Glycolysis Lab Report

In order to perform their vital functions, our cells require a continual input of energy. Aerobic cellular respiration is the process in which our cells, in the presence of oxygen, break down glucose and turn it into usable high-energy ATP molecules. It is a highly complex process that involves many individual processes and it takes place predominantly in the mitochondria.

Glycolysis is the first process of aerobic respiration, it starts in the cytosol of the cell where it converts glucose into pyruvate molecules, NADH and a small number of ATP molecules. The pyruvate is transported into the mitochondrial matrix where it will undergo pyruvate decarboxylation, a reaction that involves a cofactor called coenzyme A and a NAD+ molecule to convert pyruvate into acetyl coenzyme A (acetyl-CoA), CO2, NADH and H+. Acetyl-CoA is used as a fuel source in the next process of aerobic respiration, the citric acid cycle.

In the citric acid cycle, also known

as the Krebs cycle, acetyl-CoA combines with oxaloacetic acid to form citric acid, which is then decarboxylated to release CO2 and NADH. The oxaloacetic acid is re-formed to restart the cycle and in the process, NADH and FADH2 molecules are also formed, which act as electron carriers to the electron transport chain. Like glycolysis, the citric acid cycle does not produce a significant amount of ATP molecules, but instead, works as a gateway to forming ATP by harvesting high-energy electrons that can be used in the electron transport chain to generate the proton gradient needed to form ATP molecules.

The oxidative phosphorylation process along the electron transport chain is the final process of aerobic cell respiration, in which the highest amount of ATP molecules is formed. The energy-rich molecules NADH, formed in glycolysis and NADH and FADH2, formed in the citric acid cycle, contain electrons with high transfer potential. These electrons are used to reduce diatomic oxygen molecules to water, releasing a large amount of free energy that can be used to generate high-energy ATP molecules.

The electron transport chain is located on the inner mitochondrial membrane, between the intermembrane space and the mitochondrial matrix. It consists of five protein complexes (Complex I-V), which are involved in receiving the electrons from NADH and FADH2 molecules and moving them along the electron transport chain. Some of these protein complexes also work as pumps, forcing the hydrogen ions out of the mitochondrial matrix and into the intermembrane space, establishing a hydrogen ion gradient.

Complex I creates a hydrogen ion gradient by pumping the hydrogen ions, received from the NADH molecule, across the membrane from the mitochondrial matrix into the intermembrane space while complex II receives the electrons from FADH2 and sends them directly to the electron transport chain.

Ubiquinone, also known as coenzyme Q10, collects these electrons and delivers them to complex III, which sequentially pumps the protons through the membrane into the intermembrane space and passes its electrons to cytochrome c for transport to the next protein complex. In complex IV, more hydrogen ions are forced out of the electron transport chain and into the intermembrane space. It also pumps the electrons down into the mitochondrial matrix where diatomic oxygen is reduced, making it free to react with the hydrogen ions present in the surrounding to produce water molecules. This is the oxygen-requiring step of cellular respiration and it consumes virtually all of the oxygen that is acquired through ventilation (Alberts et al,

2010).

Complex V, also called ATP synthase, is the final step in the production of ATP. The electrical current created by the movement of the electrons through the electron transport chain and the large difference in proton concentration across the membrane created by the previous four protein complexes, has formed a proton-motive force which provides the energy for the enzyme ATP synthase to make ATP from ADP and inorganic phosphate (Cooper, 2009). As the intermembrane space has a higher amount of hydrogen ions than the mitochondrial matrix, this causes the intermembrane space to have a positive charge due to its high concentration of protons. The proton gradient created is a form of stored energy and this gradual difference in concentration of protons allows the protons to flow back down their gradient from the intermembrane space through complex V into the mitochondrial matrix, releasing the free energy needed by the enzyme ATP synthase to phosphorylate ADP, turning it into ATP.

ATP synthase, however, is a reversible device that can convert the energy of the proton gradient into chemical-bond energy or vice versa and its direction of action greatly depends on the free energy change. If the proton gradient falls below a certain level, the net-free energy available for hydrogen ion transport into the mitochondrial matrix is no longer enough to drive ATP production. Instead, ATP synthase hydrolyses ATP and uses its energy to rebuild the proton gradient (Alberts et al, 2010).
The availability of oxygen, the final electron acceptor in the electron transport chain, plays an extremely important role in maintaining the hydrogen gradient across the membranes, as without oxygen to attract electrons down the electron transport chain, hydrogen ions would not be pumped into the intermembrane space and consequently the proton gradient would not be established (Reece et al, 2011).
If oxygen supplies is reduced, an oxygen-independent process called anaerobic respiration occurs. Glycolysis is considered an anaerobic pathway to ATP production and only a small amount of ATP is produced (Karp, 2008). During this process, NAD+ must be regenerated through the metabolism of pyruvate, where the reduction of pyruvate by NADH forms lactate. This process is also known as fermentation and it yields only a fraction of the energy available from the complete combustion of glucose (Berg et al, 2006), nonetheless, glycolytic reactions occurs at rapid rates and this allows cells to produce a significant amount of ATP using this pathway (Karp, 2008).
In conclusion, the variations in the hydrogen ion gradient significantly influences the amount of energy available for ATP synthase to make ATP molecules. In order to keep the flow in the electron transport chain, the availability of glucose and oxygen, as well as other molecules, is crucial to keep feeding the ATP producing cycle. The greater the hydrogen ion gradient, the more energy is available for the production of ATP and the decrease in free energy available could result in the hydrolysis rather than in the production of ATP, affecting the energy yield for a cell to perform its functions.