MCAT Biological and Biochemical Foundations of Living Systems Quiz: 1d Carbohydrate Metabolism

What this quiz covers

This quiz focuses on 1d Carbohydrate Metabolism, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Contraction stimulates translocation of GLUT4 to the membrane via insulin-independent signaling pathways (correct answer)
2. Contraction induces expression of glucose-6-phosphatase to increase intracellular free glucose
3. Contraction decreases AMP, inhibiting PFK-1 and forcing cells to import more glucose
4. Contraction activates glucagon receptors on muscle to increase glucose uptake

Explanation: The skill being tested is non-hormonal regulation of muscle glucose uptake during exercise. In carbohydrate metabolism, glucose uptake in skeletal muscle is facilitated by GLUT4 transporters, which can be translocated to the cell membrane through both insulin-dependent and insulin-independent pathways, the latter activated by cellular energy demands. In the scenario of contracting skeletal muscle, increased ATP consumption during exercise triggers signaling cascades like AMPK activation, promoting glucose uptake to fuel glycolysis without insulin involvement. The correct answer, that contraction stimulates translocation of GLUT4 to the membrane via insulin-independent signaling pathways, follows this principle by explaining the enhanced glucose import observed in contracting muscle under identical blood glucose levels. A distractor like contraction activates glucagon receptors on muscle to increase glucose uptake fails because glucagon primarily targets liver cells for glycogenolysis, not muscle glucose uptake, highlighting a misconception about hormone receptor distribution across tissues. For similar questions, verify the tissue-specific mechanisms of metabolic regulation to avoid confusing hepatic and muscular responses. Additionally, recall that rising AMP levels during exercise activate AMPK, providing a general strategy to distinguish insulin-independent from hormonal controls in metabolism.

Question 2

1. Glucose-6-phosphate accumulates and is exported as free glucose to maintain blood glucose
2. Glucose-1-phosphate accumulates, limiting entry of glycogen-derived carbon into glycolysis (correct answer)
3. Fructose-1,6-bisphosphate accumulates, inhibiting glycogen phosphorylase by feedback
4. Pyruvate accumulates because phosphoglucomutase directly converts pyruvate to glucose-6-phosphate

Explanation: The skill being tested is interconversion of glucose-1-phosphate and glucose-6-phosphate. Phosphoglucomutase converts glucose-1-phosphate from glycogen breakdown to glucose-6-phosphate for entry into glycolysis. In this muscle with enzyme inhibition post-epinephrine, glycogen remains high as the product accumulates without conversion. Choice B is correct because glucose-1-phosphate buildup limits glycolytic substrate availability. Choice A fails by suggesting glucose-6-phosphate export, a misconception as muscle lacks glucose-6-phosphatase. For similar blocks, identify downstream pathway impacts. A transferable rule is that mutase enzymes enable flexible carbon routing in metabolism.

Question 3

1. Insulin activates glycogen phosphorylase, generating glucose-1-phosphate that exits the cell as glucose
2. Insulin directly activates glucose-6-phosphatase, increasing intracellular free glucose
3. Insulin inhibits hexokinase, preventing phosphorylation and trapping of glucose
4. Insulin triggers translocation of GLUT4 to the plasma membrane, increasing facilitated diffusion of glucose (correct answer)

Explanation: The skill being tested is hormonal regulation of glucose entry into cells. Insulin stimulates glucose uptake in insulin-sensitive tissues like adipocytes by promoting translocation of GLUT4 transporters to the plasma membrane, facilitating glucose diffusion. In this experiment, insulin addition to adipocytes rapidly increases intracellular glucose and glucose-6-phosphate levels. Choice D is correct because GLUT4 translocation enhances glucose entry, aligning with insulin's role in postprandial glucose disposal. Choice B fails by claiming insulin activates glucose-6-phosphatase, a misconception as this enzyme is inhibited by insulin to prevent glucose release. For similar questions, distinguish between GLUT transporters and their regulation by hormones. A transferable rule is that insulin promotes storage pathways, including uptake and phosphorylation of glucose in peripheral tissues.

Question 4

1. Glycogen synthase phosphorylated; glycogen phosphorylase dephosphorylated
2. Glycogen synthase dephosphorylated; glycogen phosphorylase dephosphorylated
3. Glycogen synthase dephosphorylated; glycogen phosphorylase phosphorylated
4. Glycogen synthase phosphorylated; glycogen phosphorylase phosphorylated (correct answer)

Explanation: The skill being tested is reciprocal regulation of glycogen synthesis vs breakdown. Glucagon increases cAMP and PKA, which phosphorylates glycogen synthase (inactivating it) and glycogen phosphorylase (activating it) to promote breakdown. In these hepatocytes with high glucagon and low insulin, phosphorylation favors net glycogen mobilization. Choice D is correct because both enzymes are phosphorylated, inhibiting synthesis and activating breakdown. Choice B fails by suggesting both dephosphorylated, a misconception mimicking insulin-dominant states. For similar hormone effects, recall phosphorylation states and activity. A transferable rule is that reciprocal regulation prevents futile cycling in glycogen metabolism.

Question 5

1. Increased release of free glucose from hepatocytes because glucose-6-phosphate is directly transported out
2. Decreased hepatic glycogen synthesis because glucose-6-phosphate cannot be formed from glucose
3. Accumulation of glucose-6-phosphate in hepatocytes with reduced net glucose export despite active gluconeogenesis (correct answer)
4. Increased lactate production because glucose-6-phosphatase is required for glycolysis to proceed

Explanation: The skill being tested is the role of glucose-6-phosphatase in maintaining blood glucose. Glucose-6-phosphatase dephosphorylates glucose-6-phosphate to free glucose in the liver, enabling export during gluconeogenesis or glycogenolysis. In these mutated hepatocytes under low-glucose and glucagon, active gluconeogenesis produces glucose-6-phosphate but cannot convert it to exportable glucose. Choice C is correct because reduced enzyme activity causes glucose-6-phosphate accumulation and impairs net glucose release, despite upstream gluconeogenic function. Choice A fails by suggesting direct export of glucose-6-phosphate, a misconception as it is not transported out and requires dephosphorylation. To evaluate similar scenarios, confirm the final step for glucose release in gluconeogenic tissues. A general strategy is to trace pathway endpoints and identify bottlenecks in metabolic defects.

Question 6

1. Phosphofructokinase-1
2. Fructose-1,6-bisphosphatase (correct answer)
3. Aldolase
4. Hexokinase

Explanation: The skill being tested is key irreversible steps in gluconeogenesis. Gluconeogenesis bypasses irreversible glycolytic steps, including conversion of fructose-1,6-bisphosphate to fructose-6-phosphate by fructose-1,6-bisphosphatase (FBPase). In this knockout mouse with fasting hypoglycemia despite high glucagon, the defect blocks a critical gluconeogenic reaction. Choice B is correct because FBPase deficiency prevents glucose production from precursors, leading to low plasma glucose. Choice A fails by implicating PFK-1, a misconception as it catalyzes the opposite reaction in glycolysis. To verify similar defects, map enzymes to pathway direction and irreversibility. A general strategy is to remember the three bypass enzymes in gluconeogenesis: pyruvate carboxylase, PEPCK, and FBPase.

Question 7

1. Glucagon strongly stimulates glycogen breakdown in skeletal muscle via abundant glucagon receptors
2. Epinephrine stimulates glycogen breakdown in muscle via -adrenergic signaling, increasing glucose-6-phosphate for glycolysis (correct answer)
3. Glucagon stimulates muscle glycogen synthesis by activating glycogen synthase through phosphorylation
4. Epinephrine increases hepatic glycogen synthesis by lowering cAMP and activating protein phosphatase-1

Explanation: The skill being tested is hormone receptor distribution and tissue specificity. Epinephrine acts on muscle via beta-adrenergic receptors to stimulate glycogen breakdown for local energy, while glucagon primarily targets liver receptors without significant muscle effects. In this ex vivo setup, epinephrine on muscle increases glucose-6-phosphate from glycogen, but glucagon has minimal impact on muscle. Choice B is correct because epinephrine's signaling in muscle raises cAMP, activating phosphorylase for glycolytic fuel. Choice A fails by claiming glucagon stimulates muscle glycogenolysis, a misconception ignoring receptor specificity. For similar questions, confirm hormone-tissue interactions and downstream effects. A transferable rule is that muscle responds to catecholamines for fight-or-flight, while liver responds to glucagon for fasting glucose maintenance.

Question 8

1. Increased hepatic conversion of pyruvate to oxaloacetate, increasing glucose production
2. Decreased gluconeogenesis from lactate/alanine because pyruvate cannot be efficiently converted to oxaloacetate (correct answer)
3. Decreased glycolysis because pyruvate carboxylase is required for the glyceraldehyde-3-phosphate dehydrogenase step
4. Increased muscle glucose export because pyruvate carboxylase normally traps glucose in muscle as glycogen

Explanation: The skill being tested is anaplerotic entry into gluconeogenesis. Pyruvate carboxylase converts pyruvate to oxaloacetate, providing a key intermediate for gluconeogenesis from lactate or alanine. In this newborn with pyruvate carboxylase mutation, short-fast hypoglycemia occurs due to impaired glucose production. Choice B is correct because deficient conversion blocks gluconeogenesis from pyruvate-forming precursors, reducing blood glucose. Choice A fails by suggesting increased conversion, a misconception ignoring the loss-of-function. For similar defects, trace substrate entry points into pathways. A transferable rule is that mitochondrial pyruvate carboxylase is essential for net glucose synthesis from non-carbohydrate sources.

Question 9

1. Activation of glycogen synthase via dephosphorylation, increasing incorporation of glucose into glycogen (correct answer)
2. Activation of glycogen phosphorylase via phosphorylation, increasing glycogen breakdown to store more glycogen
3. Activation of glucose-6-phosphatase to increase free glucose for glycogen synthesis
4. Inhibition of GLUT4 translocation to prevent excessive glucose uptake during insulin infusion

Explanation: The skill being tested is insulin-driven storage of carbohydrate in muscle. Insulin activates protein phosphatase-1, dephosphorylating glycogen synthase to promote synthesis and storage. In this clamp study with insulin infusion, muscle glycogen increases due to enzymatic activation. Choice A is correct because dephosphorylation activates synthase, incorporating glucose into glycogen. Choice B fails by suggesting phosphorylase activation for storage, a misconception confusing breakdown with synthesis. To assess hormonal effects, trace signaling to enzyme states. A general strategy is to remember insulin favors dephosphorylation for anabolic pathways in muscle.

Question 10

1. Decreased fructose-1-phosphate because fructokinase is inhibited by insulin
2. Accumulation of fructose-1-phosphate with depletion of inorganic phosphate and reduced ATP availability (correct answer)
3. Increased gluconeogenesis because fructose-1-phosphate directly activates fructose-1,6-bisphosphatase
4. Increased muscle glycogenolysis because aldolase B is required for hepatic glycogen breakdown

Explanation: The skill being tested is metabolic consequences of blocking fructose metabolism. Aldolase B cleaves fructose-1-phosphate (F1P) to trioses; deficiency causes F1P accumulation, trapping phosphate and depleting ATP. In this patient with post-fructose hypoglycemia, liver metabolism is disrupted. Choice B is correct because F1P buildup sequesters phosphate, impairing energy status and gluconeogenesis. Choice A fails by claiming fructokinase inhibition by insulin, a misconception unrelated to the defect. For similar intolerances, consider intermediate trapping effects. A transferable rule is that aldolase deficiencies lead to phosphate depletion in sugar metabolism pathways.

Question 11

1. Lactate
2. Acetyl-CoA
3. Glucose
4. 2-phosphoglycerate (correct answer)

Explanation: The skill being tested is mapping pathway blocks to intermediate changes. In carbohydrate metabolism, glycolysis is a sequential pathway where each enzyme catalyzes the conversion of specific intermediates, and inhibition of an enzyme leads to accumulation of its substrate upstream while depleting products downstream. In this scenario, inhibiting enolase, which converts 2-phosphoglycerate to phosphoenolpyruvate, disrupts the later stages of glycolysis, causing a buildup of upstream metabolites and reduced pyruvate formation. The correct answer, 2-phosphoglycerate, follows this principle as it is the direct substrate of enolase and would accumulate most immediately upon inhibition. A distractor like lactate fails because it is a downstream product of pyruvate under anaerobic conditions, and its levels would decrease rather than increase, reflecting a misconception about the directionality of pathway flux in blocked reactions. For similar questions, always trace the pathway linearly from the point of inhibition to identify the immediate substrate as the accumulating metabolite. This approach highlights the general rule that enzyme inhibition causes proximal upstream accumulation and distal downstream depletion in metabolic pathways.

Question 12

1. Reduced lactate production from muscle due to limited glucose-6-phosphate entry into glycolysis (correct answer)
2. Increased hepatic ketone body production immediately supplying ATP to working muscle
3. Increased muscle glucose export to blood due to activation of glucose-6-phosphatase
4. Increased net gluconeogenesis in muscle to restore ATP during sprinting

Explanation: This question tests understanding of tissue-specific glycogen metabolism during anaerobic exercise. Muscle glycogen phosphorylase deficiency (McArdle disease) prevents the breakdown of muscle glycogen to glucose-1-phosphate, which normally feeds into glycolysis during intense exercise. Without this rapid glucose source, muscles cannot generate sufficient ATP through anaerobic glycolysis during sprinting, leading to reduced lactate production. The correct answer (A) accurately describes this metabolic consequence. Answer B is incorrect because ketone bodies require aerobic metabolism and take time to produce, C is wrong because muscle lacks glucose-6-phosphatase and cannot export glucose, and D is incorrect because muscle cannot perform net gluconeogenesis. The key principle: during brief, intense exercise, muscle relies heavily on glycogenolysis to fuel rapid ATP production through glycolysis, distinguishing it from liver's role in maintaining blood glucose.

Question 13

1. Increased net glucose production because blocking FBPase-1 pushes flux through glycolysis
2. Decreased net glucose production because conversion of fructose-1,6-bisphosphate to fructose-6-phosphate is impaired (correct answer)
3. Unchanged glucose production because FBPase-1 functions only in glycogen breakdown
4. Increased lactate-to-glucose conversion in muscle because gluconeogenesis is primarily muscular

Explanation: This question tests understanding of fructose-1,6-bisphosphatase's essential role in gluconeogenesis. FBPase-1 catalyzes a key irreversible step in gluconeogenesis, converting fructose-1,6-bisphosphate to fructose-6-phosphate, effectively bypassing the glycolytic enzyme phosphofructokinase. When FBPase-1 is inhibited, the gluconeogenic pathway from lactate to glucose is blocked at this critical control point, preventing net glucose production. The correct answer (B) accurately describes this metabolic block. Answer A incorrectly suggests blocking FBPase-1 would increase glucose production, C wrongly limits FBPase-1 to glycogen metabolism, and D incorrectly places gluconeogenesis in muscle. The key principle: FBPase-1, along with PEPCK, G6Pase, and pyruvate carboxylase, represents one of the four bypass reactions essential for gluconeogenesis, distinguishing this pathway from simple glycolysis reversal.

Question 14

1. More rapid glycogen breakdown because fewer branch points reduce steric hindrance for phosphorylase
2. Impaired rapid glycogen mobilization because fewer nonreducing ends limit simultaneous phosphorylase action (correct answer)
3. Increased muscle glucose export because the liver compensates by inducing muscle glucose-6-phosphatase
4. Increased gluconeogenesis from fatty acids because glycogen branching directly controls beta-oxidation enzymes

Explanation: This question tests understanding of glycogen structure's functional importance in glucose mobilization. Glycogen branching creates multiple nonreducing ends where glycogen phosphorylase can simultaneously act, allowing rapid glucose release when needed. With reduced branching, there are fewer sites for phosphorylase action, limiting the rate of glucose mobilization during periods of high demand. The correct answer (B) accurately describes this impaired mobilization. Answer A incorrectly suggests fewer branches would accelerate breakdown, C wrongly implies muscle has G6Pase and that liver controls its expression, and D incorrectly links glycogen branching to fatty acid metabolism. The key principle: glycogen's highly branched structure is optimized for both storage efficiency and rapid mobilization, with branch points occurring approximately every 10 glucose residues to maximize accessible ends for phosphorylase.

Question 15

1. Decreased glycogen phosphorylase activity due to dephosphorylation, with reduced glucose-1-phosphate production
2. Increased glycogen phosphorylase activity, increasing glucose-1-phosphate formation from glycogen (correct answer)
3. Increased muscle glucose uptake via GLUT4 translocation, lowering intracellular free glucose
4. Increased net glycogen synthesis by activating glycogen synthase through phosphorylation

Explanation: This question tests understanding of glucagon's role in stimulating hepatic glycogenolysis. Glucagon, a counter-regulatory hormone, binds to hepatocyte receptors and activates a cAMP-dependent signaling cascade that ultimately phosphorylates and activates glycogen phosphorylase. In the liver, this enzyme cleaves glucose residues from glycogen as glucose-1-phosphate, which can be converted to glucose-6-phosphate and then to free glucose for export. The correct answer (B) reflects glucagon's primary action of increasing glycogen phosphorylase activity to mobilize hepatic glucose stores. Answer A incorrectly suggests dephosphorylation would occur with glucagon (phosphorylation activates the enzyme), while C describes muscle-specific GLUT4 translocation that doesn't occur in hepatocytes, and D incorrectly states that phosphorylation activates glycogen synthase (phosphorylation actually inhibits it). A key principle for similar questions: glucagon promotes glucose production pathways (glycogenolysis, gluconeogenesis) while inhibiting glucose storage pathways in the liver.

Question 16

1. Increased glycogen phosphorylase activity to raise hepatic glucose output
2. Increased fructose-1,6-bisphosphatase activity to promote gluconeogenesis
3. Decreased glycogen synthase activity because insulin suppresses glycogen storage
4. Increased glycogen synthase activity, favoring glycogen storage and reduced hepatic glucose release (correct answer)

Explanation: This question tests understanding of insulin's anabolic effects on hepatic carbohydrate metabolism. Insulin promotes glucose storage and suppresses glucose production in the liver by activating glycogen synthase (through dephosphorylation) and inhibiting enzymes of glycogenolysis and gluconeogenesis. Under euglycemic clamp conditions with insulin infusion, the liver shifts toward glycogen storage and reduced glucose output. The correct answer (D) accurately describes insulin's activation of glycogen synthase. Answer A incorrectly suggests insulin would activate glycogen phosphorylase (it inhibits it), B wrongly indicates insulin would promote gluconeogenesis (it suppresses it), and C incorrectly states insulin suppresses glycogen storage (it promotes it). The fundamental principle: insulin is the primary anabolic hormone, promoting glucose uptake and storage while suppressing hepatic glucose production through coordinated enzyme regulation.

Question 17

1. Activation of glycogen synthase by phosphorylation, increasing glycogen storage
2. Inhibition of glycogen phosphorylase by phosphorylation, decreasing glycogen breakdown
3. Activation of glycogen phosphorylase via phosphorylation, promoting glycogen breakdown and hepatic glucose availability (correct answer)
4. Translocation of GLUT4 to the hepatocyte membrane to increase glucose uptake

Explanation: This question tests understanding of cAMP-mediated regulation of hepatic glucose metabolism. The cAMP-dependent protein kinase A (PKA) pathway, activated by glucagon, phosphorylates key metabolic enzymes to promote glucose production. PKA phosphorylates and activates glycogen phosphorylase while simultaneously phosphorylating and inactivating glycogen synthase, creating a coordinated response that mobilizes glucose from glycogen stores. The correct answer (C) accurately describes the activation of glycogen phosphorylase through phosphorylation. Answer A incorrectly states phosphorylation activates glycogen synthase (it inhibits it), B wrongly suggests phosphorylation inhibits glycogen phosphorylase (it activates it), and D incorrectly mentions GLUT4 translocation which is insulin-mediated and doesn't occur in hepatocytes. The fundamental principle: cAMP signaling creates reciprocal regulation where catabolic enzymes are activated and anabolic enzymes are inhibited through phosphorylation cascades.

Question 18

1. Decreased fructose-6-phosphate with increased fructose-1,6-bisphosphate due to upstream acceleration
2. Increased fructose-6-phosphate with decreased downstream glycolytic intermediates because flux past the committed step is reduced (correct answer)
3. Increased pyruvate because PFK-1 is required for gluconeogenesis, not glycolysis
4. Unchanged glycolytic intermediates because PFK-1 only functions in the liver, not in other tissues

Explanation: This question tests understanding of phosphofructokinase-1 as the committed step of glycolysis. PFK-1 catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, the rate-limiting step that commits glucose to glycolysis. Inhibition of PFK-1 causes upstream metabolites like glucose-6-phosphate and fructose-6-phosphate to accumulate while downstream intermediates become depleted, ultimately reducing ATP production. The correct answer (B) accurately describes this metabolic bottleneck. Answer A incorrectly suggests F6P would decrease, C wrongly assigns PFK-1 to gluconeogenesis rather than glycolysis, and D incorrectly limits PFK-1 to liver tissue. The key principle: PFK-1 represents the major control point of glycolysis, subject to allosteric regulation by ATP (inhibitor) and AMP (activator), allowing cells to match glycolytic flux to energy needs.

Question 19

1. Increased conversion of phosphoenolpyruvate to pyruvate with increased ATP formation in the cytosol (correct answer)
2. Decreased pyruvate formation because pyruvate kinase catalyzes the reverse reaction in gluconeogenesis
3. Increased glucose release to the extracellular space because pyruvate kinase activates glucose-6-phosphatase
4. Increased glycogen synthesis in skeletal muscle because pyruvate kinase directly activates glycogen synthase

Explanation: This question tests understanding of pyruvate kinase's role in glycolysis and ATP production. Pyruvate kinase catalyzes the final ATP-generating step of glycolysis, converting phosphoenolpyruvate (PEP) to pyruvate while producing ATP through substrate-level phosphorylation. Activation of PKM2 would increase this conversion rate, generating more pyruvate and ATP in the cytosol. The correct answer (A) accurately describes this increased flux through the terminal glycolytic step. Answer B incorrectly suggests pyruvate kinase works in reverse during gluconeogenesis (PEPCK performs the bypass), C wrongly connects pyruvate kinase to G6Pase activation, and D incorrectly links pyruvate kinase to direct glycogen synthase activation. The key concept: pyruvate kinase is irreversible under physiological conditions and represents a major control point in glycolysis, with the M2 isoform being particularly important in rapidly dividing cells like tumors.

Question 20

1. Decreased glucose release because glucose-6-phosphate is trapped for glycolysis
2. Increased conversion of glucose to glycogen because G6Pase directly synthesizes glycogen
3. Increased glucose export to the medium by converting intracellular glucose-6-phosphate to free glucose (correct answer)
4. Increased GLUT4-mediated glucose uptake because G6Pase is a membrane glucose transporter

Explanation: This question tests understanding of glucose-6-phosphatase's unique role in hepatocytes. G6Pase catalyzes the final step of both gluconeogenesis and glycogenolysis in the liver, converting glucose-6-phosphate to free glucose that can exit the cell. Overexpression of this enzyme would increase the conversion of intracellular G6P to free glucose, allowing more glucose export even in the presence of insulin. The correct answer (C) accurately describes this increased glucose release. Answer A incorrectly suggests glucose would be trapped (the opposite occurs), B wrongly attributes glycogen synthesis to G6Pase, and D confuses G6Pase with a glucose transporter. The critical concept: G6Pase is expressed only in liver, kidney, and intestine, enabling these tissues to release free glucose into circulation, while its absence in muscle prevents glucose export.