Investigating Photosynthesis and Respiration using Algal Balls
Teaching Notes
Introduction and context
Photosynthesis, a chemical reaction, is dependent on several factors. These are known as rate limiting
factors and most 14-16 students need to be able to identify/discuss these
factors.
In this protocol, the green alga Scenedesmus quadricauda is immobilised in
alginate beads. Changes in the colour of hydrogen carbonate indicator solution
(also called bicarbonate indicator) can then be used to investigate the rate of
photosynthesis under different environmental conditions.
Like flowering plants, S. quadricauda cells contain the photosynthetic pigments
chlorophyll a and b. This enables the green algae to photosynthesis and means
that they are currently classified in the Plant Kingdom 1 (along with
angiosperms, conifers, ferns and mosses), although there is still debate
amongst biologists on this.
Scenedesmus , a green
alga, contains chlorophyll
a and b. They are often
found in small colonies of
2-4 cells.
Scenedesmus are described as colonial, normally being found in colonies of 2 or 4 cells. The end cells of the
colonies typically have 2 long spines protruding from the outer corners. Each cell is approximately 15µm long
and contains a single, plate-like chloroplast. There are approximately 80 recognised species of
Scenedesmus1.
Scenedesmus species have recently been investigated for production of biodiesel and have shown to be
promising in terms of high lipid and oleic acid content2.
References
1. M.D. Guiry in Guiry, M.D. & Guiry, G.M. 2012. AlgaeBase. World-wide electronic publication, National
University of Ireland, Galway. http://www.algaebase.org, searched on 10 November 2012
(specific page: http://www.algaebase.org/search/genus/detail/?genus_id=43474)
2. Pandian Prabakaran and A. David Ravindran, Current Science, Vol. 102, NO. 4, 25 February 2012 online
at: http://www.currentscience.ac.in/Volumes/102/04/0616.pdf
Video
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
Overview
This document will explain how to use the immobilised algae to demonstrate / investigate:
1. Photosynthesis only proceeds in the light
2. Light intensity affects the rate of photosynthesis
3. The compensation point between photosynthesis and respiration
4. Wavelength (colour of light) affects the rate of photosynthesis
5. Concentration of algae will affect the rate of photosynthesis
Copyright Science & Plants for Schools: www.saps.org.uk
Photosynthesis with Algal Balls: Teaching Notes (Revised 2012)
1
How photosynthesis is measured in these investigations
Using an indicator solution to obtain quantitative results
Hydrogen carbonate indicator solution is sensitive to changes in pH caused by CO2 from the air
dissolving in water to form carbonic acid. When hydrogen carbonate indicator solution is
equilibrated to atmospheric CO2 levels, it is red. When CO2 levels increase it changes from red
through orange to yellow. When CO2 levels decrease, it changes from red through magenta to
deep purple. This makes it an ideal indicator to use with photosynthesis investigations. When
photosynthesis is proceeding more quickly than respiration, CO2 will be removed from the solution
and it will change from red to magenta/deep purple. When respiration is proceeding more quickly
than photosynthesis, the solution will change from red to orange/yellow.
This colour change can be measured using a colorimeter measuring absorbance at 550nm or by
comparing the colour of the indicator with a set of standard buffered solutions.
NB By getting students to gently breathe through a straw into a test tube of the indicator solution,
you can demonstrate that CO2 causes the solution to change colour from red to yellow (eye
protection should be worn, and care taken to ensure students don’t suck up the indicator).
Indicator colour / pH
Standard buffered solutions or indicator colour charts can be used to allow students to match the
indicator in their bijou bottles to a pH value.
Colour changes that may occur during the investigation, and the corresponding pH values.
← Increasing CO
2
yellow
in indicator
Atmospheric level of CO2 (0.04%)(pH 8.4)
orange
red
Decreasing CO2 in indicator
magenta
pH 7.6
pH 7.8
pH 8.0
pH 8.2
pH 8.4
pH 8.6
pH 8.8
pH 9.0
(Download full colour chart from http://www.saps.org.uk/attachments/article/235/
Colour%20chart%20showing%20hydrogencarbonate%20indicator.pdf)
→
purple
pH 9.2
Hydrogen carbonate indicator can vary in colour depending on supplier. You can prepare your own
standard solutions for comparison rather than using this chart. These solutions can be made up
using standard boric acid/borax buffer solutions (see technical notes). In this way, a range of
desired colours / pH values can be displayed in the classroom for students to check their bijou
bottles against. Students should be supplied with indicator that has been equilibrated to
atmospheric CO2 concentration (see technical notes) for their investigations.
This continuous pH data can be plotted on a line graph against the independent variable that is
being investigated. Each investigation can lead to good discussions about how science works and
the scientific method.
Absorbance of indicator at 550nm
The indicator can be removed from the bijou bottles using clean pipettes, placed into cuvettes, and
the absorbance at 550nm (wavelength of green light) measured using a colorimeter.
The relationship between absorbance and pH is linear at 550nm with this range of pH values. The
absorbance of green light increases with increased pH of bicarbonate indicator.
Using a colorimeter will give absorbance data accurate to one or two decimal places, whereas
using colour charts or buffer solutions will give colour changes that are more subjective.
Copyright Science & Plants for Schools: www.saps.org.uk
Photosynthesis with Algal Balls: Teaching Notes (Revised 2012)
2
Investigation 1: Light and Dark
Using algal balls to show that light is needed for photosynthesis
Approximately 20 algal balls are placed into a bijou bottle filled to the top (to reduce the buffering
effect of the air trapped inside the bottle) with hydrogen carbonate indicator solution equilibrated to
atmospheric CO2 concentration.
If near a suitable lamp (see technical notes), the indicator will change to purple within 30 minutes.
This occurs as CO2 has been removed from the indicator solution during the process of
photosynthesis. If in the dark, the indicator will change to yellow as carbon dioxide produced in
respiration enters the solution.
Placing bijou bottles at different distances from a lamp, leaving them for differing times, changing
the number of algal balls are all methods that demonstrate photosynthesis in action and can
produce colour changes in the indicator solution from red through to purple.
Typical simple qualitative results
Start point
In the dark
(covered with black sugar paper paper removed for photograph)
pH 8.4 (cherry red);
0.04% CO2 in
atmosphere
Near a lamp
pH 8.0 (yellow); more yellow because of
increased CO2 produced in respiration
(creates carbonic acid).
pH 9.2 (purple); more purple because
CO2 is being removed from the indicator
for use in photosynthesis.
This CO2 is not removed as
photosynthesis does not proceed in the
dark.
This reduces the amount of carbonic
acid in the solution so it becomes more
alkaline
Copyright Science & Plants for Schools: www.saps.org.uk
Photosynthesis with Algal Balls: Teaching Notes (Revised 2012)
3
Investigation 2a: Light intensity (distance from a lamp)
Using algal balls to show that the rate of photosynthesis increases with increasing light
intensity
Setting up algal balls at different distances from a suitable lamp for a set amount of time can show
that light intensity affects the rate of photosynthesis. The colour change is slower further from the
lamp as photosynthesis proceeds more slowly at lower light levels.
Students who have compared their colour change to charts or standard indicator solutions can plot
line graphs of pH value vs. distance from the lamp.
If using a colorimeter, students can plot absorbance at 550nm against distance from the lamp.
With high ability students, you may prefer to convert distance to an approximate value for light
intensity by using the inverse square law, 1/D2, where D is distance from the lamp.
As distance from the lamp/light intensity increases the absorbance and pH values decrease
showing that the rate of photosynthesis decreases as light intensity becomes a rate limiting
factor.
This experiment works best in a darkened room so that other illumination sources are reduced as
much as possible. However with very bright light sources this effect is more limited.
Typical data obtained (when using a 150W halogen lamp in a darkened room)
Data will vary according to the type of lamp used and the concentration of algae in the algal balls.
However, the trends and patterns should remain the same. Distances as large as those noted
below are not needed if using an energy saving compact fluorescent lamps, as light levels drop
more quickly over shorter distances than Halogen lamps (see technical notes re lighting).
Distance from
lamp (cm)
(optional column)
Relative light intensity
(1/D2) (x10-5)*
(if using colourimeter)
Absorbance of indicator (550nm)
pH value
250
350
500
780
1250
1.60
0.81
0.40
0.16
0.006
0.81
0.74
0.56
0.39
0.25
8.8
8.6
8.4
8.2
8.0
*1/D2 produces numbers with many decimal places so that adjustment may be required to give a
sensible number for analysis/plotting on a graph. For instance in this example, at a distance of
250cm, 1/D2 is equal to 0.000016 and so multiplying this by 105 gives a suitable number (1.6) for
analysis.
Copyright Science & Plants for Schools: www.saps.org.uk
Photosynthesis with Algal Balls: Teaching Notes (Revised 2012)
4
Investigation 2a: Light intensity (neutral density filters)
Using algal balls to show that the rate of photosynthesis increases with increasing light
intensity.
Neutral density filters reduce transmittance of all wavelengths of light and so can be used to
demonstrate the effects of light intensity on photosynthetic rates. As the ND rating increases, the
indicator solution containing the algal balls goes from red to yellow as respiration is proceeding
more quickly than photosynthesis due to decreasing light intensity (light is becoming a rate
limiting factor). Neutral density filters are obtainable from the NCBE (www.ncbe.reading.ac.uk).
If you are using halogen lamps that give off a lot of heat, using neutral density filters allows you to
choose a distance from the lamp where temperature effects will not adversely affect the
investigation. Any heating effect that occurs, will therefore affect all bottles of algae equally.
We suggest using filters with these values: 0.15, 0.3 and 0.6. (This is because they can also be
used to find the compensation point between photosynthesis and respiration (see investigation 3).
Leaving one bottle unwrapped is a ND value of 0.0, and wrapping one bottle in thick black sugar
paper will effectively produce a ND value of 1.0.
Typical Data Obtained
Absorbance after 50 mins in front of a 42W CFL portable floodlamp (column two shows how much
light gets through each type of filter).
Filter on bottle
None (0.0 ND)
Amount of light transmitted
into the bottle (%)
100
0.15 ND
71
0.3 ND
50
0.6 ND
25
Black paper (1.0 ND)
0
(if using colourimeter)
Absorbance of indicator (550nm)
pH value
0.34
0.30
0.17
-0.03
-0.15
9.0
8.6
8.4
8.2
7.8
Line graphs can be drawn of ND filter value/light transmitted into the bottle against absorbance at
550nm/pH value of indicator solution. Lines (curves) of best fit can then be added and discussed.
Copyright Science & Plants for Schools: www.saps.org.uk
Photosynthesis with Algal Balls: Teaching Notes (Revised 2012)
5
Investigation 3: Compensation point between photosynthesis and respiration
Neutral density (ND) filters can be used to find the compensation point between photosynthesis
and respiration. Respiration produces CO2 and photosynthesis uses CO2. When the two processes
are in balance there is no net production of CO2 and we call this point the compensation point. We
can use ND filters to find the light level at which this compensation point is reached.
Neutral density filters reduce transmittance of all wavelengths of light. As the ND rating increases,
the amount of light transmitted into the bottle decreases and so the indicator solution containing
the algal balls goes more yellow.
We suggest using filters with the values 0.15, 0.3 and 0.6ND that allow 71, 50 and 25% of light,
respectively, to transmit through the filter. Leaving one bottle unwrapped (ND value 0.0) will equate
to 100% transmittance, and wrapping one bottle in thick black sugar paper (ND value of 1.0) will
equate to 0% light transmittance.
When a graph of absorbance at 550nm is plotted against % light transmitted, the compensation
point for photosynthesis (expressed as the % of light transmitted into the bijou bottle) can be
estimated by reading the % light transmitted value at an absorbance value of 0.
Typical data obtained and example graph showing how to work out the compensation point
Absorbance after 50 mins in front of a 42W CFL portable lamp (equivalent to a 200W Halogen lamp)
(column two shows how much light gets through each type of filter)
Filter on
bottle
Amount of light
transmitted into
the bottle (%)
Absorbance
of indicator
(at 550 nm)
None (0.0
ND)
0.15 ND
100
0.34
9.00
71
0.3 ND
50
0.6 ND
25
Black paper
(1.0 ND)
0
0.30
0.17
-0.03
-0.15
8.95
8.75
8.35
8.00
pH value
In the table, the negative values for absorbance
show that there has been a colour change
towards the more yellow/acidic end of the spectrum caused by an increase in CO2. This shows that
at these values for light transmitted into the bijou bottles (0 and 25%), respiration was the dominant
process.
In the bottles with positive results for absorbance of indicator, there has been a colour change to
the more purple/alkaline end of the spectrum because photosynthesis is the dominant process.
The point at which there is no net change in the concentration of dissolved CO2 is the
compensation point.
From the graph, this point is estimated to be when the % transmitted light into the bottle is 29%.
Similar results can be obtained by plotting absorbance against pH although this will not produce as
accurate or precise results if pH is being estimated based on indicator colour rather than a pH
meter.
Copyright Science & Plants for Schools: www.saps.org.uk
Photosynthesis with Algal Balls: Teaching Notes (Revised 2012)
6
Investigation 4: Wavelength of light
Using acetate / photographic filters to show that the wavelength (colour) of light affects the
rate of photosynthesis
The wavelength range for visible light is 400 (blue) – 750 nm (red). The
green part of the spectrum is 510 – 555nm.
Plants do not use all wavelengths of light equally in photosynthesis.
Plants appear to be green because they are reflecting green light which
shows that little green light can be used in photosynthesis. The
Scenedesmus algae used in this investigation, like flowering plants, ferns
and mosses, contain chlorophyll a and b and so appear green.
Scenedesmus contain
chlorophyll a and b
Most plants prefer red and blue light for photosynthesis, although this will
vary from species to species according to the environment in which they
are found. If plants don’t receive enough light of a suitable wavelength then this could become a
rate limiting factor for photosynthesis.
Acetate filters
Green, red, blue or clear (control) acetate filters wrapped round the bijou bottles can be used to
show which colour/wavelength of light is used by the algae in photosynthesis. For instance, if a
filter does not transmit any light that can be used in photosynthesis, then photosynthesis will stop
and the indicator solution will change towards the more yellow/acidic end of the spectrum. If
photosynthesis can proceed at a rate that exceeds respiration, the indicator solution will change
towards the more purple/alkaline end of the spectrum showing that the wavelength being tested is
sufficient for photosynthesis to proceed and overtake the rate of respiration.
More accurate colour filters can be used (Primary Red, Primary Green and Bray Blue) (see
suppliers details on technical notes pages)
Typical data obtained
After 30 mins in front of a 42W portable CFL floodlamp. Column two shows which light in the range
400-680nm gets through each filter.
Type of filter
on bottle
Clear acetate
Primary red
Bray Blue
Primary Green
Black paper
Colour of light allowed
into the bottle
(if using colourimeter)
Absorbance of indicator (550nm)
pH value
All (400-680nm)
Red (600-680 nm)
Blue (425-500 nm)
Green (475-590 nm)
None
0.94
0.74
0.56
0.32
0.24
9.0
8.6
8.4
7.8
7.6
These data are best represented as categoric data using a bar chart of filter colour against
absorbance at 550nm or pH value.
Copyright Science & Plants for Schools: www.saps.org.uk
Photosynthesis with Algal Balls: Teaching Notes (Revised 2012)
7
Investigation 5: Number of algal balls / concentration of algae in the balls
Students can vary the number of algal balls in their bijou bottles. The more algal balls in a bottle,
the more algal cells there will be and the more photosynthesis can occur.
Students can also vary the concentration of the algal culture in the balls by diluting the
concentrated algal culture that they start with.
The number of algal balls/concentration of algae can be plotted against absorbance at 550nm/pH
value to create a line graph with a line/curve of best fit.
When there are more algal balls or when the algae are more concentrated, the time taken for the
the indicator to change to the purple/ alkaline end of the spectrum will decrease.
Copyright Science & Plants for Schools: www.saps.org.uk
Photosynthesis with Algal Balls: Teaching Notes (Revised 2012)
8
Questions / discussions for after the practical
Questions that apply to all of the investigations
1. Which of your algal balls will be getting the most light? Which will be getting the least?
This indicator solution goes purple when carbon dioxide is removed from it
2. What process in plants uses carbon dioxide?
3. In which of your bottles will this process be taking place most quickly? Explain.
This indicator solution goes yellow when carbon dioxide is added to it
4. What process in living things (including plants) produces carbon dioxide?
5. In which of your bottles will this process be taking place?
6. What are your overall predictions? (Use the terms photosynthesis and respiration)
7. What do your results show? (Do they fit predictions, trends or patterns, explain anomalies)
8. Can you answer the question that you were investigating?
9. How will you show your results on a graph/chart?
10. Was this a fair test? What were your variables (dependent, independent, control)?
11. Can you think of a method that would have enabled you to collect more reliable data?
Questions that apply to specific investigations:
Investigation 4. Wavelength of light
1. Which colour of light appears to be best for photosynthesis? Explain.
2. Was photosynthesis occurring in the bottle with the green filter? Explain.
(It should not proceed quickly, although the Primary Green filter does let some blue light
through.)
3. Students could be asked to look up the absorbance of chlorophyll a and b and relate these
to the investigation
Acknowledgements
This protocol was developed by Debbie Eldridge from King Ecgbert School (Sheffield) during a
secondment as a SAPS/Robinson College Schoolteacher Fellow.
These resources were updated and extended by Vicki Cottrell of Didcot Girls School during a
secondment as Nuffield Education Fellow.
Copyright Science & Plants for Schools: www.saps.org.uk
Photosynthesis with Algal Balls: Teaching Notes (Revised 2012)
9