Energy Investment Phase: Two Atps Explained

During the energy investment phase of cellular respiration, two ATP molecules are used to activate and phosphorylate glucose, a 6-carbon molecule. This phosphorylation process adds phosphate groups to the glucose molecule, making it more reactive and facilitating its breakdown. Hexokinase and phosphofructokinase are the enzymes responsible for utilising one ATP molecule each during this preparatory phase of glycolysis. The investment in these two ATP molecules is repaid with interest in the subsequent energy payoff phase, where ATP is produced through substrate-level phosphorylation, resulting in a net gain of two ATP molecules.

Phosphorylation of glucose

The phosphorylation of glucose is a process that occurs during the energy investment phase of glycolysis. Glycolysis is the process by which glucose is broken down into pyruvate, and it involves nine distinct reactions. The phosphorylation of glucose is the first step of this process.

Phosphorylation is the process of adding a phosphate group to a molecule, in this case, glucose. This is achieved with the help of the enzyme hexokinase, which catalyses the reaction. Hexokinase is an enzyme that facilitates the phosphorylation of six-membered ring structures like glucose. In this reaction, hexokinase catalyses the addition of a phosphate group to the sixth carbon of the glucose molecule, resulting in a molecule called glucose-6-phosphate (G6P). This reaction consumes one molecule of adenosine triphosphate (ATP).

The phosphorylation of glucose is essential for several reasons. Firstly, it prevents the diffusion of glucose out of the cell. The addition of a charged phosphate group makes it difficult for glucose-6-phosphate to pass through the cell membrane. Secondly, it serves as the initial step in two major metabolic pathways: glycolysis and the pentose phosphate pathway.

During glycolysis, a second molecule of ATP is used to phosphorylate fructose-6-phosphate, which is derived from glucose-6-phosphate. This results in the formation of fructose-1,6-bisphosphate. Thus, a total of two ATP molecules are consumed during the energy investment phase of glycolysis, with one ATP molecule being used in the initial phosphorylation of glucose to form glucose-6-phosphate.

Glucose activation

The first step of glucose activation involves the phosphorylation of the glucose ring. One molecule of ATP is consumed as a result of this process, which occurs with the help of the enzyme hexokinase. This enzyme catalyses the phosphorylation of the glucose molecule, with atomic magnesium (Mg) also playing a role in shielding the negative charges from the phosphate groups on the ATP molecule. The outcome of this step is a molecule called glucose-6-phosphate (G6P), so named because the 6' carbon of the glucose acquires the added phosphate group.

The second step of glucose activation involves the conversion of glucose-6-phosphate to fructose-6-phosphate (F6P). This isomerization reaction is facilitated by the enzyme phosphoglucose isomerase (PI), which rearranges the carbon-oxygen bond, transforming the six-membered ring into a five-membered ring. This rearrangement occurs when the six-membered ring opens and then closes, with the first carbon now external to the ring.

The third step of glucose activation involves the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate (FBP). In this step, a second molecule of ATP provides the phosphate group that is added to the F6P molecule. This reaction mirrors the process that occurred in the first step of glycolysis.

Overall, the preparatory phase of glycolysis, or glucose activation, consumes two ATP molecules and sets the stage for the subsequent breakdown of glucose.

Energy investment and payoff phases

During cellular respiration, two ATP molecules are used in the energy investment phase to activate glucose, a 6-carbon molecule. This process involves the phosphorylation of glucose, which adds phosphate groups to it. The addition of phosphate groups makes glucose more reactive and allows it to be broken down further.

The energy investment phase, also known as the preparatory phase, is the first of two parts of glycolysis, the primary pathway for the breakdown of glucose. In this phase, two ATP molecules are used for the phosphorylation of glucose, which then splits into two 3-carbon compounds, each containing a phosphate. Hexokinase converts glucose to glucose-6-phosphate, using one ATP, and phosphoglucose isomerase then converts this into fructose-6-phosphate. Phosphofructokinase uses another ATP molecule to convert fructose-6-phosphate to fructose-1,6-bisphosphate, which is then turned into ADP.

The energy payoff phase, also known as the energy-yielding phase or energy generation phase, is the second phase of glycolysis. In this phase, the energy invested in the previous phase is harvested. An additional phosphate is added to each of the 3-carbon compounds through an endergonic reaction, with energy provided by the oxidation (removal) of two electrons from each 3-carbon compound. The removal of phosphates from these compounds then contributes to the production of four ATP molecules. Therefore, while two ATPs are initially consumed, there is an overall net gain of two ATP molecules as four are produced.

At the end of glycolysis, the glucose has been converted into two molecules of pyruvate, which can be considered to hold the majority of the original energy stored in the chemical bonds of glucose. Pyruvate plays a critical role in further energy extraction in cellular respiration.

Glycolysis

The first half of glycolysis involves an energy investment of two ATP molecules. This phase is sometimes called chemical priming, as it prepares glucose for further breakdown. Hexokinase catalyses the first step, where glucose and ATP are substrates for the reaction, producing glucose 6-phosphate and ADP as products. The second step involves an isomerase enzyme, which converts glucose 6-phosphate into fructose 6-phosphate. The third step is the phosphorylation of fructose 6-phosphate, which requires another ATP molecule. This results in the production of fructose 1,6-bisphosphate and ADP. This step is important as it commits the molecule to further oxidation in the glycolytic pathway. The fourth step cleaves 1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.

The second half of glycolysis is the energy payoff phase, where the energy invested earlier is harvested. In this phase, an additional phosphate is added to each of the three-carbon compounds. This process is powered by the removal (oxidation) of two electrons from each three-carbon compound. The removal of phosphates from these compounds then contributes to the production of four ATP molecules. Thus, despite the initial consumption of two ATP molecules, the overall reaction yields a net gain of two ATP molecules.

At the end of glycolysis, glucose is converted into two molecules of pyruvate, which can be considered to hold most of the original energy stored in the chemical bonds of glucose. Pyruvate then undergoes further breakdown, producing more ATP and capturing more of glucose's original stored energy.

Preparatory phase

The preparatory phase, also known as the energy investment phase, is the first of two stages in the process of glycolysis. This phase involves the investment of two ATP molecules, which are used to activate and prepare a six-carbon glucose molecule for further breakdown. The two ATP molecules are converted into ADP during this process.

During the preparatory phase, hexokinase uses one ATP to convert glucose to glucose-6-phosphate, which is then transformed into ADP. Phosphoglucose isomerase, an enzyme, then transforms glucose-6-phosphate into fructose-6-phosphate. Phosphofructokinase utilises another ATP molecule to convert fructose-6-phosphate into fructose-1,6-bisphosphate, which is subsequently turned into ADP.

The preparatory phase is essential for initiating the breakdown of glucose, a critical process in cellular respiration. By investing two ATP molecules, the glucose molecule becomes more reactive and can be split into two three-carbon compounds, setting the stage for the next phase of glycolysis, known as the energy-yielding or payoff phase.

Frequently asked questions