Cellular Metabolism - GRE

If glucose was not phosphorylated, it would be free to diffuse through the plasma membrane and leave the cell. This situation would not be good for the cell because the reaction cannot continue outside of the cytosol. The negative charge created by the phosphorylation prevents the glucose molecule from crossing the plasma membrane due to the similar charge at the plasma membrane.

Mannose enters glycolysis by first being phosphorylated by hexokinase. The newly formed mannose-6-phosphate is then isomerized into fructose-6-phosphate by the enzyme phosphomannose isomerase. The sugar is now in a form that can follow the normal glycolytic pathway.

Glycolysis can be divided into two parts: the energy investment phase and the energy payoff phase. The energy investment phase comes first when glucose is phosphorylated twice, requiring the use of two molecules of ATP. After the glucose is split, four molecules of ATP will be made in the final steps. This results in a net gain of two ATP in glycolysis, but ATP must be spent prior to being made.

During glycolysis, a total of two molecules of NAD+ are reduced in order to form two NADH molecules. These NAD+ molecules need to be regenerated in order for more glycolytic reactions to take place; otherwise, the process would come to a halt. Fermentation takes care of this problem in anaerobic environments by oxidizing excess NADH (since it is no longer utilized in the electron transport chain) into NAD+, which is then returned to the cytosol where it can be used again in glycolysis.

The initial energy investment required for conversion of one glucose to pyruvate is 2 ATP in the preparatory phase. In the pay-off phase, substrate level phosphorylation produces a total of 4 ATP per initial glucose.

Both phases of glycolysis occur in the cytosol of the cell. The products of glycolysis are moved for further processing into the mitochondria, but the conversion of glucose to pyruvate is a cytosolic reaction.

and are the resultant molecules from this conversion, not the inputs. and are important molecules in the citric acid cycle, but are not required for this particular oxidation step. must be reduced to , and coenzyme A is a crucial modulator of these reactions. Thus, and coenzyme A are the required inputs for the oxidation of pyruvate to acetyl CoA.

Starch is a complex carbohydrate that is digested by enzymes in the small intestine. These digestive enzymes, called glucosidases, are released by exocrine glands in humans and are involved in breakdown of complex carbohydrates to their individual monomers (glucose). Recall that energy production in cell begins with glycolysis, where a molecule of glucose is metabolized to produce intermediates for subsequent metabolic steps. Cells can’t use starch or glycogen during glycolysis; therefore, the student must add glucosidase to break down starch into individual glucose molecules.

Energy can be produced in anaerobic conditions (like in glycolysis). It might not have a high yield of energy such as aerobic respiration, but the cells can still produce energy when they are oxygen deficient. As mentioned, glycogen is a complex carbohydrate; therefore, adding it without glucosidase will not help facilitate energy production.

Anaerobic metabolism, such as glycolysis and fermentation, occur in the cellular cytoplasm. The products of glycolysis are transported to the mitochondria where they undergo Krebs cycle (in mitochondrial matrix) and oxidative phosphorylation (on the inner mitochondrial membrane). Both Krebs cycle and oxidative phosphorylation require oxygen and are, therefore, called aerobic metabolism.

Glycolysis is an anaerobic process that produces 2 net ATP, 2 pyruvate molecules, and 2 NADH. Pyruvate is a three-carbon molecule. Recall that glucose is a six-carbon molecule; therefore, the six-carbon glucose is broken down to two three-carbon pyruvate molecules. This means that all the carbons in glucose are transferred to the pyruvate molecules. ATP is produced and consumed in glycolysis. There is a total of four ATP molecules synthesized in the glycolysis; however, glycolysis consume two ATP molecules so you get a net of 2 ATP molecules. Finally, glycolysis involves the reduction two molecules to yield two NADH molecules (not FAD).

The starting molecule in glycolysis is glucose, a six-carbon molecule while the ending molecule in glycolysis is pyruvate, a three-carbon molecule. During glycolysis glucose is split and its six carbons are used to make 2 molecules of three-carbon pyruvate because. Note that no carbon dioxide is released during glycolysis, but since aerobic metabolism (starting with the Krebs cycle) uses acetyl-CoA as a substrate, which is two carbons long, one molecule of carbon dioxide is released for each molecule of pyruvate produced during glycolysis.

The first step of glycolysis consumes a molecule of ATP, removing one of the phosphate groups to make ADP. This phosphate group is added to glucose to make Glucose-6-phosphate, therefore glucose is phosphorylated.

Glycolysis is the first step in producing ATP. Glycolysis is an anaerobic process that occurs in every cell. Certain cells, such as red blood cells, only rely on glycolysis for energy. In most of the other cells, glycolysis produces ATP and few intermediates that will be used in subsequent steps to generate more ATP; therefore, glycolysis occurs in every cell.

The major input for glycolysis is glucose. Glycogen, a storage form of glucose, needs to be broken down into individual glucose units before undergoing glycolysis. The net products of glycolysis are 2 NADH, 2 pyruvate molecules, and 2 ATP. There is a total of 4 ATP produced in glycolysis; however, two of the ATP molecules are consumed, leaving behind only 2 net ATP.

The question states that the cell undergoes aerobic respiration. This means that the products from anaerobic respiration (glycolysis) will go through Krebs cycle and electron transport chain (aerobic respiration) to generate ATP. Lactate dehydrogenase is an enzyme important for converting the pyruvate molecules (from glycolysis) to lactate and oxidizing NADH. This reaction occurs in anaerobic fermentation when there is tissue hypoxia (decrease in oxygen).

If this inhibitor was placed in a cell that is deprived of oxygen, then there would be a buildup of pyruvate and NADH; however, since the inhibitor is added to cells undergoing aerobic respiration there will be no buildup. The pyruvate and NADH will undergo aerobic respiration and generate ATP. Note that red blood cells (RBCs) are unique in that they only use anaerobic respiration for ATP; therefore, adding lactate dehydrogenase inhibitor to RBCs will lead to a buildup of pyruvate and NADH.

Thiamine is an important vitamin required for the conversion of pyruvate to acetyl-CoA. It is an important cofactor for the pyruvate dehydrogenase, an enzyme important for the conversion of pyruvate to acetyl-CoA. Pyruvate and NADH from glycolysis will continue to be produced; however, they cannot go any further without thiamine. This means that cells can’t undergo Krebs cycle and oxidative phosphorylation to produce ATP.

The buildup of pyruvate and NADH will cause the pyruvate molecules to undergo fermentation and produce lactic acid. It also will oxidize NADH, the product of which is essential for several cellular processes and needs to be regenerated.

The correct answer is glyceraldehyde-3-phosphate dehydrogenase. This enzyme sequesters a hydrogen from carbon 4 on glyceraldehyde 3-phosphate to reduce to , and in its place, an inorganic phosphate molecule is transferred to this position. Fructose-bisphosphate aldolase converts Fructose-1, 6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. Pyruvate kinase converts phosphoenolpyruvate to pyruvate whereas enolase converts 2-phosphoglycerate to phosphoenolpyruvate. Phosphoglycerate kinase converts 1, 3-bisphosphoglycerate to 3-phosphoglycerate.

Glycolysis has three irreversible enzymatic steps that help the substrate intermediates proceed in one direction through the glycolytic pathway: the enzymes are hexokinase, phosphofructokinase 1, and pyruvate kinase. Of these three enzymes, the most important enzyme that controls the rate of glycolysis is phosphofructokinase 1, or PFK-1. Pyruvate carboxylase is not used in glycolysis, but in gluconeogenesis.

Photosystems I and II are each capable of conducting electrons, with photosystem II handing off electrons to photosystem I. This is accomplished by the electron carrier molecule plastocyanin.

Photosystems I and II are responsible for the light-dependent reactions of photosynthesis. These two photosystems work in tandem to create ATP and NADPH products. ATP is created in photosystem II, while NADPH is created in photosystem I.

Excitation of photosystem II splits water in the thylakoid space into hydrogen and oxygen. The hydrogen then passes through ATP synthase to move down its concentration gradient and into the stroma. Excitation of photosystem I passes electrons to NADP+ reductase to convert NADP+ to NADPH. Regeneration of NADPH is necessary for the Calvin cycle.