The volume of gas released by an aquatic plant can be used to measure the rate of photosynthesis. The apparatus shown was used to measure the rate of photos...
Question 1 Report
The volume of gas released by an aquatic plant can be used to measure the rate of photosynthesis. The apparatus shown was used to measure the rate of photosynthesis of a piece of pondweed placed in a test tube of water. The gas released from the cut end of the stem was collected in an inverted tube and its volume was measured over a fixed period of time. Before starting, sodium hydrogencarbonate solution was added to the water surrounding the plant. A bright lamp provided a constant light intensity and the temperature of the water was kept steady throughout the investigation. A student used this arrangement to measure the rate of photosynthesis and to confirm which factors were needed for the process to take place.
(a) Name the gas collected in the apparatus. [1]
(b) Describe how you would use this apparatus to measure the rate of photosynthesis. [3]
(c) Explain why sodium hydrogencarbonate solution was added to the water. [2]
(d) Suggest a control that would show the gas is produced by photosynthesis. [2]
(e) Name the green pigment needed for photosynthesis. [1]
Answer Details
(a) Gas collected [1]. The gas given off by the illuminated pondweed is oxygen [1], the by-product of photosynthesis.
(b) How to measure the rate [3]. Measure the volume of gas collected in the inverted tube [1]; over a fixed, measured period of time (using a stopwatch) [1]; then calculate \(\text{rate}=\dfrac{\text{volume of gas}}{\text{time}}\), repeating and taking a mean for reliability [1]. (An equivalent accepted method is to count the bubbles released per minute.)
(c) Why sodium hydrogencarbonate was added [2]. It slowly releases/supplies carbon dioxide to the water [1]; this ensures carbon dioxide is plentiful so that it does not become the limiting factor, and the rate then reflects the factor being studied (light) rather than a shortage of \(\text{CO}_2\) [1].
(d) A control to show the gas is from photosynthesis [2]. Set up identical apparatus but place it in the dark / cover it so no light reaches the plant [1]; little or no gas is then produced, showing that light (and therefore photosynthesis) is needed for the gas to be released [1].
(e) Green pigment [1]. Chlorophyll [1], which absorbs the light energy used to drive photosynthesis.
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