Why glycolysis in cytoplasm

Glucose first converts to glucosephosphate by hexokinase or glucokinase, using ATP and a phosphate group. Glucokinase is a subtype of hexokinase found in humans. Glucokinase has a reduced affinity for glucose and is found only in the pancreas and liver, whereas hexokinase is present in all cells.

Glucose 6-phosphate is then converted to fructosephosphate, an isomer, by phosphoglucose isomerase. Phosphofructose-kinase then produces fructose-1,6-bisphosphate, using another ATP molecule. Dihydroxyacetone phosphate DHAP and glyceraldehyde 3-phosphate are then created from fructose-1,6-bisphosphate by fructose bisphosphate aldolase. DHAP will be converted to glyceraldehydephosphate by triosephosphate isomerase, where now the two glyceraldehydephosphate molecules will continue down the same pathway.

Due to the unstable state of PEP, pyruvate kinase will facilitate its loss of a phosphate group to create the second ATP in glycolysis. Thus, PEP will then undergo conversion to pyruvate.

Glycolysis occurs in the cytosol of the cell. It is a metabolic pathway that creates ATP without the use of oxygen but can occur in the presence of oxygen. In cells that use aerobic respiration as the primary energy source, the pyruvate formed from the pathway can be used in the citric acid cycle and go through oxidative phosphorylation to undergo oxidation into carbon dioxide and water.

Even if cells primarily use oxidative phosphorylation, glycolysis can serve as an emergency backup for energy or as the preparation step before oxidative phosphorylation. In highly oxidative tissue, such as the heart, pyruvate production is essential for acetyl-CoA synthesis and L-malate synthesis.

It serves as a precursor to many molecules, such as lactate, alanine, and oxaloacetate. Glycolysis precedes lactic acid fermentation; the pyruvate made in the former process serves as the prerequisite for the lactate made in the latter process. Lactic acid fermentation is the primary source of ATP in animal tissues with low metabolic requirements and little to no mitochondria.

In erythrocytes, lactic acid fermentation is the sole source of ATP, as they lack mitochondria and mature red blood cells have little demand for ATP. Another part of the body that relies entirely or almost entirely on anaerobic glycolysis is the eye's lens, which is devoid of mitochondria, as their presence would lead to light scattering. Though skeletal muscles prefer to catalyze glucose into carbon dioxide and water during heavy exercise where oxygen is inadequate, the muscles simultaneously undergo anaerobic glycolysis and oxidative phosphorylation.

The amount of glucose available for the process regulates glycolysis, which becomes available primarily in two ways: regulation of glucose reuptake or regulation of the breakdown of glycogen. Glucose transporters GLUT transport glucose from the outside of the cell to the inside.

Cells containing GLUT can increase the number of GLUT in the cell's plasma membrane from the intracellular matrix, therefore increasing the uptake of glucose and the supply of glucose available for glycolysis.

There are five types of GLUTs. GLUT3 is present in neurons. GLUT4 is in adipocytes, heart, and skeletal muscle. GLUT5 specifically transports fructose into cells. Another form of regulation is the breakdown of glycogen. Cells can store extra glucose as glycogen when glucose levels are high in the cell plasma.

Conversely, when levels are low, glycogen can be converted back into glucose. Two enzymes control the breakdown of glycogen: glycogen phosphorylase and glycogen synthase.

As described before, many enzymes are involved in the glycolytic pathway by converting one intermediate to another. Control of these enzymes, such as hexokinase, phosphofructokinase, glyceraldehydephosphate dehydrogenase, and pyruvate kinase, can regulate glycolysis. The amount of oxygen available can also regulate glycolysis. The mechanisms responsible for this effect include allosteric regulators of glycolysis enzymes such as hexokinase.

Still, this effect is not universal in oxidative tissue, such as pancreatic cells. Both of these molecules will proceed through the second half of the pathway, and sufficient energy will be extracted to pay back the two ATP molecules used as an initial investment and produce a profit for the cell of two additional ATP molecules and two even higher-energy NADH molecules.

Step 6. The sugar is then phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate. Note that the second phosphate group does not require another ATP molecule. Here again is a potential limiting factor for this pathway. If oxygen is available in the system, the NADH will be oxidized readily, though indirectly, and the high-energy electrons from the hydrogen released in this process will be used to produce ATP.

Step 7. In the seventh step, catalyzed by phosphoglycerate kinase an enzyme named for the reverse reaction , 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP.

This is an example of substrate-level phosphorylation. A carbonyl group on the 1,3-bisphosphoglycerate is oxidized to a carboxyl group, and 3-phosphoglycerate is formed. Step 8. In the eighth step, the remaining phosphate group in 3-phosphoglycerate moves from the third carbon to the second carbon, producing 2-phosphoglycerate an isomer of 3-phosphoglycerate.

The enzyme catalyzing this step is a mutase isomerase. Step 9. Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate PEP. Step Many enzymes in enzymatic pathways are named for the reverse reactions, since the enzyme can catalyze both forward and reverse reactions these may have been described initially by the reverse reaction that takes place in vitro, under non-physiological conditions.

Gain a better understanding of the breakdown of glucose by glycolysis by visiting this site to see the process in action.

Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and 2 NADH molecules for its use. If the cell cannot catabolize the pyruvate molecules further, it will harvest only two ATP molecules from one molecule of glucose. Mature mammalian red blood cells are not capable of aerobic respiration —the process in which organisms convert energy in the presence of oxygen—and glycolysis is their sole source of ATP.

Starting with glucose, one ATP is used to donate a phosphate to glucose to produce glucose 6-phosphate. With the help of glycogen phosphorylase, glycogen can change into glucose 6-phosphate as well.

During energy metabolism, glucose 6-phosphate turns into fructose 6-phosphate. With the help of phosphofructokinase, an additional ATP can be used to turn phosphorylate fructose 6-phosphate into fructose 1, 6-diphosphate. Fructose 1, 6-diphosphate then splits into two phosphorylated molecules with three carbon chains that later degrades into pyruvate. Some archaea, the most notable ones being halobacteria, make proton gradients by pumping in protons from the environment.

They are able to do this with the help of the solar-driven enzyme bacteriorhodopsin, which is used to drive the molecular motor enzyme ATP synthase to make the necessary conformational changes required to synthesize ATP. By running ATP synthase in reverse, proton gradients are also made by bacteria and are used to drive flagella. Large enough quantities of ATP cause it to create a transmembrane proton gradient.

This is used by fermenting bacteria, which lack an electron transport chain, and which hydrolyze ATP to make a proton gradient. Bacteria use these gradients for flagella and for the transportation of nutrients into the cell.

In respiring bacteria under physiological conditions, ATP synthase, in general, runs in the opposite direction. This creates ATP while using the proton motive force created by the electron transport chain as a source of energy. The overall process of creating energy in this fashion is termed oxidative phosphorylation. Privacy Policy. Skip to main content. Microbial Metabolism. Search for:. Importance of Glycolysis Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism.

Learning Objectives Explain the importance of glycolysis to cells. Key Takeaways Key Points Glycolysis is present in nearly all living organisms. Glucose is the source of almost all energy used by cells. Key Terms glycolysis : the cellular metabolic pathway of the simple sugar glucose to yield pyruvic acid and ATP as an energy source heterotroph : an organism that requires an external supply of energy in the form of food, as it cannot synthesize its own.

Electron Donors and Acceptors Electrons can enter the electron transport chain at three levels: dehydrogenase, the quinone pool, or a mobile cytochromeelectron carrier. Learning Objectives Recognize the various types of electron donors and acceptors. Key Takeaways Key Points Bacterial electron transport chains may contain as many as three proton pumps. The most common electron donors are organic molecules. There are a number of different electron acceptors, both organic and inorganic.

If oxygen is available, it is invariably used as the terminal electron acceptor. Key Terms organotroph : An organism that obtains its energy from organic compounds. ATP Yield The amount of energy as ATP gained from glucose catabolism varies across species and depends on other related cellular processes. Learning Objectives Describe the origins of variability in the amount of ATP that is produced per molecule of glucose consumed. Key Takeaways Key Points While glucose catabolism always produces energy, the amount of energy in terms of ATP equivalents produced can vary, especially across different species.

The number of hydrogen ions the electron transport chain complexes can pump through the membrane varies between species. The use of intermediates from glucose catabolism in other biosynthetic pathways, such as amino acid synthesis, can lower the yield of ATP. Key Terms catabolism : Destructive metabolism, usually including the release of energy and breakdown of materials. Respiration and Proton Motive Force Respiration is one of the key ways a cell gains useful energy to fuel cellular activity.

Learning Objectives Describe the role of the proton motive force in respiration. Key Takeaways Key Points The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process as they break high-energy bonds. Aerobic respiration requires oxygen in order to generate energy ATP. Aerobic metabolism is up to 15 times more efficient than anaerobic metabolism which yields two molecules ATP per one molecule glucose.

With the help of the solar-driven enzyme bacteriorhodopsin, some bacteria make proton gradients by pumping in protons from the environment. Key Terms exothermic : releasing energy in the form of heat redox : a reversible process in which one reaction is an oxidation and the reverse is a reduction. Licenses and Attributions. CC licensed content, Shared previously.