Glucose acts as a starting material in the formation of amylose. First, glucose is converted into glucose 1-phosphate (G 1-P). Starch phosphorylase is an intracellular enzyme that can catalyse the synthesis of amylose from G 1-P, which is the substrate for the reaction: $(\text{glucose 1-phosphate})_n \rightarrow \text{amylose} + (\text{phosphate ions})_n$, where $n$ is a large number. Students used a colorimeter to monitor the progress of the reaction. They prepared a reaction mixture containing $0.01\ \text{mol dm}^{-3}$ G 1-P in a buffer solution at $\text{pH }6.0$. A very small quantity of amylase was added to start the reaction. A solution of starch phosphorylase was then added to the reaction mixture and samples were collected at $1$-minute intervals. Each sample was mixed with dilute iodine solution, stirred, and then transferred to a cuvette. The absorbance of each solution was measured. The findings of the investigation are shown in Fig. 2.1.
(a(i))[1]
Explain why the absorbance gets larger in Fig. 2.1.
(a(ii))[2]
The students collected their last sample at 12 minutes.
Predict the absorbance results if the students had carried on sampling for another 10 minutes. Explain your answer.
(a(iii))[1]
State one advantage of using a colorimeter to track the progress of the reaction.
(b)[4]
The reaction catalysed by starch phosphorylase takes place at the ends of amylose molecules.
Describe the sequence of events that happens when starch phosphorylase catalyses the addition of a molecule of glucose to the end of an amylose molecule.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “As more starch or amylose is formed, the iodine solution darkens, changing from yellow-brown to blue-black.” …