(a)    
(i)
Graph A: The rate of reaction is decreasing as the other factor increases.

Graph B: The rate of reaction increases, reaches an optimum and then decreases rapidly as x increases.

(ii) pH

(b)    
(i) Enzymes are proteins that speed up a reaction without being used up in that reaction.  Enzymes are very specific and each one catalyses just one particular reaction. This specificity is related to their 3-D shape and in particular, the shape of the active site, which matches just one substrate.  The shape of the active site can be changed by changes in pH or temperature, denaturing the enzyme.

When the substrate enters the active site of the enzyme, the shape of the active site changes slightly to allow the substrate fit more precisely; this is called the induced fit model. The enzyme and substrate together form an enzyme-substrate complex and the substrate is changed into the products, which leave the active site.  The active site returns to its original shape and is able to join with other substrate molecules.

(ii) Digestion and DNA replication

(iii)      

1. Enzymes are included to break down food stains.
2. This is the optimum temperature for these enzymes (37°C being body temperature).
3. The enzymes would be denatured and unable to work so the stains would not be removed.

(c)    
(i)   

(ii) Advantages:

1. It is easy to separate the yeast cells from the alcohol.     
2. It is very easy to reuse the immobilised yeast.

(iii) The dichromate test
Put 5cm³ of potassium dichromate solution in a test tube.
Add 5 drops of the test solution.
Add 5 drops of concentrated sulphuric acid.
Warm the test tube gently in a beaker of hot water.
Potassium dichromate turns from orange to green in the presence of alcohol.