Population growth curve: yeast in a flask sim

What the sim shows

Population growth curve: yeast in a flask. A 250 cm³ conical flask holds 100 cm³ of glucose broth (1 % glucose: 1 g of glucose), closed with a cotton wool plug and kept at 25 °C. Press Fresh flask to add the yeast (about 1 000 live cells per mm³): that is 0 h. Drag the glucose syringe onto the flask to add 2 cm³ of 50 % glucose solution (1 g of glucose) whenever you like: that changes the food supply. Every 8 h, drag the dropping pipette into the flask to take a sample, choose a dilution (made with methylene blue solution), and drag the pipette onto the counting chamber on the microscope stage. Count the live cells in the central grid: it is 1 mm × 1 mm and 0.1 mm deep, so it holds 0.1 mm³. A usual rule of thumb: count only when the grid holds 30 to 300 cells, live and dead together; if it is more crowded, dilute more. Live yeast cells turn methylene blue colourless; dead cells cannot, so they stay blue. Live cells per mm³ = live cells counted × dilution × 10: type it into the table, then press Wait 8 h. The graph plots your counts against time. The curve is S-shaped (sigmoid): a lag phase while the cells adjust and make the enzymes they need, a log (exponential) phase while food is plentiful and the number doubles in equal times, a stationary phase when the glucose (a limiting factor) runs out and new cells are made only about as fast as cells die, and a death phase when cells die faster than new ones are made. More glucose gives a higher stationary phase. Simplified on purpose: the cell numbers are illustrative (≈): they come from a model in which a cell divides every 6 h at 25 °C, the yeast first spends a few hours adjusting to the broth, and the cells starve once the glucose runs out; real cultures vary; here only the glucose limits the yeast; in a real flask the ethanol and other wastes the yeast makes also build up and can add to the deaths; each count varies by a seeded amount (up to 5 %), so the same sample gives the same count if you do it again; a real count of only a few cells varies far more, so you would count more squares or a second chamber and take the mean; the counter counts the live cells in the grid for you: in a real count you count them yourself, square by square; the flask is kept at exactly 25 °C and is swirled before every sample, so the cells are spread evenly; the samples are too small to change the flask; each syringe of glucose adds 2 cm³ of liquid, and the model dilutes the culture by that amount; dead cells stay whole and stay blue, and only the dead cells are drawn settling on the flask's floor; real dead cells slowly break apart, and in an unshaken flask live cells settle too; the phase boundaries on the graph are model rules: lag until the population grows at half its fastest rate, log while it keeps that up, stationary until it falls below 90 % of its peak, then death; the bench apparatus is drawn to scale; in the close-up the cells are drawn about 3 times larger than they would look, so you can see them.

• Lit: the dropping pipette's teat
• Close-up: the counting grid under the microscope, the tally counter, and the flask at every sample
• Dashed ring: where the carried pipette or syringe goes

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