W4 Energy, Enzymes, Metabolism

Potential Energy Definition & Chemical Potential Energy Examples

Potential energy is stored energy available to do work; chemical potential energy is stored specifically within covalent bonds holding atoms together in organic molecules, such as covalent bonds in glucose, fatty acids, and amino acids, as well as the high-energy phosphoanhydride bonds connecting phosphate groups in ATP.

Electrochemical Gradients as Potential Energy

Electrochemical gradients store energy in the unequal distribution of ions across a biological membrane, establishing both a chemical concentration gradient and an electrical charge gradient; key biological examples include the proton (H+) gradient generated across the inner mitochondrial membrane during cellular respiration, the proton gradient across the chloroplast thylakoid membrane during photosynthesis, and the sodium/potassium (Na+/K+) gradient across animal plasma membranes maintained by the Na+/K+-ATPase pump.

Thermal Energy as Kinetic Energy

Kinetic energy is the energy associated with active motion; thermal energy (heat) represents the random kinetic movement of atoms and molecules within a system, which in biological systems is generated as a byproduct of metabolic chemical reactions to help maintain body temperature in homeotherms.

Mechanical Energy Examples in Biological Systems

Mechanical energy is kinetic energy involved in the physical movement of cellular structures or whole organisms; examples include cytoskeletal motor proteins (kinesin, dynein, and myosin) walking along microtubules or actin filaments to transport intracellular vesicles, muscle contraction via sliding actin and myosin filaments, and the beating motion of cellular cilia and flagella.

Radiant / Light Energy in Biological Systems

Radiant or light energy is kinetic energy carried by electromagnetic waves or photon particles; in biology, photons are absorbed by chlorophyll and accessory pigment molecules located within plant photosystems to initiate the light-dependent reactions of photosynthesis.

Gibbs Free Energy (G) Definition

Gibbs free energy (G) is the portion of a system's total energy that is available to perform biological work under conditions of constant temperature and pressure.

Exergonic Reaction Definition & Free Energy State

An exergonic reaction is a chemical reaction that results in a net release of free energy (ΔG < 0), meaning the final products hold less free energy than the starting reactants, making the reaction energetically favorable and spontaneous.

Endergonic Reaction Definition & Free Energy State

An endergonic reaction is a chemical reaction that requires a net input of free energy (ΔG > 0), meaning the final products hold more free energy than the starting reactants, making the reaction energetically unfavorable and non-spontaneous.

Gibbs Free Energy Equation & Parameters

Reaction spontaneity is evaluated using the equation ΔG = ΔH - TΔS, where ΔG is the change in free energy, ΔH is the change in enthalpy or potential energy, T is the absolute temperature in Kelvin, and ΔS is the change in entropy.

Role of Enthalpy / Potential Energy Change (ΔH) in Spontaneity

Enthalpy change (ΔH) reflects net changes in heat and bond potential energy; exothermic reactions (ΔH < 0) release energy because the new bonds formed are stronger and more stable than the bonds broken, and this lowering of potential energy promotes a negative ΔG and favors reaction spontaneity.

Role of Entropy Change (ΔS) in Spontaneity

Entropy change (ΔS) measures the change in system disorder, randomness, or energy dispersion; processes that increase system disorder (ΔS > 0), such as breaking a single complex molecule down into multiple smaller molecules, promote a negative ΔG and favor reaction spontaneity.

Absolute Conditions Guaranteed for Reaction Spontaneity

A chemical reaction is guaranteed to be spontaneous (ΔG < 0) whenever there is a decrease in potential energy/enthalpy (ΔH < 0) combined with an increase in system entropy (ΔS > 0).

Free Energy Diagram Components (Reactants, Products, Activation Energy)

On a free energy diagram (Free Energy vs. Reaction Progress), the starting energy level represents Reactants, the final level represents Products, the energy difference between Reactants and Products is the overall free energy change (ΔG), and the peak height measured from Reactants to the peak top represents Activation Energy (Ea).

Free Energy Diagram Transition State & Enzyme Effect

The peak apex of a free energy diagram represents the unstable Transition State where chemical bonds are breaking and forming; adding an enzyme lowers the height of the Activation Energy (Ea) peak without shifting the free energy levels of the Reactants or Products, leaving the net ΔG completely unchanged.

Exergonic vs. Endergonic Free Energy Diagram Profiles

In an exergonic free energy diagram, the Reactants start at a higher energy level than the Products yielding a net downward slope (ΔG < 0); in an endergonic free energy diagram, the Reactants start at a lower energy level than the Products yielding a net upward slope (ΔG > 0).

ATP Structural Components & High-Energy Bonds

Adenosine Triphosphate (ATP) is a nucleotide composed of a nitrogenous adenine base, a five-carbon ribose sugar, and a chain of three sequentially linked phosphate groups (alpha, beta, gamma); adjacent negative charges on oxygen atoms within the phosphate groups exert strong electrostatic repulsion, making the terminal phosphoanhydride bonds highly unstable and rich in chemical potential energy.

ATP Hydrolysis Reaction Equation & Standard Free Energy

Water breaks the terminal phosphoanhydride bond of ATP via hydrolysis to convert ATP into ADP and inorganic phosphate (Pi)

Energetic Reaction Coupling Mechanism

Cells power energetically unfavorable endergonic reactions (ΔG > 0) by coupling them directly to the highly exergonic hydrolysis of ATP (ΔG < 0), combining both reactions into a single shared enzymatic pathway.

Phosphorylated Intermediate Role in Reaction Coupling

During reaction coupling, an enzyme transfers the terminal phosphate group shed from ATP directly onto a reactant molecule rather than releasing it as free heat; this covalent addition creates a high-energy Phosphorylated Intermediate that is structurally unstable and reactive enough to drive the subsequent endergonic step forward.

Net Spontaneity Rule for Coupled Reactions

Energetic reaction coupling is thermodynamics-compliant and proceeds spontaneously only as long as the combined net sum of free energy changes yields an overall negative value

Enzyme Definition & Ribozyme Exception

An enzyme is a biological catalyst—almost always a protein, though catalytic RNA molecules called ribozymes also exist—that significantly accelerates the rate of a chemical reaction without being consumed or permanently altered in the process.

Enzyme Active Site & ES Complex Formation

An enzyme contains a specialized 3D pocket or groove called an active site where specific substrate molecules bind non-covalently to form a temporary Enzyme-Substrate (ES) complex.

Induced Fit Model

Substrate binding to the active site induces a dynamic conformational shape change in the enzyme protein, wrapping the active site tightly around the substrate to strain chemical bonds, properly orient reactive functional groups, or establish a favorable catalytic microenvironment.

Enzyme Activation Energy (Ea) Lowering Mechanism

Enzymes increase reaction rates specifically by binding and stabilizing the high-energy, unstable transition state, which reduces the total activation energy (Ea) barrier required for reactants to proceed to products.

Enzyme Thermodynamic Boundaries (ΔG and Keq Invariance)

Enzymes accelerate both forward and reverse reaction rates equally, but they cannot alter the net free energy change (ΔG) of a reaction, convert an endergonic reaction into an exergonic one, or change the final chemical equilibrium constant (Keq).

Biological Necessity of Enzymes

Metabolic reactions (such as breaking down carbohydrates or synthesizing nucleic acids) have activation energies far higher than what normal cell temperatures can supply; without enzymes, these vital reactions would proceed over weeks or years, which is far too slow to sustain cellular life.

Temperature Effects on Enzyme Kinetics (Cold to Optimal)

At sub-optimal cold temperatures, low thermal energy reduces molecular motion, leading to infrequent physical collisions between substrate molecules and enzyme active sites; rising temperatures increase thermal kinetic energy and collision frequency, driving up enzymatic reaction velocity until an optimal temperature is reached.

High Temperature Denaturation Mechanism

Exceeding an enzyme's optimal temperature imparts excessive thermal energy that disrupts delicate non-covalent interactions (hydrogen bonds, ionic bonds, hydrophobic interactions) holding the protein's tertiary and quaternary structures together, causing the active site to unfold (denature) and permanently destroying catalytic activity.

pH Effects on Enzymatic Activity & Protonation States

Enzymes function within a specific optimal pH range tailored to their location (e.g., pepsin at pH 2 in stomach acid, trypsin at pH 8 in the small intestine, cytosolic enzymes at pH 7.2); deviating from optimal pH changes the protonation state (H+ concentration) of acidic and basic amino acid side chains, disrupting ionic and hydrogen bonds holding active site geometry, which alters substrate binding or causes denaturation.

Enzyme Concentration Effects on Reaction Rate

Increasing total enzyme concentration increases reaction velocity in a direct linear fashion, provided that substrate molecules are present in excess to saturate newly added active sites.

Substrate Concentration Effects & Vmax Saturation

Increasing substrate concentration initially increases reaction velocity due to higher active site occupancy; however, as active sites become continuously occupied, the rate levels off and reaches a maximum velocity plateau (Vmax), where the enzyme is completely saturated with substrate.

Maximum Velocity (Vmax) Definition

Vmax is the maximum achievable reaction velocity reached when an enzyme population is fully saturated with substrate molecules, meaning every active site is continuously engaged in catalysis.

Michaelis Constant (Km) Definition & Affinity Relationship

Km is the specific substrate concentration at which reaction velocity reaches exactly half of its maximum speed (1/2 Vmax); Km acts as an inverse measure of substrate binding affinity, where a low Km indicates strong substrate affinity (requires little substrate to reach 1/2 Vmax) and a high Km indicates weak substrate affinity.

Competitive Inhibitor Binding Site & Mechanism

A competitive inhibitor structurally resembles the natural substrate and directly binds to the active site of the enzyme, physically blocking substrate access.

Competitive Inhibitor Reversibility & Effects on Vmax and Km

Competitive inhibition is reversible by adding excess substrate; because high substrate concentrations outcompete the inhibitor, Vmax remains UNCHANGED, but Km is INCREASED because a higher substrate concentration is required to reach 1/2 Vmax (reflecting an apparent decrease in substrate binding affinity).

Noncompetitive (Allosteric) Inhibitor Binding Site & Mechanism

A noncompetitive (allosteric) inhibitor binds to an allosteric site—a location distinct from the active site—inducing a conformational shape change in the enzyme protein that alters active site geometry and impairs catalytic turnover.

Noncompetitive Inhibitor Reversibility & Effects on Vmax and Km

Adding excess substrate CANNOT overcome noncompetitive inhibition; because functional enzyme capacity is permanently reduced, Vmax is DECREASED, while Km remains UNCHANGED because the remaining active enzyme molecules retain their normal substrate affinity.

Metabolic Pathway Definition

A coordinated, multi-step sequence of specific biochemical reactions in a cell, where the chemical product of one enzyme serves as the immediate substrate for the next enzyme in the sequence, progressing through metabolic intermediates to yield a final end-product.

Feedback Inhibition (End-Product Inhibition) Mechanism

Feedback inhibition occurs when the final end-product of a metabolic pathway accumulates and binds allosterically to an enzyme operating early in the pathway (typically the first committed step), shutting down pathway activity to prevent the overproduction of molecules and conserve cellular energy and resources.

Allosteric Pathway Regulation

Regulator molecules bind non-covalently to allosteric sites on key rate-limiting enzymes in a pathway, inducing structural shifts that stabilize either the active high-affinity conformation (allosteric activators) or the inactive low-affinity conformation (allosteric inhibitors).

Covalent Modification Regulation (Kinases and Phosphatases)

Reversible covalent attachment or removal of functional groups modulates enzyme activity; specifically, protein kinase enzymes attach phosphate groups (phosphorylation) to switch regulatory enzymes on or off, whereas protein phosphatase enzymes remove phosphate groups (dephosphorylation).

Gene Expression Control of Metabolic Pathways

Cells regulate metabolic pathways over longer time frames by controlling gene transcription and mRNA translation, which adjusts the net synthesis rate and total quantity of specific enzyme proteins present in the cell.

Cellular Compartmentalization of Pathways

Cells segregate opposing metabolic pathways into distinct, membrane-bound organelle compartments to prevent futile cycles; for example, fatty acid breakdown via beta-oxidation occurs inside the mitochondria, whereas fatty acid synthesis takes place in the cytosol.

Redox Reaction Core Principle & OIL RIG Mnemonic

Oxidation-reduction (redox) reactions are coupled chemical reactions involving the physical transfer of one or more electrons from an electron donor to an electron acceptor; remembered by the mnemonic OIL RIG

Oxidation Definition, Hydrogen/Oxygen Changes & Energy Role

Oxidation is the loss of electrons from an atom or molecule; in biological systems, oxidation frequently manifests as the loss of hydrogen atoms (dehydrogenation) or the gain of oxygen atoms, causing the oxidized molecule to release energy and act as a reducing agent.

Reduction Definition, Hydrogen/Oxygen Changes & Energy Role

Reduction is the gain of electrons by an atom or molecule; in biological systems, reduction frequently manifests as the gain of hydrogen atoms or the loss of oxygen atoms, causing the reduced molecule to gain chemical energy and act as an oxidizing agent.

Major Biological Electron Carriers (NAD+, FAD, NADP+ Equations)

Specialized coenzymes act as temporary electron carriers to transfer high-energy electrons during metabolic redox reactions; key reduction equations include

Energy Management via Pathway Coupling

Cells maintain energy balance by directly coupling energy-releasing catabolic pathways with energy-consuming anabolic pathways, utilizing ATP and reduced electron carriers (NADH, NADPH) as transferable energy currency.

Catabolism vs. Anabolism Primary Function

Catabolic pathways function to break down complex organic macromolecules into simpler, smaller molecular components; anabolic pathways function to construct and assemble complex macromolecules from simpler precursor building blocks.

Catabolism vs. Anabolism Free Energy (ΔG) and Entropy (ΔS) Changes

Catabolic pathways are overall exergonic (ΔG < 0) with a net release of free energy and increase system entropy (ΔS > 0) by breaking ordered structures down; anabolic pathways are overall endergonic (ΔG > 0) with a net consumption of free energy and decrease system entropy (ΔS < 0) by building ordered molecular structures.

Catabolism vs. Anabolism ATP Balance and Electron Carrier Usage

Catabolic pathways generate ATP (via substrate-level and oxidative phosphorylation) and reduce coenzymes to produce NADH and FADH2; anabolic pathways consume ATP (to drive endergonic bond formation) and oxidize coenzymes, consuming NADPH as reducing power.

Representative Examples of Catabolism vs. Anabolism

Catabolic pathway examples include cellular respiration, glycolysis, the citric acid cycle, beta-oxidation of fatty acids, and glycogenolysis; anabolic pathway examples include photosynthesis, protein synthesis, DNA/RNA replication, fatty acid synthesis, and glycogenesi