Textbook Solutions: Microorganisms | IGCSE Cambridge Science for Year 6 - Class 6 PDF Download

Types of Microorganisms

Q1. List seven processes that all microorganisms do.

Ans: Movement, sensitivity, growth, respiration, reproduction, excretion, nutrition.

Q2. What piece of apparatus (equipment) do you need to study microorganisms?

Ans: Microscope.

Q3. Give another word for 'microorganism'.

Ans: Microbe.

Q4. a) What type of organism is a mould?

Ans: Mould is a type of fungus.

Q4. b) What is the mould in figure 2.2 feeding on?

Ans: Organic material.

Q4. c) What is the name of the life process that involves feeding?

Ans: Nutrition.

Q4. d) Describe how this mould started to grow on this food.

Ans: Mould spores landed on the food and began to germinate under suitable conditions of moisture and temperature.

Q4. e) State the conditions in which the mould will grow fastest.

Ans: Warm, moist environments.

Q5. Give one difference between fungi and plants.

Ans: Fungi do not perform photosynthesis while plants do.

Q6. a) Give one similarity between yeasts and moulds.

Ans: Both are types of fungi.

Q6. b) Give one difference between yeasts and moulds.

Ans: Yeasts are unicellular, while moulds are multicellular.

Q7. a) Reproduction is a life process. What happens in reproduction?

Ans: Organisms produce new individuals of their species.

Q7. b) How do you know that the bacteria in figure 2.4 have reproduced?

Ans: There is an increase in the number of cells.

Q8. The cells in figure 2.4 are magnified x 6000. What does this mean?

Ans: The cells are enlarged 6000 times their actual size.

Q9. Which do you think are smaller, bacteria or yeasts? Give your reasoning.

Ans: Bacteria are generally smaller than yeasts because bacteria are usually a few micrometers in length while yeasts are about 10 micrometers in diameter.

Q10. Which do you think are smaller, bacteria or viruses? Give your reasoning.

Ans: Viruses are smaller than bacteria as viruses are typically about one hundredth the size of most bacteria.

Q11. Why are viruses not like all other microorganisms?

Ans: Viruses are not considered living organisms because they cannot reproduce without a host cell and do not carry out life-sustaining functions by themselves.

Q12. a) The images of microorganisms on these pages are not taken with a light microscope. Find out what kind of microscope has been used.

Ans: Electron microscope.

Q12. b) Why has this microscope been used and not a light microscope?

Ans: An electron microscope allows for higher magnification and resolution, necessary for viewing the fine details of microorganisms, which are not visible with light microscopes.

Microorganisms and Decay

Q1. Use the evidence in figure 2.8 to say which of the soups has 'gone bad'.

Ans. The soup in the tube on the right, which is cloudier, has gone bad, indicating the presence of microorganisms as suggested by its appearance compared to the clearer soup on the left.

Q2. Describe two nineteenth century ideas to explain why soup goes bad.

Ans. One idea was the theory of spontaneous generation, which posited that life could arise from non-living matter. Another was the germ theory, proposed by scientists like Pasteur, which argued that microorganisms from the environment are responsible for food spoilage.

Q3. Explain why 'Shall we paint the classroom blue?' is not a scientific question.

Ans. It is not a scientific question because it does not ask about a natural phenomenon nor can it be tested through scientific investigation; it is rather a matter of personal or aesthetic preference.

Q4. Describe Pasteur's evidence.

Ans. Pasteur's evidence included observations that the broth turned cloudy and spoiled only when exposed to air, suggesting that microorganisms in the air were responsible for spoilage.

Q5. For each sentence, say whether it is a scientific question, a testable hypothesis, a prediction or evidence.

Ans. (a) Testable hypothesis (b) Evidence (c) Scientific question (d) Prediction

Q6. What is a decomposer? Explain why decomposers are important.

Ans. A decomposer is an organism that breaks down dead organic material. They are important because they recycle nutrients back into the ecosystem, making them available for other organisms.

Q7. A scientist took four containers, each containing 100 g of dead plants. The scientist added decomposers to each container and put them at different temperatures for 30 days. The table shows the results.

Ans. (a) The hypothesis being tested might be: "The effectiveness of decomposers depends on the temperature of the environment." (b) The evidence shows that higher temperatures increase the efficiency of decomposers, as seen by a greater reduction in plant mass.

Q8. Use the food chain shown in the diagram to answer the following questions:

Ans. (a) The producer is phytoplankton. (b) The primary consumer is krill. (c) Two predators are the chinstrap penguin and the seal.

Q9. Describe how the energy is transferred through the food chain from producer to tertiary consumer. Explain why the amount of energy transferred to the tertiary consumer is not the same as the amount of energy made by the producer.

Ans. Energy is transferred from producers to consumers through feeding. The energy transfer is not 100% efficient at each step, leading to less energy being available at each successive level due to metabolic processes and heat loss.

Q10. What role do decomposers have in food chains?

Ans. Decomposers break down dead organisms and wastes, recycling essential nutrients back into the ecosystem, supporting the growth of producers.

Q11. Look at figure 2.13 on the previous page. Which apple do you think fell from the tree first? What is happening to the mineral salts from the apple?

Ans. The apple that is more decomposed likely fell first. The mineral salts from the decomposing apple are being recycled into the soil by decomposers.

Q12. Use figure 2.14 to draw a food chain which includes snakes and fungi.

Ans. The food chain might look something like this: Plants → Insects → Snakes → Fungi (as decomposers breaking down dead snakes).

Q13. State where decomposers get their energy from. All the energy in a food chain comes from a single source. What is this source?

Ans. Decomposers get their energy from breaking down organic matter. The single source of all energy in a food chain is the sun, which provides the energy for producers to make food via photosynthesis.

Q14. Describe two benefits of having decomposers in a habitat.

Ans. Decomposers help in nutrient cycling by breaking down dead matter and waste, thereby enriching the soil. They also help in reducing the buildup of waste materials in the ecosystem, which can prevent the spread of disease.

Q15. Using the internet and/or textbooks to help, draw a food chain that includes a tertiary consumer, for a habitat in your country. Add decomposers to your food chain (to create a food web).

Ans. An example food chain might be: Grass → Rabbit → Fox (tertiary consumer) → Decomposers (such as fungi and bacteria). Decomposers break down the organic matter from all levels, returning nutrients to the soil and supporting plant growth.

End of the Chapter Review

Q1. All organisms are made of cells. How many cells are most microorganisms made of?

Ans: d) millions

Q2. Fungi are not like plants because fungi cannot:

Ans: b) make their own food

Q3. A hypothesis is:

Ans: d) an idea about how or why something happens

Q4. The first organism in a food chain provides energy for all the other organisms in the chain. This type of organism is called:

Ans: a) a producer

Q5. Which of these is a discovery made by Louis Pasteur?

Ans: b) Microorganisms from the air cause foods to go bad.

Q6. Explain why we wash our hands with soap after using the toilet.

Ans: Washing hands with soap after using the toilet helps remove germs and prevents the spread of diseases.

Q7. Louis Pasteur asked: 'Do microorganisms from the air cause clear soup to go bad?' Why is this a scientific question?

Ans: It is a scientific question because it can be tested through experimentation.

Q8. Plants make an energy-rich substance called glucose. (a) Give the name of the process that plants use to do this. (b) State where the energy in glucose comes from. (c) Give one way in which plants use the glucose that they make. (d) Explain why there are usually a smaller number of secondary consumers in a food chain than there are primary consumers.

Ans: (a) Photosynthesis. (b) The energy comes from sunlight. (c) Plants use glucose for growth and energy. (d) Energy is lost at each step of the food chain, so there are fewer organisms as you move up.

Q9. For each sentence, say whether it is a prediction, a scientific question, evidence or a conclusion. (a) Do yeast cells reproduce faster when given more sugar? (b) If I add more sugar to the water, then the yeast will reproduce faster. (c) There were more yeast cells found in the water with more sugar. (d) Increasing the amount of sugar makes yeast cells reproduce faster.

Ans: (a) Scientific question. (b) Prediction. (c) Evidence. (d) Conclusion.

Q10. (a) What is a decomposer? (b) Name two types of decomposers. (c) As decomposers feed on wastes, they respire. What gas do they make? (d) Describe one way in which decomposers are useful. (e) Describe one way in which decomposers are not useful.

Ans: (a) Decomposers are organisms that break down dead materials. (b) Fungi and bacteria. (c) Carbon dioxide. (d) They recycle nutrients back into the ecosystem. (e) They can cause rot and decay in unwanted places.

Q11. Manure is rotting animal waste mixed with parts of dead plants. Explain why manure helps crops to grow.

Ans: Manure enriches the soil with nutrients that are essential for crop growth.

Q12. (a) Figure 2.16 shows a yeast cell. What are parts X, Y, and Z? (b) Give one way the yeast cell is similar to a plant cell but not to an animal cell. (c) What green structures are in some plant cells but not in yeast cells? (d) What group of organisms do yeast belong to? (e) The image is magnified × 4000. What does this mean?

Ans: (a) X is the nucleus, Y is the vacuole, Z is the cell wall. (b) Both have a cell wall. (c) Chloroplasts. (d) Fungi. (e) The image is enlarged 4000 times its actual size.

Q13. (a) Draw and label a food chain for the following information. Water fleas eat algae (small green producers) in the water. Small stickleback fish eat water fleas, and they are eaten by large fish called pike. (b) Name a type of organism that breaks down dead organisms and their wastes. (c) Add this organism to your food chain, to show the feeding relationship between it and the primary consumer.

Ans: (a) Algae → Water fleas → Stickleback fish → Pike. (b) Decomposer. (c) Algae → Water fleas → Stickleback fish → Pike → Decomposer.

Q14. If floods hit an area, sewage (dirty water containing waste materials) can get into the water supply. Why will boiling water before drinking it prevent diseases?

Ans: Boiling water kills most pathogens present in the water, making it safe to drink.

Q15. A carton of milk is on top of a fridge. Another carton of milk is inside the fridge. (a) Predict which milk will spoil first. (b) Explain your prediction.

Ans: (a) The milk on top of the fridge will spoil first. (b) It is exposed to higher temperatures which promote faster bacterial growth.

Q16. Yeast is used to make bread dough rise. The yeast feed on energy-rich substances in the dough, and their respiration produces carbon dioxide as a waste. A student made six batches of bread dough, and left each at a different temperature for 30 minutes. The student then measured the increase in height of each dough. The table shows the results. (a) What sort of microorganism is yeast? (b) Explain why the height of the dough increased. (c) Suggest what hypothesis the student was investigating. Using the phrase 'depends on' may help you. (d) Use the evidence in the table to make a conclusion.

Ans: (a) Yeast is a fungus. (b) The height of the dough increased because yeast respiration produced carbon dioxide that made the dough rise. (c) The student was investigating the hypothesis that the rate of yeast respiration depends on temperature. (d) The evidence suggests that higher temperatures increase yeast activity and dough rise, up to a certain point.

Q17. Explain why decomposers are an essential part of a healthy habitat.

Ans: Decomposers break down dead organic matter, returning nutrients to the soil and maintaining the natural cycle of nutrients within the ecosystem.

Q18. A baker added some yeast cells to water containing sugar. The number of yeast cells increased for 10 hours, and then stopped increasing. (a) Which life process caused an increase in the number of cells? (b) Suggest why the number of cells stopped increasing.

Ans: (a) Reproduction caused the increase in the number of cells. (b) The number of cells likely stopped increasing due to the depletion of nutrients or accumulation of waste products that inhibited further growth.

Q19. Some scientists do not think that viruses are organisms. Explain why not.

Ans: Viruses are not considered organisms by some scientists because they cannot reproduce on their own and lack cellular structures, relying entirely on infecting a host cell to replicate.

Q20. Measure the labelled yeast cell in figure 2.16 and calculate its size in real life.

Ans: If the cell measures 5 mm on the magnified image and the magnification is ×4000, the actual size of the cell would be 5 mm / 4000 = 0.00125 mm or 1.25 micrometers.