Plants use sunlight as well as carbon dioxide and water to make glucose, and it is all of this glucose, among other biomolecules, that becomes the starting material for metabolic processes in our bodies.
This degradation of biomolecules to generate energy that cells can use is called cellular respiration, or sometimes more specifically, aerobic respiration. looking at glucose as the substrate.
Aerobic respiration requires oxygen so any organism that breathes in oxygen from the atmosphere is doing so in order to facilitate aerobic respiration.
Glucose,which we can either consume as starch or break off from glycogen stored in the cell, can be converted through metabolic pathways in the presence of oxygen into carbon dioxide, which we breathe out, water, which is most of what we are, and energy, the energy we need to think and move.
The electron exchanges that occur throughout these metabolic pathways utilise the electron carrier NAD+ and it's other form, NADH. This is a dinucleotide with an interesting base, Nicotinamide that can exist either as NAD+, with a positively charged nitrogen atom, or if reduced it can become NADH.
This transfer, facilitated by an enzyme called dehydrogenase, helps catalyze the breakdown of glucose. Cellular Respiration happens over three major pathways. There is Glycolysis, the Citric Acid Cycle, and Oxidative Phosphorylation.
Process of Glycolysis
Glycolysis occurs in the Cytoplasm of the cell. This is the process by which glucose molecules are split into two pieces called pyruvate. This first pathway is actually anaerobic, meaning it does not require oxygen, so it is the most evolutionarily ancient metabolic pathway, occurring in even the simplest cells.
In this pathway, one glucose molecule can yield a net of 2 ATPs. It requires 10 enzymes to happen which catalyse each of the 10 steps, as well as an investment of two ATP molecules in the preparatory phase to get 4 ATPs back over several steps in the payoff phase.
Steps of Glycolysis
Step 1: Phosphorylation
First reaction is called Hexokinase reaction.The enzyme Hexokinase phosphorylates the oxygen on carbon 6 of the glucose to make Glucose 6-phosphate.
The polar phosphate group traps the molecule inside the cell and also reduces the concentration of regular glucose inside the cell, which encourages more glucose to enter by diffusion. This step costs 1 ATP, which provides the necessary phosphate group for the reaction.
Step 2: Isomerisation
Glucose-6-phosphate isomerizes to become Fructose-6-phosphate, a process which is catalysed by enzyme phosphoglucoisomerase.Step 3: Second Phosphorylation
That is another phosphorylation, this time on the carbon 1 hydroxyl which gives us Fructose-1,6-bisphosphate.This step is catalysed by phosphofructokinase 1 and it will cost another ATP.
Step 4: Cleavage
Now fructose-1,6 bisphosphate molecule is ready to be cleaved into two smaller ones.
Fructose-bisphosphate aldolase is a lyase enzyme that will split fructose-1,6-bisphosphate into a molecule of Glyceraldehyde-3-phosphate, or GADP, and a molecule of Dihydroxyacetone phosphate or DHAP.
Step 5: Conversion of DHAP into GADP
The DHAP will be converted into another molecule of GADP by the enzyme Triosephosphate - isomerase, which leaves us with two molecules of GADP.That is the end of the five-step preparatory phase, with two ATPs spent to achieve the two phosphorylations.
Step 6: Oxidation
Now it's time for the payoff phase. One of our two GADP molecules from the preparatory phase, and we see that the first thing that will happen is an oxidation to become 1,3-bisphosphoglycerate. This requires NAD+ and a free phosphate, or inorganic phosphate to occur, and the enzyme involved is called glyceraldehyde phosphate dehydrogenase.Step 7: Dephosphorylation
Next, a Phosphoglycerate kinase will catalyse transfer of a phosphate group to ADP to become 3-phosphoglycerate producing 1 ATP in the process. Since each of the 2 GADP molecules will make 1 ATP, that's a total of 2 ATPs, for half the total payoff of glycolysis.Step 8: Phosphate transfer
Then, phosphoglycerate mutase transfers the remaining phosphate from this hydroxyl to the next one over to make 2-phosphoglycerate.Step 9: Dehydration
Then, Enolase catalyses a dehydration, resulting in the loss of hydroxyl group (OH) which will produce Phosphoenolpyruvate.Step 10: Second Dephosphorylation
The remaining phosphate group is transferred to an ADP by pyruvate kinase, generating another ATP and the Pyruvate.So altogether it's a10-step process. The first five steps comprise the preparatory phase, which take one molecule of glucose and produce two molecules of GADP( glyceraldehyde-3-phosphate).This will cost 2 ATP.
Then the other five steps make up the payoff phase, in which each molecule of GADP will be converted into Pyruvate, producing 2 ATP each in the process, for a total of 4 ATP, meaning the net energy production from one molecule of glucose is 2 ATP.
Certainly the main thing to remember is that in glycolysis, glucose in the cytoplasm of the cell is converted into pyruvate, which will then move on to the next stage of cellular respiration, which is TCA Cycle cycle.
