Glycolysis is a term that literally means taking sugar (glyco) and splitting it (lysis.) Glycolysis is the first stage of both aerobic and anaerobic respiration.
Glycolysis occurs in the cytoplasm (a thick liquid that bathes the organelles) of the cell. During glycolysis, glucose splits into two 3-carbon molecules that then transform into pyruvate through a series of reactions.
Fig. 1 - A step by step diagram of glycolysis
What is the equation for glycolysis?
The overall equation for glycolysis is:
Sometimes pyruvate is referred to as pyruvic acid, so don’t get confused if you are doing any extra reading! We use the two names interchangeably.
What are the different stages of glycolysis?
Glycolysis occurs in the cytoplasm, and involves splitting a single, 6-carbon glucose molecule into two 3-carbon pyruvate molecules. There are multiple, smaller, enzyme-controlled reactions during glycolysis. These occur in ten stages. The general process of glycolysis follows these different phases:
- Two phosphate molecules are added to glucose from two molecules of ATP. This process is called phosphorylation.
- Glucose is split into two molecules of triose phosphate, a 3-carbon molecule.
- One molecule of hydrogen is removed from each triose phosphate molecule. These hydrogen groups are then transferred to a hydrogen-carrier molecule, NAD. This forms reduced NAD/NADH.
- Both of the triose phosphate molecules, now oxidised, are then converted into another 3-carbon molecule known as pyruvate. This process also regenerates two ATP molecules per pyruvate molecule, resulting in the production of four ATP molecules for every two ATP molecules used up during glycolysis.
Fig. 2 - A step by step diagram of glycolysis
We will now look at this process in more detail and explain the different enzymes involved during each stage of the process.
The investment phase
This phase refers to the first half of glycolysis, in which we invest two molecules of ATP in order to split glucose into two 3-carbon molecules.
1. Glucose is catalysed by hexokinase into glucose-6-phosphate. This uses one molecule of ATP, which donates a phosphate group. ATP is converted to ADP. The role of phosphorylation is to make the glucose molecule reactive enough to proceed with subsequent enzymatic reactions.
2. the enzyme phosphoglucose isomerase catalyses Glucose-6-phosphate. This isomerises (same molecular formula but different structural formula of a substance) glucose-6-phosphate, which means that it changes the molecule’s structure into another 6-carbon phosphorylated sugar. This creates fructose-6-phosphate.
3. Fructose-6-phosphate is catalysed by the phosphofructokinase-1 (PFK-1) enzyme which adds a phosphate from ATP into fructose-6-phosphate. ATP is converted to ADP and fructose-1,6-bisphosphate is formed. Again, this phosphorylation increases the reactivity of the sugar to allow the molecule to proceed further in the glycolysis process.
4. The enzyme aldolase splits the 6-carbon molecule into two 3-carbon molecules. These are Glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP.)
5. Between G3P and DHAP, only G3P is used in the next step of glycolysis. Therefore, we need to convert DHAP into G3P, and we do this using an enzyme called triose phosphate isomerase. This isomerises DHAP into G3P. Therefore, we now have two molecules of G3P which will both be used in the next step.
The pay-off phase
This second phase refers to the final half of glycolysis, which generates two molecules of pyruvate and four molecules of ATP.
From step 5 of glycolysis onwards, everything happens twice, as we have two 3-carbon molecules of G3P.
6. G3P combines with the enzyme Glyceraldehyde-3-phosphate Dehydrogenase (GAPDH), NAD+, and inorganic phosphate. This produces 1,3-biphosphoglycerate (1,3-BPh). As a by-product, NADH is produced.
7. A phosphate group from 1,3-biphosphoglycerate (1,3-BPh) combines with ADP to make ATP. This produces 3-phosphoglycerate. The enzyme phosphoglycerate kinase catalyses the reaction.
8. the enzyme phosphoglycerate mutase converts 3-phosphoglycerate into 2-phosphoglycerate.
9. An enzyme called enolase converts 2-phosphoglycerate into phosphoenolpyruvate. This produces water as a by-product.
10. Using the enzyme pyruvate kinase, phosphoenolpyruvate loses a phosphate group, gains a hydrogen atom, and converts into pyruvate. ADP takes up the lost phosphate group and becomes ATP.
In total, Glycolysis produces 2 pyruvate molecules, 2 molecules of ATP, and 2 NADH molecules (which go to the electron transport chain.)
You don’t have to know the chemical structures of the molecules involved in glycolysis. Exam boards would only expect you to know the names of the molecules and enzymes involved, how many ATP molecules are gained/lost, and when NAD/NADH is formed during the process.
Glycolysis and energy yields
The overall yield from a single glucose molecule after glycolysis is:
Glycolysis has been used as indirect evidence for evolution. The enzymes involved in glycolysis are found in the cytoplasm of cells, so glycolysis doesn’t require an organelle or membrane for it to take place. It also does not require oxygen to occur as anaerobic respiration takes place in the absence of oxygen, through converting pyruvate into lactate or ethanol. This step is necessary in order to re-oxidise NAD. In other words remove the H+ from NADH, so that glycolysis can continue to occur.
In Earth’s very early days, there was not as much oxygen in the atmosphere as there is now, so some (or maybe all) of the earliest organisms used reactions that resemble glycolysis in order to gain energy!
Glycolysis - Key takeaways
- Glycolysis involves splitting glucose, a 6-carbon molecule, into two 3-carbon pyruvate molecules.
- Glycolysis occurs in the cytoplasm of the cell.
- The overall equation for glycolysis is:
- Glycolysis involves a series of enzyme-controlled reactions. These include phosphorylation of glucose, splitting of phosphorylated glucose, oxidation of triose phosphate, and ATP production.
- Overall, glycolysis produces two molecules of ATP, two molecules of NADH, and two H+ ions.
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