Cellular Energy, Metabolism, and Enzyme Function

Vocabulary practice flashcards covering cellular energetics, thermodynamics, metabolic pathways, enzyme structure and kinetics, and high-energy electron carriers.

Metabolism

The basic chemistry and sum of all chemical reactions that take place in an organism, including both processes that build molecules to store energy and processes that break down molecules to release energy.

Cellular Respiration

A catabolic pathway occurring in both animals and plants where a high-energy sugar, such as glucose, is broken down with oxygen ($\text{aerobic respiration}$) to release energy in a stepwise manner and capture it as ATP.

Photosynthesis

An anabolic process exclusive to plants in which carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$) are combined using solar energy to synthesize glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) and release oxygen ($\text{O}_2$).

Potential Energy

The stored energy or energy of position in a system, such as the chemical energy contained within the covalent bonds of glucose or fatty acids.

Kinetic Energy

The energy of motion or movement, released in biological systems when chemical bonds are broken, or rapidly and uncontrollably during combustion.

First Law of Thermodynamics

The law of conservation of energy stating that within a closed system like the universe, energy cannot be created or destroyed, but can only be transferred or transformed from one form to another.

Second Law of Thermodynamics

A physical law stating that energy transfers increase the disorder of a system, meaning total entropy can only increase or remain constant, with biological systems continuously losing unrecoverable energy as heat.

Entropy

A measure of the disorder or randomness in a system; in living organisms, energy transfers cause an increase in entropy as heat is lost to the atmosphere at approximately $37^\circ\text{C}$ ($98.6^\circ\text{F}$).

Enthalpy

A thermodynamic property representing the total heat and total energy present in a system.

Gibbs Free Energy Equation

The mathematical relationship $\Delta G = \Delta H - T\Delta S$, which measures the change in usable energy within biological systems where absolute energy levels are impractical to determine.

Exergonic Reaction

A chemical reaction with a negative change in free energy ($\Delta G < 0$) that proceeds spontaneously, releases energy, and breaks down molecules.

Endergonic Reaction

A non-spontaneous chemical reaction with a positive change in free energy ($\Delta G > 0$) that requires energy input, stores energy, builds chemical bonds, and creates mass.

Catabolic Pathway (Catabolism)

A metabolic pathway that breaks down complex molecules into simpler compounds, breaking chemical bonds and releasing energy (e.g., breaking down glucose into $\text{CO}_2$ and $\text{H}_2\text{O}$).

Anabolic Pathway (Anabolism)

A metabolic pathway that consumes energy to construct complex molecules from simpler ones, such as assembling amino acids into proteins or storing glucose as glycogen or starch.

Substrate-Level Phosphorylation

A mechanism of ATP synthesis wherein an enzyme directly transfers a phosphate group from a phosphorylated substrate molecule to ADP.

Chemiosmosis

The generation of ATP driven by an electrochemical gradient of hydrogen ions pumped across a membrane, which flow back through a specialized valve via facilitated diffusion (passive transport) to bond ADP and inorganic phosphate.

Activation Energy ($E_a$)

The initial investment of energy required to contort, stress, and break bonds to initiate a chemical reaction.

Enzyme

A biological organic catalyst, typically protein-based, that speeds up chemical reactions by lowering the activation energy ($E_a$) without being consumed in the process.

Active Site

The specific region on an enzyme that binds substrates and changes shape to induce stress on chemical bonds during catalysis.

Enzyme-Substrate Complex

The temporary intermediate molecular structure formed when substrate molecules are bound within the active site of an enzyme.

Cofactor

A non-protein chemical component, such as a metal ion or carrier, required by an enzyme to execute its catalytic function.

Apoenzyme

An inactive enzyme that lacks its required cofactor.

Holoenzyme

A biochemically active, functional enzyme that is assembled with its required cofactor.

Coenzyme

An organic cofactor that aids an enzyme in performing catalysis.

NAD+ / NADH

A high-energy electron carrier located in the mitochondria of plants and animals; NAD+ is the oxidized form, and NADH is the reduced form that carries two electrons and a hydrogen ion.

FAD / FADH

A high-energy electron carrier found in the mitochondria of plants and animals; FAD is the oxidized form and FADH is the reduced form carrying hydrogen and electrons.

NADP+ / NADPH

A high-energy electron carrier exclusive to plants and located in chloroplasts; NADP+ is the oxidized form and NADPH is the reduced form carrying hydrogen ions and electrons.

Maximum Velocity ($V_{\max}$)

The point of saturation where the rate of reaction levels off because substrate concentration exceeds the processing capacity of the available enzymes.

Pepsin

A protein-digesting enzyme found in the stomach that operates optimally in a highly acidic environment of $\text{pH}\ 2\text{--}3$.

Trypsin

A digestive enzyme found in the small intestine that functions optimally at a slightly basic $\text{pH}$ of approximately $8.35$.

Energy Coupling

The biological process in which energy released from an exergonic catabolic reaction, like ATP hydrolysis ($\Delta G = -7.3\,\text{kcal/mol}$), is directly used to drive an endergonic anabolic reaction (such as phosphorylating glucose at $3\,\text{kcal/mol}$).

Trophic Energy Loss

The continuous reduction of available energy across trophic levels, where roughly $20\text{--}30\%$ of energy is lost at each consumer step through body heat, waste, and metabolites of respiration.