Cellular Respiration and Energy Metabolism

Comprehensive vocabulary flashcards covering the concepts, stages, biochemical mechanisms, and regulatory enzymes of cellular respiration and energy metabolism.

Metabolism

All of the chemical reactions that occur within living cells to sustain life, including acquiring nutrients and expelling wastes.

Adenosine Triphosphate (ATP)

The primary energy currency of the cell that stores potential energy in unstable high-energy phosphate bonds, which release energy when broken via hydrolysis.

Autotrophs

Organisms that obtain carbon from $\text{CO}_2$ and synthesize glucose using energy from sunlight (photoautotrophs) or inorganic chemical reactions (chemoautotrophs).

Heterotrophs

Organisms that obtain both carbon and energy from reduced organic compounds by consuming living or decaying autotrophs and other organisms.

Aerobic Respiration

A catabolic pathway that completely oxidizes glucose using oxygen ($\text{O}_2$) as the ultimate electron acceptor of the electron transport chain, generating a high yield of ATP.

Anaerobic Respiration

A catabolic pathway that digests glucose using an inorganic molecule other than oxygen as the terminal electron acceptor, resulting in lower ATP production than aerobic respiration.

Fermentation

An anaerobic energy-yielding pathway where electrons are dumped onto pyruvate or its derivatives to regenerate $\text{NAD}^+$, producing end products such as lactic acid or ethanol and $\text{CO}_2$.

Oxidation

The loss of electrons from an atom or molecule during a chemical reaction, typically accompanied by protons ($\text{H}^+$), resulting in lower potential energy.

Reduction

The gain of electrons by an atom or molecule during a chemical reaction, typically accompanied by protons ($\text{H}^+$), resulting in higher potential energy.

Redox Reaction

A coupled chemical reaction where one substance undergoes oxidation by transferring electrons and protons to another substance that undergoes reduction.

$\text{NAD}^+$ (Nicotinamide Adenine Dinucleotide)

An oxidized coenzyme and electron carrier that accepts high-energy electrons and a proton from glucose breakdown, transforming into $\text{NADH}$.

$$\text{NADH}$$

The reduced form of nicotinamide adenine dinucleotide that carries reducing power and transfers electrons to the electron transport chain.

Hydride Ion ($\text{H}^-$)

A chemical unit consisting of one proton and two electrons ($2e^- + \text{H}^+$) transferred onto $\text{NAD}^+$ during oxidation-reduction reactions.

Electron Carrier

A specialized molecule, such as $\text{NAD}^+$ or $\text{FAD}$, capable of accepting and transferring high-energy electrons between metabolic pathways.

Substrate-Level Phosphorylation

A mode of ATP synthesis where an enzyme directly transfers a phosphate group from a phosphorylated organic substrate onto $\text{ADP}$, occurring during glycolysis and the citric acid cycle.

Oxidative Phosphorylation

The synthesis of ATP powered by an electrochemical proton gradient created as electrons move through the electron transport chain to terminal oxygen, activating ATP synthase.

Glycolysis

The initial step of cellular respiration taking place in the cytosol that splits one glucose into two pyruvate molecules, producing a net yield of $2\,\text{ATP}$ and $2\,\text{NADH}$ anaerobically.

Energy Investment Phase

The early phase of glycolysis that consumes $2\,\text{ATP}$ to phosphorylate glucose intermediates before energy can be harvested.

Pyruvate Processing

The second stage of respiration occurring in the mitochondrial matrix where each pyruvate is oxidized to produce one acetyl CoA, one $\text{NADH}$, and one $\text{CO}_2$.

Pyruvate Dehydrogenase

The enzyme complex in the mitochondrial matrix that catalyzes the oxidation and decarboxylation of pyruvate to form acetyl CoA and $\text{NADH}$.

Acetyl CoA (Acetyl Coenzyme A)

A two-carbon acetyl group coupled to coenzyme A, synthesized from pyruvate, which donates its acetyl unit into the citric acid cycle.

Citric Acid Cycle (Krebs Cycle)

An eight-step cyclic pathway in the mitochondrial matrix that oxidizes acetyl CoA to $\text{CO}_2$, yielding $3\,\text{NADH}$, $1\,\text{FADH}_2$, and $1\,\text{ATP}$ per acetyl CoA turn.

$\text{FADH}_2$ (Flavin Adenine Dinucleotide)

A reduced electron carrier generated during the citric acid cycle that transfers electrons directly to complex II of the electron transport chain.

Electron Transport Chain (ETC)

A collection of multi-protein complexes in the inner mitochondrial membrane that sequentially transfer electrons, using the released energy to pump protons into the intermembrane space.

Chemiosmosis

The passive diffusion of protons ($\text{H}^+$) across the inner mitochondrial membrane down their electrochemical gradient into the matrix through ATP synthase to generate ATP.

ATP Synthase

A rotary membrane-bound enzyme complex in the inner mitochondrial membrane that harnesses proton motive force to synthesize ATP from $\text{ADP}$ and inorganic phosphate ($\text{P}_i$).

Feedback Inhibition

A cellular control mechanism in which high concentrations of metabolic products (such as ATP or NADH) bind to and inhibit key regulatory enzymes to downregulate respiration rates.

Phosphofructokinase (PFK)

A key regulatory glycolytic enzyme that phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate; inhibited by high ATP and citrate, and activated by ADP.

Theoretical ATP Yield vs. Actual ATP Yield

The theoretical maximum yield of aerobic respiration is $36\text{--}38\,\text{ATP}$ per glucose, whereas the actual biological yield is approximately $25\text{--}30\,\text{ATP}$ due to proton leaks and active transport costs.