Plant cells contain enzymes which catalyse some of their metabolic reactions. Some of these enzymes catalyse the release of oxygen from hydrogen peroxide. A cylinder of potato tissue will have these enzymes on the surface. When hydrogen peroxide solution and a cylinder of potato tissue are mixed, oxygen bubbles are released. You will need to investigate the effect of surface area by: • changing the surface area • counting the number of bubbles of oxygen released in a set time (dependent variable). You are provided with the materials shown in Table 1.1 and Table 1.2. Table 1.1 labelled | contents | hazard | volume / cm³ ---|---|---|--- H | hydrogen peroxide solution | moderate | 40 W | water | none | 100 If any of H comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection and gloves. Table 1.2 labelled | contents | details | quantity ---|---|---|--- P | potato cylinders | same cross-sectional area | 4 (a) To investigate the effect of surface area, other variables need to be standardised. Each potato cylinder has been provided with the same diameter but with different lengths. Each cylinder of potato tissue must be cut to the same length. 1. Cut each of the four potato cylinders in the beaker labelled P, to a length of 20 mm. To investigate the effect of surface area, the surface area can be changed by cutting each of these four cylinders into a different number of pieces. The formula for calculating the total surface area of a cylinder is shown in [Figure 1.1]. Total surface area of a cylinder = curved surface area + surface area of all the circular ends. The curved surface area of a cylinder can be calculated by using the formula: curved surface area = 2πrl π = 3.14 r = radius of cylinder l = length of cylinder All the cylinders start with the same length (20mm) and have the same radius, so the curved surface area is standard. The total curved surface area is the same for all four cylinders, even when a cylinder is cut into several pieces, as shown in the example on page 4. To change the total surface area, each cylinder is cut into a different number of pieces. The change in surface area depends on the number of the circular ends, n. surface area of all the ends = πr² n EXAMPLE One cylinder has 2 circular ends as shown in [Figure 1.2]. • number of circular ends, n = 2 • area of one circular end = πr² = 3.14 x 2² = 12.56 = 13 mm² (to the nearest whole number) • surface area of all the circular ends = πr² n = 13 x 2 = 26 mm² [Figure 1.3] shows another cylinder with the same radius and length, which is cut into two pieces. There are then 4 circular ends. • total number of circular ends, n = 4 • area of circular end = 13 mm² to the nearest whole number • surface area of all the circular ends = πr² n = 13 x 4 = 52 mm² (to the nearest whole number)