Maths for A Level Biology - updated edition

2.1.2 Per cent increase and decrease A change in mass can be expressed as a percentage change. If mass has been gained, the percentage change is positive and if mass has been lost, the percentage change is negative. To ϐind a percentage change, you must ϐirst ϐind the actual change. Then you work it out as a percentage of what you started with. Here are examples of the change of mass of different desert rodents in response to the same diet of dry food with no supplementary water, as is often the case in their habitats. Gerbillus increases its mass from 35.0 g to 37.1 g but the mass of Acomys decreases from 37.4 g to 27.7 g. You may be asked to ϐind the per cent change in mass. For Gerbillus , % mass increase = actual mass increase intial mass × 100 = 37.1 − 35.0 35.0 × 100 = 2.1 35.0 × 100 = +6% For Acomys , %mass increase = actual mass increase intial mass × 100 = 37.4 − 27.7 37.4 × 100 = 9.7 37.4 × 100 = −25.9% For Acomys , the change is negative, because it lost mass. But it is important to be sure of the wording of the question: If you were asked, instead, to ϐind, not the per cent change, but the per cent increase in mass for Gerbillus and the per cent decrease for Acomys , it would be the same for Gerbillus because unless a ϐigure is qualiϐied, it is assumed to be positive, so 6% means the same as +6%, in other words, an increase. For Acomys , however, you would not need the minus sign, because the question has asked for the decrease, not the change. For Gerbillus , % mass increase = actual mass increase intial mass × 100 = 37.1 − 35.0 35.0 × 100 = 2.1 35.0 × 100 = 6% For Acomys , %mass decrease = actual mass decrease intial mass × 100 = 37.4 − 27.7 37.4 × 100 = 9.7 37.4 × 100 = 25.9% The per cent increase for Gerbillus is 6.0% but the percentage change can be written +6.0%. The per cent decrease for Acomys is 25.9% but the percentage change is −25.9%. Notice how the words are used. For Gerbillus , the increase is positive and the change is positive as it has gained mass. But for Acomys , the decrease is positive but the change is negative because it has lost mass. It is important that you pay attention to the words so that you express your answer properly. As this is Biology, you could be asked about the meaning of the results, such as to suggest which rodent is better adapted to desert life. If you picture the organism and think about its niche, you are more likely to produce an answer that is biologically sensible. The answer would be Gerbillus because it can increase its mass even without water being available, which is presumably a good thing in the desert, whereas Acomys, on the same diet, decreases its mass. You can deduce that Acomys is not thriving so well and is therefore less well adapted. QUICKFIRE 2.2 Raw cabbage contains 49mg vitamin C per 100g. Boiled cabbage contains 20 mg vitamin C per 100g. Calculate the percentage loss of mass. Pointer Mass gain is a positive mass change; mass loss is a negative mass change. Gerbillus 2 Processed numbers 33