1. What is the effect of changing concentration of the substrate for an enzyme catalysed reaction?

When investigating the effect of concentration on enzyme activity, three factors must be kept constant, (i) either the concentration of substrate or enzyme, (ii) the temperature and (iii) the pH of the solution (see experimental methods).

Concentration: If the substrate reactant e.g. sugar, concentration is increased, the rate of reaction increases in a simple proportional way as long as the enzyme concentration is constant.

Kinetic particle theory:

The greater the concentration of substrate (or enzyme, see below) the greater the probability of a fruitful collision leading to the formation of products.

For more details see Effect on rate of changing reactant concentration

Assume the grey background is the water solvent.

The graph on the above-left shows what happens as you gradually increase the substrate molecule concentration for a fixed constant concentration of enzyme.

Initially the rate of reaction steadily increases, in fact, usually, it rises proportionately with increase in substrate concentration, but ....

What happens if the enzyme gets overloaded with substrate molecules?

However, if the concentration of enzyme is low but the substrate concentration becomes very high, the rate of reaction rises to a maximum and then stays constant.

This is because the maximum number of catalyst active sites for the 'key and lock' mechanism are all in use and the maximum rate of reaction depends on the rate of diffusion of substrate in, and diffusion of product out from the active sites.

This shows up as the later horizontal portion on the graph - meaning the speed of the reaction has become constant because there are no more active sites to bring into operation.

What happens if you change the enzyme concentration?

At constant substrate concentration, the rate of reaction (usually) increases linearly with increase in enzyme concentration. The greater the concentration of enzyme, the greater the probability of a fruitful collision leading to the formation of products. Hence the initial linear part of the graph.

However, if the enzyme concentration is very high and the substrate concentration low, the rate tails off and becomes constant. This is because there are lots of 'active sites' available, in excess of those needed, and adding more of the enzyme makes no difference.

See a 'decay' investigation using milk and lipase  gcse biology revision notes

Part of the section Carbon cycle, nitrogen cycle, water cycle, decomposition - decay investigation