Identifying the conditions needed for photosynthesis
Class practical
This protocol isolates the contributions of three of the four requirements for photosynthesis in leaves: it shows that chlorophyll, light and carbon dioxide are all necessary for starch to form in leaves.
Lesson organisation
Practise the technique of testing leaves for starch before setting up this practical. If you can set it up at the end of one session, you will be able to test the leaves the following day.
If you have enough plants of each type and enough time, each pair of student could test leaves that have been exposed to all four treatments. Keeping track of four leaves with different treatments for starch testing can be a challenge! If not, each pair could test leaves for one condition being investigated and results could be pooled for the class.
Apparatus and Chemicals
Details of the apparatus and chemicals needed to test the leaves for starch are in Testing leaves for starch: the technique
Additional apparatus only is listed here.
For each group of students:
Black paper or aluminium foil, in strips 2–3 cm wide and long enough to wrap around both sides of a leaf
Paperclips, 2
Marker pen to label tubes if electric hot water bath is used
For the class – set up by technician/ teacher:
Plants, variegated and fully green, de-starched by keeping in the dark for 48 hours (Note 1)
Plants for investigation 3
Bell jar, covering a de-starched plant, and a beaker of soda lime (Note 2), secured to a glass plate or plastic tray with Vaseline or silicone grease
Bell jar, covering a de-starched plant, and a beaker containing hydrochloric acid and marble chips (Note 3), secured to a glass plate or plastic tray with vaseline or silicone grease
Health & Safety and Technical notes
See also Health and safety guidelines in Testing leaves for starch: the technique.
1 You can de-starch a plant by placing it in a dark cupboard, or placing it under a bell jar and draping black paper or cloth over the bell jar. It takes about 48 hours for a pelargonium (pot geranium) to use up the starch stored in its leaves.
2 Soda lime absorbs carbon dioxide from the air. Refer to CLEAPSS Hazcard 91 for more details. It is corrosive. Check the first aid procedure if anyone should get soda lime in their eye. 1 cm depth of granular soda lime in a 100 cm3 beaker, or in a Petri dish (which has a larger surface area if there is room) under the bell jar will absorb the carbon dioxide from the atmosphere. Carbosorb® is a self-indicating carbon dioxide absorber which is more efficient than soda lime, but more expensive.
3 Marble chips (10–20 chips) will produce carbon dioxide if you add some hydrochloric acid. Add about 50 cm3 of 0.1 M acid to a 250 cm3 beaker containing marble chips. The chips will effervesce for a while as the carbon dioxide is released. Refer to Hazcard 47A: hydrochloric acid is an irritant at concentrations about 2.0 M but is low hazard at this concentration.
Ethical issues
There are no ethical issues associated with this procedure.
Procedure
SAFETY: Only the teacher/ technician should handle the soda lime.
Preparation
a Keep enough plants for the investigation in a dark place for 48 hours so that they use up their stored starch.
b Test a few leaves from the de-starched plants to show that they contain no starch before the different treatments.
Investigation 1
c Place a de-starched variegated plant on a sunny windowsill for 24 hours. In the winter you may need additional illumination from an artificial source such as a halogen lamp.
d Remove a variegated leaf. Trace its outline on a piece of paper and draw the pattern of white/green patches.
e Test the leaf for starch using the procedure practised before.
f Draw the pattern of starch-containing cells that show with iodine solution. Compare this pattern with the original pattern of white/green patches.
Investigation 2
g Several groups of students can use the same plant for this procedure, by covering parts of different leaves.
h Take a strip of black paper or aluminium foil and fold it in half. Mark the paper or foil with your initials, or, if there is time, cut a shape out of the paper or foil.
i Place the paper or foil over the leaf so that it covers part of the leaf on both sides. Use two paperclips to secure it gently in place, so that it is in close contact with the leaf surface. If the paper or foil does not actually touch the leaf, light can get in around the edges and make the result less clear.
j Put the plant on a sunny windowsill for 24 hours.
k Remove the covered leaves. Remove the paper or foil and test the leaves for starch using the procedure practised before.
l Draw the pattern of starch-containing cells that show with iodine solution. Compare this pattern with the position of the paper or foil.
Investigation 3
Set up by teacher/ technician
j Think about the differences between the two bell jars set up by the teacher/ technician. One contains carbon dioxide in the air and the other doesn’t. Place both is a sunny position for 24 hours.
k Test a leaf from each plant and note which contains starch.
Teaching notes
Expected results
Investigation 1 – the pattern of starch-containing cells should follow the pattern of chlorophyll containing cells – starch will only be made where the leaf contains chlorophyll and can photosynthesise.
Investigation 2 – the pattern of starch-containing cells should follow the pattern of the paper or foil used to cover the leaf. Areas left uncovered should contain starch, and areas that were covered should not. Starch will only be made where the leaf has received light and has photosynthesised.
Investigation 3 – the plant kept in an atmosphere of reduced carbon dioxide will not have been able to produce starch by photosynthesis but the plant in the enriched atmosphere will be able to produce starch.
Health & Safety checked, December 2008
Downloads
Download the student sheet 
Identifying the conditions needed for photosynthesis (57 KB) with questions and answers.
Related experiments and Standard procedures
Testing leaves for starch: the technique
Practise the techniques for testing leaves for starch in a teaching session before running the practical that identifies the conditions needed for photosynthesis.
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This procedure uses a more sophisticated method to follow the process of photosynthesis by directly measuring the changes in carbon dioxide levels caused by photosynthetic activity.