Cellular Respiration: Glycolysis, Pyruvate Processing, and the Krebs Cycle

Vocabulary flashcards covering the key terms, enzymes, substrates, locations, and reactions of cellular respiration, including glycolysis, pyruvate oxidation, and the Krebs cycle.

Glycolysis

A $10$-step catabolic pathway occurring in the cytosol where $1$ molecule of glucose ($6$ carbons) is oxidized to produce $2$ molecules of pyruvate ($3$ carbons), yielding a net of $2\,ATP$ directly and $2\,NADH$ indirectly.

Energy Investment Phase

The beginning stage of glycolysis in which $2\,ATP$ molecules are used ($2\,ATP \rightarrow 2\,ADP + 2\,P$) to break down glucose.

Energy Payoff Phase

The second stage of glycolysis in which $4\,ADP + 4\,P$ are converted into $4\,ATP$, $2\,NAD^+ + 4\,e^- + 4\,H^+$ form $2\,NADH + 2\,H^+$, and $2\text{ Pyruvate} + 2\,H_2O$ are produced.

Cytosol

The fluid portion of the cytoplasm where glycolysis occurs in the cell.

Pyruvate Processing

The second stage of cellular respiration occurring in the mitochondrial matrix, converting $2\text{ Pyruvate}$, $2\,NAD^+$, and $2\,CoA$ into $2\,CO_2$, $2\,NADH$, and $2\text{ Acetyl CoA}$.

Pyruvate Dehydrogenase

An enzyme complex in the mitochondrial matrix that processes pyruvate into Acetyl CoA; it is regulated such that it becomes phosphorylated and changes shape to stop the process when ATP is abundant.

Pyruvate Carrier

A transport membrane protein located in the inner mitochondrial membrane that actively transports pyruvate into the mitochondrial matrix using ATP.

Coenzyme A (CoA)

A compound involved in acetylation that reacts with pyruvate during pyruvate processing, transferring an acetyl group to the substrate to generate Acetyl CoA.

Decarboxylation

A chemical reaction that removes a carboxyl group from a substrate and releases carbon dioxide ($CO_2$).

Citric Acid Cycle (Krebs Cycle)

A cyclical series of $8$ reactions in the mitochondrial matrix outside cristae in which $8$ small carboxylic acids are oxidized rapidly in sequence, yielding $3\,NADH$, $1\,FADH_2$, $2\,CO_2$, and $1\,GTP$ or $ATP$ per Acetyl-CoA molecule.

Hans Krebs

The scientist after whom the Krebs cycle is named, identifying the sequence of rapid oxidations of eight small carboxylic acids.

Citrate Synthase

The enzyme that combines Acetyl-CoA ($2$ carbons) with oxaloacetate ($4$ carbons) to produce citrate, a $6$-carbon compound.

Aconitase

The enzyme that facilitates step 1 of the Krebs cycle by dehydrating citrate and then hydrating the intermediate product to form isocitrate.

Isocitrate Dehydrogenase

The enzyme that facilitates step 2 of the Krebs cycle, catalyzing the oxidation and decarboxylation of isocitrate to produce $CO_2$, the first molecule of $NADH$, and $\alpha\text{-Ketoglutarate}$.

$$\alpha\text{-Ketoglutarate Dehydrogenase}$$

The enzyme that facilitates step 3 of the Krebs cycle, catalyzing the oxidative decarboxylation of $\alpha\text{-ketoglutarate}$ to produce $CO_2$, the second molecule of $NADH$, and Succinyl-CoA.

Succinyl-CoA Synthase

The enzyme facilitating step 4 of the Krebs cycle that carries out substrate-level phosphorylation on Succinyl-CoA to produce succinate along with either GTP (in liver cells) or ATP (in muscle cells).

Succinate Dehydrogenase

The enzyme facilitating step 5 of the Krebs cycle that oxidizes succinate to fumarate while reducing FAD to produce one molecule of $FADH_2$.

Fumarase

The enzyme facilitating step 6 of the Krebs cycle that catalyzes the hydration of fumarate to produce malate.

Malate Dehydrogenase

The enzyme facilitating step 7 of the Krebs cycle that oxidizes malate to regenerate oxaloacetate and produce the third molecule of $NADH$.

Oxaloacetate

The four-carbon starting and ending compound of the Krebs cycle that combines with Acetyl-CoA in the presence of citrate synthase to form citrate.

Nicotinamide Adenine Dinucleotide ($NAD^+$)

A coenzyme electron carrier that accepts electrons to become reduced to $NADH$ during glycolysis, pyruvate processing, and the Krebs cycle.

Mitochondrial Matrix

The internal fluid space of the mitochondrion where pyruvate processing and all $8$ reactions of the Krebs cycle occur.

Cristae

The inner folds of the mitochondrial membrane across which electron transport and chemiosmosis take place.

Overall Glucose Oxidation Equation

The net chemical formula representing respiration stages through the Krebs cycle: $C_6H_{12}O_6 + 10\,NAD^+ + 2\,FADH + 4\,ADP + 4\,P_i = 6\,CO_2 + 10\,NADH + 2\,FADH_2 + 4\,ATP$.