Metabolism, Oxidative Phosphorylation, and Reactive Oxygen Species

Vocabulary flashcards covering cellular energy metabolism, mitochondrial electron transport, oxidative phosphorylation, reactive oxygen species (ROS), and antioxidant defense systems.

Metabolism

A dynamic, highly coordinated network of interconnected reactions continuously adjusting to nutritional state, hormones, and tissue demands to provide energy, building blocks, and metabolic flexibility for homeostasis.

Catabolism

The metabolic oxidation of complex molecules that yields ATP, NADH, and FADH2, active primarily during fasting to extract energy.

Anabolism

The metabolic synthesis of complex macromolecules (such as glycogen and proteins) that consumes ATP and precursor molecules, active primarily in the fed state.

Adenosine Triphosphate (ATP)

The immediate energy-transfer molecule of the cell, composed of adenine, ribose, and three phosphate groups, which releases energy via hydrolysis ($ATP + H_2O \rightarrow ADP + P_i$).

Thiophorase

An enzyme required for ketone body utilization that is absent in the liver, preventing the liver from consuming the ketone bodies it produces.

GLUT2

The glucose transporter located in the liver through which incoming glucose enters hepatocytes during the fed state.

GLUT4

An insulin-responsive glucose transporter in skeletal muscle and adipose tissue that translocates to the cell membrane upon insulin signaling.

Gluconeogenesis

The liver pathway active during the fasting state that synthesizes new glucose from non-carbohydrate precursors, specifically lactate, glycerol, and amino acids.

Inner Mitochondrial Membrane

A mitochondrial membrane highly impermeable to protons, featuring folds called cristae that increase surface area for the Electron Transport Chain and ATP Synthase.

Intermembrane Space

The mitochondrial compartment between the inner and outer membranes where protons pumped by the electron transport chain accumulate to form stored potential energy.

Complex I

The respiratory chain complex that accepts electrons from NADH and mechanically pumps protons ($H^+$) from the matrix into the intermembrane space.

Complex II

The respiratory chain complex where $FADH_2$ donates electrons; it does not pump protons, resulting in a lower ATP yield (~1.5 ATP vs ~2.5 ATP for NADH).

Complex IV (Cytochrome c Oxidase)

The terminal respiratory chain complex that transfers electrons to molecular oxygen ($O_2$) to form water while pumping protons across the inner mitochondrial membrane.

Proton-Motive Force

The stored potential energy gradient across the inner mitochondrial membrane that is both chemical (concentration difference) and electrical (charge separation).

Cyanide Poisoning

A toxicological inhibition where cyanide directly inhibits Complex IV, halting electron transport and ATP synthesis despite oxygen being available in the blood.

ATP Synthase

An enzyme complex composed of $F_0$ and $F_1$ components that converts the rotational energy from proton flow into chemical synthesis of ATP.

Chemiosmotic Coupling

The process where protons flow down their electrochemical gradient into the matrix through $F_0$, driving rotational changes in $F_1$ to force ADP and inorganic phosphate together into ATP.

Thermogenin (UCP1)

An uncoupling protein in brown adipose tissue that creates an alternative proton pore in the inner mitochondrial membrane, bypassing ATP Synthase to generate heat.

Superoxide ($O_2^{\bullet-}$)

A primary reactive oxygen species and true free radical with an unpaired electron, formed by electron leak at Complex I and Complex III of the respiratory chain.

Hydrogen Peroxide ($H_2O_2$)

A non-radical reactive intermediate generated by superoxide dismutase that can diffuse across membranes and undergo the Fenton reaction to generate hydroxyl radicals.

Hydroxyl Radical ($\bullet OH$)

An extremely reactive and destructive free radical generated via the Fenton reaction ($Fe^{2+}$) that attacks lipids, proteins, and DNA directly at its site of formation.

Respiratory Burst

The deliberate generation of ROS ($O_2^{\bullet-}$ and $H_2O_2$) by NADPH oxidase within phagosomes of immune cells for antimicrobial defense.

Superoxide Dismutase (SOD)

The first-line antioxidant enzyme that converts superoxide ($O_2^{\bullet-}$) into hydrogen peroxide ($H_2O_2$) and molecular oxygen ($O_2$).

Glutathione Redox Cycle

The antioxidant system in which reduced glutathione (GSH) donates reducing equivalents to detoxify peroxides, becoming oxidized glutathione (GSSG), which is then regenerated using NADPH.

Vitamin C (Ascorbic Acid)

A water-soluble non-enzymatic antioxidant that neutralizes reactive molecules in aqueous cytosol and extracellular fluid, and serves as an essential cofactor for proline and lysine hydroxylation in collagen synthesis.

Vitamin E (Tocopherols)

A lipid-soluble non-enzymatic antioxidant incorporated into cellular membranes that protects polyunsaturated fatty acids by interrupting lipid peroxidation chain reactions.

Oxidative Stress

The breakdown of redox homeostasis occurring when reactive oxidizing activity exceeds the cell's capacity to control, neutralize, and repair resulting damage.

Lipid Peroxidation

The oxidative modification of membrane polyunsaturated fatty acids that alters physical membrane properties, impairs protein function, and produces secondary reactive products.

Ischemia-Reperfusion Injury

Biphasic cellular damage where initial blood flow loss (ischemia) impairs electron transport and ATP production, followed by blood restoration (reperfusion) triggering a massive ROS surge from damaged mitochondria.