 1: Hexokinase reaction - hexokinase catalyses the phosphorylation or the addition of phosphate group to a molecule, in this case is the carbon-6 to form glucose-6-phosphate. This step requires one glucose and an ATP to produce the glucose-6-phosphate and an ADP. The phosphate groups traps the glucose inside the cell and encourages more glucose to enter by diffusion.
Step 2: Phosphoglucoisomerase reaction - this reaction is the isomerization of the glucose-6-phosphate to its isomer, fructose-6-phosphate. This process is catalysed by Phosphoglucoisomerase or glucose-6-phosphate isomerase; this is vital for further phosphorylation and reactivity in the glycolysis.
Step 3: Phosphofructokinase 1 reaction - the second phosphorylation, but this time the phosphate group is added in the carbon-1 hydroxyl producing the fructose-1,6-phosphate. Catalysed by Phosphofructokinase 1, it uses another ATP and produces 1 ADP.
Step 4: Fructose bisphosphate aldolase reaction - The fructose-1,6-phosphate are ready to be cleaved into two molecules, glyceraldehyde-3-phosphate (GADP or G3P) and dihydroxyacetone phosphate (DHAP) with the aid of fructose bisphosphate aldolase.
Step 5: Triosephosphate isomerase reaction - DHAP will be converted into another GADP through isomerization aided by the Triosephosphate isomerase leaving us with two GADP.
Payoff stage:
Step 6: Glyceraldehyde phosphate dehydrogenase reaction - This oxidation is catalysed by Glyceraldehyde phosphate dehydrogenase to produce 1,3 -bisphosphoglycerate. It requires NAD+ which carries the electron and an inorganic phosphate.
Step 7: Phosphoglycerate kinase reaction - phosphoglycerate kinase catalyses the 1,3-bisphosphoglycerate to produce 3-phosphoglycerate. The transfer of a phosphate group to an ADP forms an ATP. Since each of one of the 2 GADP will make one ATP, that's a total of 2 ATP in the process.
Step 8: Phosphoglycerate mutase reaction - the remaining phosphates in the 3-phosphoglycerate are transferred forming 2-phosphoglycerate.
Step 9: Enolase reaction - enolase catalyses dehydration of the 2-phosphoglycerate resulting in a loss of a hydroxyl group and forming phosphoenolpyruvate
Step 10: Pyruvate kinase reaction - this is dephosphorylation reaction where phosphates are removed from phosphoenolpyruvate and transferred to an ADP resulting in another ATP as well as the Pyruvate. This process yields 2 more ATP because of 2 phosphoenolpyruvates.
Summary:
Step 1: addition of phosphate to glucose forming Glucose-6-phosphate
Step 2: atom rearrangement of glucose-6-phosphate forming the fructose-6-phosphate
Step 3: addition of phostphate to fructose-6-phosphate creating the fructose-1,6-phosphate
Step 4: cleavage or splitting of fructose-1,6-phosphate into two molecules the GADP or PGAL and DAHP
Step 5: atom rearrangement of DAHP into GADP/PGAL
Step 6: oxidation or loss of electron of GADP/PGAL, generating NADH and 1,3-BPG
Step 7: transfer of phosphate from 1,3-BPG to ADP, generating ATP and 3-PG
Step 8: rearrangement of atom, from 3-PG to 2-PG
Step 9: dehydration of 2-PG forming the PEP
Step 10: transfer of phosphate from 2-PG to ADP, creating another ATP and the Pyruvate.
Conditions
The amount of pyruvate that enters the mitochondrion matrix are based on the amount of oxygen available in the mitochondria.
When pyruvate enters the mitochondrion matrix, it goes under decision making whether it would go through aerobic or anaerobic pathway.
If oxygen is available pyruvate continues in the aerobic pathway, but if oxygen is unavailable the pyruvate exits the matrix and undergoes fermentation in the cytosol.
Transition Reaction
Pyruvate enters the mitochondrion matrix, where it undergoes decarboxylation (if oxygen is available). Electrons from one of the carbon in the pyruvate are transferred in the NAD+, releasing CO2 and NADH. The final product is an Acetyl-CoA.
If oxygen is unavailable, the pyruvate goes back outside the matrix and undergoes fermentation in the cytosol.
Fermentation
Alcoholic fermentation
Plants and microorganisms
Pyruvate converted into ethanol and carbon dioxide generating energy in the process
Lactic acid fermentation
Animal cells and human muscle cells
Pyruvate is converted into lactic acid, quick but less efficient energy source
Can also be transported to liver to be glucose again through gluconeogenesis