MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 1d Glycolysis Gluconeogenesis Ppp

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This deck focuses on 1d Glycolysis Gluconeogenesis Ppp, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What is the purpose of fermentation in human cells during anaerobic glycolysis?

Answer: Regenerate NAD$^+$ to sustain glycolysis. Fermentation recycles NADH to NAD$^+$, preventing depletion that would halt glycolysis at the glyceraldehyde-3-phosphate dehydrogenase step.

Flashcard 2: What is the rate-limiting enzyme of the PPP oxidative phase?

Answer: Glucose-$6$-phosphate dehydrogenase (G6PD). It catalyzes the first committed step, regulated by the NADPH/NADP$^+$ ratio to control flux.

Flashcard 3: What is the rate-limiting enzyme of glycolysis?

Answer: Phosphofructokinase-$1$ (PFK-$1$). It catalyzes the slowest step in the pathway and is subject to extensive allosteric regulation based on cellular energy status.

Flashcard 4: Which glycolysis steps generate ATP by substrate-level phosphorylation?

Answer: 1,3-BPG $o$ 3-PG and PEP $o$ pyruvate. These reactions involve direct transfer of phosphate groups from high-energy intermediates to ADP, yielding ATP.

Flashcard 5: Which tissues are the principal sites of gluconeogenesis?

Answer: Liver (major) and kidney cortex (important in prolonged fasting). These organs express the necessary enzymes and can release glucose into circulation to support systemic needs.

Flashcard 6: What are the key allosteric regulators of PFK-$1$ (activators and inhibitors)?

Answer: Activated by AMP and F-$2,6$-bisP; inhibited by ATP and citrate. These effectors modulate enzyme activity to accelerate glycolysis when energy is low or inhibit it when energy is abundant.

Flashcard 7: What molecule allosterically activates pyruvate carboxylase?

Answer: Acetyl-CoA. It signals abundant energy from fatty acid oxidation, promoting the initiation of gluconeogenesis from pyruvate.

Flashcard 8: Which glycolysis step produces NADH, and what is the enzyme?

Answer: G3P dehydrogenase: G3P $o$ 1,3-BPG + NADH. This oxidation-reduction step generates reducing power by transferring electrons from the substrate to NAD$^+$.

Flashcard 9: Which gluconeogenesis enzyme requires biotin and uses ATP to carboxylate pyruvate?

Answer: Pyruvate carboxylase (biotin-dependent). Biotin serves as a cofactor for carboxyl group transfer, facilitating the ATP-dependent conversion to oxaloacetate.

Flashcard 10: What are the main cellular uses of NADPH produced by the PPP?

Answer: Reductive biosynthesis and glutathione reduction (antioxidant defense). NADPH supplies electrons for anabolic reactions and maintains antioxidant systems by regenerating reduced glutathione.

Flashcard 11: What enzyme converts fructose-$1,6$-bisphosphate to fructose-$6$-phosphate in gluconeogenesis?

Answer: Fructose-$1,6$-bisphosphatase (FBPase-$1$). It hydrolyzes the phosphate group, providing a bypass for the irreversible phosphofructokinase-1 reaction in glycolysis.

Flashcard 12: What molecule allosterically activates pyruvate kinase in glycolysis?

Answer: Fructose-$1,6$-bisphosphate (feed-forward activation). This upstream product enhances enzyme affinity for its substrate, promoting flux through the latter part of the pathway.

Flashcard 13: What is the net energy cost to form 1 glucose from 2 pyruvate in gluconeogenesis?

Answer: Consumes $4$ ATP + $2$ GTP + $2$ NADH. Energy input is required to drive the endergonic synthesis of glucose from pyruvate against the thermodynamic gradient.

Flashcard 14: What is the primary function of the pentose phosphate pathway (PPP)?

Answer: Generate NADPH and ribose-$5$-phosphate. The pathway provides reducing power for biosynthesis and pentoses for nucleotide synthesis.

Flashcard 15: What are the three irreversible (regulated) enzymes of glycolysis?

Answer: Hexokinase (or glucokinase), PFK-$1$, pyruvate kinase. These enzymes catalyze steps with large negative free energy changes, making them irreversible and primary sites for pathway regulation.

Flashcard 16: Which glycolysis intermediate is the substrate that enters the PPP, and what is it called?

Answer: Glucose-$6$-phosphate (G6P). This early glycolytic intermediate can be diverted to the PPP for NADPH and pentose production when needed.

Flashcard 17: What is the net ATP yield of glycolysis per 1 glucose under anaerobic conditions?

Answer: Net: $2$ ATP (NADH is reoxidized by fermentation). Without oxygen, NADH cannot enter the electron transport chain, so fermentation recycles it without additional ATP production.

Flashcard 18: What is the primary physiologic role of gluconeogenesis?

Answer: Maintain blood glucose during fasting. It synthesizes glucose from non-carbohydrate precursors to prevent hypoglycemia when glycogen stores are depleted.

Flashcard 19: What enzyme converts pyruvate to lactate and regenerates NAD$^+$ in anaerobic muscle?

Answer: Lactate dehydrogenase. It catalyzes the reduction of pyruvate using NADH, thereby regenerating NAD$^+$ for continued glycolysis.

Flashcard 20: What is the net yield of glycolysis per 1 glucose under aerobic conditions?

Answer: Net: $2$ ATP, $2$ NADH, $2$ pyruvate. Glycolysis converts glucose to pyruvate through a series of reactions that produce these molecules as net outputs for energy transfer and further oxidation.

Flashcard 21: Identify the direct effect of fructose-$2,6$-bisphosphate on glycolysis vs gluconeogenesis.

Answer: Activates PFK-$1$; inhibits FBPase-$1$. This molecule reciprocally regulates the opposing pathways to favor glycolysis or gluconeogenesis based on hormonal cues.

Flashcard 22: What is the key antioxidant role of NADPH in red blood cells?

Answer: Maintain reduced glutathione (GSH) to detoxify peroxides. It regenerates GSH via glutathione reductase, enabling peroxide neutralization and protecting cells from oxidative damage.

Flashcard 23: What are the two major fates of pyruvate in human metabolism (excluding gluconeogenesis)?

Answer: Pyruvate $o$ acetyl-CoA (aerobic) or $o$ lactate (anaerobic). Oxygen availability determines whether pyruvate is oxidized for energy or reduced to regenerate NAD$^+$ for anaerobic glycolysis.

Flashcard 24: What glycolysis step commits glucose to glycolysis and is strongly regulated?

Answer: PFK-$1$: fructose-$6$-P $o$ fructose-$1,6$-bisP. This irreversible reaction, regulated by energy indicators, commits the substrate to the glycolytic pathway without reversal.