Q.Explain the mode of enzyme action on the basis of lowering of Activation energy. Draw a labelled diagram to show activation energy. OR Explain Apoplast pathway of water in plants. Draw a diagram to show major paths of water flow in plant cells.

An enzyme accelerates a reaction by reducing the energy barrier (activation energy) that reactants must cross to form products, via formation of an enzyme-substrate complex that stabilises the transition state.

Every chemical reaction, even one that is energetically favourable overall, needs an initial input of energy to get the reactant molecules to an unstable, high-energy 'transition state' before they can be converted to products - this energy requirement is called the activation energy (Ea). Enzymes are biological catalysts that dramatically speed up reactions by lowering this activation energy: the substrate binds to the enzyme's active site, forming an enzyme-substrate (ES) complex; the enzyme's active site stabilises the transition state of the substrate (by straining bonds, providing a favourable microenvironment, or bringing reactive groups together), so a much smaller energy input is needed to reach the transition state than in the uncatalysed reaction. The enzyme itself is not consumed and is released unchanged after the product is formed, and importantly, the enzyme does not alter the overall free-energy change (ΔG) of the reaction (i.e. it does not make an unfavourable reaction favourable) - it only speeds up how fast equilibrium is reached.

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