A student makes jelly using two different methods. Both methods use a protein called gelatin which causes the jelly to set. Canned papaya is fresh papaya which has been heat-treated during processing. Method A: Fresh papaya is added to the gelatin. It is left for 2 hours. The jelly does not set. Method B: Canned papaya is added to the gelatin. It is left for 2 hours. The jelly sets. Papaya contains an enzyme called papain. Papain digests proteins.
Explain why the jelly made with fresh papaya does not set but the jelly made with canned papaya does set. Use your knowledge of enzymes.
The graph below shows the rate of an enzyme-catalysed reaction as the concentration of substrate is changed. 
Draw an X on the graph to show where approximately 50% of the enzymes' active sites are occupied with substrate. Explain your decision.