Cellular Respiration and Energy Harvesting

Vocabulary flashcards covering the key terms, enzymes, pathways, and mechanisms of cellular respiration, including glycolysis, pyruvate oxidation, the citric acid cycle, electron transport, oxidative phosphorylation, and fermentation.

Substrate-level phosphorylation

A mechanism of generating $\text{ATP}$ during glycolysis and the citric acid cycle via the direct enzymatic transfer of a phosphate group from a phosphorylated organic substrate to $\text{ADP}$.

Oxidative phosphorylation

The pathway occurring in the inner mitochondrial membrane that produces the largest amount of $\text{ATP}$, where $\text{ATP}$ synthase phosphorylates $\text{ADP}$ to $\text{ATP}$ using energy derived from a proton ($\text{H}^+$) gradient.

ATP synthase

A multi-part enzyme complex consisting of a rotor, stalk, and catalytic head embedded in the inner mitochondrial membrane that utilizes the flow of protons to synthesize $\text{ATP}$ from $\text{ADP}$ and $\text{P}_i$.

Chemiosmosis

The movement of protons diffusing across a membrane through $\text{ATP}$ synthase down their electrochemical gradient to generate chemical energy in the form of $\text{ATP}$.

$\text{H}^+$ pumps

Active transport protein complexes in the electron transport chain that utilize energy from electrons to pump protons against their gradient from the mitochondrial matrix (lower $[\text{H}^+]$) to the intermembrane space (higher $[\text{H}^+]$).

Proton gradient

An uneven distribution of protons ($\text{H}^+$) established across the inner mitochondrial membrane of eukaryotes or the plasma membrane of prokaryotes.

Electron Transport Chain (ETC)

A series of inner mitochondrial membrane protein complexes—including $\text{NADH}$ dehydrogenase, the $bc_1$ complex, and the cytochrome oxidase complex—that transfer electrons and pump protons across the membrane.

Mobile electron carriers

Molecules including ubiquinone ($\text{Q}$) and cytochrome c ($\text{C}$) that shuttle electrons between the major protein complexes of the electron transport chain.

Oxygen ($\text{O}_2$)

The final electron acceptor in aerobic cellular respiration, which combines with electrons and protons to form water ($\text{H}_2\text{O}$).

$$\text{NAD}^+$$

A coenzyme and electron shuttle that binds with an enzyme and substrate, accepting $2$ electrons and $1$ proton ($\text{H}^+$) to form $\text{NADH}$, while donating a second proton to solution.

$$\text{FAD}$$

A coenzyme and electron carrier that accepts $2\text{ H}$ atoms ($2\text{ H}^+$ and $2e^-$) to become reduced to $\text{FADH}_2$.

Glycolysis

A metabolic pathway located in the cytoplasm that breaks down $1$ glucose ($6$ carbons) into $2$ pyruvate ($3$ carbons), yielding a net of $2\text{ ATP}$ via substrate-level phosphorylation and $2\text{ NADH}$, operating in both the presence and absence of oxygen.

Hexokinase

The enzyme catalyzing the initial step of glycolysis by transferring a phosphate group from $\text{ATP}$ to carbon-$6$ of glucose to form glucose $6$-phosphate.

Phosphoglucose isomerase

The glycolytic enzyme that rearranges the atoms of glucose $6$-phosphate to convert it into fructose $6$-phosphate.

Phosphofructokinase

The key regulatory enzyme of glycolysis that transfers a phosphate group from $\text{ATP}$ to carbon-$1$ of fructose $6$-phosphate; it is allosterically inhibited by $\text{ATP}$ and citrate, and activated by $\text{ADP}$.

Glyceraldehyde 3-phosphate dehydrogenase

The glycolytic enzyme that removes $2\text{ H}$ atoms ($2\text{H}^+$ and $2e^-$) from $\text{G3P}$, transferring $1\text{H}^+$ and $2e^-$ to $\text{NAD}^+$ to form $\text{NADH}$, producing $1,3$-bisphosphoglycerate ($\text{BPG}$).

Phosphoglycerate kinase

An enzyme in glycolysis that transfers a phosphate group from $1,3$-bisphosphoglycerate ($\text{BPG}$) to $\text{ADP}$, yielding $\text{ATP}$ and $3$-phosphoglycerate ($\text{3PG}$).

Enolase

The glycolytic enzyme that catalyzes the dehydration reaction of $2$-phosphoglycerate ($\text{2PG}$), releasing $\text{H}_2\text{O}$ to produce phosphoenolpyruvate ($\text{PEP}$).

Pyruvate kinase

The enzyme catalyzing the final step of glycolysis, transferring a phosphate group from phosphoenolpyruvate ($\text{PEP}$) to $\text{ADP}$ to yield $\text{ATP}$ and pyruvate.

Lactic acid fermentation

An anaerobic pathway occurring in animal skeletal muscle cells where electrons from $\text{NADH}$ are transferred directly to pyruvate to regenerate $\text{NAD}^+$, producing lactate ($2\text{ Pyruvate} + 2\text{ NADH} \rightarrow 2\text{ Lactate} + 2\text{ NAD}^+$).

Ethanol fermentation

An anaerobic pathway in yeast where $2$ pyruvate are converted to $2$ acetaldehyde and $2\text{ CO}_2$, followed by the transfer of electrons from $\text{NADH}$ to acetaldehyde to regenerate $\text{NAD}^+$ and form ethanol.

Pyruvate oxidation

The conversion occurring in the mitochondrial matrix where $2$ pyruvate ($3$ carbons each) are decarboxylated and oxidized to $2$ acetyl-CoA ($2$ carbons each), producing $2\text{ CO}_2$ and reducing $2\text{ NAD}^+$ to $2\text{ NADH}$.

Pyruvate dehydrogenase

The multienzyme complex located in the mitochondrial matrix responsible for decarboxylating and oxidizing pyruvate into acetyl-CoA, which is inhibited by high levels of $\text{NADH}$.

Citric Acid Cycle (Krebs cycle)

A cyclical series of reactions in the mitochondrial matrix where each acetyl-CoA combines with $4\text{-C}$ oxaloacetate to form citrate; each turn produces $2\text{ CO}_2$, $3\text{ NADH}$, $1\text{ FADH}_2$, and $1\text{ ATP}$ via substrate-level phosphorylation.

Theoretical net ATP yield

The total maximum number of $\text{ATP}$ generated per glucose molecule from complete aerobic respiration, equaling $32\text{ ATP}$ overall ($30\text{ ATP}$ net in eukaryotes).

Deamination

The biochemical removal of an amino group from an amino acid, allowing its remaining carbon skeleton to enter glycolysis or the citric acid cycle while releasing ammonia ($\text{NH}_3$).

$$\beta\text{-oxidation}$$

The catabolic pathway that breaks down fatty acids into two-carbon acetyl-CoA units, which subsequently feed directly into the citric acid cycle.