Method
Put a piece of pondweed (Elodea or Cabomba), cut end upwards, in a
boiling tube of water containing sodium hydrogencarbonate , which
provides carbon dioxide so it is not the limiting factor
Stand the tube in a beaker of water to act as a heat shield ,
keeping the temperature constant, and use an LED lamp , which gives
off little heat
Put the lamp 10 cm from the tube and leave for a few minutes to
acclimatise
Count the bubbles of oxygen given off in one minute. Repeat twice
more and take a mean Move the lamp to 20, 30, 40 and 50 cm and repeat
Plot rate (bubbles per minute) against distance, or (HT) against
1 d 2 \frac{1}{d^2} d 2 1
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Required practical — the rate of photosynthesis lamp measure the distance from the lamp to the pondweed beaker of water — keeps the temperature constant bubbles of oxygen rise from the cut stem — count them for a fixed time pondweed in sodium hydrogencarbonate solution, which supplies carbon dioxide count bubbles per minute Move the lamp to different distances and count the bubbles at each one. Light intensity is proportional to 1 ÷ distance², so doubling the distance quarters the intensity. Keep the temperature and the carbon dioxide supply the same throughout — the beaker of water absorbs the heat from the lamp so that only the light intensity changes. The set-up: a lamp at a measured distance from a boiling tube of pondweed in sodium hydrogencarbonate solution, standing in a beaker of water, bubbles rising from the cut stem, with a ruler and stopwatch
Variables: independent distance from the lamp (light intensity);
dependent number of bubbles per minute (rate of photosynthesis);
control temperature (water bath), carbon dioxide concentration
(same solution), the same piece of pondweed, and other lights in the room
(darkened)
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In the full topic
• Method• Results and evaluation• The leaf disc methodRead the free biology topics →