Light Intensity and the rate of Photosynthesis
Teaching and Technical Notes
Introduction
The aim is to construct an experimental set up that allows a quantitative analysis of the rate of photosynthesis under
different light intensities. Students should be aware that light is needed for photosynthesis and so should easily
conclude that a higher light intensity leads to faster photosynthesis. This investigation takes this concept further
allowing students to explore the principles of “limiting factors” and “compensation points”.
This experimental set up actually measures the rate of photosynthesis in comparison to the rate of respiration.
Because of this it is possible to determine the light intensity at which the photosynthesising material is using the
same quantity of carbon dioxide in photosynthesis as it is producing in respiration (the light compensation point).
This level of light intensity is a critical environmental tipping point for photosynthesising organisms. Above this point
the light intensity is high enough to allow for growth; below this point the organism has to use up energy stores to
supply the raw material for respiration.
This practical provides the opportunity for skills development in
 immobilisation of organic material (in this case algae but the technique is often used for enzymes as well);
 the use of a colorimeter ;
 the use of a smart phone as a lux meter (optional);
 graphical skills (including plotting graphs with lines of best fit and reading off intercept points).
The data from the experiment can also be used for simple computer modelling to estimate the light compensation
point.
Safety notes
All of the pieces of equipment and chemicals used in this experiment are low hazard. Standard laboratory safety
procedures are sufficient to provide an appropriate level of safety when conducting this experiment.
Safely locate power cables of light sources to avoid them being a trip hazard.
If using a light source that generates a lot of heat, students should be careful with regard to setting it up, placing
items in front of it and packing it away (see below).
Apparatus
Quantities for one student group – it is suggested that students work in pairs
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5cm3 concentrated algal (suspension Scenedesmus quadricauda) in a bijou bottle
5cm3 3% sodium alginate solution in a bijou bottle
12.5cm3 fine-nosed syringe with plunger removed. (A standard 10cm3 syringe could be used but it produces
fewer, larger beads)
Clean test tube and stopper
2x 50cm3 beaker
25cm3 2% calcium chloride solution
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Tea strainer
Spatula
Distilled water bottle
Distilled water
Desk lamp or equivalent – about 1000 lumens (either 20 Watt CFL /86 Watt equivalent or 11 Watt LED/75
Watt equivalent)
12 bijou bottles and lids (white)
100 cm3 Hydrogen Carbonate indicator solution (cherry red colour (pH 8.4) – equilibrated with the carbon
dioxide in the air)
13 clean 2cm3 plastic Pasteur pipettes
Graph paper for drawing out grids
Sticky tape
30cm ruler
Box or light proof cupboard to place ‘Dark/light intensity=0’ bijou bottle into
Light meter (see notes on light meters)
Colorimeter (at least 1 per class)
12 cuvettes
Other apparatus for extension activities
 Access to data handling software
Hydrogen carbonate indicator solution
The hydrogen carbonate indicator solution is very sensitive to pH change and it is important that the solution is
equilibrated with the air before the experiment (pH 8.4 – cherry red) and that all bijou bottle lids, glassware and
pipettes have been washed in distilled water and are dry before use. The indicator supplied must be diluted with
distilled water to provide a 10% solution for use in this investigation, prior to equilibration with the air.
Selecting appropriate light bulbs
In order to obtain data across a wide enough range of light intensities to be able to investigate limiting factors as
well as light compensation points, bright bulbs are needed for the experiment.
This investigation was trialled using 20 Watt CFL /86 Watt equivalent or 11 Watt LED/75, and consistent results were
obtained with both lamps.
If tungsten filament lamps are used, a heat sink must be placed between the bulb of the lamp and the first tube. As
this will reduce the light intensity of the readings closest to the lamp, students are unlikely to be able to obtain data
to investigate limiting factors.
If CFL bulbs are used, the bulb must be allowed to warm up for at least 6 mins, as this is the time taken for the bulb
to emit its maximum light intensity.
It has been suggested that students should use the grid layout in the separate file to arrange their bijou bottles,
providing consistent sampling points within and between successive investigations.
Algae
Scenedesmus quadricauda algae are suitable for this practical.
Method for bulking up the algae (3-4 weeks before the lesson)
You will need a small starter culture of algae (see supplier information) and a large round bottomed flask (5dm3) or
volumetric flask (2dm3).
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Add the starter culture to a large round bottomed flask (5dm3) or a volumetric flask (2dm3) containing a two litre
solution of enrichment medium (see supplier information).
This culture will need to be aerated gently with a pump (a simple aquarium pump, tube and air-stone is suitable for
this) and maintained at approximately 18-22°C. Growth will be fastest when 24h illumination is supplied, preferably
a compact fluorescent lamp (CFL) of approximately 42W supplying about 2500 Lumens). After 3-4 weeks, when this
bulked algal culture is dark green, like pea soup, it is ready for use.
Prior to the lesson, this bulked algal culture will need to be allowed to settle. Pour off the supernatant and dispense
5cm3 of the settled algae into each of the required bijou bottles. With effective bulking up of the algal culture 5cm3
of settled algae should be obtainable from 50cm3 of algal culture.
Buying the culture ready concentrated
If you need the algae at short notice, concentrated cultures can be purchased from the supplier (see supplier notes)
in 50cm3 quantities. Each student group needs 5cm3 of this concentrated culture.
Sodium alginate
This should be made up the day before the lesson. A 2-3%* w/v solution should be made by adding the sodium
alginate powder to water. A magnetic stirrer should be used to mix the solution.
The powder does not mix in immediately and will need to be left overnight to fully dissolve. The alginate should not
be heated as this compromises the gel strength. Once made, it can be kept in bottles/beakers covered with cling film
for up to two weeks at 4°C.
*As sodium alginate is a natural product, its viscosity can vary from batch to batch. The alginate required for this
investigation should be fluid enough to pour slowly, so it is advisable to test your alginate for viscosity in advance of
the lesson.
Once made 5cm3 of sodium alginate can be dispensed into each bijou bottle using a 3ml plastic pipette with its tip
cut off.
Information about smart phones and apps
It might be worth checking that there are enough smart phones in the class for each group of students to have one
student with a smart phone and to allow suitable group allocation to ensure this. The type of smart phone is not
important so long as the student can find a free light meter app. These apps shouldn’t take long to acquire but it
might be worth getting students to do this before the lesson. An Android phone was used when trialling the
experiment and the app used was “Physics Toolbox Sensor Suite” (find light meter in the toolbox menu) by Vieyra
Software. The students will need to spend some time locating the position of the light sensor on their phones so that
they can place their phone’s light sensor in the correct position on the grid to measure light intensity at each bijou
bottle location.
Colorimeter s
The sample data provided at the end of this section was obtained using a’ WPA Colourware colorimeter’, using light
of a wavelength of 550nm (green filter).
A wireless colorimeter is available from Mystrica™ which can be used with a wireless connection to iPads and
iPhones, USB connection to a laptop or stand alone. Please note that in the stand alone mode, this Mystrica™,
colorimeter does not give negative Absorbance values (which are needed for calculations of light compensation
point) as readouts on the LCD. These negative values will only be visible when linked to a PC, iPad or iPhone.
Suppliers
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Most of the chemicals and equipment you will need for these investigations are easily sourced from standard school
science suppliers.
The following information is given to help you source less common apparatus and materials.
Scenedesmus quadricauda algae can be obtained from Sciento, 61 Bury Old Road, Whitefield, Manchester M45 6TB,
http://www.sciento.co.uk/catalog/category/49/
You can order 30 ml starter culture or a 50 ml culture (both under £10) and the powdered enrichment media (£6).
NCBE, based at Reading University, have put together a kit for this investigation. It contains a starter culture of algae,
powdered enrichment media, a bottle of hydrogen carbonate indicator solution, bijou bottles (75), fine-nosed
syringes (15) and light filters. This is enough equipment for a class of 30 students to conduct several experiments and
the equipment can be reused. Details for ordering can be found at http://www.ncbe.reading.ac.uk/. The cost is £65 +
VAT
A wireless colorimeter is available from Mystrica™, http://www.mystrica.com/Wireless/index for approximately
£116.00.
Teaching Notes
Light bulbs and lighting
It is worth trialling this experiment with the bulbs that are going to be used so that the students can obtain a range
of data that shows light becoming no longer limiting and has several data points either side of the light
compensation point. Suggested distances from the bulb might need to be altered depending on the bulb type.
The CFL, “energy saving” bulbs have a short warm up time (only a few minutes) that is worth students being aware
of when they are measuring the light intensities of their locations but shouldn’t be a problem for running the
experiment. Currently (2015) there is a wider range of bright CFL bulbs than LED bulbs but this will probably change.
The lighting in the laboratory should be controlled as much as possible. It is suggested that window blinds are closed
and doors shut so that when lit from the ceiling lights the conditions in the laboratory are relatively constant. This is
important for measuring light intensities for the different distances from the bulb.
General practical tips
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Making and storing algal balls – it would be a push to get the whole of this investigation done within an hour
lesson. It is possible to make algal balls in one lesson and store them in distilled water at 4°C. Stored in this
way the algal balls will last for several months and the experiment should work well so long as the algal balls
are returned to room temperature before the start of the experiment.
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The colour change is actually a measure of pH change caused by a change in CO2 concentration rather than
the CO2 concentration directly. This means that anything that changes the pH away from the equilibrium,
starting, pH will cause a colour change. This starting pH is 8.4 so even distilled water with pH7 is a
contaminant. This however is unavoidable as the algal balls must stay damp in order for the algae to remain
alive. The relatively small volume of water added with the algal balls, compared to the total volume of
indicator in the bijou bottle means that this effect is negligible in this experiment. To minimise
contamination from other sources, it is important that the bijoux bottles, Pasteur pipettes and cuvettes used
in this investigation are clean and dry. The students must rinse their bijou bottles and lids with the diluted
(10%) hydrogen carbonate indicator prior to setting them up. At the end of the experiment, a separate,
clean, plastic Pasteur pipette is used to transfer the indicator from each bijou bottle to a separate, clean, dry
cuvette. In order to minimise any potential re-equilibration between the indicator in the bijou bottles and
the air in the lab, the students should keep the lids on their bijou bottles until they take their turn to use the
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colorimeter. The lids should only be removed just prior to transferring the indicator to the cuvettes and the
absorbance of the indicator must be measured as quickly as possible after the indicator has been placed into
the cuvette.
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Making algal balls – the suggested quantities and method produces about 250 balls with the fine nosed
12.5cm3 syringe. In this experiment 220 algal balls are needed. If the algal balls contain sufficient algae they
should look pea green or darker, if paler the experiment may take longer to get results.
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The whole practical involves a reasonable amount of setting up time and recording data time as well as 510min to make algal balls, 5-10mins for the balls to solidify in the calcium chloride, and 45minutes for the
balls to be in the light. This is manageable in a 90min or longer lesson. As suggested previously it is possible
to separate the algal ball production from the experimental procedure.
One possibility for speeding up the practical would be to give students the protocol the lesson before, talk it
over and get them to plan their time before the next lesson, as well as downloading suitable apps. Students
could get into their pairs or groups the lesson before and plan for an effective practical – this would get
students thinking about the practicalities of investigative work and the best use of each member of the
group.
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Use of the smart phone – As well as making their own algal balls, students using their own smart phones for
data collection should give them more ownership of the investigation as well as possibly leading them to
realise that they have investigative tools available to them at any time should they wish to study something.
This investigation does require that at least one student in each group has a smart phone that they are
prepared to put apps on and use in the study.
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Light measuring apps – School light meters and light meter apps measure light in lux which is a measure of
perceived illuminance. This means that it is a measure of how much light (luminous flux – measured in
lumens) is illuminating a certain area and so lux is lumens per square metre. Light bulbs now give their
“brightness” in lumens, showing how much light they give out – how “illuminating” this bulb is depends on
how far away the object being illuminated is (because the total lumens being emitted are spread out over a
greater area the further the object is from the light source. Lux, the only really viable unit for measuring
illuminance in schools is not actually ideal for use in ecological or photosynthesis studies as it is a measure of
perceived illuminance and hence each wavelength of light is weighted based on how bright the human eye
perceives it. This weighting is very different to the action spectrum for photosynthesis and so lux isn’t used
in scientific studies of photosynthesis, which tend to prefer PAR (photosynthetically active radiation)
measured in moles of photons (between 400 and 700nm) per metre squared per second. In these school
experiments lux provides a suitable comparative measurement which would only be likely to skew findings if
photosynthesis was being compared under different types of light source.
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Students need to run the experiment with the laboratory ceiling lights off and the blinds down, however
they need to use the colorimeter to quantify the colour change just after the experiment ends with the
ceiling lights on. This means that all students need to be ready to start their experiment at the same time.
Colours produced by hydrogen carbonate indicator
The hydrogen carbonate indicator solution produces a very clear colour change from yellow at pH 7.6, through
orange to red at pH8.4 (when in equilibrium with the carbon dioxide in the air), to purple at pH 9.2.
← Decreasing photosynthesis
← Increasing CO2 in indicator
Photosynthesis and respiration in balance
Atmospheric level of CO2 (0.04%)(pH 8.4)
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Increasing photosynthesis →
Decreasing CO2 in indicator →
yellow
pH 7.6
pH 7.8
orange
pH 8.0
pH 8.2
red
pH 8.4
pH 8.6
magenta
pH 8.8
pH 9.0
purple
pH 9.2
Figure 4: the range of colours and pHs seen in investigations into photosynthesis using hydrogen carbonate
indicator.
Students may need to be taught or reminded about
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The study organism here is algae and not a plant with leaves so, although the principles of photosynthesis
and respiration remain the same, the route carbon dioxide takes is different. Here the carbon dioxide just
diffuses between the algal cells and the solution they are in. There are no leaves or stomata.
All living organisms respire all the time. So even when well lit, the algae are still respiring. There is a net
absorption of carbon dioxide because photosynthesis is using more carbon dioxide than respiration is
producing.
The link between carbon dioxide in solution and pH as well as the link to colour change of the indicator.
Limiting factors for photosynthesis.
Plotting graphs, drawing lines of best fit.
The equation for a straight line and how to calculate it.
Solving equations.
Notes on student tasks
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As a minimum, students should conduct the experiment, tabulate their results, plot a graph of their data and
add a line of best fit in order to read off the light saturation point and the compensation point from the
graph.
If time is severely limiting then algal balls can be made for the students in advance. However, it is preferable
for students to take more ownership of their experiment by doing more of it for themselves.
To get students really thinking about what they have done and what their data shows they can answer the
questions in the Students’ sheet document or complete the student worksheet. You can extend either of
these using some of the questions in the Possible questions for students list below.
When analysing data there are possibilities here to get students finding the equation for a straight line and
solving the equation for y=0 to find the light compensation point – as suggested in the questions section of
the student sheet, the student worksheet and in the Possible questions for students below.
Inputting data into Excel and using this to draw a trend line and show the equation of the trend line is a
simple way to do some computer modelling of the data and to discuss the assumptions made when
modelling in this way and how these might affect the calculation of the light compensation point if they
were broken.
As an extension investigation students could develop methods to test the reliability and accuracy of the apps
used. Although not adding to the learning of limiting factors and light compensation points, this extension
would be very relevant to interpreting the results of this investigation and would provide students with the
opportunity to be creative and truly exploratory whilst learning about the principles of accuracy and
reliability.
Possible questions for students
1. Follow the protocol and answer the following questions
a) What organism/species did you investigate?
b) Record the conditions of your experiment – date, room temperature, number of days between making algal
balls and the experiment.
c) What are the advantages and disadvantages of immobilising the algae?
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d) Were your algal balls spherical and a consistent size? If so did you have to modify anything to ensure this? If
not how might this have affected your results?
e) Why did you have to thoroughly mix the algae and the sodium alginate? What effect would an uneven mix
have had on your experiment?
f) What happens to the sodium alginate and algae mix when it drops into the calcium chloride? Explain how
this happens.
g) Why is it necessary to rinse the algal balls with distilled water?
h) Why is the hydrogen carbonate indicator necessary?
i) Why is it useful to use a colorimeter to get a numerical value for colour in this experiment?
j) How was the colorimeter set up, what settings were used and what values were recorded? Explain.
k) What other environmental factors (as well as light intensity) might vary with distance from the light source?
How could you check whether they do vary and how could you control them?
l) Why do you need a control tube that contained only indicator solution?
m) Why is it important to use a clean pipette to remove the indicator from each bijou bottle at the end of the
experiment?
n) Why is it important to end the experiment by switching the light source off, the laboratory ceiling lights on
and removing the box covering the lid in the dark before starting to record the final colour of the solution in
each bottle?
o) Why is it important to measure the final colour in each bijou bottle relatively quickly?
2. Your findings
a) Record your results in a suitable table using the column suggestions in the instructions for this investigation.
b) Describe the shape of your line of best fit in as much detail as possible (imagine there was an exam
questions to describe this graph).
c) Is there a light intensity above which there is no (or very little) increase absorbance? If so what is this value?
d) Record the light intensity at which the line of best fit crosses the x-axis.
e) Using the section of data that crosses the x-axis, draw a second, enlarged graph and add a straight line of
best fit through the section of data that crosses the x-axis.
f) Record the light intensity at which this straight line of best fit crosses the x-axis.
g) Comment on the two values you have obtained for where the lines of best fit cross the x-axis.
h) Calculate the equation for the straight line of best fit you have drawn (in the form y=mx+c) and use it to
calculate where this line intercepts the x-axis. How does this value compare with the value you got when
reading off the intercept point from your line?
i) Input your data used to plot the graph into Excel and use it to draw a graph. See if you can get Excel to draw
a trend line – there are various options for the general shape of the trend line and each is effectively a model
of your data. Is there an appropriate option for your data?
j) Use a subset of your data (the same subset that you used in e) to plot another graph just of the section of
data that crosses the x-axis. Add a straight trend line to this graph and get Excel to show you the equation
for this line. Solve the equation of the line for y=0 to identify the point where the line intercepts the x-axis
(the light compensation point). Compare your hand calculated equation and intercept point with the Excel
derived one.
k) Your Excel generated straight trend line is a simple computer model of your data. What assumptions does
this modelling make? What are the advantages and disadvantages of modelling data in this way?
l) If the experiment was left for much too long you might expect to see that all of the tubes contained either
yellow or dark purple liquid but no bottles with an intermediate colour except for the control bottle. Why
would this occur and what precaution does it suggest you should take with respect to when to finish your
experiment? If this did happen what would now not be possible to work out and what conclusions would
you still be able to draw?
m) If the experiment was stopped too quickly how might this affect the shape of the graph and how would this
affect the confidence you have in your identification of key points in the graph (the light intensities at which
the line of best fit levels off and where it crosses the x-axis)?
3. The biology of what you see
a) Write a simplified word equation and balanced symbol equation for photosynthesis.
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b) Explain the purpose of photosynthesis and respiration in organisms, as well as the relationship between the
two processes.
c) Explain what is happening in the solution when you get a positive absorbance reading using the colorimeter.
d) Explain what is happening in the solution when you get a negative absorbance reading using the colorimeter.
e) Explain what is happening to the rate of photosynthesis and why this is occurring as light intensity increases
in the part of your graph that has a relatively steep positive correlation.
f) What is a limiting factor (or rate limiting factor)? Give 4 examples of potential limiting factors for the rate of
photosynthesis.
g) What is limiting the rate of photosynthesis at the light intensities where your graph shows a steep positive
correlation between light intensity and Absorbance at 550nm?
h) What is a “light saturation point”?
i) If there is a light intensity above which there is no (or very little) increase in absorbance, explain why this
happens. If not, do you think that if light intensity was increased further and further above what you
managed in your experiment that absorbance would continue to increase and increase? Explain your
thoughts.
j) What could be limiting the rate of photosynthesis at light intensities where a graph shows no increase in
absorbance with an increase in light intensity?
k) What is the name of the point at which your line of best fit crosses the x-axis, what is happening at this light
intensity, and what is its significance for the photosynthesising organism?
4.
Further experiments and exploring the use of smart phones for data collection
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Design a series of experiments, and explain what you would be looking for, to identify what the limiting
factor for the rate of photosynthesis is, in this experiment, when light intensity is no longer limiting (don’t
worry too much about how you would change or measure different factors just say what you would change
or measure).
Design a series of investigations to advise about the suitability of different plant species for use as pot plants
in a particular, relatively dimly lit office space. Explain what you would be looking for and the biology behind
it.
Design a series of investigations to advise a commercial horticulture business about whether they would
benefit from investing in brighter lighting in their environmentally controlled greenhouses. Explain what you
would be looking for and the biology behind it.
Design (and conduct) a study into the accuracy and reliability of the colorimeter that you used.
Design (and conduct) a study into the accuracy and reliability of the use of a smart phone as a light meter.
Getting learning value from the practical
It is important to consider the purpose of practical work in Biology lessons and the following information provides
suggested areas where this practical can be used to help students develop their practical, mathematical and subjectknowledge-based skills and understanding. The latter part of this section shows how this practical (or an extension of
it) can also be used to meet some of the A-level specification requirements for the Use of apparatus and techniques
as well as the Common practical assessment criteria (CPAC).
PRACTICAL SKILL DEVELOPMENT
You should be able to:
 follow instructions to make uniform (in size, colour and shape) algal balls
 use a light source and ruler to standardise a way of exposing the bijou bottle lids to different light intensities
 measure light intensity using a smart phone app
 use a smart phone app to obtain quantitative values for colour change (acting as a colorimeter for very small
quantities of liquid)
 work in an organised way to obtain colour change data from several samples, controlling variables, avoiding
contamination, and using appropriate labelling or systems to track the samples
 explain why a colorimeter that provides a numerical value for colour is a useful tool for collecting data on the colour
of liquid samples
 explain why hydrogen carbonate indicator solution is used in this investigation
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 explain why a control tube is needed and what the absorbance reading from it is used for
 record data in a suitable table
EXTENSION TO PRACTICAL SKILL DEVELOPMENT
You should be able to:
 modify the method to produce uniform algal balls if necessary
 explain the connection between varying light intensity and the measures of absorbance in this investigation
 design a series of experiments, and explain what you would be looking for, to identify what the limiting factor for
the rate of photosynthesis is, in this experiment, when light intensity is no longer limiting
 suggest possible experiments to investigate the effect of other factors on the rate of photosynthesis (or
respiration) in these algal balls.
 compare the advantages and disadvantages of immobilising algae in this experiment.
 compare the advantages and disadvantages of using smart phone apps as data collection tools rather than
standard school laboratory equipment in this experiment.
 explain how to use a colorimeter and explain how to ensure appropriately consistent and objective readings using
this piece of apparatus
 explain how to use a smart phone app to measure light intensity and explain how to ensure appropriately
consistent and objective readings
 design (and conduct) a study into the accuracy and reliability of the colorimeter that was used.
 design (and conduct) a study into the accuracy and reliability of the use of a smart phone as a light meter.
MATHS SKILLS DEVELOPMENT
You should be able to:
 Explain why some absorbance values obtained with a colorimeter in this practical are positive whilst others are
negative
 draw an appropriate graph of the data collected
 draw an appropriate line of best fit for your data
 read off the intercept of the line of best fit with the x-axis to find the compensation point
 suggest, from the graph, a light intensity above which it is no longer the limiting factors for the rate of
photosynthesis
EXTENSION TO MATHS SKILL DEVELOPMENT
 draw a straight line of best fit just for the data in the section of the graph that crosses the x-axis and work out the
equation for this line in the format y=mx+c
 explain how to use the equation for the straight line of best fit to work out the compensation point
 solve the equation for the straight line of best fit for y=0 to calculate the compensation point
 input your data points used to draw your straight line of best fit into a graph drawing package, use this to
calculate the equation for the straight line of best fit and then find the compensation point using this equation.
 comment on the 3 values for the light compensation point – read off the graph, calculated using a hand drawn
straight line of best fit, calculated from the “computer model” of the data
ASSOCIATED SUBJECT KNOWLEDGE DEVELOPMENT
You should be able to:
1. write a simplified word equation and balanced symbol equation for photosynthesis
2. explain the purpose of photosynthesis and respiration in organisms, as well as the relationship between the
two processes
3. state 4 factors that affect the rate of photosynthesis and for each explain how (and why) they affect the rate
of photosynthesis
4. define the terms “limiting factor” and “light saturation point”
5. sketch and annotate (in relation to limiting factors) a graph to show the relationship between light intensity
and the rate of photosynthesis
6. define the term “light compensation point”
7. explain the significance of the light compensation point to the photosynthesising organism (and more
broadly to agriculture)
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8. explain the significance of the light saturation point to the photosynthesising organism (and more broadly to
agriculture)
RECORDING EVIDENCE OF STUDENTS WORK
Students can use the worksheet provided to produce evidence of their work in this practical that can contribute to
their practical skills accreditation.
MEETING ASPECTS OF THE USE OF APPARATUS AND TECHNIQUES
The full practical can allow students to meet various components of Appendix 5c of the GCE AS and A level subject
content for biology, chemistry, physics and psychology (DfE, April, 2014) in bold (with clarification where needed):
1. Use appropriate apparatus to record a range of quantitative measurements (to include mass, time, volume,
temperature, length and pH) (distance from light source as well as light intensity).
2. Use appropriate instrumentation to record quantitative measurements, such as a colorimeter (if the smart
phone app counts as a colorimeter) or potometer.
3. Safely and ethically use organisms to measure:
a. Plant or animal responses (measuring algal responses to light intensity)
b. Physiological functions.
4. Use ICT such as computer modelling, or use software to process data. (Using a graph drawing package to
calculate a line of best fit for the “steep positive correlation” part of the graph and then using this to
calculate the light compensation point.)
Sample data
1.Light Intensity vs Absorbance, LED Bulb (1055 lumens), 30 minutes Illumination
Light intensity measured using phone app/lux
115301
52680
18778
7828
4061
2234
871
618
575
0
Absorbance at 550nm
0.36
0.29
0.25
0.22
0.15
0.1
0.06
0.03
0.02
-0.06
Science & Plants for Schools: www.saps.org.uk
2a.Light Intensity vs Absorbance, LED Bulb (1055 lumens), 45 minutes illumination
Light intensity measured using phone app/lux
122747
54885
21068
9865
4609
2388
1334
899
562
210
0
Absorbance at 550nm
0.71
0.71
0.68
0.59
0.45
0.25
0.26
0.11
0.13
0.15
-0.04
2b.Light Intensity vs Absorbance, LED Bulb (1055 lumens), 45 minutes illumination
Light intensity measured using data harvest light
sensor/lux
46121
28532
17551
11354
7635
5688
4359
Absorbance at 550nm
0.71
0.71
0.68
0.59
0.45
0.25
0.26
Science & Plants for Schools: www.saps.org.uk
3400
2659
2318
0
0.11
0.13
0.15
-0.04
1
3
2
Line 1. The light intensity at which light saturation is reached by the photosynthesising algae.
Line 2. The range of light intensities that are limiting the rate of photosynthesis.
Line 3. The light intensity at which the algae reach their light compensation point.
Further References
For a video demo showing how to create the algal balls and how best to use this in the classroom, see our YouTube
channel: www.youtube.com/watch?v=fI3x68CkKW0.
Acknowledgements
Based on a protocol developed by Debbie Eldridge.
Support materials developed by Dr Chris Graham, Hills Road Sixth Form College, Cambridge, Gail Webdell, Faculty of
Education, University of Cambridge.
Support material and protocol modified by Bev Goodger, West Derby School, Liverpool, supported by Jane
Martindale and Vicki Doronina, Department of Science Education, Manchester Metropolitan University.
Science & Plants for Schools: www.saps.org.uk