Cellular Respiration: Glycolysis, Citric Acid Cycle, and Oxidative Phosphorylation

Vocabulary flashcards covering the individual steps, enzymes, yields, and mechanisms of glycolysis, pyruvate processing, the citric acid cycle, the electron transport chain, chemiosmosis, and fermentation.

Hexokinase

The glycolytic enzyme in Step 1 that uses ATP to phosphorylate glucose, converting it into glucose-6-phosphate.

Phosphoglucose isomerase

The enzyme in Step 2 of glycolysis that rearranges glucose-6-phosphate into its isomer, fructose-6-phosphate.

Phosphofructokinase (PFK)

The glycolytic enzyme in Step 3 that uses ATP to add a phosphate to fructose-6-phosphate, producing fructose-1,6-bisphosphate; it is allosterically inhibited by high ATP levels.

Fructose-bisphosphate aldolase

The enzyme in Step 4 of glycolysis that cleaves the six-carbon sugar fructose-1,6-bisphosphate into two three-carbon sugars: DHAP and G3P.

Triose phosphate isomerase

The enzyme in Step 5 of glycolysis that converts DHAP into G3P so that it can continue into the payoff phase.

Glyceraldehyde-3-phosphate dehydrogenase

The enzyme in Step 6 of glycolysis that oxidizes G3P to 1,3-bisphosphoglycerate while adding an inorganic phosphate and reducing NAD to NADH.

Phosphoglycerate kinase

The enzyme in Step 7 of glycolysis that produces ATP by transferring a phosphate group directly from 1,3-bisphosphoglycerate to ADP, forming 3-phosphoglycerate.

Substrate-level phosphorylation

A mechanism of ATP synthesis in which a phosphate group is transferred directly from a phosphorylated metabolic intermediate to ADP.

Phosphoglycerate mutase

The enzyme in Step 8 of glycolysis that relocates the phosphate group from carbon 3 to carbon 2, converting 3-phosphoglycerate to 2-phosphoglycerate.

Enolase

The enzyme in Step 9 of glycolysis that removes a molecule of water from 2-phosphoglycerate to generate phosphoenolpyruvate (PEP).

Pyruvate kinase

The enzyme in Step 10 of glycolysis that catalyzes the transfer of a phosphate group from PEP to ADP, generating ATP and pyruvate.

Energy investment phase

Steps 1 through 5 of glycolysis, during which two molecules of ATP are consumed to phosphorylate intermediate sugars.

Energy payoff phase

Steps 6 through 10 of glycolysis, during which ATP and NADH are synthesized from the oxidation of three-carbon sugars.

Pyruvate dehydrogenase

The enzyme complex in the mitochondrial matrix that oxidizes pyruvate to acetyl-CoA, releasing $CO_2$ and reducing NAD to NADH.

Pyruvate processing products (per glucose)

The net production of 2 acetyl-CoA, 2 $CO_2$, and 2 NADH from two molecules of pyruvate.

Citrate synthase

The enzyme in Step 1 of the citric acid cycle that joins a two-carbon acetyl group to four-carbon oxaloacetate to produce six-carbon citrate.

Aconitase

The enzyme in Step 2 of the citric acid cycle that isomerizes citrate into isocitrate via the sequential removal and addition of water.

Isocitrate dehydrogenase

The enzyme in Step 3 of the citric acid cycle that oxidizes six-carbon isocitrate to five-carbon alpha-ketoglutarate, producing NADH and releasing $CO_2$.

Alpha-ketoglutarate dehydrogenase

The enzyme in Step 4 of the citric acid cycle that oxidizes alpha-ketoglutarate, producing NADH, releasing $CO_2$, and attaching CoA to yield succinyl-CoA.

Succinyl-CoA synthetase

The enzyme in Step 5 of the citric acid cycle that converts succinyl-CoA into succinate, driving substrate-level phosphorylation to make ATP or GTP.

Succinate dehydrogenase

The enzyme in Step 6 of the citric acid cycle that converts succinate to fumarate and reduces FAD to $FADH_2$; it also serves as Complex II of the electron transport chain.

Fumarase

The enzyme in Step 7 of the citric acid cycle that hydrates fumarate by adding a water molecule to form malate.

Malate dehydrogenase

The enzyme in Step 8 of the citric acid cycle that oxidizes malate to regenerate oxaloacetate while producing NADH.

Citric acid cycle products (one turn)

The metabolic yield consisting of 3 NADH, 1 $FADH_2$, 1 ATP or GTP, and 2 $CO_2$.

Citric acid cycle products (per glucose)

The total metabolic yield resulting from two turns of the cycle: 6 NADH, 2 $FADH_2$, 2 ATP or GTP, and 4 $CO_2$.

Electron transport chain (ETC)

A series of inner mitochondrial membrane-associated protein complexes and electron carriers that transfer electrons via redox reactions to generate a proton gradient.

ETC Complex I (NADH dehydrogenase)

The protein complex that oxidizes NADH, passes electrons through FMN and Fe-S cofactors to ubiquinone, and pumps protons across the inner mitochondrial membrane.

Flavin mononucleotide (FMN)

An electron-carrying cofactor located in Complex I of the electron transport chain.

ETC Complex II (Succinate dehydrogenase)

The protein complex that transfers electrons from succinate oxidation through FAD/$FADH_2$ and Fe-S cofactors to ubiquinone without pumping protons.

ETC Complex III (Cytochrome c reductase)

The protein complex that shuttles electrons from ubiquinol through heme groups and Fe-S cofactors to cytochrome c while pumping protons.

ETC Complex IV (Cytochrome c oxidase)

The terminal protein complex of the ETC that transfers electrons from cytochrome c to oxygen, forming water and pumping protons.

Ubiquinone (Q)

A lipid-soluble, mobile electron carrier within the inner mitochondrial membrane that shuttles electrons from Complex I or Complex II to Complex III.

Cytochrome c

A mobile electron carrier protein that transfers electrons from Complex III to Complex IV.

Redox potential

A measure of the tendency of a molecule to accept electrons; carriers in the ETC are arranged so electrons move toward oxygen, which has a higher electron affinity, releasing energy.

Chemiosmosis

The biological process of utilizing energy stored in an electrochemical proton gradient across a membrane to drive the synthesis of ATP.

ATP synthase

A membrane protein complex that couples the movement of protons down their electrochemical gradient to the synthesis of ATP from ADP and inorganic phosphate.

$F_0$ subunit

The membrane-bound, rotor-containing channel of ATP synthase through which protons flow.

$F_1$ subunit

The catalytic portion of ATP synthase that undergoes conformational changes to synthesize ATP as the rotor turns.

Stator

The structural component of ATP synthase that anchors the catalytic unit in place while the rotor and central shaft rotate.

Oxidative phosphorylation

ATP synthesis driven by the electrochemical proton gradient that is generated by electron transport along the inner mitochondrial membrane.

Aerobic respiration

A metabolic pathway of cellular respiration that uses oxygen as the final electron acceptor, yielding approximately 29 ATP per glucose.

Anaerobic respiration

Respiration that uses an inorganic molecule other than oxygen (such as nitrate or sulfate) as the final electron acceptor in an electron transport chain.

Fermentation

An anaerobic pathway whose primary role is to re-oxidize NADH back to NAD so glycolysis can continue producing ATP in the absence of an ETC electron acceptor, yielding 2 ATP net per glucose.

Lactic acid fermentation

A fermentation process wherein pyruvate accepts electrons directly from NADH, generating lactate and regenerating NAD.

Alcohol fermentation

A fermentation process wherein pyruvate is decarboxylated to acetaldehyde, which then accepts electrons from NADH to produce ethanol and NAD.

Facultative anaerobes

Organisms that can sustain energy production through aerobic cellular respiration when oxygen is present and switch to fermentation when oxygen is unavailable.

Catabolic pathways

Metabolic pathways that break down complex molecules into simpler compounds, generally releasing free energy.

Anabolic pathways

Metabolic pathways that build complex molecules from simpler precursors, typically requiring the expenditure of ATP.