Practical 5: Investigating photosynthesis

Aim

To show that a water plant gives off oxygen in the light, that a leaf makes starch, and that photosynthesis needs light, carbon dioxide and chlorophyll.

Background

Photosynthesis is the process by which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen. The word equation is: carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll).

Part A uses a water plant such as Elodea (pondweed). The bubbles of oxygen given off from the cut stem are a measure of the rate of photosynthesis. Moving a lamp closer increases the light intensity, which should increase the rate until another factor becomes limiting.

Parts B and C use the starch test. Plants store the glucose they make as starch, so a leaf that contains starch has been photosynthesising. Before the investigation the plant is destarched by leaving it in the dark for 24 to 48 hours, so that any starch found afterwards was made during the investigation. Changing one condition at a time shows that light, carbon dioxide and chlorophyll are each needed.

Hypothesis

The rate of oxygen production by pondweed will fall as the lamp is moved further away, because the light intensity is lower. Only the parts of a leaf that receive light, carbon dioxide and have chlorophyll will contain starch.

Variables

Equipment

• Part A: pondweed (Elodea or Cabomba), 250 cm³ beaker of dilute (about 1%) sodium hydrogencarbonate solution to supply carbon dioxide, inverted filter funnel and test tube, forceps or a glass rod, scissors, lamp (preferably LED), metre rule, clear container of water as a heat shield, thermometer, stopwatch, wooden splint
• Parts B and C: destarched potted plants (geranium or similar) and a plant with variegated leaves (for example variegated geranium or Coleus), aluminium foil or black paper, two conical flasks with bungs and soda lime (or sodium hydroxide solution) and petroleum jelly
• Starch test: Bunsen burner, tripod, gauze and 250 cm³ beaker of water (to boil, then use as the hot water bath), forceps, boiling tube, ethanol, white tile, iodine solution with dropping pipette
• Eye protection and heat-resistant mat

Risk assessment

Method

1. Part A (oxygen from pondweed): put on eye protection. Cut the stem of a piece of pondweed at an angle and place it, cut end upwards, in a beaker of water containing a little sodium hydrogencarbonate, under an inverted funnel with a test tube filled with water over the end.
2. Place the beaker a measured distance (for example 10 cm) from the lamp, with a clear container of water between them as a heat shield. Switch off other lights and check that the temperature stays constant.
3. Leave the pondweed for 2–3 minutes to adjust, then count the bubbles from the cut stem in 1 minute.
4. Repeat the count twice more at the same distance and move the lamp to 20, 30, 40 and 50 cm, leaving the plant to adjust each time.
5. Calculate the mean number of bubbles per minute at each distance. To show that the gas is oxygen, collect it in the test tube and test it with a glowing splint, which relights.
6. Part B (starch test on a leaf): remove a leaf from a plant that has been in the light. Heat a beaker of water to boiling and dip the leaf in it for about 1 minute, using forceps, to kill the cells and soften it.
7. Turn off the Bunsen burner. Put the leaf in a boiling tube of ethanol and stand the tube in the hot water for about 5–10 minutes, until the leaf is pale.
8. Rinse the leaf in warm water to soften it, spread it on a white tile and add drops of iodine solution. Record the colour: blue-black means starch is present; orange-brown means no starch.
9. Part C (requirements): to test light, cover part of a leaf on a destarched plant with foil or black paper, leave the plant in bright light for several hours, then remove the leaf and carry out the starch test.
10. To test carbon dioxide, enclose one leaf of a destarched plant in a flask with soda lime (seal the neck with petroleum jelly), and another leaf in a control flask without soda lime. Leave in bright light for several hours, then carry out the starch test on both leaves.
11. To test chlorophyll, put a destarched plant with variegated leaves in bright light for several hours. Draw the leaf to show its green and white areas, then carry out the starch test and draw the result.
12. Compare the iodine colour in the treated area with the control area for each requirement and record your results.

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

Part A: plot mean bubbles per minute on the y-axis against distance of the lamp (cm) on the x-axis, with a smooth curve of best fit. (As an extension, calculate light intensity as 1 ÷ distance² and plot the rate against this.) Parts B and C: record the starch test results in a table (area tested, iodine colour, starch present or absent) and draw the leaves before and after testing.