Glycolysis is a metabolic pathway that occurs in the cytoplasm of all respiring cells. It is the process by which a glucose molecule, via a number of different reactions, get broken down into two molecules of pyruvate. Below I will briefly outline the stages of glycolysis and the products of it.
In the first stages of glycolysis a lot of phosphorylation takes place. Phosphorylation is the addition of a phosphate group to a protein or an organic molecule (in this case it's the glucose molecule). This occurs in the steps detailed below:
- In the first step a molecule of ATP (adenosine triphosphate) is hydrolysed and the phosphate group that is released from this then attaches to the glucose molecule at the 6th carbon to form a molecule called glucose 6-phosphate.
- Glucose 6-phosphate is then converted into fructose 6-phosphate which is catalysed by the enzyme Glucose-6-phosphate isomerase
- Yet another molecule of ATP is hydrolysed and this time the phosphate group that is released attaches to the fructose 6-phosphate molecule at carbon 1.
- The molecule that has been created by this addition of the phosphate molecule is called fructose 1,6-biphosphate.
- the energy released from the hydrolysis of the ATP molecule activates fructose 1,6-biphosphate and prevents it from being transported out of the cell.
- This activated and phosphorylated fructose 1,6-biphosphate molecule is referred to as hexose 1,6- biphosphate (it's called hexose because it's a sugar containing 6 carbon atoms).
- Each molecule of the hexose 1,6-biphosphate is split into two to form triose phosphate (triose because it has 3 carbon atoms in the molecule).
- Dehydrogenase enzymes catalyse the removal of 2 hydrogen atoms from each of the triose molecules.
- The coenzyme NAD accepts the hydrogen atoms and becomes reduced NAD.
- 2 molecules of glucose are formed by substrate level phosphorylation.
- 4 enzyme catalysed reactions then convert each triose molecule into a molecule of pyruvate.
- Substrate level phosphorylation occurs again converting 2 molecules of ADP into ATP by adding a phosphate group.
Products of glycolysis per molecule of glucose.
Net gain of 2 x molecules of ATP
2 x reduced NAD
2 x pyruvate molecules
The link reaction takes place in the mitochondrial matrix and involves converting the pyruvate produced in glycolysis into acetate.
- The enzyme pyruvate dehydrogenase remove the hydrogen atoms from the pyruvate molecules.
- The enzyme pyruvate decarboxylase removes the carboxyl group from the pyruvate.
- NAD accepts the removed hydrogen atoms and acetate is produced which is then accepted by coenzyme A (coA) which then becomes acetyl coenzyme A.
- CoA carries the acetate to the next stage of aerobic respiration (the Krebs cycle).
Products of the link reaction per molecule of glucose
2 x reduced NAD
2 x carbon dioxide (from the removed carboxyl groups)
The Kreb's cycle also takes place in the mitochondrial matrix. The cycle involves the following steps:
- The acetate molecule from acetyl coA (produced in the link reaction) joins with a molecule called oxaloacetate (4 carbon compound) to form citric acid (6 carbon compound). The coA molecule then goes back into the link reaction so that it can accept more acetyl and continue the process of respiration.
- One molecule of carbon dioxide and 2 hydrogen atoms are removed (decarboxylated and dehydrogenated) from the citric acid molecule by specific enzymes and a 5 carbon compound is formed.
- The pair of hydrogen atoms is accepted by NAD to produce reduced NAD.
- The 5 carbon molecule produced once again has a carbon dioxide molecule and 2 hydrogen atoms removed to form a 4 carbon compound. During this time a molecule of ATP is produced during substrate level phosphorylation.
- The 4 carbon molecule is then converted to another 4 carbon molecule which then has 2 hydrogen atoms removed which are then accepted by a coenzyme called FAD.
- The FAD molecule then becomes reduced (much like when NAD accepts hydrogen atoms).
- This 4 carbon molecule is then further dehydrogenated and amolecule of NAD accepts the removed hydrogen atoms and a molecule of oxaloacetate is produced.
Products of the Krebs cycle per molecule of glucose
6 x reduced NAD
2 x reduced FAD
4 x carbon dioxide
2 x ATP
The Final Stage - Oxidative Phosphorylation and Chemiosmosis
- Embedded in the inner membrane of the mitochondria are electron carriers that form the electron transport chain.
- Reduced NAD and FAD are re-oxidised when they reach the dehydrogenase protein complex that removes the hydrogen atoms from them and then split the atoms into an electron and a H+ ion.
- The H+ ion is pumped from the matrix of the mitochondria to the intermembrane space and the electron is passed along the chain of electron carriers.
- As the electrons flow along the chain energy is released and used to pump the proteins (H+ ions) across the inner mitochondrail membrane.
- This build up of H+ ions in the intermembrane space creates an electrochemical gradient.
- The protons then move down the electrochemical gradient through channels in the inner membrane that are associated with ATP synthase enzymes.
- As the protons flow through the ATP synthase enzyme they drive the rotation of part of it and ADP is joined to ATP from this rotation.
- When the electrons in the transport chain reach the last electron carrier they are passed to oxygen which is known as the final electron acceptor. The hydrogen ions also bond to the oxygen to form water.
- Theoretically 30 molecules of ATP can be produced during oxidative phosphorylation, however there are many things that can prevent that amount from being produced, for example some protons will leak across the mitochondrial membrane and reduce the electrochemical gradient and thus the amount of proton motive force.