echoesof.science · Ages 15–16
Microbial growth and fermentation
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the game and the quiz
echoesof.science/school/microbes/ages-15-16/
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Bacteria are prokaryotes: single cells without a membrane-bound nucleus, usually 0.5–5 μm wide. Fungi such as yeasts and moulds are eukaryotes with rigid cell walls and no chlorophyll.
Viruses are acellular. A virus is DNA or RNA (never both) inside a protein coat called a capsid, sometimes with a lipid envelope. Most are about 20–900 nm across, too small for a light microscope, and they can only replicate inside a host cell.
Bacteria usually reproduce by binary fission: one cell divides into two. The generation time is the time for the population to double; E. coli can double in as little as 20 minutes in ideal laboratory conditions.
Experiment: Rate of yeast fermentation and temperature
- Mix a set mass of yeast with a set volume of sugar solution. Divide equally into three boiling tubes.
- Place each tube in a water bath at a different temperature and let it settle for 5 minutes. Record the actual temperatures.
- Fit the bung and delivery tube and collect the gas over water in an upturned measuring cylinder (or count bubbles per minute).
- Record the volume of gas every 2 minutes for 16 minutes.
- Plot volume against time for each temperature. Calculate the mean rate (cm³/min) from the steepest straight part. Repeat if time allows and estimate the uncertainty from the spread.
| Time (min) | Gas volume at ~20 °C (cm³) | Gas volume at ~30 °C (cm³) | Gas volume at ~40 °C (cm³) |
|---|---|---|---|
| 0 | Predict the order of rates and sketch the shape of each graph. | ||
| 4 | Predict the order of rates and sketch the shape of each graph. | ||
| 8 | Predict the order of rates and sketch the shape of each graph. | ||
| 12 | Predict the order of rates and sketch the shape of each graph. | ||
| 16 | Predict the order of rates and sketch the shape of each graph. |
Questions
Compare the structure of a bacterium, a yeast cell and a virus.
A culture starts with 500 bacteria and the generation time is 20 minutes. How many cells after 2 hours, assuming log-phase growth? Show your working.
Use your graphs to describe how temperature affected the rate of fermentation. Quote a rate with units.
Give two sources of uncertainty in your method and one improvement.
Sources for every fact on this sheet: echoesof.science/school/microbes/sources/