Fig. 1.1 shows the apparatus a class used to compare the rate of photosynthesis of a shoot of pondweed at different distances from a bench lamp. The oxygen ...
Question 1 Report
Fig. 1.1 shows the apparatus a class used to compare the rate of photosynthesis of a shoot of pondweed at different distances from a bench lamp. The oxygen released collected as bubbles, which a student counted for one minute at each lamp distance. A ruler beside the beaker gave the scale. The counts are shown in Table 1.1.
| Distance / cm | Trial 1 bubbles per minute | Trial 2 bubbles per minute | Mean bubbles per minute |
|---|---|---|---|
| 10 | 48 | 52 | 50 |
| 20 | 26 | 24 | |
| 30 | 13 | 11 | 12 |
| 40 | 7 | 5 |
(a) Measure, using the scale on Fig. 1.1, the distance in cm between the lamp and the near side of the beaker. [1]
(b) Measure the length of the pondweed shoot drawn in Fig. 1.1. [1]
(c) State the independent variable in this investigation. [1]
(d) State the dependent variable. [1]
(e) Complete Table 1.1 by calculating the mean number of bubbles per minute for the 20 cm and the 40 cm distances. [2]
(f) Describe the relationship shown between the lamp distance and the rate of bubble production. [2]
(g) Explain, using the idea of a limiting factor, why the rate changed as the lamp was moved closer to the beaker. [3]
(h) State two variables that must be kept constant for the results to be valid. [2]
(i) Suggest why counting bubbles by eye may give unreliable results, and describe one change that would improve the reliability. [3]
(j) The student assumed the gas was oxygen. Describe a test that would confirm the gas is oxygen. [2]
(k) Calculate the percentage decrease in the mean rate when the lamp was moved from 10 cm to 30 cm. Show your working. [2]
Answer Details
Labelled answer diagram:
This question tests the practical skills of measuring, reading data tables, calculating means and percentages, and explaining photosynthesis in terms of a limiting factor. Light intensity from a point source falls off with distance, so moving the lamp changes the light reaching the pondweed and therefore the rate at which oxygen bubbles are released.
(a) Read the distance from the lamp to the near side of the beaker directly off the ruler in the figure. Any value close to 10 cm (accept 9-11 cm) earns the mark. [1]
(b) Read the length of the drawn shoot off the same scale, e.g. about 6 cm (any reasonable value read from the scale is accepted). [1]
(c) The independent variable is the one you deliberately change: the distance of the lamp from the pondweed / beaker. [1]
(d) The dependent variable is what you measure in response: the number of bubbles produced per minute (the rate of bubbling). [1]
(e) The mean is the sum of the two trials divided by 2:
\[ \text{20 cm mean} = \frac{26 + 24}{2} = 25 \quad \textbf{[1]} \qquad \text{40 cm mean} = \frac{7 + 5}{2} = 6 \quad \textbf{[1]} \](f) Describe the trend using the data: as the distance increases, the rate decreases [1], and it falls steeply, more than halving each time the distance is doubled (50 to 25 to 12 to 6) [1]. [2]
(g) Moving the lamp closer increases the light intensity reaching the pondweed [1]. Here light was the limiting factor - the single factor in shortest supply that was holding the rate back [1] - so supplying more of it lets the rate of photosynthesis increase [1]. [3]
(h) Any two variables that would otherwise change the rate and confuse the results, for example the temperature of the water and the carbon dioxide / hydrogencarbonate concentration (also acceptable: the same piece of pondweed, the length of shoot, or the time counted). Controlling them ensures only lamp distance affects the rate. [2]
(i) Counting by eye is unreliable because bubbles vary in size, so an equal number of bubbles is not an equal volume of gas, and very fast bubbling is easy to miscount [1]. Improve it by collecting the gas in a capillary tube or measuring cylinder and measuring its volume instead of counting [1], and by repeating each reading and taking a mean [1]. [3]
(j) To confirm the gas is oxygen, collect a test tube of it [1] and hold a glowing splint inside: oxygen makes it relight [1]. [2]
(k) Percentage decrease uses the change over the original value:
\[ \frac{50 - 12}{50} \times 100 = \frac{38}{50} \times 100 = 76\,\% \]Working [1], answer 76 % [1]. [2]
Exam tip: when a lamp is the source, always link "closer / brighter" to "higher light intensity", then name light as the limiting factor before saying the rate rises - stating the rate change alone will not gain the explanation marks.
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