Enzymes, Metabolism and Bioenergetics: every key term you need (+ practice quiz)
25 flashcard terms for General Biology I Topic 4, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 15-question quiz — free, no account needed.
The total network of chemical reactions in a cell, divided into catabolic routes that break molecules down and release energy and anabolic routes that consume energy to build complex products.
First law of thermodynamics
Energy is conserved: it can be transferred or converted but not created or destroyed. Cells therefore do not make energy, they capture it from food or light and redirect it into useful work.
Second law of thermodynamics
Every energy transfer increases the total entropy of the universe. Living cells maintain internal order only by exporting more disorder, largely as heat, into their surroundings.
Gibbs free energy
The portion of a system's energy available to do work at constant temperature and pressure, combining enthalpy change with the temperature-weighted entropy change of the system.
Exergonic reaction
A reaction with a negative free energy change, which proceeds spontaneously in the forward direction and can be harnessed to drive other processes. Spontaneous refers to direction only, never to speed.
Endergonic reaction
A reaction with a positive free energy change, which will not run on its own and must be pushed by coupling to a larger exergonic process such as ATP hydrolysis.
Chemical equilibrium
The state at which forward and reverse rates are equal and the free energy change is zero. A cell at equilibrium is dead, because open pathways depend on continuous flux away from equilibrium.
Adenosine triphosphate
The cell's short-term energy currency, whose value lies in the mutual repulsion and poor resonance stabilization of its three phosphates, making transfer of the terminal phosphate strongly favorable.
Energy coupling
Linking an unfavorable reaction to a favorable one so the pair has a negative overall free energy change. In cells this usually means phosphorylating a substrate to raise its reactivity.
Phosphorylated intermediate
A substrate that has received a phosphate group from ATP, making it less stable and more reactive. This intermediate is the concrete mechanism by which coupling actually works.
Activation energy
The energy barrier that reactants must climb to reach the transition state. Its height, not the overall free energy change, sets the rate at which a spontaneous reaction actually proceeds.
Transition state
The highest energy, most strained arrangement along a reaction path, in which old bonds are partly broken and new ones partly formed. Enzymes bind this fleeting state most tightly of all.
Catalyst
A substance that lowers activation energy and speeds a reaction without being consumed and without changing the equilibrium position. It accelerates the forward and reverse rates equally.
Active site
The pocket where substrate binds and chemistry happens, lined by a precise arrangement of side chains that position reactants, strain bonds, and sometimes donate or accept protons.
Induced fit
The observation that substrate binding reshapes the enzyme, tightening the pocket around the substrate. It explains specificity better than a rigid lock and key and helps exclude water.
Michaelis constant
The substrate concentration at which an enzyme runs at half its maximum rate. A low value means the enzyme reaches saturation at low substrate levels, so it works efficiently in scarce conditions.
Maximum velocity
The reaction rate approached when every active site is occupied, so adding more substrate cannot help. Reaching this plateau is the signature of catalysis by a finite number of sites.
Competitive inhibitor
A molecule resembling the substrate that binds the active site and blocks it. Because enough substrate can outcompete it, the maximum velocity is unchanged while the apparent Michaelis constant rises.
Noncompetitive inhibitor
An inhibitor that binds away from the active site and distorts the enzyme's shape. Extra substrate cannot displace it, so the maximum velocity falls and cannot be restored.
Allosteric regulation
Control of an enzyme by a molecule binding a site other than the active site, shifting the protein between more active and less active conformations. It is the basis of rapid, reversible metabolic tuning.
Cooperativity
Behavior of a multi-subunit enzyme or carrier in which binding at one site eases binding at the others, producing a sigmoidal response curve and switch-like sensitivity to small concentration changes.
Feedback inhibition
Regulation in which the end product of a pathway allosterically inhibits an early, committed enzyme. This prevents waste by shutting down production as soon as the product accumulates.
Cofactor and coenzyme
Nonprotein partners required for catalysis. Inorganic ions such as magnesium or zinc are cofactors, while organic helpers such as the vitamin-derived electron carriers are coenzymes.
Redox reaction
A paired transfer in which one substance loses electrons and is oxidized while another gains them and is reduced. In biology electrons usually travel with protons, so oxidation often means losing hydrogen.
Nicotinamide adenine dinucleotide
The principal electron shuttle of catabolism, accepting two electrons and one proton to become its reduced form and delivering them to the respiratory chain for ATP production.