factors affecting reaction rate (temperature, pH, concentration)
/ ree-AK-shun rayt FAK-torz /
Picture a small kitchen turning out dishes. How fast the plates come out depends on a few things: how warm and busy the kitchen is, whether the conditions are comfortable for the cooks, and how many orders are waiting. Enzyme reactions are similar. Their speed — the reaction rate — is set chiefly by temperature, pH (acidity), and the amounts of enzyme and substrate present.
Temperature: warming things up makes molecules move and collide faster, so up to a point the rate rises. But push too far and the heat shakes the enzyme apart, destroying its shape (denaturation), and the rate crashes. Each enzyme has an optimum temperature — for human enzymes, near body temperature. pH: every enzyme has an optimum pH where its active site holds the right shape and charge; stray too far in either direction (too acidic or too alkaline) and activity falls, eventually denaturing the enzyme. Concentration: with plenty of enzyme, adding more substrate speeds the reaction — until every active site is busy (saturated), after which extra substrate does nothing; adding more enzyme (when substrate is plentiful) raises the rate roughly in proportion.
These factors explain why your body works hard to hold temperature and pH steady (homeostasis): enzymes only perform well within narrow windows. They also explain practical things — why fevers are dangerous, why stomach enzymes need acid while blood enzymes need near-neutral conditions, and why cooling or salting food slows the enzymes (microbial and otherwise) that spoil it. A common misconception is that 'hotter is always faster': beyond the optimum, more heat means less activity, not more.
Pepsin, the protein-digesting enzyme in your stomach, works best around pH 2 — the stomach's strong acid. Move it into the near-neutral small intestine and it shuts down, while a different enzyme, trypsin, takes over because trypsin's optimum pH is around 8.
Each enzyme has its own best temperature and pH; outside that window it slows or stops.
Warming helps only up to the optimum; past it the enzyme denatures and the rate falls sharply — and cold merely slows enzymes, it does not destroy them.