EK
Proteins
Proteins have a lot of different functions. It is the shape of the protein which determines it's function. Proteins are made of amino acids and they determine their shape/function.
Here are some examples:
Enzymes
Enzymes are biological catalysts. They are made in every cell and they are made from protein. Chemical reactions would be too slow without enzymes to allow life to exist. Each enzyme is specific.
The amalyse breaks up the starch (the substrate) into small pieces (the product) and the enzymes is unchanged afterwards.
Effect of pH and temperature on enzyme function
Changes in temperature can have a big effect on enzymes. Every enzyme has its own optimum temperature. This is when it is as warm as possible (as much energy as possible) without changes on the active site of the enzyme. The warmer it is, the more energy there is and the faster do the molecules move. The enzyme changes shape if there is too much energy ( it denatures). The enzyme can't turn back once it's denatured, even if you lower the temperature.
pH concentration can also have a big effect on enzymes. Like with temperature, each enzyme has its own pH optimum. If the concentration is too acidic or otherwise, the enzyme denatures. With pH concentration the enzyme can turn back to its former shape if the pH concentration is turned back.