(10) Measuring the rate of photosynthesis: experimental method 2 - timing movement of a bubbles of gas in a capillary tube

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

At the end of method 2 the inverse square low of light intensity is explained.

Method2.

• Method 2. Following the gas evolution from a gas bubble in a capillary tube

  • The 'set-up'

    • I've seen this sort of set-up in textbooks and on the internet and it seems ok in principle, but I have doubts about its use in practice?

    • In this the Canadian pondweed (elodea) is enclosed in a boiling tube and placed in a large beaker of water that acts as a simple thermostated bath to keep the temperature constant. Again a thermostated water bath would be ideal.

    • A lamp is positioned at suitable distances with a ruler.

    • The oxygen bubbles are channelled into a capillary tube.

    • From the rate of movement of the bubbles you get an estimate of the rate of production of oxygen as a measure of the rate of photosynthesis.

    • It might ok just to measure the speed of a bubble down the capillary tube, BUT what happens if it fills with oxygen gas - you won't see any movement.

    • The general points about investigating the three variables were described in method 1. should be no need to repeat them.

  • How do you measure the rate?

    • You can measure the speed of an air bubble by the scale,

    • If you used a gas syringe here you would get a mixture of gas and liquid in the syringe - not very satisfactory, liquid in the syringe might make it quite stiff in movement and difficult to measure an accurate volume of oxygen gas formed.

 

• Further thoughts on the experimental methods described in methods 1. and 2. above for determining the rate of photosynthesis in Canadian pondweed experiment.

• The 'set-up' probably the best system I can devise sitting at home in front of the computer screen!

  • In method 2 the pondweed tube could be enclosed in a large beaker of water that acts as a simple thermostated bath to keep the temperature constant - ideally a thermostated water bath.

  • The tube of pondweed is immersed in NaHCO₃ solution is subjected to a lamp emitting bright white light at a specific distance from tube of pondweed.

  • You can again 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 ie 0.1g to 5g per 100 cm³ of water.

  • • (i) The oxygen bubbles are still channelled into a capillary tube but the gases and liquids allowed to freely exit from the capillary tube - no problem with liquid in the syringe which might quite stiff anyway and difficult to measure an accurate volume.

    • (ii) A T junction in the tubing allows the 'injection' of water into the gas stream to make bubbles of gas visible.

  • 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.

• What can you measure and vary?

• Measuring the rate of photosynthesis by measuring the rate of oxygen gas production in the gas syringe is more accurate but requires more time to get a set of readings to plot a graph.

• Measuring the speed of the horizontal movement of the gas bubbles is quite easy via the accurate linear scale and stopwatch.

• You can use quite a long uniform capillary tube to increase the sensitivity and hence accuracy of the experiment.

• • For each set of experimental conditions get at least three reasonably consistent readings and compute an average for the best accuracy.

  • The speed of bubbles in cm/s gives you a relative measure of the rate of the overall reaction of photosynthesis to produce oxygen.

• With increasing concentration (of NaHCO₃) you should see an increase in the rate of oxygen bubbles, but you must keep the temperature constant eg lab. temp. 20-25^oC, and the light intensity constant by keeping the lamp a fixed distance from the flask. The light from the laboratory itself will contribute, but the total light should be constant and you can use a light meter to ensure the same light intensity.

  • Try to use a range of concentrations eg 1% to 5% solutions (1g - 5g NaHCO₃ per 100 cm³ of water).

• To vary temperature you need to immerse the boiling tube in water baths of different carefully controlled and constant temperatures - ideally using a thermostated water bath.

  • You should be able to get enough results eg 5^o increments from 15^oC to 50^oC to show maximum the maximum rate of photosynthesis expected to be around 35-40^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 pondweed tube.

• You can calculate the relative intensity using the inverse square law, see light intensity section on this page.

• BUT, for accurate results you should take a light meter reading by the flask in the direction of the lamp (see the discussion on the inverse square law further down the page).

• You must choose, and keep constant, both the temperature and sodium hydrogencarbonate concentration of appropriate values eg a 2% solution of NaHCO₃ and 25^oC.

• Problems!

  • Although I think this is an improvement on method 2, its still quite difficult to get accurate results.

  • I think a light meter is essential for accurate results - changing the lamp distance is relevant to changing the light intensity, BUT, intensity is NOT a simple function of distance.

  • You need to use the same sample of pondweed, but is it always the same leaf area towards the light?

  • The experimental runs should not take too long as the NaHCO₃/CO₂ concentration is falling all the time.

• Graphs of experimental data and their interpretation

  • See graphs in photosynthesis Part 6.

• The relative intensity of the light from a fixed power is governed by an inverse square law.

  • When investigating the influence of light intensity on the rate of photosynthesis you must appreciate the inverse square law applied to light intensity for a fixed lamp power and light emission.

  • As you move the lamp further away, the light intensity falls, and so should the rate of photosynthesis.

  • BUT the light intensity is inversely proportional to the distance between the light source and the experiment tube squared.

  • From a specific light source ...

    • relative light intensity = 1 / d²

    • ... the light intensity is in arbitrary units, d = distance of the lamp from experiment.

  • The effect of the law can be demonstrated by some simple calculations ...

    • ... treating this idea as both predictions and ideal theoretical results!

distance from lamp d	10	20	30	40	distance to the experiment flask in cm
1 / d	0.1	0.05	0.033	0.025	reciprocal of distance
d2	100	400	900	1600	distance squared
1 / d2	0.01	0.0025	0.00111	0.000625	reciprocal of distance squared
relative intensity 1 / d2	1.0	0.25	0.111	0.0625	arbitrary units calculated by the inverse square law equation
relative distance	x1	x 2	x 3	x 4	distance from lamp to experiment
relative intensity as a fraction	1	1/4	1/9	1/16	decreasing with the inverse square law
		not 1/2	not 1/3	not/1/4	this is what it would be if intensity = 1 / d
relative rate of photosynthesis (see graphs)	1.0	0.25	0.111	0.0625	assuming rate of photosynthesis is proportional to light intensity

• The inverse square law for relative light intensity means that the relative brightness that the plant experiences falls away quite dramatically as the lamp is move further and further from the experiment tube.

• Graphs of rate of photosynthesis versus distance of the lamp from experiments such as method 3.

• 

  • These graphs are plots of the theoretical data used in the table above assuming a constant light source (a lamp!).

  • Graph (a) shows how rapidly the light intensity decreases as you move the experiment tube/flask away from the light source, shown by the equally rapid decline in the rate of photosynthesis. This is a consequence of the inverse square law of light intensity. You can show by experiment the rate of photosynthesis is proportional to the light intensity where it is the limiting factor. The graph also shows that the relationship between rate of photosynthesis and lamp distance is not linear.

  • Graph (b) shows that the rate photosynthesis is not proportional to reciprocal of the lamp distance, but it is a more linear graph than (a).

  • Graph (c) shows (for ideal results) that the rate of photosynthesis is proportional to the reciprocal of the lamp distance squared (and the lamp light intensity is proportional to 1 / d²). Therefore in graph (c) the horizontal axis could be also labelled relative light intensity, a proportional linear relationship with the rate of photosynthesis.

• See graphs of factors in photosynthesis Part 6.

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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 timing movement of a gas bubble of oxygen formed moving along a capillary tube investigation method.

Be able describe the experimental method, apparatus and observation data for timing the movement of a gas bubble of oxygen formed moving along a capillary tube experiment of photosynthesis and also describe how to get experiment results for different temperatures, varying light intensities and changing carbonate concentrations (= varying CO₂ concentration)..

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