Biology · Biomolecules · NEET
An enzyme LOWERS the activation energy. This is the whole reason enzymes speed up reactions. NCERT says enzymes bring down the activation-energy barrier so that a much larger fraction of molecules can cross it in a given time. It never raises the barrier. In NEET graphs, the SMALLER hump is 'with enzyme' and the BIGGER hump is 'without enzyme'.
The transition state is the highest-energy, unstable arrangement of the substrate at the very top of the reaction curve. The substrate must first reach this peak before it can become product. The energy gap from the substrate's normal level UP to this peak is the activation energy. Once past the peak, the reaction rolls down to product on its own.
No. An enzyme does NOT change the energy level of the substrate (start) or the product (end). It also does not change whether the reaction is exothermic or endothermic. It only lowers the HEIGHT of the barrier (the transition state peak). This is a very common NEET trap, so remember: enzyme = shorter barrier only, same start and same end.
They are linked but not the same. The transition state is the unstable peak arrangement of atoms. Activation energy is the AMOUNT of energy needed to climb from the substrate up to that peak. So the transition state is a 'point' on the curve, and activation energy is the 'height' you must climb to reach it.
Molecules have different energies. Only the ones with enough energy to cross the barrier can react. When the enzyme lowers the barrier, MANY more molecules already have enough energy to cross it. So per second, far more substrate turns into product, meaning the reaction rate becomes very high (enzymes can be thousands of times faster than inorganic catalysts).
An energy-profile (reaction-coordinate) graph shows a substrate S at higher energy and product P at lower energy, with two activation-energy barriers labelled 'A' (smaller) and 'B' (larger). Which of the following describes the graph correctly?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Enzymes only make the reaction reach equilibrium FASTER. They lower the activation energy for both forward and backward directions equally, so the final equilibrium position stays the same. This point is often mixed with 'transition state' in NEET, so keep them separate.
It is the vertical height from the substrate's energy level up to the top of the hump (the transition state). Without enzyme this height is large; with enzyme this height is small. The starting and ending levels do not move.
No, it is the most UNSTABLE point of the whole reaction. It exists for only an instant at the peak of the curve. From there the atoms either fall forward to product or fall back to substrate.
Enzymes are far more efficient. NCERT notes an enzyme-catalysed reaction can proceed thousands to millions of times faster than the same reaction without a catalyst, because the enzyme lowers the activation-energy barrier so strongly.
