Microorganisms: Helpful and Harmful — Answers
Answer sheet with model answers and success criteria for the 6th Grade living things & life cycles worksheet.
Success TutoringWorksheetsAnswer sheet6th Grade · Science · Living Things & Life CyclesMicroorganisms: Helpful and Harmful — Answers
Part A: Tiny living things
Read the passage. Then circle the best answer.
Microorganisms (also called microbes) are living things that are too small to see without a microscope. Two important groups are bacteria and some fungi, such as yeast and mold. A patch of mold on old bread is a mass of fungus that has grown big enough for us to see.
Helpful microorganisms: yeast makes bread rise by giving off bubbles of carbon dioxide gas. Some bacteria turn milk into yogurt and cheese. Decomposers (bacteria and fungi) break down dead plants and animals and return nutrients to the soil. Bacteria living in our gut help us digest food.
Harmful microorganisms: some bacteria and fungi spoil food, making it unsafe to eat. Some microbes cause illnesses. Germs is an everyday word for microbes that can make us sick (viruses, which cause colds and flu, are germs too).
Most microorganisms grow fastest when they have food, moisture (water) and warmth. That is why we keep food in the fridge, cover it, and cook it properly. Washing our hands with soap and water, and covering coughs and sneezes, helps stop germs spreading.
- Aa living thing too small to see without a microscope
- Ba very small insect
- Ca type of plant seed
- Da tiny piece of dust
Answer: a living thing too small to see without a microscope
Micro means very small. Microorganisms are living things so tiny that we need a microscope to see one.
- Amold
- Byeast
- Ca virus
- Dalgae on a pond
Answer: yeast
Yeast is a tiny fungus. It feeds on sugar and gives off carbon dioxide gas, which makes bubbles in the dough.
- AThe cold kills every microorganism.
- BIt makes the milk thicker.
- CThe cold slows down the growth of microorganisms that spoil it.
- DMicroorganisms only grow in the dark.
Answer: The cold slows down the growth of microorganisms that spoil it.
Cold temperatures slow down the growth of bacteria and fungi, so food lasts longer. They are not all killed, which is why food still spoils in the end.
- AThey make the compost hot and dry.
- BThey keep insects away.
- CThey turn compost into plastic.
- DThey break down dead plant material into nutrients for soil.
Answer: They break down dead plant material into nutrients for soil.
Bacteria and fungi are decomposers. They break down dead plants and food scraps, returning nutrients to the soil.
- AWash them well with soap and water.
- BRinse them quickly with cold water only.
- CWipe them on your clothes.
- DBlow on them.
Answer: Wash them well with soap and water.
Washing well with soap and water, then drying your hands, removes most germs.
Part B: Helpful or harmful?
- yeast making bread rise
- mold growing on old bread
- bacteria turning milk into yogurt
- harmful
- helpful
- helpful
Answer: yeast making bread rise → helpful; mold growing on old bread → harmful; bacteria turning milk into yogurt → helpful
Answer: decomposers
Answer: carbon
Answer: Any two of: warmth, moisture (water), food.
Part C: True or false?
Write True or False.
Answer: False
Many microorganisms are helpful, such as yeast for bread, bacteria for yogurt and decomposers that recycle nutrients.
Answer: True
Cold slows them down, so food stays fresh for longer.
Answer: True
Yeast is a tiny, single-celled fungus.
Answer: False
They are too small to see without a microscope. A microscope makes them big enough to see.
Part D: Working scientifically: balloon yeast
You can try this at home with a grown-up. Use warm water from the tap, not hot water.
Sam puts one teaspoon of dried yeast and one teaspoon of sugar into each of two small plastic bottles. He adds half a cup of warm tap water to bottle A and half a cup of cold tap water to bottle B, then stretches a balloon over the top of each bottle. After 20 minutes he compares the balloons.
Answer: The temperature of the water (warm or cold).
Answer: How much the balloon blows up (inflates).
Answer: Any two of: the same amount of yeast, the same amount of sugar, the same amount of water, the same size bottles and balloons, the same waiting time.
Model answer: Yeast is more active in warm water than in cold water. The yeast fed on the sugar and gave off carbon dioxide gas, which filled the balloon. In the warm bottle the yeast was more active, so it made more gas and the balloon blew up more.
Success criteria:
- States that the yeast was more active in warm water
- Explains that yeast feeds on sugar and gives off carbon dioxide gas
- Links more gas to a bigger balloon
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.
Great work. Here's what to try next: