Micro-organisms and the Growth of Yeast, Grade 8
The prescribed yeast practical asks for six bottles labelled A to F, and then gives you a results table with five rows.
Bottle F is named in the apparatus list, named in the method, and marked in question 13. Its condition is never stated anywhere in the book. A teacher setting this up has six bottles, five conditions and a question about a variable that does not exist.
This page gives you the missing sixth condition, the temperature the yoghurt practical actually needs, and the four things in this topic that a learner will be marked wrong on if they copy the textbook.
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, prescribed. The question marks in the book sum to 20 and we have kept that allocation exactly |
Practical task 1: the factors that affect the growth of yeast
The missing bottle, and what to put in it
Question 13 groups bottles B, D, E and F together as one investigation. B, D and E are the temperature series: room temperature, fridge, warm place. So F is the fourth temperature, and the obvious one missing from that set is hot.
Use 15 ml of sugar in water at about 60 °C. It completes the series, and it gives the class the result the other three cannot: yeast is a living organism and heat kills it. The balloon on F stays flat, and it stays flat for a different reason from bottle A.
| Bottle | Condition | Testing |
|---|---|---|
| A | No sugar, room temperature | Is food needed? |
| B | 15 ml sugar, room temperature | The baseline |
| C | 25 ml sugar, room temperature | Does more food mean more activity? |
| D | 15 ml sugar, in the fridge | Cold |
| E | 15 ml sugar, in a warm place | Warm |
| F, ours | 15 ml sugar, water at about 60 °C | Hot. Does heat kill it? |
We have supplied bottle F because the book does not. If your school's edition prints a condition for F, use theirs. Ours completes the temperature series that question 13 marks.
Measure the sugar in grams, not millilitres
The book asks for "15 ml sugar" and "25 ml sugar". Sugar is a solid. A loosely spooned 15 ml and a packed 15 ml differ by a third, and every group gets a different answer.
Use 15 g and 25 g on a balance, or three and five level teaspoons. Say which you used, and make the whole class use the same one.
Method
- Wash six one-litre plastic bottles and label them A to F
- One packet of dry yeast into each, then the sugar according to the table
- Add 300 ml of water at the right temperature for that bottle. Cap, shake to dissolve, then remove the cap
- Stretch a balloon over the neck, a different colour for each bottle, and record the colour
- Put each bottle in its place and leave for one hour
- Tie the balloon off tightly with string, then lift it off the bottle
- Fill a large measuring jug and record the water level
- Push the balloon right under and record the new level
- Subtract: after minus before is the volume of gas
- Repeat for each bottle
What you should see
| Bottle | Balloon | Why |
|---|---|---|
| A, no sugar | Barely moves | No food, so almost no fermentation. Yeast is alive but has nothing to work on |
| B, 15 g, room temp | Inflates | The baseline |
| C, 25 g, room temp | Inflates more | More food, more activity, up to a limit |
| D, fridge | Almost flat | Cold slows the reactions right down. The yeast is not dead, it is slow |
| E, warm | The biggest | Warmth speeds the reactions up. Yeast works best around 30 to 35 °C |
| F, 60 °C | Flat | The yeast has been killed. Warm it up again and nothing happens |
D and F look the same and mean opposite things. That is the best question in the practical. Take bottle D out of the fridge and stand it in a warm place: it starts up. Do the same with F and it stays dead. Slowed down is not the same as killed.
One thing to be careful of when you write it up
The book says "the more carbon dioxide produced, the faster the yeast cells are growing". Carbon dioxide measures how fast the yeast is fermenting, not how fast it is multiplying. They usually go together, but 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: the temperature the book gets wrong
The printed method says to let the milk cool to room temperature and then leave it overnight in a warm place. That will not set.
Yoghurt cultures need 42 to 45 °C. Below about 35 °C they work too slowly, and a tepid overnight stand is exactly the window in which spoilage organisms outgrow them.
- 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. If you have none, 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, or a 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. 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. The dry one does not or does so far more slowly. Water is the variable, and the bags let you look without touching.
The bag stays sealed. Always. Mould spores are a real asthma and allergy trigger in a classroom of thirty. Examine the bread through the clear bag with a hand lens, and put the whole thing in the bin still sealed. The textbook does not say this and it should.
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 the textbook gets wrong here
These are all things a learner would be marked down for if they copied them into an exam.
| The book | Correct |
|---|---|
| The bacterial cell diagram labels the surface projections "cilia" | Bacteria do not have cilia. Cilia belong to complex cells. The short projections on a bacterium are pili. The long tail for swimming is a flagellum, and that part is right |
| "All bacteria are unicellular although some live together in colonies that are multicellular" | A colony is not multicellular. It is many separate single cells living together. Multicellular means one organism made of cells that depend on each other |
| Micro-organisms are visible with "a hand lens or a microscope" | A hand lens will not show you a bacterium. At 10x it is still invisible. That is what the microscope is for |
| Plasmodium travels to the liver "where it lays eggs" | Plasmodium does not lay eggs. It is a single-celled protist and it multiplies by dividing, inside liver cells and then inside red blood cells |
Two more worth knowing: the anthrax bacterium is Bacillus anthracis, not "Bacillucis", and the kingdom name Monera is no longer used by biologists, although CAPS material still prints it.
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 fungus and it is unicellular, which is why the prescribed practical sits in this topic. When it ferments sugar it 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 use a rubber band as well |
| Bottle A inflates as much as B | The bottles were not rinsed. Sugar residue from a cool drink bottle feeds the yeast in your control |
| Displacement readings make no sense | The balloon was not tied tightly, or it was 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 of bread | Too cold, too dry, or the bread was heavily preserved. 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
- Hot water for bottle F is prepared 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.
- Practical task 1 worksheet, 20 marks. The prescribed mark allocation exactly, with the six-bottle table completed and the aim, hypothesis and variables laid out
- Marking memorandum, with the full answers, the mark split question by question, 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 practical task itself is almost free. Six recycled bottles, six packets of supermarket yeast, sugar, balloons and string. That is the point of the way it is written, 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 without one bottle F cannot be set up safely or repeatably. 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 schools 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.
The book also names a micro-viewer at 20x and 200x. We do not stock one yet and we are sourcing it, because it sits exactly in the gap between a R35 hand lens and a R1 990 microscope.