how is the rate of enzyme activity affected by increasing the concentration of the substate/
To understand this you need to know about the lock+key model of substrate fitting into the enzyme's active site and that enzymes work by lowering the ativation energy of a reaction ( the amount of energy the reactants have to have to form products)
1. Temperature - increases enzyme activity because higher temperature means higher kinetic energy which means the molecules move faster which means the enzymes will bump into the substrate more ofter, therefore form more enzymes-substrate complexes and the reaction will proceed faster. Also with higher kinetic energy, when they collide, more of the reactants will have high enough energy to overcome the activation energy and form products.
The enzyme denatures if the temperature is too high because the high kinetic energy causes big vibrations and disrupts the bonds.
2. pH
pH affects the ionisation of the R group of the amino acids. Changes in ionisation affects electrostatic bonding of the protein. Affecting bonding means affecting the shape. If the shape of the active site is affected, the substrate will not be able to bind as easily causing reduced enzyme activity. If the pH change is extreme enough, the bonding will be so disrupted that the enzyme denatures and becomes non-functional.
3. Increasing substrate or enzyme concentration will increase the reaction rate because there is more of it to react with. Simple as that. If the amount of enzyme stays the same but you increase substrate concentration, you will reach a point where there is no more enzyme to react with (all active sites are occupied). The enzyme activity will plateux and this point is called the Vmax which will vary depending on the other factors e.g pH. The substrate concentration that gives half the Vmax is called the Km and this is constant for an enzyme.