Microorganisms: Friend and Foe

Microorganisms are living organisms so small that they can be seen only with a microscope. They are classified into bacteria, fungi, protozoa and algae, while viruses reproduce only inside the cells of a host. In this chapter you will learn where microorganisms live and how many of them are our friends: Lactobacillus turns milk into curd, yeast makes bread dough rise and ferments sugar into alcohol, as Louis Pasteur showed in 1857, and the fungus from which Alexander Fleming discovered penicillin in 1929 gave us the first antibiotic. Vaccines prepared from killed or weakened microbes protect us from diseases, and Rhizobium and blue-green algae fix nitrogen in the soil. Microorganisms can also be harmful. You will study pathogens that cause diseases in humans, such as cholera, typhoid, tuberculosis, malaria and dengue, in animals, such as anthrax, and in plants, such as citrus canker and rust of wheat, and how they spread through air, water, food and carriers like mosquitoes. You will also learn methods of food preservation, including pasteurisation, and the nitrogen cycle.

Detailed Worksheet: Microorganisms: Friend and Foe Section A - Definitions (10 marks) 1. What are microorganisms? Name the four major groups of microorganisms, giving one example of each. (2 marks) 2. Why are viruses considered different from other microorganisms? Name two diseases caused by viruses. (2 marks) 3. What is fermentation? Who discovered it, and in which year? (2 marks) 4. Define antibiotics and vaccines. Who discovered penicillin? (2 marks) 5. What are pathogens? What are communicable diseases? Give two examples of each. (2 marks) Section B - Calculations and Applications (15 marks) 6. A bacterium divides into two every 20 minutes under suitable conditions. Starting from a single bacterium, how many bacteria will be present after 2 hours? Why must cooked food not be left in a warm place for long? (3 marks) 7. Match each disease with its causative microorganism and its mode of transmission: tuberculosis, cholera, malaria, chicken pox, typhoid. Microorganisms: bacteria, protozoa, virus. Modes: air, water, mosquito, contact or air. (3 marks) 8. In a village of 2000 people, 150 people fell ill with cholera after drinking water from a contaminated well. (i) What percentage of people were affected? (ii) Name the microorganism group that causes cholera. (iii) Suggest two measures to prevent the spread of the disease. (3 marks) 9. A dairy heats milk to about 70 degrees C for 15 to 30 seconds and then chills it suddenly before storing it in the cold. (i) Name this process. (ii) Who discovered it? (iii) Why is the milk cooled suddenly after heating? (3 marks) 10. Match each food with its preservation method: pickles, jams and jellies, fish and meat, packaged dry fruits, fruit juices sold in bottles. Methods: preservation with oil and vinegar, with sugar, with common salt, in airtight packets, with chemical preservatives such as sodium benzoate. (3 marks) Section C - Diagrams (10 marks) 11. Draw labelled diagrams of the shapes of bacteria (rod-shaped, spherical and spiral), a fungus such as bread mould, and a protozoan such as Amoeba or Paramecium. (4 marks) 12. Draw a labelled flowchart of the nitrogen cycle, showing nitrogen fixation by bacteria and blue-green algae, absorption by plants, eating by animals, decomposition, and the return of nitrogen to the atmosphere. (3 marks) 13. Draw a labelled diagram of a female Anopheles mosquito and explain its role as a carrier of malaria. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. A student says that all microorganisms are harmful and should be destroyed. Evaluate this statement by giving five useful roles of microorganisms in food, medicine, agriculture and the environment. (5 marks) 15. Explain how dengue and malaria spread, and suggest five measures a community can take to prevent them. Why is it important to complete the full course of antibiotics prescribed by a doctor, and why do antibiotics not work against the common cold? (5 marks) 16. Explain how a vaccine protects the body from a disease. Name the scientist who developed the smallpox vaccine and the year, and explain how vaccination campaigns have helped to eradicate smallpox and nearly eradicate polio. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part.