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AQA GCSE Biology (9-1): Getting the most out of Required Practicals - Photosynthesis

Posted by James Simms on November 25, 2019

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Specification location: AQA GCSE Biology Paper 1, Section 4.4 - Bioenergetics

Summary of method: A cut piece of an aquatic plant (usually pondweed) is placed in a beaker under the water (usually with an inverted funnel submerged over it and a filled measuring cylinder placed over the funnel to collect and measure the volume of gas produced). A light source (usually a lamp) is shone on the pondweed from a distance and the rate of bubble production is measured (as is the total volume of gas produced over a set time). The distance is then varied (e.g. 100cm, 80cm, 60cm, 40cm, 20cm) and the experiment repeated.

For more detail on the set-up of this experiment, watch our high-quality required practical tutorial on YouTube.

Photosynthesis required practical variables:

Independent variable: Distance of the plant from the light source (light intensity)

Dependent variable: Rate of photosynthesis (rate of gas production)

Control variables: Temperature, light colour, carbon dioxide concentration

Common issues	Potential solutions
Time	With a 60-minute lesson, it can be challenging to explain the experiment, get students to effectively set it up and carry it out and to analyse the results. This is especially a problem when considering high[performing students, where discussion of the results alongside the inverse square law is desirable as part of extended analyses. It is, therefore, necessary to be as efficient with class time as possible, to avoid splitting the practical over two lessons. By far, the best solution I have found is for pupils to preload the investigation’s methodology through a flipped learning technique (e.g. viewing and note-taking tutorials and undertaking quizzes on https://www.theeverlearner.com/). This minimises lag time at the start through misunderstanding and also the time needed to be spent on covering the required practical method and photosynthesis content again.
Setting up equipment	Students seem to have a lot of difficulty filling the measuring cylinder with water and inverting it over the top of the submerged funnel fast enough to keep all the water in it. Teacher modelling and student practice is thoroughly advised.
Slow rate of photosynthesis	Depending on the conditions, the rate of photosynthesis in this experiment may be extremely slow, perhaps even too slow for the investigation. The rate can be accelerated by using sodium hydrogen carbonate solution (to increase the carbon dioxide available to the plant) and to use warm water (to accelerate the chemical reactions). I would still advise having a set of pre-prepared results, in case this experiment does not work as intended.
Variables	As the light source approaches the pond weed, it is likely that the water will begin to heat slightly over the course of the experiment (especially when the lamp is in proximity). It is advisable then to place a transparent heat shield in between the lamp and the pond weed (a large beaker full of water should work well).

Sample photosynthesis required practical results:

	Distance from light source (cm)
	100	80	60	40	20
Number of gas bubbles/min	0	1	3	6	10
Volume of gas collected in 5 mins (cm³)	0.00	0.75	2.25	4.50	7.50

Source: The EverLearner greenhouse

AQA GCSE Science subject-specific terminology:

As the content supporting this required practical is more straightforward, this is a good opportunity for students to discuss predictions and hypotheses.
It may also be desirable as part of students’ predictions to draw a sketch graph of what they think the results will be.