OPOpenPithMake curiosity visibleBiology lab / Life processes

About this lab

Five investigations: from one enzyme, to one population, to one law of inheritance.

The investigations in a biology textbook all ask the same kind of question: change one condition, and how does a living process respond? Five of them share one bench here. Each topic changes one or two conditions and holds the rest fixed.

Every reading and every curve comes from one model. The yeast counts and the ball draws are random, but the same settings always give the same sequence, so a whole class can compare notes.

Five findings

Five findings

  • Enzyme activity: at pH 7, call the activity at 37 °C 100 %. It is 53.4 % at 20 °C, 23.7 % at 0 °C, only 12.8 % at 60 °C and 0.2 % at 80 °C. At 37 °C the activity is 51.0 % at both pH 5 and pH 9, and only 1.0 % at pH 3. 20 °C is slower than 37 °C but still gives 21.6 mL of oxygen within 60 seconds.
  • Photosynthesis: in 0.5 % sodium bicarbonate with the lamp at 20 cm, the net rate is 1.74 mL/h and you count 14 bubbles in 5 minutes. At 10 cm the light is four times stronger but the count only reaches 24. At 50 cm the light is about 32 µmol/(m²·s), below the compensation point of 32.9, and there are no bubbles at all.
  • Osmosis: with 0.3 mol/L sucrose in the funnel and pure water in the beaker, the level starts at 1.95 mm/min and climbs 5.69 cm in 30 minutes. At 0.1 mol/L the starting rate is 0.65 mm/min, exactly one third. With 0.3 mol/L on both sides the level does not move.
  • Population: with 5 % glucose at 28 °C the yeast reaches K/2 at hour 9.1, where growth is fastest. Growth then slows and the population settles near 60 million cells per millilitre. Raise the glucose to 10 % and K becomes 120 million per millilitre.
  • Segregation ratio: with 20 draws group 1 gets 6 : 9 : 5 and group 2 gets 3 : 11 : 6. With 400 draws the dominant shares of the three groups are 78.0 %, 78.8 % and 73.3 %, and all three pass the chi square test.

Model and sources

Model and sources

  • Enzyme: 2 mL of 3 % hydrogen peroxide (0.88 mol/L) in the tube. The substrate is used up by the Michaelis–Menten equation with Km = 0.1 mol/L. Each 10 °C rise speeds the reaction 1.5 times, and half the enzyme is denatured at 48 °C. The pH curve is the bell shape of two ionising groups with its best value at pH 7. Oxygen is converted at 24.5 L/mol for 25 °C.
  • Photosynthesis: the lamp is a point source, so light falls with the square of the distance, set to 800 µmol/(m²·s) at 10 cm. Gross photosynthesis saturates with light along a hyperbola whose ceiling depends on the bicarbonate concentration. Respiration is 0.8 mL/h and one bubble is 0.01 mL.
  • Osmosis: osmotic pressure follows the van ’t Hoff equation π = cRT at 20 °C. The membrane radius is 2 cm, the stem bore radius is 3 mm and the funnel holds 30 mL of solution, which the incoming water dilutes. The equation is stepped once a second with the fourth order Runge–Kutta method.
  • Population: the exact solution of the logistic equation, seeded with 1 million cells per millilitre. The liquid above one counting square is 0.2 mm × 0.2 mm × 0.1 mm, and the cell counts are drawn from a Poisson distribution.
  • Segregation ratio: each bucket holds 10 D balls and 10 d balls, and each ball goes back after it is drawn. The chi square test uses the 0.05 level, where the critical value for 2 degrees of freedom is 5.99.

The enzyme Km, the denaturation temperature and the membrane permeability are teaching values chosen to give sensible speeds. Bubble size, foam height and the cloudiness of the culture are sketches and take no part in the calculation.

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