A substrate doesn't 'know' anything. The interaction of enzymes with substrates depends on random collisions to take effect.
A substrate is when the enzyme can only join onto certain substances
The enzyme substrate complex
enzyme-substrate complex
in an enzyme-substrate complex, the enzyme acts on the substrate .
When an enzyme and substrate come together, it is called the enzyme-substrate complex. This complex is a temporary intermediate state in which the enzyme binds to the substrate to catalyze a chemical reaction.
On one part of an enzyme is an active site (which is what the substrate binds to) that is shaped a certain way, say a triangle. A substrate that's in the shape of a square won't fit onto the triangle/the enzyme - but a substrate that has an indent in the shape of a triangle will. The way an enzyme recognizes its substrate is if it can attach itself to the enzyme's active site.
The substrates are converted into products, which are released.
A substrate is the substance acted upon by an enzyme. The enzyme substrate complex is when an enzyme molecule combines with its substrates.
When a substrate fits into the active site of an enzyme, an enzyme-substrate complex is formed. This complex allows for the enzyme to catalyze a specific chemical reaction on the substrate.
The lock and key model means that the substrate must perfectly fit the enzyme, and the enzyme does not change. The induced fit model is different as when the substrate fits together with the enzyme, the enzyme itself will change to either join substrates together or break a substrate down.
complex
If the reactant is affected by an enzyme, it then referred to as a SUBSTRATE.