Passage
Physical activity produces an increased energy requirement in the muscles as well as in the brain. During periods of intense exercise, glycogenolysis is activated in muscles to provide them with the energy needed for contractions, and research indicates that astrocytes also consume glycogen at increased levels during exercise. It is hypothesized that during periods of physical exertion, astrocytes convert glycogen to lactate, which is then used as a fuel source in neurons.Monocarboxylate transporters (MCTs) are responsible for the movement of lactate across membranes, and MCT2 is expressed at increased levels in neuronal cells during exercise. Scientists proposed that lactate is secreted from astrocytes and taken up by neurons through MCT2. Lactate is then converted to pyruvate and enters the mitochondria, where it is further metabolized. To test their predictions, scientists examined the effect of exercise on glycogen metabolism in rodents.Experiment 1Rats were either allowed to rest or exercised to exhaustion. The levels of several metabolites were then quantified in both the plantaris muscle and the hippocampus.
Figure 1 Levels of several metabolites in plantaris muscle (A) and hippocampus (B) from rats at rest or after exercising to exhaustionExperiment 2Rats were injected with 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) or saline, and glycogen and lactate levels were measured in the hippocampus of rats at rest or after exercise to exhaustion.
Figure 2 Levels of glycogen (A) and lactate (B) in rats injected with DAB or saline, after rest or upon exhaustion
Adapted from Matsui T, Omuro H, Liu Y-F, et al. Astrocytic glycogen-derived lactate fuels the brain during exhaustive exercise to maintain endurance capacity. Proceedings of the National Academy of Sciences. 2017;114(24) :6358-6363.
-If the proposed mechanism in the passage is correct, which of the following metabolic reactions must occur for astrocytes to convert glycogen into a fuel for neurons?Glucose-1-phosphate to glucose-6-phosphateGlucose to glucose-6-phosphate3-phosphoglycerate to 2-phosphoglyceratePyruvate to acetyl-CoAPyruvate to lactate

Question

Passage
Physical activity produces an increased energy requirement in the muscles as well as in the brain. During periods of intense exercise, glycogenolysis is activated in muscles to provide them with the energy needed for contractions, and research indicates that astrocytes also consume glycogen at increased levels during exercise. It is hypothesized that during periods of physical exertion, astrocytes convert glycogen to lactate, which is then used as a fuel source in neurons.Monocarboxylate transporters (MCTs) are responsible for the movement of lactate across membranes, and MCT2 is expressed at increased levels in neuronal cells during exercise. Scientists proposed that lactate is secreted from astrocytes and taken up by neurons through MCT2. Lactate is then converted to pyruvate and enters the mitochondria, where it is further metabolized. To test their predictions, scientists examined the effect of exercise on glycogen metabolism in rodents.Experiment 1Rats were either allowed to rest or exercised to exhaustion. The levels of several metabolites were then quantified in both the plantaris muscle and the hippocampus.

Figure 1 Levels of several metabolites in plantaris muscle (A) and hippocampus (B) from rats at rest or after exercising to exhaustionExperiment 2Rats were injected with 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) or saline, and glycogen and lactate levels were measured in the hippocampus of rats at rest or after exercise to exhaustion.

Figure 2 Levels of glycogen (A) and lactate (B) in rats injected with DAB or saline, after rest or upon exhaustion
Adapted from Matsui T, Omuro H, Liu Y-F, et al. Astrocytic glycogen-derived lactate fuels the brain during exhaustive exercise to maintain endurance capacity. Proceedings of the National Academy of Sciences. 2017;114(24) :6358-6363.
-If the proposed mechanism in the passage is correct, which of the following metabolic reactions must occur for astrocytes to convert glycogen into a fuel for neurons?Glucose-1-phosphate to glucose-6-phosphateGlucose to glucose-6-phosphate3-phosphoglycerate to 2-phosphoglyceratePyruvate to acetyl-CoAPyruvate to lactate

Quizplus · Accepted Answer

11ecba2d_0ed3_f7f3_a3bf_f9d288d1a8a2_MD0008_00 Glycogen serves as a form of energy storage in several tissues, including muscle, liver, and in glial cells such as astrocytes. The passage presents the hypothesis that during exercise, astrocytes convert glycogen to lactate, which is then sent to neurons for use as a fuel. This conversion requires three metabolic pathways: glycogenolysis, glycolysis, and fermentation. Glycogenolysis is the degradation of glycogen into individual glucose-6-phosphate units. The first step in glycogenolysis produces glucose-1-phosphate, which is then converted to glucose-6-phosphate units through the enzyme phosphoglucomutase (Number I). Glucose-6-phosphate then enters glycolysis, which involves the conversion of 3-phosphoglycerate to 2-phosphoglycerate by the enzyme phosphoglycerate mutase (Number III). The final step of glycolysis produces pyruvate, which is then fermented to lactate by lactate dehydrogenase (Number V). (Number II) In glycogenolysis, glycogen is converted to glucose-1-phosphate and then to glucose-6-phosphate (G6P). G6P can then enter glycolysis directly, and free glucose is not converted to G6P in this process. (Number IV) Pyruvate is converted into acetyl-CoA (a 2-carbon molecule) prior to its entry into the citric acid cycle. This step includes the irreversible loss of a carbon atom as CO₂. The fuel that astrocytes produce for neurons is lactate (a 3-carbon molecule), but once acetyl-CoA is formed, it cannot be converted to lactate. Educational objective: Glycogen is a form of energy storage. When energy is needed, individual glucose units may be removed from glycogen through glycogenolysis. The glucose-6-phosphate generated by glycogenolysis may then enter other metabolic pathways, including glycolysis and fermentation.

Passage Physical Activity Produces an Increased Energy Requirement in the Muscles