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

Vocabulary practice flashcards covering cellular respiration, redox chemistry, glycolysis phases, pyruvate oxidation, fermentation pathways, the citric acid cycle, and oxidative phosphorylation with electron transport.

Oxidation

The removal or loss of electrons from a molecule in a chemical reaction.

Reduction

The addition or gain of electrons to a molecule in a chemical reaction.

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Nicotinamide Adenine Dinucleotide ($NAD^+$ / $NADH$)

An organic coenzyme that functions as an electron shuttle to transfer high-energy electrons between metabolic reactions, existing in an oxidized state ($NAD^+$) and a reduced state ($NADH$).

Catabolic Reactions

Metabolic pathways that break down complex organic molecules into simpler compounds, releasing stored potential chemical energy and high-energy electrons.

Cellular Respiration

The controlled catabolic process by which living cells break down organic fuel to harvest energy in the form of ATP, summarized by the equation: $C_6H_{12}O_6 + 6 O_2 \rightarrow 6 CO_2 + 6 H_2O + \text{Energy}$, with $\Delta G = -685\,\text{kcal/mol}$.

Glycolysis

A 10-step metabolic pathway located in the cytosol that oxidizes one 6-carbon glucose molecule into two 3-carbon pyruvate molecules, yielding a net of $2\,\text{ATP}$ and $2\,\text{NADH}$.

Energy Investment Phase

The first phase of glycolysis during which the cell consumes $2\,\text{ATP}$ molecules to phosphorylate glucose compounds prior to cleavage.

Cleavage Phase

The second phase of glycolysis in which an activated 6-carbon diphosphate intermediate is enzymatically cleaved into two 3-carbon sugar molecules.

Payoff Phase

The third phase of glycolysis that runs twice per glucose molecule, producing a total of $4\,\text{ATP}$, $2\,\text{NADH}$, and $2\,\text{pyruvate}$ molecules (yielding $2\,\text{ATP}$ net).

Substrate-Level Phosphorylation

The formation of ATP from ADP and inorganic phosphate via the direct enzymatic transfer of a phosphate group from a phosphorylated metabolic intermediate (such as phosphoenolpyruvate).

Phosphofructokinase (PFK)

The key regulatory enzyme of glycolysis that catalyzes the conversion of fructose-6-phosphate into fructose-1,6-bisphosphate using ATP.

$$H^+/\text{Pyruvate Symporter}$$

A secondary active transport protein in the inner mitochondrial membrane that imports pyruvate into the mitochondrial matrix coupled to the inward flow of a proton ($H^+$).

Pyruvate Dehydrogenase

The multienzyme complex situated in the mitochondrial matrix that catalyzes the oxidative decarboxylation of pyruvate into a 2-carbon acetyl group and $CO_2$, transferring electrons to $NAD^+$ to generate $NADH$.

Acetyl-CoA

The activated carrier molecule consisting of a 2-carbon acetyl group attached via a high-energy bond to coenzyme A, which delivers acetyl carbons into the citric acid cycle.

Oxaloacetate

A 4-carbon dicarboxylic acid intermediate in the citric acid cycle that combines with the 2-carbon acetyl group of acetyl-CoA to regenerate citrate.

Citrate

The 6-carbon tricarboxylic acid intermediate produced in the first step of the citric acid cycle when oxaloacetate condensation occurs with an incoming acetyl group.

Citric Acid Cycle (TCA Cycle)

A cyclic pathway in the mitochondrial matrix that completes the oxidation of acetyl-derived carbons to $CO_2$, generating $3\,\text{NADH}$, $1\,\text{FADH}_2$, and $1\,\text{ATP}$ per cycle turn ($6\,\text{NADH}$, $2\,\text{FADH}_2$, and $2\,\text{ATP}$ per glucose).

Fermentation

An anaerobic metabolic process that breaks down organic nutrients without an electron transport chain, reducing pyruvate or its derivatives to regenerate the $NAD^+$ required to keep glycolysis operating.

Lactic Acid Fermentation

An anaerobic pathway occurring in human muscle cells where pyruvate accepts electrons directly from $NADH$ without forming an intermediate, yielding lactate and regenerated $NAD^+$.

Alcohol Fermentation

A two-step anaerobic pathway occurring in yeast in which pyruvate is decarboxylated to acetaldehyde with the release of $CO_2$, and acetaldehyde is subsequently reduced by $NADH$ to ethanol, regenerating $NAD^+$.

Acetaldehyde

The 2-carbon organic intermediate produced by the decarboxylation of pyruvate in alcohol fermentation that acts as the terminal electron acceptor for $NADH$.

Oxidative Phosphorylation

The metabolic process in which ATP is synthesized from ADP and $P_i$ by ATP synthase using energy derived from the oxidation of $NADH$ and $FADH_2$ via an electron transport chain.

Electron Transport Chain (ETC)

A sequence of multiprotein complexes and mobile electron shuttles embedded within the inner mitochondrial membrane that transfer electrons from reduced carriers to $O_2$ while translocating protons ($H^+$) into the intermembrane space.

Complex I (NADH Dehydrogenase)

The first multiprotein complex of the mitochondrial ETC that oxidizes $NADH$ to $NAD^+$, transfers electrons to ubiquinone, and pumps $4\,H^+$ into the intermembrane space.

Complex II (Succinate Reductase)

An inner membrane complex participating in both the citric acid cycle and the ETC that transfers electrons from succinate to ubiquinone via a covalently attached FAD group, pumping zero protons across the membrane.

Flavin Adenine Dinucleotide (FAD)

A prosthetic redox group covalently attached to Complex II that accepts two electrons and two protons to form $FADH_2$ during the oxidation of succinate to fumarate.

Ubiquinone (Coenzyme Q)

A small, hydrophobic non-protein electron carrier localized inside the inner mitochondrial lipid bilayer that collects electrons from Complex I and Complex II and shuttles them to Complex III.

Complex III (Cytochrome $b\text{-}c_1$)

A respiratory chain complex that accepts electrons from reduced ubiquinone ($QH_2$), pumps protons ($H^+$) into the intermembrane space, and transfers single electrons to cytochrome $c$.

Cytochrome

An electron-transfer heme protein containing an iron-porphyrin prosthetic group that cycles between reduced ($Fe^{2+}$) and oxidized ($Fe^{3+}$) states.

Cytochrome c

A mobile peripheral membrane protein located on the intermembrane space side of the inner mitochondrial membrane that carries single electrons between Complex III and Complex IV.

Complex IV (Cytochrome Oxidase)

The terminal enzyme complex of the mitochondrial ETC that accepts electrons from cytochrome $c$, pumps protons ($H^+$), and reduces molecular oxygen ($O_2$) to form water ($H_2O$).

Chemiosmosis

The process by which energy stored in a transmembrane proton electrochemical gradient (proton motive force) is used to perform cellular work, specifically ATP synthesis via ATP synthase.

ATP Synthase

A rotary molecular motor spanning the inner mitochondrial membrane that uses the spontaneous flow of protons ($H^+$) down their electrochemical gradient into the matrix to drive the phosphorylation of ADP to ATP.

$\beta$ Subunit Conformations of ATP Synthase

The three successive conformational states of each catalytic $\beta$ subunit: Conformation 1 (binds ADP and $P_i$ with good affinity), Conformation 2 (binds ADP and $P_i$ tightly to synthesize ATP), and Conformation 3 (binds ATP weakly to release it).

$\gamma$ Subunit of ATP Synthase

The central shaft subunit that physically rotates within the catalytic $\alpha_3\beta_3$ ring when protons flow through the membrane base, sequentially inducing conformational changes in the $\beta$ subunits.

Anaerobic Respiration

A cellular respiration mechanism utilizing an electron transport chain and chemiosmosis wherein an inorganic compound other than molecular oxygen ($O_2$), such as nitrate ($NO_3^-$), functions as the terminal electron acceptor.