Below you will find notes about some simple experiments on photosynthesis and respiration in plants. [Based on experiments decribed in: N. P. O. Green, J. M. Potter & G. W. Stout, Biology, 1980, Pan Study Aids.]
Write reports of these experiments in a style which could be part of a published paper; but do not write a full paper. Make your reports short, easy to understand, accurate, and complete. Your reports should include:
Think carefully about how each experiment must be done to ensure that the objective of the experiment is achieved and the conclusion drawn is true.
Reports of experiments are normally written in the past tense. In these exercises you may choose the past tense (as if you are reporting experiments you have done), or you may choose the simple present tense (to show how the experiment must be done).
Green plants produce carbohydrates (sugars and starch) by photosynthesis. To do this plants need carbon dioxide, water, light and chlorophyll.
Carbon dioxide enters the plant through small holes in the leaves; water enters through the roots. The carbon dioxide and water then produce sugars and oxygen with the help of light and chlorophyll in the leaves. Chlorophyll absorbs red light and violet light for photosynthesis and reflects green light.
Photosynthesis has two steps:
The net sum of the two reactions is
6CO2 + 6H2O = C6H12O6 + 6O2.
The sugar (C6H12O6) is then changed to starch (nC6H10O5) and stored temporarily in the leaf.
Respiration is the process by which a carbohydrate is broken down by oxygen to produce energy, carbon dioxide and water. The net chemical reaction is the reverse of the photosynthetic reaction.
Respiration occurs in plants all the time (day and night). It is slower than photosynthesis, so photosynthesis is the main reaction in the daytime. At night respiration is the only reaction; photosynthesis is not possible in the dark.
Oxygen gas ignites a glowing stick of wood.
A potassium hydroxide solution absorbs carbon dioxide from air.
The color of a bicarbonate indicator shows changes in the carbon dioxide level in air as follows:
To test a leaf for the presence of starch, the leaf is:
A blue-black color then indicates the presence of starch.
Leaves are destarched by keeping the plant in the dark for 48 hours.
Fig. 1. Apparatus.
The green water plant was first kept in the dark for several hours and then exposed to sunlight for several hours.
Before exposure no gas was produced. During the exposure a gas was produced which ignited a glowing piece of wood.
Exposure of the green water plant to sunlight caused oxygen to be produced.
For an example of how to write a report of this experiment click here.
A plant was kept in the dark for 48 hours and several of its leaves were then tested with iodine. No blue-black color was observed.
The plant was then exposed to sunlight for 6 hours and several of its leaves were tested with iodine. A blue-black color was observed.
Starch is produced when a plant is exposed to sunlight.
Fig. 2. Apparatus.
No blue-black color was observed in the leaf covered with light-proof paper. A blue-black color was observed in the control leaf covered with plain glass.
Light is necessary for starch production.
Fig. 3. Apparatus.
No blue-black color was observed in the leaf covered with the flask containing potassium hydroxide. A blue-black color was observed in the control leaf covered with the flask containing plain water.
Carbon dioxide is necessary for starch production.
Fig. 4. A varigated leaf.
A plant with varigated (partly green and partly white) leaves was used.
The plant was exposed to sunlight for 6 hours. Then several of its leaves were tested with iodine.
A blue-black color was observed only in the green parts of the leaves, not in the white parts.
Chlorophyll is needed for photosynthesis.
Fig. 5. Apparatus.
Three glass tubes, with two tubes containing leaves as shown, were exposed to sunlight for two hours. The colors of the indicators in the tubes were observed at the start and at the end of the two hours.
|Time (h)||Tube A (dark)||Tube B (in sunlight)||Tube C (control)|
Discuss what the results show about the variation of carbon dioxide in each tube, and what the results imply about the occurrence of photosynthesis and respiration in each tube.
By R. H. B. Exell, 2001. King Mongkut's University of Technology Thonburi.