6 3 Humans are surrounded by microorganisms in the air, water and food. Some microorganisms are pathogenic. The human body has several barriers to prevent infection by pathogens. (a) Complete the table by putting a tick (ü) in the box if the type of barrier is correct. If the type of barrier is not correct, place a cross (X) in the box. (2) Type of barrier Keratin in the skin Lysozyme in mucus Hydrochloric acid in the stomach Physical Chemical (b) Explain why the presence of microorganisms on the skin and in the gut helps to prevent pathogenic organisms multiplying in the body. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 7 Turn over (c) The human gut contains more than a thousand species of bacteria. Only 30 to 40 of these species are found in the stomach. Explain why there are relatively few species of bacteria in the stomach. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 3 = 7 marks)