MCAT Biology : Other Metabolic Pathways
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Example Question #341 : Organic Chemistry, Biochemistry, And Metabolism
The body attempts to maintain a steady concentration of glucose in the blood, promoting consistent brain function and red blood cell survival. When glucose levels fall, however, the body breaks down glycogen to replenish stores for a short period of time before new glucose molecules are made through the process of gluconeogenesis.
In which organ does gluconeogenesis occur?
Heart
Brain
Liver
Skeletal muscle
Liver
Gluconeogenesis, the process of creating new glucose from precursors, occurs in the liver and to a very small extent in the cortex of the kidney. The largest stores of glycogen are also located in the liver, but become quickly depleted in situations of low blood glucose.
Example Question #1 : Other Metabolic Pathways
The process of glycolysis is used by all cells of the body to turn glucose into ATP for cellular energy. When stores of glucose are low, however, the body can break down a form of stored glucose in the liver to increase glucose reserves.
What molecule is broken down by a phosphorylase in the liver to yield glucose-1-phosphate?
Glycogen
RNA
Triacylglycerol
Glycosylated protein
Glycogen
Glycogen is the polymer form of glucose, stored in the liver and other tissues when glucose is abundant. When glucose levels are high, glucose-1-phosphate is assembled into branching chains of glycogen. When glucose levels fall, glycogen is broken down by glycogen phosphorylase back into glucose-1-phosphate units. These monomers can be used in glycolysis and cellular respiration. Glycogen is the first source of energy that is used when glucose stores are low.
Example Question #1 : Other Metabolic Pathways
Which of the following cannot be directly converted to acetyl-CoA?
Alpha-keto acid
Fatty acids
Glucose
Pyruvate
All of these answers can be directly converted to acetyl-CoA
Glucose
Pyruvate can be converted to acetyl-CoA by decarboxylation. Beta oxidation can convert fatty acids to acetyl-CoA. Transaminases can be used to make alpha-keto acids, which can be converted to acetyl-coA. Glucose cannot be directly converted to acetyl-CoA; it must be transformed into pyruvate first.
Example Question #2 : Other Metabolic Pathways
Which of the following is not an adequate alternative energy source for humans?
Fatty acids
Triglycerides
Glycogen
Cellulose
Alpha-keto acids
Cellulose
Carbohydrates can be stored as glycogen in the liver, fats can be stored as triglycerides or fatty acids in adipose tissue, and proteins can be made into alpha-keto acids. Hence, all of these are forms of energy storage that can be used as alternative energy sources.
Cellulose is a polysaccharide that is found in plants. Humans cannot digest cellulose due to its beta-glycosidic linkages.
Example Question #3 : Other Metabolic Pathways
When the body is unable to renew its glucose stores through glycogenolysis and gluconeogenesis, it makes ketone bodies derived from beta-oxidation of free fatty acids. Which of the following is not a ketone body utilized by the brain during periods of starvation?
Acetoacetate
Acetone
Aldehyde
Beta-hydroxybutyrate
Aldehyde
The three ketone bodies utilized by the body are acetoacetate, beta-hydroxybutyrate, and acetone. These are produced from acetyl-CoA during beta-oxidation. Acetyl-CoA undergoes conversion reactions to the three ketone bodies in the liver.
Even if you did not know the names of the ketone bodies, you should know that aldehyde is not a ketone because its carbonyl moiety does not have carbons connected from both sides to the carbonyl carbon.
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