1.7 MANDATORY PRACTICAL. Overview: The phenolphthalein in the agar cubes reacts with the acid, changing the colour of the cube from pink to clear. After the cubes are exposed to the acid you will see how far the acid diffused based on the change in colour which it caused. This will allow you to determine the relationship between diffusion and the surface area and volume of the cubes. Procedure 1. Cut 8 cubes 2 of each size. 2. Calculate the surface area and volume of each agar cube and record these values in the table below. 3. Carefully fill 3 beakers with acid so that the cubes will be completely submerged when placed in the beaker. 4. Place the 2 agar cubes of the same size into each beaker. After 10 minutes, remove one cube of each size them and place them on a paper towel; keep timing the other cubes. 5. Using the paper towel, blot the cubes dry. 6. Use the scalpel to carefully cut each cube in half. 7. Measure the distance travelled into each cube by the acid. Then calculate the volume of the coloured proportion of the cube, and the uncoloured portion. 8. Finally calculate the percentage of uncoloured cube. 9. Whilst you are doing the above calculations watch the remaining cubes and time how long each one takes to go completely clear. Record each time in the table below. Results Cube Size (mm) 10 20 30 40 Surface Area (A) Volume (Y) Surface area / Volume Ratio (A/Y) Dimensions of Coloured Cube Volume of Coloured Cube (X) Volume of Uncoloured Portioned (Y-X= Z) Percentage of Block Uncoloured (Z/Yx100) Time Taken for all of the blocks to go completely clear (s) Questions 1. Explain why parts of the cube became clear 2. If the length of the side of a cube is decreased, what happens to the surface area to volume ratio of the cube? 3. Which cube has the least volume remaining not reached by the acid (the least pink colour)? 4. In which size cube would substances be able to reach the middle fastest? Why is this? 5. Imagine that ,on a smaller scale, the gelatine block represents a single celled organism a) What substances need to penetrate to all parts of the cell b) What substances need to diffuse out of the cell in the opposite direction 6. Most single celled animals are less than 1mm across and the largest are only about 2mm.By reference to your results in this experiment suggest one reason why single celled organisms are no larger than this? 7. On the basis of the results of this experiment the penetration of substances into a large cellular organism such a fish should take a long time. What aspects of structure and organisation in a fish allow it to survive and be active despite such a slow rate of diffusion? 8. Suggest a shape for a volume of one cm3 of gelatine, which would increase the rate of penetration of substances.