Micro-organisms and the Growth of Yeast, Grade 8
Yeast is alive, and the two ways of stopping it look identical in the results.
Put a bottle of yeast in the fridge and the balloon stays flat. Mix yeast into water at 60 °C and the balloon also stays flat. One of those bottles starts working again when you warm it. The other is dead.
That contrast is the whole point of the prescribed investigation, and most versions of it never set up the bottle that makes the point. This page gives our six-bottle design, the yoghurt temperature that actually works, and four things in this topic that cost learners marks.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 8 |
| Term | 1 |
| Strand | Life and living |
| Topic | Topic 3, Micro-organisms |
| Status | PRESCRIBED PRACTICAL TASK 1 |
| Marks | 20 for the task. The question split on our worksheet is ours |
Our design for the investigation
The curriculum asks the class to investigate the factors that affect the growth of yeast. It does not dictate the bottles. Two factors are worth testing, so six bottles covers both properly: three for food and four for temperature, sharing one baseline.
| Bottle | Sugar | Where it stands | What it tests |
|---|---|---|---|
| A | None | Room temperature | Is food needed at all? |
| B | 15 g | Room temperature | The baseline, used by both halves |
| C | 25 g | Room temperature | Does more food mean more activity? |
| D | 15 g | Fridge | Cold |
| E | 15 g | Warm place, about 30 to 35 °C | Warm |
| F | 15 g | Water at about 60 °C | Hot. Does heat kill it? |
Bottle F is the one worth insisting on. A lot of versions of this investigation run only three temperatures, cold, room and warm, and then ask a question that needs a fourth. Without a hot bottle the class never finds out that heat does something different from cold, and that is the most useful thing in the whole practical.
Weigh the sugar
Sugar is a solid, so measure it in grams. Recipes and worksheets often give it as a volume in millilitres, and a loosely spooned spoonful against a packed one differs by about a third. Every group then gets a different answer for reasons that have nothing to do with yeast.
15 g and 25 g on a balance, or three and five level teaspoons. Whichever you choose, the whole class uses the same one.
Method
- Wash six one-litre plastic bottles and label them A to F. Rinse properly: sugar left in a cool drink bottle will feed the yeast in your control
- One sachet of dry yeast into each, then the sugar from the table
- Add 300 ml of water at the right temperature for that bottle. Cap it, shake to dissolve, then take the cap off
- Stretch a balloon over the neck, a different colour per bottle, and write the colour in your table
- Stand each bottle in its place and leave them for one hour
- Tie the balloon off tightly with string, then lift it off
- Fill a large measuring cylinder or jug and read the water level
- Push the balloon right under and read the level again
- Subtract. After minus before is the volume of gas the yeast made
- Repeat for every bottle
What you should see
| Bottle | Balloon | Why |
|---|---|---|
| A, no sugar | Barely moves | No food, so almost no fermentation. The yeast is alive and has nothing to work on |
| B, 15 g | Inflates | The baseline |
| C, 25 g | Inflates more | More food, more activity, up to a limit |
| D, fridge | Almost flat | Cold slows every reaction right down. The yeast is not dead, it is slow |
| E, warm | The biggest | Yeast works best at about 30 to 35 °C |
| F, 60 °C | Flat | The yeast has been killed. Warm it up and nothing happens |
D and F look the same and mean opposite things. Stand bottle D somewhere warm and it starts working within twenty minutes. Do the same with F and it stays dead. Slowed down is not the same as killed, and a class that has seen both never confuses them again.
One thing to watch in the write-up
It is easy to write that more carbon dioxide means the yeast is growing faster. Carbon dioxide measures how fast the yeast is fermenting, not how fast it is multiplying. Usually the two go together. In bottle A they come apart: the cells are alive and not fermenting.
Write "rate of fermentation" or "how active the yeast is" and the answer is safe either way.
Making yoghurt, at the temperature that works
Yoghurt cultures need 42 to 45 °C. That single number decides whether this practical succeeds, and methods that say to cool the milk to room temperature will not set.
Below about 35 °C the culture works too slowly, and a tepid overnight stand is exactly the window in which spoilage organisms outgrow it.
- Heat the milk to just below boiling, then take it off. This kills competing bacteria
- Cool it to 45 °C, not to room temperature. Use a thermometer. Without one, it should feel warm but not hot on the inside of your wrist
- Stir in three tablespoons of live yoghurt
- Hold it at 40 to 45 °C for six to eight hours. A cooler box with a bottle of hot water in it does this. So does a vacuum flask, or a pot wrapped in blankets in a warm cupboard
- Refrigerate as soon as it has set
If it does not set, do not eat it. Milk that has stood warm for eight hours without a culture taking hold has grown something else instead. Bin it and start again.
Bread mould: grow it, but do not open it
Two slices of bread, one dry and one lightly dampened, each sealed in its own clear bag or jar, left in a dark cupboard for a few days.
The damp slice grows mould and the dry one does not, or does far more slowly. Water is the variable, and the bag lets you look without touching.
The bag stays sealed. Always. Mould spores are a genuine asthma and allergy trigger in a classroom of thirty, and this is a practical that deliberately grows them. Examine the bread through the clear bag with a hand lens, and put the whole thing in the bin still sealed. Very few published versions of this activity say so.
Through the bag at 10x you can see the fine white threads, the hyphae, and the black dots on stalks, which are the sporangia full of spores.
Four things that cost marks in this topic
All four turn up in classroom notes and diagrams, and a learner who repeats any of them loses the mark.
| Commonly written | Correct |
|---|---|
| The hair-like projections on a bacterial cell labelled cilia | Bacteria do not have cilia. Cilia belong to complex cells. The short projections on a bacterium are pili. The single long tail it swims with is a flagellum, and that name is right |
| Bacteria living together in colonies described as multicellular | A colony is not multicellular. It is many separate single cells living side by side. Multicellular means one organism whose cells depend on each other and do different jobs |
| Micro-organisms seen with a hand lens or a microscope | A hand lens cannot show you a bacterium. At 10x it is still invisible. A lens is for whole specimens; a microscope is for cells |
| The malaria parasite laying eggs in the liver | It does not lay eggs. Plasmodium is a single-celled protist and it multiplies by dividing, first inside liver cells and then inside red blood cells |
One more worth checking before a test: the five-kingdom system many notes still use puts bacteria in a kingdom called Monera. Biologists stopped using that name some time ago, so if your class is examined on the five kingdoms, teach it as the name in their material while saying plainly that it is out of date.
The four groups of micro-organism
| Group | Cells | Alive? | Example |
|---|---|---|---|
| Bacteria | Single, simple | Yes | E. coli, Vibrio cholerae, TB |
| Protists | Single, complex | Yes | Amoeba, Plasmodium, diatoms |
| Fungi | Single or many | Yes | Yeast, bread mould, mushrooms |
| Viruses | None. Not cells at all | Generally counted as non-living | HIV, influenza, the common cold |
Yeast is a single-celled fungus, which is why the prescribed practical sits in this topic. Fermenting sugar releases carbon dioxide, which is what inflates the balloon and what makes bread rise.
Bacteria are grouped by shape: rod-shaped are bacilli, round are cocci, spiral are spirilli.
If it does not work
| What happens | What caused it |
|---|---|
| No balloon inflates at all | Dead yeast. Check the expiry date, and check the water was not too hot when you mixed. Test one spoon in warm sugar water first: it should froth within ten minutes |
| Every balloon inflates the same amount | The bottles were not in genuinely different places. A fridge bottle left on the bench is just another room-temperature bottle |
| The balloon slips off | Wet bottle neck. Dry it before stretching the balloon on, and add a rubber band |
| Bottle A inflates as much as B | The bottles were not rinsed. Sugar residue feeds the yeast in your control |
| Displacement readings make no sense | The balloon was not tied tightly, or not pushed fully under. Hold it right down with a spoon |
| The yoghurt is thin and sour-smelling | Too cool for too long. 42 to 45 °C, and do not eat it if it did not set |
| No mould on either slice | Too cold, too dry, or heavily preserved bread. Use plain bread and dampen it properly |
Safety
- Mould bags stay sealed, are examined through the plastic, and go in the bin sealed. Spores trigger asthma
- Never eat yoghurt that did not set. Warm milk that failed to culture has grown something else
- Bottles are not capped while fermenting. A balloon can stretch; a sealed bottle builds pressure
- The hot water for bottle F is poured by the teacher, not by learners
- Wash hands after handling yeast, bread and soil, and before eating
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 20 marks. Our questions on our six-bottle design: the results table, the aim and variables, the food comparison, the temperature comparison and a conclusion
- Marking memorandum, with the full answers, expected gas volumes, and the bottle D against bottle F discussion
Related practicals
- Photosynthesis and respiration, Grade 8. Respiration again, this time in a plant
- Food chains and food webs, Grade 8. Where the decomposers sit
- Plant and animal cells, Grade 9. Using the microscope properly, next year
What you need to run it
The investigation itself is almost free, and that is deliberate. Six recycled bottles, six sachets of supermarket yeast, sugar, balloons and string. It is designed to run in a school with no laboratory, and we are not going to pretend otherwise.
Two things do come from us and both improve the result. A laboratory thermometer is what turns "a warm place" into a number, and bottle F cannot be set up safely or repeatably without one. A plastic measuring cylinder reads the displacement far more accurately than a kitchen jug.
Weigh the sugar rather than spooning it. Any classroom balance that reads to a gram is enough.
For the rest of the topic, the microscope is the practical. Bacteria need 400x and oil immersion to see properly, but yeast cells, mould hyphae and pond protists are all comfortable at 100x to 400x on a school instrument. Our student microscopes start at R1 990, with slides and cover slips from R16 a box.
A prepared slide set is the honest answer for a school that cannot culture anything. A beginner slide set at R400 for twelve mounted specimens shows the class real cells with no risk and no waiting.
One gap in our range, and we would rather name it. Grade 8 work of this kind calls for a handheld micro-viewer at about 20x and 200x, and we do not stock one. We sell hand lenses from R35 and microscopes from R1 990, with nothing in between. We are sourcing it.