Catabolic Pathways and Cellular Respiration

Comprehensive vocabulary flashcards covering the biochemical pathways of catabolism, including glycolysis, pyruvate oxidation, the citric acid cycle, oxidative phosphorylation (ETC and chemiosmosis), and fermentation pathways.

Catabolic Pathways

Metabolic pathways that release energy by breaking down complex molecules into simpler compounds.

Aerobic Respiration

A catabolic pathway in which oxygen ($O_2$) is consumed as a reactant along with organic fuel to produce energy, carbon dioxide, and water.

Oxidation

The loss of electrons from a substance during a chemical redox reaction.

Reduction

The gain of electrons by a substance during a chemical redox reaction.

Reducing Agent

The electron donor in a redox reaction that gives up electrons and becomes oxidized.

Oxidizing Agent

The electron acceptor in a redox reaction that receives electrons and becomes reduced.

$$NAD^+$$

Nicotinamide adenine dinucleotide; an organic coenzyme and oxidizing agent that functions as an electron shuttle during the breakdown of glucose, cycling between its oxidized ($NAD^+$) and reduced ($NADH$) forms.

Substrate-Level Phosphorylation

The mode of $ATP$ synthesis in which an enzyme directly catalyzes the transfer of a phosphate group from an organic substrate intermediate to $ADP$, accounting for approximately $10\%$ of $ATP$ produced during cellular respiration.

Oxidative Phosphorylation

The production of $ATP$ powered by the redox reactions of the electron transport chain, which creates a proton ($H^+$) gradient across the inner mitochondrial membrane to drive $ATP$ synthase; accounts for approximately $90\%$ of $ATP$ generated.

Glycolysis

A ten-step metabolic pathway in the cytosol that splits one six-carbon glucose molecule into two three-carbon molecules of pyruvate, yielding a net of $2\,ATP$ and $2\,NADH$ without consuming oxygen or releasing carbon as $CO_2$.

Hexokinase

The enzyme in step 1 of glycolysis that transfers a phosphate group from $ATP$ to glucose to produce glucose 6-phosphate, chemically activating the sugar and trapping it inside the cell.

Phosphofructokinase

The enzyme in step 3 of glycolysis that transfers a phosphate group from a second $ATP$ molecule to fructose 6-phosphate, producing fructose 1,6-bisphosphate; serves as a major key regulatory step of glycolysis.

Aldolase

The enzyme in step 4 of glycolysis that cleaves the six-carbon sugar fructose 1,6-bisphosphate into two different three-carbon isomers: dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P).

Enolase

The glycolytic enzyme in step 9 that extracts a water molecule from 2-phosphoglycerate to introduce a double bond, forming the high-energy intermediate phosphoenolpyruvate (PEP).

Pyruvate Kinase

The enzyme in step 10 of glycolysis that catalyzes the substrate-level phosphorylation of $ADP$ to $ATP$ by transferring the phosphate group from phosphoenolpyruvate (PEP), producing pyruvate.

Pyruvate Dehydrogenase

The multienzyme complex inside the mitochondrion that converts pyruvate into acetyl CoA by releasing a molecule of $CO_2$, oxidizing the remaining two-carbon fragment to form $NADH$, and attaching Coenzyme A.

Acetyl CoA

The two-carbon entry compound for the citric acid cycle, formed when an acetyl group derived from pyruvate oxidation attaches to the sulfur atom of coenzyme A.

Citric Acid Cycle

An eight-step metabolic cycle in the mitochondrial matrix (also called the Krebs or TCA cycle) that fully oxidizes acetyl CoA to $CO_2$, generating $1\,ATP$ (or GTP), $3\,NADH$, and $1\,FADH_2$ per acetyl group.

Oxaloacetate

The four-carbon dicarboxylic acid that condenses with the two-carbon acetyl group of acetyl CoA in step 1 of the citric acid cycle to form citrate, and is regenerated in step 8.

$$FAD$$

Flavin adenine dinucleotide; an electron carrier derived from riboflavin that accepts two electrons and two protons during step 6 of the citric acid cycle to become reduced to $FADH_2$.

Electron Transport Chain (ETC)

A series of multiprotein complexes (I through IV) and electron carriers embedded in the inner mitochondrial membrane that shuttle electrons from $NADH$ and $FADH_2$ down an exergonic pathway to molecular oxygen.

Ubiquinone (Q)

A small, non-protein hydrophobic electron carrier (also called coenzyme Q) that is mobile within the inner mitochondrial membrane, shuttling electrons from Complexes I and II to Complex III.

Cytochromes

Iron-containing proteins with a heme prosthetic group that accept and donate electrons sequentially in Complexes III and IV of the electron transport chain before transferring them to oxygen.

ATP Synthase

A membrane-spanning protein complex consisting of a stator, rotor, internal rod, and catalytic knob that uses the flow of protons down their electrochemical gradient to synthesize $ATP$ from $ADP$ and inorganic phosphate ($P_i$).

Chemiosmosis

An energy-coupling mechanism that uses energy stored in the form of a transmembrane hydrogen ion ($H^+$) gradient to drive cellular work, such as the synthesis of $ATP$.

Proton-Motive Force

The potential energy stored in the electrochemical proton gradient generated across the inner mitochondrial membrane when Complexes I, III, and IV pump $H^+$ ions from the matrix into the intermembrane space.

Anaerobic Respiration

A form of cellular respiration that uses an electron transport chain but terminates at an electronegative inorganic final electron acceptor other than oxygen, such as sulfate ($SO_4^{2-}$).

Fermentation

An anaerobic catabolic process that enables continuous generation of $ATP$ via substrate-level phosphorylation in glycolysis by transferring electrons from $NADH$ to an organic molecule to regenerate $NAD^+$ without an electron transport chain.

Alcohol Fermentation

A two-step fermentation pathway carried out by yeast and certain bacteria in which pyruvate is converted to acetaldehyde with the release of $CO_2$, and acetaldehyde is then reduced by $NADH$ to form ethanol.

Lactic Acid Fermentation

A fermentation pathway carried out by certain fungi, bacteria, and human muscle cells under oxygen debt, in which pyruvate is reduced directly by $NADH$ to form lactate with no production of $CO_2$.

Obligate Anaerobes

Organisms that survive exclusively through fermentation or anaerobic respiration and cannot survive in the presence of molecular oxygen.

Facultative Anaerobes

Organisms that can generate sufficient $ATP$ to survive using either aerobic cellular respiration (in the presence of $O_2$) or fermentation (in the absence of $O_2$).

Beta Oxidation

A metabolic sequence that breaks down fatty acids into two-carbon fragments that enter the citric acid cycle as acetyl CoA.