Q.Enzymes are biocatalysts. They catalyse biochemical reactions. In general they reduce activation energy of reactions. Many physico-chemical processes are enzyme mediated. Which of the following reactions is not enzyme-mediated in biological system?

Physical vs Chemical Change

Melt an ice cube and you still have water — just in a different form. Burn a piece of paper and you no longer have paper at all — you have ash, carbon dioxide and water vapour, genuinely new substances that didn't exist before. That distinction — does the substance itself change into something new, or does it merely change shape or state — is exactly what separates a physical change from a chemical reaction, and it's a distinction that turns out to matter enormously for understanding how enzymes work.

The core test: are bonds broken or formed?

A physical change never touches the actual covalent bonds holding a molecule together — it's purely a change in shape, phase, or spatial arrangement. A chemical reaction, by contrast, genuinely breaks existing bonds and forms new ones, which is exactly why new substances appear at the end that weren't present at the start.

Why this distinction matters for enzymes specifically

Enzymes exist to speed up chemical reactions — processes where actual bonds are being made or broken. This gives you a genuinely useful shortcut for a whole class of exam questions that ask "which of the following is NOT enzyme-mediated": if a listed process doesn't actually involve breaking or forming a bond, it has no chemistry for an enzyme to catalyse in the first place, no matter how biologically important it sounds.

A classic exemplar question lists four processes and asks which one isn't enzyme-mediated: dissolving CO2 in water, untwining the two strands of DNA, hydrolysis of sucrose, and formation of a peptide bond. Running each through the bond-test:

• Untwining DNA strands breaks the hydrogen bonds holding the double helix together — a genuine chemical event, carried out by helicase enzymes.
• Hydrolysis of sucrose breaks a glycosidic bond with the addition of water — a hydrolase does this work.
• Formation of a peptide bond creates a brand-new covalent bond linking two amino acids — a ligase-type enzyme drives this.
• Dissolving CO2 in water, however, is pure solvation — the gas molecules simply disperse among the water molecules. No covalent bond within the CO2 molecule itself is broken or formed at that step. …