Cellular Energetics: Metabolism, Cellular Respiration, and Photosynthesis

Vocabulary practice flashcards covering thermodynamics, ATP coupling, cellular respiration, fermentation, and photosynthesis based on Units 3.1, 3.2, and 3.3.

Metabolism

The totality of an organism's chemical reactions, organized into hundreds of intersecting pathways.

Catabolic Pathways (Catabolism)

Metabolic pathways that release energy by breaking down complex molecules into simpler compounds, such as the breakdown of glucose in cellular respiration.

Anabolic Pathways (Anabolism)

Metabolic pathways that consume energy to build complex molecules from simpler ones, such as assembling amino acids into polypeptides.

First Law of Thermodynamics

The physical law stating that energy can be transferred and transformed, but it cannot be created or destroyed.

Second Law of Thermodynamics

The physical law stating that every energy transfer or transformation increases the entropy of the universe, with some energy inevitably lost as heat.

Entropy

A quantitative measure of disorder or randomness within a system or the universe.

Free Energy ($G$)

The portion of a system's energy that can perform work, serving also as a measure of the system's instability.

Exergonic Reaction

A spontaneous chemical reaction that releases free energy into its surroundings, characterized by a negative change in free energy ($\Delta G < 0$).

Endergonic Reaction

A non-spontaneous chemical reaction that absorbs free energy from its surroundings, characterized by a positive change in free energy ($\Delta G > 0$).

Activation Energy ($E_A$)

The initial investment of energy required to start a chemical reaction by breaking and forming bonds, typically supplied as heat from the surroundings.

ATP Coupling

The cellular strategy of pairing an exergonic reaction, such as ATP hydrolysis ($\Delta G = -7.3\,kcal/mol$), to power an endergonic reaction so the overall combined process has a negative $\Delta G$.

Endotherm

An organism that maintains a relatively constant body temperature independent of environmental temperatures by altering its metabolic rate to generate internal heat.

Ectotherm

An organism whose body temperature fluctuates with the ambient environmental temperature, relying predominantly on behavioral adaptations rather than metabolic heat for thermoregulation.

Torpor

A physiological state of decreased metabolic rate that allows an organism to conserve energy and avoid unfavorable environmental conditions, exemplified by hibernation and estivation.

Oxidation

The loss of electrons from a substance during a redox reaction, which corresponds to a decrease in hydrogen atoms or an increase in the proportion of oxygen.

Reduction

The addition of electrons to a substance during a redox reaction, which corresponds to an increase in hydrogen atoms or a decrease in the proportion of oxygen.

Coenzymes (NAD+ and FAD)

Nonprotein helper molecules that function as oxidizing agents during cellular respiration by capturing pairs of high-energy electrons and hydrogen atoms from metabolic intermediates.

Glycolysis

A series of $10$ enzyme-catalyzed reactions in the cytoplasm that breaks one molecule of glucose into two molecules of pyruvate, yielding a net of $2\,ATP$ and $2\,NADH$ without requiring $O_2$.

Substrate-Level Phosphorylation

The direct synthesis of ATP from ADP by an enzyme transferring a phosphate group from an organic substrate molecule.

Pyruvate Oxidation

The step in aerobic cellular respiration where two pyruvate molecules enter the mitochondrial matrix and are oxidized to form two Acetyl-CoA molecules, $2\,CO_2$, and $2\,NADH$.

Citric Acid Cycle (Krebs Cycle)

A series of $8$ reactions in the mitochondrial matrix starting when Acetyl-CoA combines with oxaloacetate; two turns per glucose molecule yield $4\,CO_2$, $6\,NADH$, $2\,FADH_2$, and $2\,ATP$.

ETC (cellular respiration)

A series of protein complexes located in the inner mitochondrial membrane that transfer electrons from $NADH$ and $FADH_2$ to oxygen while pumping protons ($H^+$) into the intermembrane space.

Chemiosmosis

The process in which the potential energy stored in an electrochemical proton gradient (proton-motive force) drives the phosphorylation of ADP to ATP via ATP synthase.

Uncoupling Proteins

Transport proteins in the inner mitochondrial membrane that allow protons to leak back into the matrix without synthesizing ATP, decoupling oxidative phosphorylation to generate heat for thermoregulation.

Feedback Inhibition

A regulatory mechanism where the end product of a metabolic pathway, such as ATP, accumulates and inhibits an earlier enzyme (such as one in glycolysis) to decrease reaction rates.

Alcohol Fermentation

An anaerobic pathway in which pyruvate is converted into ethanol and $CO_2$ in two steps, regenerating $NAD^+$ to keep glycolysis functioning.

Lactic Acid Fermentation

An anaerobic pathway in which pyruvate is directly reduced by $NADH$ to form lactate without releasing $CO_2$, regenerating $NAD^+$ for glycolysis.

Anaerobic Respiration

A form of cellular respiration used by some bacteria and archaea that employs an electron transport chain with an inorganic final electron acceptor other than oxygen, such as sulfate, nitrate, or sulfur.

Facultative Anaerobes

Organisms that survive by switching between aerobic respiration when oxygen is present and anaerobic pathways (fermentation or anaerobic respiration) when oxygen is absent.

Obligate Anaerobes

Microorganisms that survive and grow exclusively in environments devoid of oxygen, as molecular oxygen ($O_2$) is toxic and lethal to them.

Photoautotrophs

Autotrophic organisms, including plants, algae, and cyanobacteria, that utilize light energy to synthesize organic molecules from carbon dioxide.

Chemoautotrophs

Autotrophic bacteria and archaea that synthesize organic compounds from carbon dioxide using energy derived from inorganic compounds like hydrogen sulfide, iron, or ammonia.

Stomata

Microscopic pores on leaf surfaces that regulate gas exchange, allowing $CO_2$ to enter, $O_2$ to exit, and water vapor to evaporate during transpiration.

Photosystem

A protein complex embedded in the thylakoid membrane composed of light-harvesting pigments and a reaction-center chlorophyll $a$ pair that absorbs photon energy to excite electrons.

Photosystem II (P680)

The photosystem with chlorophyll $a$ molecules that absorb best at $680\,nm$, capturing light energy to drive an electron transport chain and splitting water into $O_2$, protons, and replacement electrons.

Photosystem I (P700)

The photosystem with reaction-center chlorophyll $a$ molecules that absorb light optimally at $700\,nm$, re-exciting electrons received from the ETC before delivering them to NADP+ reductase.

NADP+ Reductase

An enzyme complex in the thylakoid membrane that catalyzes the transfer of high-energy electrons from Photosystem I to $NADP^+$, forming $NADPH$ for the Calvin cycle.

Rubisco

The enzyme that catalyzes carbon fixation in the Calvin cycle by attaching carbon dioxide ($CO_2$) to ribulose bisphosphate (RuBP).

G3P

The 3-carbon carbohydrate produced by the Calvin cycle that serves as the immediate starting precursor for synthesizing glucose, fatty acids, and other organic molecules.