Bioenergetics: Energy, Enzymes, Cellular Respiration, and Photosynthesis

Comprehensive vocabulary flashcards covering energy transformations, enzyme kinetics, aerobic respiration, fermentation, and photosynthesis based on Chapters 6, 7, and 8 lecture materials.

Metabolism

The complete set of biochemical reactions in an organism that builds or breaks down molecules to transform energy.

Catabolic Pathway

A metabolic pathway that degrades complex macromolecules into smaller subunits, releasing energy ($\Delta G < 0$).

Anabolic Pathway

A metabolic pathway that synthesizes complex macromolecules from smaller precursor subunits, requiring an input of energy ($\Delta G > 0$).

Potential Energy

The stored energy an object or molecule possesses due to its position, structure, or spatial arrangement, such as chemical bonds or an object atop stairs.

Kinetic Energy

The energy associated with objects or molecules in motion.

Gibbs Free Energy ($G$)

The portion of a biological system's energy available to perform work, defined by the thermodynamic equation $G = H - TS$.

Exergonic Reaction

A spontaneous chemical reaction that releases energy into its surroundings, characterized by products having less free energy than reactants ($\Delta G < 0$).

Endergonic Reaction

A non-spontaneous chemical reaction that requires an input of free energy, resulting in products that possess higher free energy than the reactants ($\Delta G > 0$).

Adenosine Triphosphate (ATP)

The primary energy currency of the cell, composed of adenine, ribose, and three negatively charged, unstable phosphate groups that release energy upon hydrolysis.

Energetic Coupling

The thermodynamic process in which an exergonic reaction (such as ATP hydrolysis, $\Delta G = -7.3\,\text{kcal/mol}$) drives an endergonic reaction ($\Delta G > 0$) to yield an overall negative net $\Delta G$.

Activation Energy ($E_A$)

The initial amount of energy required for chemical reactants to reach the transition state; enzymes accelerate reactions by lowering this energy threshold without changing $\Delta G$.

Active Site

The catalytic pocket or cleft of an enzyme formed by specific amino acids brought together by tertiary protein folding where substrates bind.

Coenzyme

A non-protein organic helper molecule (frequently derived from water-soluble vitamins like niacin, thiamine, or riboflavin) required by an enzyme to perform catalytic activity.

Denaturation

The loss of an enzyme's native tertiary and secondary structure caused by extreme environmental factors such as high temperature or non-optimal pH, abolishing its catalytic activity.

Competitive Inhibitor

A regulatory molecule or toxin (e.g., sarin or DFP targeting acetylcholinesterase) that reduces an enzyme's rate by binding directly to the active site in competition with the substrate.

Allosteric Inhibitor

A non-competitive inhibitor that binds to a distinct regulatory (allosteric) site on an enzyme rather than the active site, inducing a conformational shape change that decreases substrate turnover.

Feedback Inhibition

A cellular control mechanism in which the accumulated end product of a multi-step biosynthetic pathway allosterically inhibits an enzyme that catalyzes an earlier step.

Oxidation

The partial or complete loss of one or more electrons from a substance during a chemical redox reaction.

Reduction

The partial or complete gain of one or more electrons by a substance during a chemical redox reaction.

Aerobic Cellular Respiration

The catabolic pathway through which organic fuels like glucose are oxidized to $\text{CO}_2$ and $\text{O}_2$ is reduced to $\text{H}_2\text{O}$, producing ATP across four stages: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

Glycolysis

A 10-step metabolic pathway located in the cytosol that splits one molecule of glucose ($6\text{C}$) into two molecules of pyruvate ($3\text{C}$), producing a net yield of $2\,\text{ATP}$ and $2\,\text{NADH}$.

Substrate-Level Phosphorylation

The direct enzymatic transfer of a phosphate group from a phosphorylated metabolic intermediate directly to ADP to form ATP.

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

A central coenzyme electron carrier in cellular respiration that accepts two electrons and one proton ($2\,e^- + \text{H}^+$) to convert from oxidized $\text{NAD}^+$ to reduced $\text{NADH}$.

Pyruvate Oxidation

The transition stage of aerobic respiration located in the mitochondrial matrix that decarboxylates pyruvate into acetyl-CoA, producing $\text{CO}_2$ and $\text{NADH}$.

Citric Acid Cycle

An 8-step cyclical pathway in the mitochondrial matrix that oxidizes acetyl-CoA to $\text{CO}_2$, producing $3\,\text{NADH}$, $1\,\text{FADH}_2$, and $1\,\text{ATP}$ (via substrate-level phosphorylation) per acetyl unit.

Electron Transport Chain (ETC)

A series of four multi-protein complexes and mobile carriers located within the inner mitochondrial membrane that sequentially transfer electrons to oxygen, translocating protons ($\text{H}^+$) into the intermembrane space.

Chemiosmosis

An energy-coupling mechanism where energy stored in a transmembrane electrochemical proton gradient ($\text{H}^+$) is harnessed by ATP synthase to drive ATP phosphorylation.

ATP Synthase

A membrane-bound catalytic rotary motor enzyme that utilizes proton flow down an electrochemical gradient across the membrane to synthesize ATP from ADP and inorganic phosphate.

Lactic Acid Fermentation

An anaerobic metabolic process occurring in various bacteria, fungi, and animal muscle cells in which pyruvate is reduced directly by $\text{NADH}$ to form lactate, regenerating $\text{NAD}^+$.

Ethanol Fermentation

An anaerobic pathway occurring in yeast and plants in which pyruvate is decarboxylated to acetaldehyde, which is then reduced by $\text{NADH}$ to ethanol, regenerating $\text{NAD}^+$ and releasing $\text{CO}_2$.

Phosphofructokinase-1 (PFK-1)

The key allosteric regulatory enzyme of glycolysis that catalyzes the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate; inhibited by high ATP and citrate, and activated by AMP and ADP.

Photoautotroph

An organism that harnesses light energy to synthesize organic carbon molecules from inorganic carbon dioxide ($\text{CO}_2$).

Stomata

Microscopic pores bordered by guard cells in the epidermis of plant leaves that facilitate gas exchange by allowing $\text{CO}_2$ to enter and $\text{O}_2$ and water vapor to exit.

Action Spectrum

A biological graph depicting the relative effectiveness of different wavelengths of light in driving a light-dependent physiological process, such as oxygen release during photosynthesis.

Photosystem II (PSII)

The multiprotein light-harvesting complex in the thylakoid membrane containing a $P_{680}$ reaction center that oxidizes and splits water molecules into protons ($\text{H}^+$), electrons ($e^-$), and oxygen ($\text{O}_2$).

Linear Electron Flow

The primary photosynthetic pathway in which electrons flow from $\text{H}_2\text{O}$ through PSII and PSI to $\text{NADP}^+$, producing both ATP and NADPH while releasing $\text{O}_2$.

Cyclic Electron Flow

A light-driven electron transport pathway involving only Photosystem I ($P_{700}$) that cycles electrons back to the cytochrome complex to pump protons and produce ATP without generating NADPH or releasing oxygen.

Calvin Cycle

The light-independent metabolic cycle in the chloroplast stroma comprising carbon fixation, reduction, and RuBP regeneration phases that converts $\text{CO}_2$ into glyceraldehyde 3-phosphate (G3P).

RuBisCO

Ribulose-1,5-bisphosphate carboxylase/oxygenase; the stroma enzyme that catalyzes the primary carbon fixation reaction by attaching $\text{CO}_2$ to ribulose 1,5-bisphosphate (RuBP).