Chapter 9: How Cells Harvest Energy

Vocabulary flashcards covering the major stages, regulatory mechanisms, reactions, and pathways of cellular respiration and fermentation.

Aerobic Respiration

An efficient catabolic pathway requiring oxygen ($O_2$) in which organic nutrients (typically glucose) are catabolized into carbon dioxide ($CO_2$) and water ($H_2O$), yielding energy stored in $36\text{--}38\text{ ATP}$ molecules.

Anaerobic Respiration

A catabolic process similar to aerobic respiration that operates without oxygen ($O_2$) and utilizes an electron transport chain with alternative terminal electron acceptors, such as $NO_3^-$, $SO_4^{2-}$, or $CO_2$.

Fermentation

An inefficient anaerobic pathway that generates a net yield of $2\text{ ATP}$ per glucose through glycolysis alone without an electron transport chain, using organic molecules to oxidize NADH back into $NAD^+$.

Substrate-Level Phosphorylation

A coupled reaction in which an enzyme directly transfers a phosphate group from an organic substrate to ADP or GDP, generating ATP or GTP.

Dehydrogenation Reactions

Redox reactions in which hydrogen atoms (electrons and protons) are transferred from substrate molecules to electron carriers such as $NAD^+$ or FAD.

Decarboxylation Reactions

Reactions in which carboxyl groups are removed from metabolic intermediates and released in the form of carbon dioxide ($CO_2$).

Glycolysis

A ten-step anaerobic metabolic sequence occurring in the cytosol that splits one molecule of glucose into two molecules of pyruvate, generating a net of $2\text{ ATP}$ and $2\text{ NADH}$.

Preparatory Phase

The initial energy investment phase of glycolysis where $2\text{ ATP}$ molecules are consumed to phosphorylate glucose and fructose intermediates, ultimately producing two molecules of glyceraldehyde 3-phosphate (G3P).

Payoff Phase

The second phase of glycolysis where two molecules of G3P are oxidized into two molecules of pyruvate, producing a gross total of $4\text{ ATP}$ (net $2\text{ ATP}$) and $2\text{ NADH} + 2\text{ H}^+$.

Pyruvate Dehydrogenase Complex (PDC)

A multienzyme complex in the mitochondrial matrix (or cytosol of prokaryotes) that catalyzes the oxidative decarboxylation of pyruvate to produce acetyl-CoA, $CO_2$, and NADH.

Citric Acid Cycle

An eight-step cyclical pathway in the mitochondrial matrix where acetyl-CoA condenses with oxaloacetate to form citrate, decomposing it back to oxaloacetate while producing $2\text{ CO}_2$, $3\text{ NADH}$, $1\text{ FADH}_2$, and $1\text{ ATP}$ (or GTP) per acetyl group.

Oxaloacetate

A four-carbon compound that joins with the acetyl group of acetyl-CoA in Step 1 of the citric acid cycle to form citrate, and is regenerated by Step 8 to sustain the cycle.

Oxidative Phosphorylation

The fourth stage of cellular respiration comprising the electron transport chain and chemiosmosis, in which electrons from NADH and $FADH_2$ reduce oxygen to water while driving ATP synthesis.

Chemiosmosis

An energy-coupling mechanism that couples the potential energy stored in an electrochemical proton gradient across a membrane to drive the phosphorylation of ADP by ATP synthase.

ATP Synthase

A molecular machine consisting of a membrane-spanning channel ($F_o$ subunit) that rotates as protons move through it, and a catalytic knob ($F_1$ subunit) that uses rotational kinetic energy to synthesize ATP from ADP and $P_i$.

Phosphofructokinase (PFK)

An essential allosteric enzyme in early glycolysis that transfers a phosphate to fructose 6-phosphate; it is allosterically inhibited by ATP and citrate and allosterically stimulated by AMP.

Deamination

The enzymatic removal of the amino group ($-NH_2$) from an amino acid during protein catabolism, allowing the carbon skeleton to enter cellular respiration and the amino group to form urea.

Beta-Oxidation ($\beta$-Oxidation)

The catabolic process by which fatty acid chains are oxidized and broken into two-carbon units that combine with coenzyme A to form acetyl-CoA for entry into the citric acid cycle.

Alcohol Fermentation

A fermentation pathway wherein pyruvate is converted to acetaldehyde and $CO_2$, and acetaldehyde is reduced by NADH to ethanol in order to regenerate $NAD^+$.

Lactic Acid Fermentation

A fermentation pathway wherein pyruvate is directly reduced by NADH to form lactate, regenerating $NAD^+$ without producing $CO_2$.

Nicotinamide Adenine Dinucleotide ($NAD^+$)

A vital dinucleotide electron carrier that accepts two electrons and one proton to form NADH, transporting high-potential-energy electrons to the electron transport chain.

Coenzyme Q (Ubiquinone)

A lipid-soluble mobile electron carrier in the inner mitochondrial membrane that accepts electrons from Complexes I and II and transfers them to Complex III.

Cytochrome c

A mobile electron carrier protein located in the intermembrane space that transports electrons from Complex III to Complex IV of the electron transport chain.