Glycolysis
55 flashcards covering Glycolysis for the MCAT Biology & Biochemistry section.
Glycolysis is the fundamental metabolic process that cells use to break down glucose, a simple sugar, into two molecules of pyruvate. This pathway occurs in the cytoplasm of cells and doesn't require oxygen, making it essential for energy production in both aerobic and anaerobic conditions. Through a series of ten enzyme-catalyzed reactions, glycolysis generates a net gain of two ATP molecules and two NADH molecules per glucose molecule, providing the cell with quick energy while also feeding into other metabolic pathways like the Krebs cycle.
On the MCAT, glycolysis frequently appears in biology and biochemistry questions, often testing your understanding of the pathway's steps, key enzymes, and regulation by factors like ATP levels or hormones. Common traps include confusing glycolysis with later stages of respiration or miscalculating energy yields, so focus on the net versus gross ATP production and how it's regulated in different tissues. A solid grasp of these details helps integrate it with larger topics like cellular respiration and metabolism.
Remember to practice sketching the pathway from memory.
Terms (55)
- 01
Glycolysis
Glycolysis is the metabolic pathway that breaks down one molecule of glucose into two molecules of pyruvate, producing a net gain of two ATP and two NADH molecules in the process, and it occurs in the cytoplasm of cells.
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Investment phase of glycolysis
The investment phase of glycolysis uses two ATP molecules to phosphorylate glucose and fructose-6-phosphate, converting them into fructose-1,6-bisphosphate, which sets up the molecule for cleavage.
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Payoff phase of glycolysis
The payoff phase of glycolysis generates four ATP molecules and two NADH molecules through substrate-level phosphorylation and oxidation, resulting in a net gain of two ATP per glucose molecule.
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Hexokinase
Hexokinase is the enzyme that catalyzes the first step of glycolysis by phosphorylating glucose to glucose-6-phosphate using ATP, and it is inhibited by its product to prevent unnecessary energy expenditure.
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Glucose-6-phosphate
Glucose-6-phosphate is the first intermediate in glycolysis, formed by the phosphorylation of glucose, and it can also enter other pathways like the pentose phosphate pathway.
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Phosphoglucose isomerase
Phosphoglucose isomerase catalyzes the isomerization of glucose-6-phosphate to fructose-6-phosphate, a reversible step that rearranges the sugar molecule for the next reaction.
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Fructose-6-phosphate
Fructose-6-phosphate is an intermediate in glycolysis that is phosphorylated to fructose-1,6-bisphosphate, serving as a key regulatory point in the pathway.
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Phosphofructokinase-1
Phosphofructokinase-1 is the key regulatory enzyme in glycolysis that phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate using ATP, and it is allosterically controlled by ATP and AMP levels.
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Fructose-1,6-bisphosphate
Fructose-1,6-bisphosphate is a six-carbon intermediate in glycolysis that is split into two three-carbon molecules, marking the end of the preparatory phase.
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Aldolase
Aldolase catalyzes the cleavage of fructose-1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate, a key step that produces the three-carbon intermediates.
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Dihydroxyacetone phosphate
Dihydroxyacetone phosphate is a three-carbon intermediate in glycolysis that is isomerized to glyceraldehyde-3-phosphate, allowing both products of aldolase to proceed in the pathway.
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Glyceraldehyde-3-phosphate
Glyceraldehyde-3-phosphate is the common three-carbon intermediate in glycolysis that undergoes oxidation and phosphorylation to produce 1,3-bisphosphoglycerate and NADH.
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Triose phosphate isomerase
Triose phosphate isomerase catalyzes the reversible isomerization of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate, ensuring both halves of the split molecule can continue.
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Glyceraldehyde-3-phosphate dehydrogenase
Glyceraldehyde-3-phosphate dehydrogenase oxidizes glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, producing NADH and requiring NAD+ as a cofactor in this redox reaction.
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,3-Bisphosphoglycerate
,3-Bisphosphoglycerate is a high-energy intermediate in glycolysis that donates a phosphate group to ADP to form ATP via substrate-level phosphorylation.
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Phosphoglycerate kinase
Phosphoglycerate kinase catalyzes the transfer of a phosphate from 1,3-bisphosphoglycerate to ADP, producing 3-phosphoglycerate and ATP in the payoff phase.
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Phosphoglycerate
Phosphoglycerate is an intermediate in glycolysis that is rearranged to 2-phosphoglycerate, continuing the pathway toward pyruvate production.
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Phosphoglycerate mutase
Phosphoglycerate mutase catalyzes the conversion of 3-phosphoglycerate to 2-phosphoglycerate by shifting the phosphate group, a necessary step before dehydration.
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Enolase
Enolase catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate, requiring a magnesium ion and producing a high-energy compound.
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Phosphoenolpyruvate
Phosphoenolpyruvate is a high-energy intermediate in glycolysis that transfers its phosphate to ADP to form pyruvate and ATP via pyruvate kinase.
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Pyruvate kinase
Pyruvate kinase catalyzes the final step of glycolysis, converting phosphoenolpyruvate to pyruvate while producing ATP, and it is regulated by factors like ATP and fructose-1,6-bisphosphate.
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Pyruvate
Pyruvate is the end product of glycolysis, a three-carbon compound that can enter the mitochondria for further oxidation or be converted to lactate under anaerobic conditions.
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Net ATP production in glycolysis
Glycolysis results in a net production of two ATP molecules per glucose molecule, as four ATP are produced but two are consumed in the investment phase.
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NADH production in glycolysis
Glycolysis produces two molecules of NADH per glucose molecule through the oxidation of glyceraldehyde-3-phosphate, which can later contribute to ATP synthesis.
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Regulation of glycolysis by phosphofructokinase
Phosphofructokinase is the primary control point of glycolysis, inhibited by high ATP and citrate levels and activated by AMP and fructose-2,6-bisphosphate to match cellular energy needs.
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Allosteric inhibitors of PFK-1
Allosteric inhibitors of phosphofructokinase-1, such as ATP and citrate, slow down glycolysis when energy is abundant, preventing unnecessary glucose breakdown.
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Allosteric activators of PFK-1
Allosteric activators of phosphofructokinase-1, like AMP and fructose-2,6-bisphosphate, stimulate glycolysis during energy deficits to increase ATP production.
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Hexokinase inhibition
Hexokinase is inhibited by its product, glucose-6-phosphate, which prevents further phosphorylation of glucose when intermediates build up, conserving ATP.
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Pyruvate kinase regulation
Pyruvate kinase is regulated by ATP inhibition and activation by fructose-1,6-bisphosphate, ensuring glycolysis aligns with the cell's energy status.
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Anaerobic glycolysis
Anaerobic glycolysis is the process where pyruvate is converted to lactate in the absence of oxygen, allowing ATP production to continue without NADH regeneration.
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Lactic acid fermentation
Lactic acid fermentation regenerates NAD+ from NADH by reducing pyruvate to lactate, enabling glycolysis to proceed in oxygen-deprived conditions like muscle cells.
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Alcoholic fermentation
Alcoholic fermentation converts pyruvate to ethanol and carbon dioxide in yeast, regenerating NAD+ to sustain glycolysis under anaerobic conditions.
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Energy yield from glycolysis
The energy yield from glycolysis is two ATP and two NADH per glucose, though NADH can yield additional ATP via oxidative phosphorylation if oxygen is available.
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Overall equation of glycolysis
The overall equation of glycolysis is C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi → 2 C3H4O3 + 2 NADH + 2 H+ + 2 ATP + 2 H2O, summarizing the conversion of glucose to pyruvate.
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Location of glycolysis
Glycolysis occurs in the cytoplasm of the cell, making it accessible to both prokaryotes and eukaryotes without the need for organelles.
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Irreversible steps in glycolysis
The irreversible steps in glycolysis are the reactions catalyzed by hexokinase, phosphofructokinase-1, and pyruvate kinase, which are key regulatory points due to their large free energy changes.
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Reversible steps in glycolysis
The reversible steps in glycolysis include isomerizations and some phosphorylations, allowing for flexibility in the pathway under varying conditions.
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Substrate-level phosphorylation
Substrate-level phosphorylation in glycolysis directly transfers a phosphate group from a substrate to ADP to form ATP, occurring in the reactions catalyzed by phosphoglycerate kinase and pyruvate kinase.
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Role of glycolysis in cellular respiration
Glycolysis is the initial stage of cellular respiration, breaking down glucose to pyruvate and generating ATP and NADH that feed into the Krebs cycle and electron transport chain.
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Glycolysis in red blood cells
In red blood cells, glycolysis is the primary source of ATP since they lack mitochondria, and it produces lactate to regenerate NAD+.
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Warburg effect
The Warburg effect describes cancer cells' preference for aerobic glycolysis, producing lactate even with oxygen present, which supports rapid cell growth and is a topic in metabolic regulation.
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Calculating ATP from glycolysis
To calculate ATP from glycolysis, subtract the two ATP invested from the four ATP produced, resulting in a net of two ATP per glucose, plus potential from NADH.
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Common mistake in glycolysis
A common mistake is confusing the net ATP yield of glycolysis with the gross yield, forgetting that the two ATP used in the investment phase must be deducted.
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Strategy for memorizing glycolysis
A strategy for memorizing glycolysis is to group the steps into the investment and payoff phases, focusing on the enzymes and intermediates in each to build a logical sequence.
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Isomerization reactions in glycolysis
Isomerization reactions in glycolysis, such as those catalyzed by phosphoglucose isomerase and triose phosphate isomerase, rearrange molecules to allow the pathway to proceed efficiently.
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Phosphorylation reactions in glycolysis
Phosphorylation reactions in glycolysis add phosphate groups to intermediates using ATP or inorganic phosphate, increasing reactivity and trapping molecules inside the cell.
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Dehydration reactions in glycolysis
Dehydration reactions in glycolysis, like the one catalyzed by enolase, remove water from 2-phosphoglycerate to form phosphoenolpyruvate, creating a high-energy bond.
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Oxidation reactions in glycolysis
Oxidation reactions in glycolysis, specifically the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, produce NADH by transferring electrons to NAD+.
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Connection to pentose phosphate pathway
Glycolysis connects to the pentose phosphate pathway at glucose-6-phosphate, allowing cells to divert intermediates for NADPH production and nucleotide synthesis when needed.
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Glycolytic flux
Glycolytic flux refers to the rate of glucose conversion through glycolysis, influenced by enzyme activity and regulators, and it adjusts based on cellular demands for energy and building blocks.
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Fate of pyruvate
The fate of pyruvate from glycolysis depends on oxygen availability: it enters the mitochondria for further oxidation if oxygen is present, or is reduced to lactate or ethanol if absent.
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Energy investment in glycolysis
Energy investment in glycolysis involves using two ATP molecules early in the pathway to prime glucose for breakdown, which is later repaid with a higher ATP yield.
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NAD+ regeneration in glycolysis
NAD+ regeneration in glycolysis is crucial for the pathway to continue, achieved through fermentation in anaerobic conditions or the electron transport chain in aerobic ones.
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Glycolysis in muscle cells
In muscle cells, glycolysis provides quick ATP during intense exercise, switching to lactic acid production when oxygen is low to maintain energy output.
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Bypass reactions in gluconeogenesis
Bypass reactions in gluconeogenesis reverse the irreversible steps of glycolysis, such as using pyruvate carboxylase to convert pyruvate to oxaloacetate instead of pyruvate kinase.