Glycolysis is a metabolic pathway that breaks down glucose to produce energy. It occurs in the cytoplasm of cells and consists of two major phases: the energy investment phase and the energy payoff phase.
1. Energy Investment Phase
In the energy investment phase, the cell invests ATP to initiate the breakdown of glucose. This phase includes the following key steps:
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Phosphorylation of Glucose: The process begins when glucose (a 6-carbon sugar) is phosphorylated by the enzyme hexokinase, which consumes one ATP molecule to convert glucose into glucose-6-phosphate. This step helps to trap glucose in the cell and prepares it for further breakdown.
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Further Phosphorylation: Glucose-6-phosphate is then converted to fructose-6-phosphate, which is subsequently phosphorylated again to fructose-1,6-bisphosphate using another ATP in a reaction catalyzed by phosphofructokinase-1 (PFK-1). This is a key regulatory step in glycolysis.
Overall, during the energy investment phase, two ATP molecules are consumed to invest energy into the system. This phase prepares the glucose for its eventual breakdown, making it easier to split into smaller molecules in the next phase.
2. Energy Payoff Phase
In the energy payoff phase, the cell generates ATP and NADH from the breakdown of glucose. Here are the main activities that occur in this phase:
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Cleavage of Fructose-1,6-bisphosphate: The 6-carbon sugar (fructose-1,6-bisphosphate) is split into two 3-carbon molecules (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate). The latter is quickly converted into glyceraldehyde-3-phosphate so that there are two molecules of glyceraldehyde-3-phosphate.
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Substrate-Level Phosphorylation: Each glyceraldehyde-3-phosphate molecule undergoes several enzymatic reactions, leading to the production of pyruvate. During these reactions, energy is released, and this energy is used to produce four ATP molecules (2 ATP per glyceraldehyde-3-phosphate) and two NADH molecules.
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Net Gain: Despite the production of four ATP molecules, the initial investment of two ATP in the energy investment phase means that the net gain of ATP from glycolysis is two molecules per glucose molecule (4 ATP produced - 2 ATP used = 2 ATP net). Additionally, two NADH molecules are produced, which can be utilized in further metabolic processes to generate more ATP during oxidative phosphorylation.
In summary, the energy investment phase of glycolysis consumes energy (ATP) to activate and break down glucose, while the energy payoff phase generates energy (ATP and NADH) as glucose is further processed into pyruvate. Together, these phases enable cells to efficiently extract energy from glucose.