Enzymes

Enzymes are biological catalysts, almost all of which are globular proteins, and understanding their action requires separating kinetics from thermodynamics. A catalyst increases reaction rate by offering an alternative pathway with a lower activation energy, so a larger fraction of substrate molecules can surmount the energy barrier at a given temperature; the enzyme binds the substrate at its active site and stabilises the transition state to achieve this. Crucially, a catalyst does not change the thermodynamics of the reaction: the equilibrium constant and the overall free-energy change of reactants to products remain unaltered, so the option claiming the equilibrium constant increases is wrong. Enzymes do not heat the substrate, ruling out the temperature-raising option, and they supply no free energy to the system, so they cannot make a non-spontaneous reaction spontaneous. They merely speed the approach to the same equilibrium in both directions. This matches the NCERT description of enzymes and the general principle of catalysis. As a plausibility check, since a catalyst lowers the activation energy of forward and reverse steps equally, it accelerates both directions and leaves the equilibrium position unchanged, consistent with the laws of thermodynamics.