This is an extremely important part of cellular respiration. It happens all the time, both with and without oxygen. And in the process, transfers some energy to ATP. The first stage of cellular respiration is glycolysis. It does not require oxygen, and it does not take place in the mitochondrion - it takes place in the cytosol of the cytoplasm. When was the last time you enjoyed yogurt on your breakfast cereal, or had a tetanus shot? These experiences may appear unconnected, but both relate to bacteria which do not use oxygen to make ATP.
In fact, tetanus bacteria cannot survive if oxygen is present. However, Lactobacillus acidophilus bacteria which make yogurt and Clostridium tetani bacteria which cause tetanus or lockjaw share with nearly all organisms the first stage of cellular respiration, glycolysis. Because glycolysis is universal, whereas aerobic oxygen-requiring cellular respiration is not, most biologists consider it to be the most fundamental and primitive pathway for making ATP.
Enzymes split a molecule of glucose into two molecules of pyruvate also known as pyruvic acid. This occurs in several steps, as shown in Figure below. The structure of ATP shows the basic components of a two-ring adenine, five-carbon ribose, and three phosphate groups.
At the heart of ATP is a molecule of adenosine monophosphate AMP , which is composed of an adenine molecule bonded to both a ribose molecule and a single phosphate group Figure 1. The addition of a second phosphate group to this core molecule results in adenosine diphosphate ADP ; the addition of a third phosphate group forms adenosine triphosphate ATP. The addition of a phosphate group to a molecule requires a high amount of energy and results in a high-energy bond. Phosphate groups are negatively charged and thus repel one another when they are arranged in series, as they are in ADP and ATP.
The release of one or two phosphate groups from ATP, a process called hydrolysis, releases energy. You have read that nearly all of the energy used by living things comes to them in the bonds of the sugar, glucose.
Glycolysis is the first step in the breakdown of glucose to extract energy for cell metabolism. Many living organisms carry out glycolysis as part of their metabolism. Glycolysis takes place in the cytoplasm of most prokaryotic and all eukaryotic cells. Glycolysis begins with the six-carbon, ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvate. Glycolysis consists of two distinct phases. In the first part of the glycolysis pathway, energy is used to make adjustments so that the six-carbon sugar molecule can be split evenly into two three-carbon pyruvate molecules.
Figure 2. In glycolysis, a glucose molecule is converted into two pyruvate molecules. If the cell cannot catabolize the pyruvate molecules further, it will harvest only two ATP molecules from one molecule of glucose. For example, mature mammalian red blood cells are only capable of glycolysis, which is their sole source of ATP.
If glycolysis is interrupted, these cells would eventually die. Glycolysis consists of two distinct phases. The first part of the glycolysis pathway traps the glucose molecule in the cell and uses energy to modify it so that the six-carbon sugar molecule can be split evenly into the two three-carbon molecules.
Step 1. The first step in glycolysis Figure is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars.
Hexokinase phosphorylates glucose using ATP as the source of the phosphate, producing glucosephosphate, a more reactive form of glucose. This reaction prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins, and it can no longer leave the cell because the negatively charged phosphate will not allow it to cross the hydrophobic interior of the plasma membrane.
Step 2. In the second step of glycolysis, an isomerase converts glucosephosphate into one of its isomers, fructosephosphate this isomer has a phosphate attached at the location of the sixth carbon of the ring. An isomerase is an enzyme that catalyzes the conversion of a molecule into one of its isomers. This change from phosphoglucose to phosphofructose allows the eventual split of the sugar into two three-carbon molecules.
Step 3. The third step is the phosphorylation of fructosephosphate, catalyzed by the enzyme phosphofructokinase.
A second ATP molecule donates a high-energy phosphate to fructosephosphate, producing fructose-1,6- bi sphosphate. In this pathway, phosphofructokinase is a rate-limiting enzyme. This is a type of end product inhibition, since ATP is the end product of glucose catabolism.
Step 4. The newly added high-energy phosphates further destabilize fructose-1,6-bisphosphate. The fourth step in glycolysis employs an enzyme, aldolase, to cleave fructose-1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone phosphate and glyceraldehydephosphate.
Step 5. In the fifth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer, glyceraldehydephosphate. Thus, the pathway will continue with two molecules of a glyceraldehydephosphate. At this point in the pathway, there is a net investment of energy from two ATP molecules in the breakdown of one glucose molecule. 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.
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