Many plants store starch in their roots. The amount of starch present in roots can vary with the seasons. In the summer months, when plants are actively growing, the starch present in roots is lower than during the winter months. In the winter months, many of the cells are storing starch in starch grains. Fig. 2.1 is a photomicrograph of a stained transverse section through a root in winter. [Figure 2.1] (b) Iodine solution can be used to test for the presence of starch in root extracts. The concentration of starch in a root extract can be determined by the colour observed when iodine solution is added. You will need to: • prepare different concentrations of starch suspension • estimate the concentration of starch in two root extracts • identify the season in which each root extract was taken. You are provided with the materials shown in Table 2.1. Table 2.1 labelled | contents | hazard | volume/cm³ --- | --- | --- | --- S | 1.0% starch suspension | none | 50 iodine | iodine solution | none | 20 W | distilled water | none | 150 R1 | root extract | none | 20 R2 | root extract | none | 20 If any solution comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. You need to carry out a serial dilution of the 1.0% starch suspension, S, to reduce the concentration by a factor of 10 between each successive dilution. You will need to prepare four concentrations of starch suspension in addition to the 1.0% starch suspension, S. After the serial dilution is completed, you will need to have 9cm³ of each concentration available to use. Carry out step 1 to step 6. step 1 Prepare the concentrations of starch suspension as decided in (b)(i) and shown in Fig. 2.2. Mix well. step 2 Label test-tubes with the concentrations of starch suspension prepared in step 1. step 3 Put 5 cm³ of each concentration of starch solution into the appropriately labelled test-tube. step 4 Put 3 drops of iodine into each of the test-tubes. Shake gently to mix. step 5 Place the white card behind the test-tubes and observe the colour of the liquid in each test-tube. You may see the same colour in more than one test-tube. step 6 Compare the colour of the liquid in each test-tube with the key in Fig. 2.3. Record your observations in (b)(ii) using only the symbols shown in the key in Fig. 2.3. [Figure 2.3 Key with colour symbol] Carry out step 7 to step 12. step 7 Label a test-tube R1. step 8 Put 5 cm³ of R1 into the test-tube you have labelled R1. step 9 Put 3 drops of iodine into the same test-tube. Shake gently to mix. step 10 Place the white card behind the test-tube and observe the colour of the liquid in the test-tube. step 11 Compare the colour of the liquid in the test-tube with the key in Fig. 2.3. Record your observation in (b)(iv) using only the symbols shown in the key in Fig. 2.3. step 12 Repeat step 7 to step 11, using R2 instead of R1. (c) Fig. 2.4 is the same photomicrograph as that shown in Fig. 2.1. [Figure 2.4] Fig. 2.5 is a photomicrograph of a stained transverse section through a different root. [Figure 2.5]