For many chemical reactions, adding more of a reactant reliably speeds the reaction up, roughly in proportion. Reactions run by enzymes — the protein catalysts behind essentially every chemical process in a living cell — behave differently past a certain point: adding more of the enzyme's target molecule, its substrate, speeds the reaction up only up to a limit, after which adding still more makes almost no difference at all.
An enzyme is a machine with a fixed number of working stations
An enzyme speeds up a reaction by binding its substrate at a specific active site, helping it convert into product, then releasing the product and binding a new substrate molecule to repeat the cycle. At low substrate concentration, most enzyme molecules are sitting empty, waiting for a substrate to bind, so adding more substrate genuinely does speed the reaction up, roughly proportionally, because more of the enzyme's total capacity gets used. But once substrate concentration is high enough that essentially every enzyme molecule is already occupied and cycling as fast as it physically can, adding more substrate just means more molecules waiting in line — the reaction rate levels off at a maximum the enzyme's own turnover speed sets, called Vmax.
A formula that names the exact halfway point
This relationship was captured mathematically by Leonor Michaelis and Maud Menten in 1913, in what's now called Michaelis-Menten kinetics. Their equation describes the whole curve — how reaction rate rises with substrate concentration and then flattens toward Vmax — and defines a specific reference point, called Km, the substrate concentration at which the reaction runs at exactly half its maximum speed. Km is a useful practical number: a low Km means an enzyme reaches near-maximum speed even at low substrate levels, a strong binder; a high Km means it needs much more substrate around before it's working near capacity.
What we're still unsure about
Michaelis-Menten kinetics assumes a simplified picture — one substrate, one enzyme, straightforward binding — and real cellular enzymes are frequently more complicated than that: many are regulated by additional molecules that speed them up or slow them down (allosteric regulation), work on multiple substrates, or operate inside a crowded cellular environment quite different from the clean, dilute test-tube conditions the classical equation was derived under. Extending simple enzyme kinetics to accurately model these messier, more realistic conditions is still an active area of biochemistry and systems biology research, not a fully closed textbook problem.
This sits inside Enzyme Kinetics & Michaelis-Menten Theory, one of seven topics in Biochemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.