(9) Measuring the rate of photosynthesis - experimental method 1 measuring the volume of oxygen produced with a gas syringe

• Possible practical work you may have encountered - methods of measuring the rate of photosynthesis

  • You can investigate the need for chlorophyll for photosynthesis with variegated leaves

  • Taking thin slices of potato and apple and adding iodine to observe under the microscope - test for starch, which gives a blue colour with iodine.

  • Investigating the effects of light, temperature and carbon dioxide levels (using Canadian pondweed, Cabomba, algal balls or leaf discs from brassicas) on the rate of photosynthesis.

  • You can use computer simulations to model the rate of photosynthesis in different conditions

  • You can use sensors to investigate the effect of carbon dioxide and light levels on the rate of photosynthesis and the release of oxygen.

• You may have done/seen experiments on the rate of photosynthesis in which the volume of oxygen formed is measured with a gas syringe connected to a flask of sodium hydrogen carbonate solution (to supply the carbon dioxide) and Canadian pondweed immersed in it.

• All experimental methods depend on measuring the rate of oxygen production as a measure of the rate of photosynthesis.

  • The faster the oxygen production the faster the photosynthesis.

  • It is assumed that the rate of oxygen production is proportional to the rate of photosynthesis.

  • So, how can we measure the rate of photosynthesis?

• Next, methods of measuring the rate of photosynthesis

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You can use this gas syringe system to measure the effects of changing temperature, light intensity and carbon dioxide level (via different concentrations of sodium hydrogencarbonate solution).

• method 1.

• Method 1. Gas syringe system

  • A lamp and thermostated water bath are not shown in this diagram, but they are in the apparatus diagram for method 2.

  • There are several aquatic plants you can use, the most popular seems to Canadian pondweed (elodea canadensis), but this is regarded as an invasive species, so perhaps some other oxygenated aquatic plant should be used!

  • In this 'set-up' you measure the rate of photosynthesis by measuring the rate of oxygen production as the gas is collected in the gas syringe.

    • From the graph of volume of oxygen versus time you measure the initial gradient to calculate the rate of production of oxygen as a measure of the rate of photosynthesis.

    • The graph should be reasonably linear at first e.g. rate of photosynthesis in cm³/min.

  • You can use sodium hydrogencarbonate (NaHCO₃) as source of carbon dioxide and vary its concentration to vary the carbon dioxide concentration. You can use from 0.1% to 5% of NaHCO₃ i.e. 0.1g to 5g per 100 cm³ of water.

    • With increasing concentration you should see an increase in the rate of oxygen bubbles (eg cm³/min), but you must keep the temperature constant eg lab. temp. 20-25^oC, and the light intensity constant by keeping the lamp (not shown in the diagram) the same distance from the flask.

    • The light from the laboratory itself will contribute, but the total light should be constant.

    • You need to use the same quantity and batch of pondweed (or other oxygenating aquatic plant).

    • You use the same volume of water/sodium hydrogencarbonate solution.

    • Using the set-up described in the diagram, at constant temperature, constant light intensity - by using same lamp at the same distance from the flask, you can investigate the effect of the concentration of carbonate/carbon dioxide on the rate of photosynthesis.

  • To vary temperature you need to immerse the conical flask in a thermostated water bath (not shown here, see method 2. Part 10 diagram) of different, but carefully controlled constant temperatures.

    • You should be able to demonstrate a maximum ~35-40^oC i.e. the rate should be significantly lower at ~20^oC and 50^oC.

    • The concentration of NaHCO₃ and the light intensity should be both kept constant.

  • Varying the light intensity is quite difficult, you need to position a lamp at different measured distances away from the flask, but for accurate results you must take a light meter reading by the flask in the direction of the lamp - but you can still use the basic set-up of apparatus described in method 1. above.

    • A lamp in position is shown in method 2. and see the discussion on the inverse square law in method 2.

  • This simple experiment can readily show in principle the effect of changing the three controlling factors of the rate of photosynthesis.

  • Problems and errors with the method

    • Ideally the experiments should be done in the dark, with the lamp the only source of light, not very convenient in a classroom situation but it is particularly important when varying the light intensity - I don't see how you can get accurate results for light intensity though using a light meter might just ok?

    • Do you swirl the flask so the NaHCO₃ concentration remains reasonably constant?, but will the same leaf area be exposed to the light in the direction of the lamp?

    • When varying the temperature it is not easy to maintain a constant temperature - if it falls a little, you could use the average temperature, not as accurate, but better than nothing! A thermostated water bath would be ideal.

  • The above apparatus is typical of that used in rate of reaction experiments in chemistry.

  • How can we measure the speed or rate of a chemical reaction?

  • See graphs in photosynthesis Part 6.

• You can use other experiment designs to look more conveniently, and hopefully more accurately at the three factors that influence the rate of photosynthesis.

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Key points about photosynthesis investigations using the gas syringe method

Photosynthesis Rate Experiment (Gas Syringe Method)

Core Practical Setup for this method of measuring the rate of photosynthesis

• Organism used: Aquatic plant (e.g., Elodea or pondweed).

• Apparatus: Conical flask with pondweed + sodium hydrogencarbonate solution, delivery tube, gas syringe.

• Process:

  • Pondweed photosynthesises → releases oxygen.

  • Oxygen collected in gas syringe → volume measured over time.

• Variables:

  • Independent:

    • Light intensity (distance of lamp).

    • Temperature (water bath).

    • Carbonate concentration (amount of sodium hydrogencarbonate).

  • Dependent: Volume of oxygen collected (rate of photosynthesis).

  • Control: Same species, same volume of solution, same apparatus, same time interval.

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Typical Exam Board Coverage of photosynthesis investigations using a gas syringe

Required Knowledge
Core practical: investigate effect of light intensity on photosynthesis using pondweed.
Practical skills: measuring oxygen production, variables, reliability, evaluation.
Photosynthesis practicals: light intensity, temperature, carbonate concentration.
Emphasis on planning, controlling variables, interpreting graphs.
Investigating photosynthesis rate with pondweed; importance of carbonate ions.
Practical work: effect of light intensity and temperature on photosynthesis.
Core practical: measuring oxygen production with pondweed; effect of light intensity, temperature, CO2 concentration.  For all above see also Part 10 Measuring the rate of photosynthesis - experimental method 2 - timing the movement of a gas bubble

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Data Collection & Analysis in photosynthesis investigations using a gas syringe

• Measure oxygen volume at regular intervals (e.g., every 2 minutes).

• Plot rate of photosynthesis against variable tested:

  • Light intensity → inverse square law relationship.

  • Temperature → bell-shaped curve (optimum ~25–35°C).

  • Carbonate concentration → increases until saturation point.

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Student Tips about photosynthesis investigations using a gas syringe

• Always repeat readings → calculate mean for reliability.

• Control extraneous variables (e.g., same lamp bulb, same pondweed length).

• Use graphical analysis:

  • Identify limiting factors.

  • Recognise plateau when another factor becomes limiting.

• Link experiment to photosynthesis equation:
   

•  carbon dioxide + water == light + chlorophyll  ==> glucose + oxygen

•  6H₂O_(l) + 6CO_2(g) == sunlight ==> C₆H₁₂O_6(aq) + 6O_2(g)

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Common Misconceptions about photosynthesis

• Bubbles ≠ oxygen: Students often assume bubbles are pure oxygen; they may contain other gases. Gas syringe is more accurate.

• Light intensity versus distance: Forgetting inverse square law (intensity 1/d²).

• Temperature extremes: Believing higher temperature always increases rate; enzymes denature above optimum.

• Carbonate concentration: Assuming unlimited increase; in reality, rate plateaus when another factor limits.

• Photosynthesis vs. respiration: Confusing oxygen release with respiration rather than photosynthesis.

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Exam Tips on photosynthesis

• Be precise: “Rate of photosynthesis measured by volume of oxygen collected per unit time.”

• Always state independent, dependent, and control variables clearly.

• When describing graphs:

  • Initial rise → factor increases rate.

  • Plateau → limiting factor changes.

  • Drop at high temperature → enzyme denaturation.

• Use scientific terminology: “limiting factor,” “denaturation,” “inverse square law.”

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Keywords, phrases and learning objectives for this part on photosynthesis

Be able to describe and analyse the results of an investigation experiment to measure rate of photosynthesis by measuring the volume of oxygen produced using a gas syringe investigation method.

Be able describe the experimental method, apparatus and observation data of a gas volume experiment of photosynthesis and also describe how to get experiment results at different temperatures, different light intensities and different carbonate concentrations (= varying CO₂ concentration)..

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