The Complete Biology Respiration in Plants | Glycolysis

The first step of respiration is glycolysis. All of the steps of glycolysis take place in the cytosol of the cell; this allows prokaryotes to perform glycolysis, as well as eukaryotes. Glycolysis is an anaerobic process that occurs in the cytoplasm. The glycolysis reaction follows two steps. The initiation requires the input of two ATP, which become converted to ADP. Later in the process, however, four ADPs are required to produce four ATP products. ADP is consumed in a greater quantity than it is produced, eliminating it from the net products.

Glucose+2ADP+2NAD+→2Pyruvate+2ATP+2NADH+2H2O

In addition to making ATP, glycolysis also generates NADH, which goes to play a role in the electron transport chain. Glycolysis produces four total ATP molecules, but only produces two net ATP. The process requires an initial investment of two ATP to initiate the glycolysis pathway. By using two ATP and producing four, there is a net production of two ATP.

The initial reactants for glycolysis are glucose, ATP, ADP, and NAD+. The final products are pyruvate, ATP, ADP, and NADH. To get from glucose to pyruvate, a number of enzymes are needed. While knowing the names of each enzyme is not usually necessary, it is important to have a general understanding of the glycolytic process. The first step is phosphorylation of the reactant glucose, which is accomplished by hexokinase in most cells, and by glucokinase in the liver and pancreas specifically. The resultant glucose-6-phosphate then continues through the remaining steps in glycolysis to produce pyruvate. In glycolysis, one glucose molecule and two NAD+ molecules yield two molecules of pyruvate, two molecules of ATP, and two molecules of NADH.

Acetyl CoA is formed from pyruvate at the beginning fo the Krebs cycle. GTP is a product of the Krebs cycle. Oxygen and glucose are both reactants in metabolic processes that are derived from external intake (respiration and digestion).

Phosphofructokinase (PFK) is a key regulatory enzyme in glycolysis, the metabolic pathway that converts glucose into pyruvate. PFK catalyzes the phosphorylation of fructose-6-phosphate (F6P) to fructose-1,6-bisphosphate (F1,6BP). The activity of PFK is tightly regulated to ensure appropriate control of glucose metabolism in response to cellular energy needs and hormonal signals.

Insulin, fructose-2,6-bisphosphate (F2,6BP), and glucagon are important regulators of PFK activity through their effects on PFK-2, an enzyme that synthesizes and degrades F2,6BP. PFK-2 has two distinct activities: a kinase activity that synthesizes F2,6BP and a phosphatase activity that degrades it.

Insulin promotes the synthesis of F2,6BP and activates PFK, thereby stimulating glycolysis. When insulin binds to its receptor on the cell surface, it initiates a signaling cascade that leads to the activation of PFK-2 kinase activity. The activated PFK-2 kinase phosphorylates F6P, converting it to F2,6BP. F2,6BP allosterically activates PFK, resulting in increased glycolytic flux and glucose metabolism.

On the other hand, glucagon, which is released during times of low blood glucose levels, inhibits PFK activity. Glucagon activates protein kinase A (PKA), which phosphorylates and inhibits PFK-2 kinase activity. As a result, less F2,6BP is synthesized, and PFK is less active. This leads to a decrease in glycolysis and a shift toward alternative metabolic pathways such as gluconeogenesis.

Overall, the regulation of PFK activity by insulin and glucagon through the modulation of PFK-2 and F2,6BP levels provides a mechanism for fine-tuning glucose metabolism in response to changes in energy demand and hormonal signals.

The Krebs cycle or Citric acid cycle is a series of enzyme-catalyzed reactions occurring in the mitochondrial matrix, where acetyl-CoA is oxidized to form carbon dioxide and coenzymes are reduced, which generate ATP in the electron transport chain.

It is a series of eight-step processes, where the acetyl group of acetyl-CoA is oxidized to form two molecules of

CO2 and in the process, one ATP is produced. Reduced high-energy compounds, NADH, and FADH2 are also produced. Two molecules of acetyl-CoA are produced from each glucose molecule so two turns of the Krebs cycle are required which yields four CO2, six NADH, two FADH₂, and two ATPs.

The Krebs cycle or Citric acid cycle is a series of enzyme-catalyzed reactions occurring in the mitochondrial matrix, where acetyl-CoA is oxidized to form carbon dioxide and coenzymes are reduced, which generate ATP in the electron transport chain.

It is a series of eight-step processes, where the acetyl group of acetyl-CoA is oxidized to form two molecules of

CO2 and in the process, one ATP is produced. Reduced high-energy compounds, NADH, and FADH2 are also produced. Two molecules of acetyl-CoA are produced from each glucose molecule so two turns of the Krebs cycle are required which yields four CO2, six NADH, two FADH₂, and two ATPs.